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Fuel Injection Tech Tips
I decided to create this document because of the difficulty I had in finding practical information about running nitro in my dragster. Many conversations and interrogations, along with a little actual experience make me confident that I've at least captured the facts here. I began reporting my exploits with light loads on my website. This prompted quite a number of people interested in doing a similar thing to write and ask for advice. I've done pretty good over the years now running 20-98%.
These notes are NOT for the person wanting to go out and run 98% in their blown ride. This is for the racer who is set up to run injected methanol and is wondering about how to run some nitro in the mix.
There are two basic ways to run nitro; the "AFR method" and the "Volume method". The AFR method is covered here and works well up to about 80% nitro. After that, things get out of control and become very unpredictable. At that point, if you don't make the jump to the volume method, you will start melting things. The AFR method uses the stoichiometric values for nitromethane and methanol and provides a reliable guide for tuning it. The math works! The volume method is a different beast entirely however and is difficult to make sense of at first. I'll cover the volume method here in a future update.
I'll try to hit all the common questions here on the AFR method so you'll know the basics.
Why run nitro?
How risky is it?
There are quite a few reasons NOT to do it. If you start pushing the percentage higher and higher, you *WILL* burn something up if you don't upgrade your hardware along the way. Stay on the safe side and only run what you can safely run. It will be tempting to run a little more to make that milestone ET or MPH, but resist. You can run any percentage you want if you buy the right parts (pump, nozzles, etc).
Don't end up spending your money replacing torched parts that could have been spent buying the correct components to run it right the first time.
Light loads do not require anything special in the bottom end, just the right fuel system components. Heavier loads will require a beefy bottom end along with aluminum rods. The aluminum absorbs the shock loads that are generated by high percentages. Steel rods transfer this shock to the crank, and you may end up running over it or spinning a bearing. Plan on running steel main caps (not stock cast caps!) if you are running more than 25%. You may consider using a main girdle too. I was running stock 4-bolt main caps on my 350 Chevy, and 50% broke the #2 & #3 cap right in two. Of course the bearings spun as well and messed up the rod bearings. If they hadn't been pinned, the rod bearings would have spun too. Don't even consider running a cast iron crank when running any nitro.
The killer when running nitro is detonation. A lean condition can melt pistons, but more commonly, a lean condition will promote detonation which can break parts, spin bearings, and blow head gaskets. You will burn a bunch more fuel volume than normal at elevated percentages, and if you don't compensate and watch closely, it could be all over for your motor. If your existing fuel system is marginal (worn pump, small nozzles, etc.), then you won't want to run any juice. You need more volume above 33% nitro, and a marginal methanol setup just won't do.
Is it legal?
Many tracks do not allow it at regular events because of the increased insurance that must often be purchased. At big events, oldies, and nostalgia events, it's usually okay. When in doubt ask the track operator. Know though, that the official answer may have to be "no". If you just run it, they may know, but won't say anything unless you ask permission: the "don't ask and we won't have to tell you no" policy. Personally, I always ask permission or at least let the track manager know what I'm doing.
If you are running more than 10%, the smell will be very obvious. You can buy scented fuel additive that will help cover it up, but at 20% or more, that won't help. If you have to hide it, you probably shouldn't be doing it.
The NHRA rulebook says nitro is illegal for anything but the pro nitro classes. The Junior Fuel rules state "Methanol only". It is okay in the Nostalgia Eliminator classes however. To run it successfully in the nostalgia brackets requires experience (quite a few laps), and a constant fuel mix (see below). Until you get a handle on it, you may find it very hard to be consistent while running it. Once you figure it out, you will be very consistent!
Aside from the possible peril to your engine parts, there is the consideration of the safety of your crew and those around you.
An engine with a very small amount of nitro in one cylinder can be extremely dangerous. Always spin the motor in the pits with the fuel off (or disconnected) and the plugs out to dry it out. If it's a hemi-style motor, back it down to get stray fuel out of the cylinders. After doing this, you can even squirt some methanol or gas in through the spark plug hole to dilute anything in there and spin that out as well.
Once the engine is running, you're ok...it's cranking over to start or rotating the engine by hand in the pits that's the problem. I once witnessed the entire front corner of an engine block being blown out at the touch of the starter button. You may have heard stories about entire cylinder heads being ripped off a motor and shot across the pits or into a trailer pitted next to someone. Who knows where those huge, heavy chunks of sharp metal will go. It's scary and it can happen to those who aren't careful.
If the engine has run on nitro and been turned off, treat it like a bomb. The easiest, most effective way of diffusing the bomb is to remove the spark plugs. Without pressure, the nitro won't hurt anyone. Consider removing the plugs from the motor at the top end before towing back. If the engine were to rotate just a bit on the tow back, you could have big problems. Your crew should be in the habit of checking to make sure the plugs are out before running the valves or even putting that breaker bar on the motor.
Once your percentage is over 30% or so, please consider the use of a system to start and run the motor on pure methanol or gas that will allow you to switch the motor over to the nitro mix for the run. There are many ways to do this. The easiest and most popular being another set of nozzles gravity fed by a small vessel quick-connected to the motor. The idle adjustment comes via a needle valve or nozzle orifice changes. The main fuel system with nitro isn't turned on at all until the motor is running. Once the cackle begins, the vessle is detached and you're ready to rock.
Can I run nitro with my setup?
When starting out with nitro, you will want to richen things up a bit. Percentages under 33% actually require a bit leaner setup (more on that later), but that is under optimal conditions. Many things determine cylinder pressure and where detonation will start to occur with your combination. Static compression ratio, camshaft overlap, piston dome, ignition timing, piston "squish" (or quench) height, cylinder head flow...it all adds up and every combination is different. If you start rich and move leaner, you will avoid trouble if you advance slowly and pay attention to detail and to all the warning signs as you go.
You must already be setup to run injected methanol (not gasoline). How big are your injector nozzles? How big is your pump? What is the smallest (richest) pill you have available? What size barrel valve spool are you using? As a rule of thumb, if you are normally running a 65 or bigger pill on methanol, then you could run 20% just fine with a smaller pill. Once you are running a 50 pill, you are just about out of available pump volume. There is such a thing as a "blank" pill with no hole in it. They call this "running the pump". At this point, you are at the mercy of the health of your pump and have no tunability at all. Just buy a bigger pump! Pumps do wear and flow less and less over time between rebuilds. That means your engine gets leaner and leaner. Eventually you'll hit trouble if you don't keep a close eye on things.
You could call an injection expert and tell them what you want to do and get help getting a system tailored to your needs. There are quite a few choices to make. Get some guidance! They will want to know the size of your motor, barrel valve, nozzles, pump, normal pill size, etc. They will do some calculations and tell you if you have enough headroom to run nitro. The very best thing is to send your pump, nozzles and bypasses to myself or some other injection expert and have them check the flow of everything and deliver some different tune-ups for you. This will quickly help you get to the sweet spot safely.
Here was my regular methanol setup when I started out:
362" Small Block Chevy
Hilborn 2-3/16" Injector
18A Hilborn nozzles
#75 pill on an average day
#54 Barrel valve rotor
5800RPM stall converter
80 passes on a rebuilt pump
It turns out my nozzles were a bit on the big side. That's good! This means they are adequate to deliver extra volume and I'm pretty safe to run up to 25%. I did need to purchase some smaller pills however. The rule of thumb generally is to go five steps richer when running 20%. I think that is horribly excessive and believe 2-3 steps are plenty. Most people start out running so rich, the motor doesn't make good power and they will barely see a performance gain with 20%. A "step" is usually considered .005" and 3 steps richer than a #75 pill is a #60 and a safe place to start. Based on the area of the orifice in the pill, that is a HUGE difference in fuel delivery! When I decided to increase the percentage to 50%, I had to move up to a bigger pump (#1) and bigger nozzles (36AS) that deliver more volume to be safe. Also recommended is a larger barrel valve rotor (#56) at that point that will flow more fuel off the line.
All of this information is very relative to your setup. In my case, I found that my baseline pill on a standard day with pure methanol was a #75. Using the same hardware and running 25% nitro, I started with a #60 and I leaned it from there ending up right back at #75 after making some adjustments to timing. After upgrading my hardware for more nitro, my baseline pill with 50% nitro became a #90. Basically, my pump and nozzle volume were doubled, but the fuel demand did not. Therefore, I needed to send more back to the tank than I used to. My bigger setup worked great up to about 75%.
If you are running mechanical fuel injection, you really need to acquire the knowledge to calculate your own tune-up on paper eventually. It'll take the purchase of a fuel injection manual and some math. After doing it a time or two, you'll see it really isn't that hard. You'll also be amazed at how little difference the weather makes on your engine and how huge of a difference a single pill change can make. We've recently put tune-up calculators here on our website to help you out. They are web based and will do all the hairy math for you! No need for a spreadsheet program or a calculator...just the facts.
What are the characteristics of nitro?
Nitromethane is made from propane, and I read somewhere that if nitro were only 4 degrees less flammable, it wouldn't even have to carry the red diamond "flammable" label. I'm not sure if that is true or not, but if you pour a little out on the pavement and throw a match to it, you'll see that it isn't very flammable at all. Nitro needs to be under pressure and at an elevated temperature, and then LOOK OUT!
Nitro is heavy stuff. While the specific gravity of water is 1.000, pure methanol is .792 @ 68°F (lighter than water) and pure nitro is 1.139 (heavier than water). More on this later in the mixing section.
Nitro and gasoline don't mix well, but people have mixed all kinds of strange things with it like toluene, benzene, xylene, and other nasty stuff. People even mixed it with hydrazine, but apparently, there are now laws against that. It seems that nitro becomes very shock sensitive when hydrazine is added. So shock sensitive, that just dropping a container of it could make a big boom. It's more commonly mixed these days with good old methanol. While running large percentages, it can be very hard to get the motor to start, but you won't notice any difficulties starting with 70% or less. Nitro can be clear or may have a yellow tinge to it. The specific gravity is very unstable, and as the temperature changes, the apparent percentage can change a bit also (see more on mixing below). We all know what nitro smells like. From a distance it's sweet stuff. Up close, it'll make your eyes water and you won't be able to breathe. Even a cheap $20 3-M respirator is nice to have while tuning the motor or sitting in the cockpit unless there's a good breeze happening.
How much difference will it make in my ET/MPH?
Can you eventually be consistent running nitromethane? YES! In fact, you have a big advantage running nitro because the amount of atmosphere it needs to run is hugely reduced.
In a gas motor, typical air/fuel ratios of 12-14:1 are common. That's a lot of air to very little fuel. As the ambient atmospheric conditions change throughout the day at the track, the air/fuel mixture is effected and the car's performance changes drastically as a result.
An alcohol motor typically likes about 5:1. It is much less effected by the weather because so much LESS air is being utilized by the motor. The fuel brings it's own oxygen to the party!
A nitro motor running 100% likes a theoretical ratio of about 1.2:1 while a motor on 50% enjoys about 2.6:1. As you can see, the weather has to change an enormous amount throughout the day for the nitro motor to suffer any effects at all. The tune-up you choose in the morning will likely last all day.
Lots of folks only have pills in increments of .005". If you change from a #100 pill to a #105 pill, you are making a 10% change to the amount of fuel going to your motor (calculate area of the orifice not the diameter: .00866 sq.in. vs. .0078 sq.in.). It takes a considerable amount of weather to make a 10% difference in the air fuel ratio! To make a pill change like this to account for changing weather at the track is pure folly. The secret is not to have a zillion pills in fine increments, the secret is to find a setting on the rich side of "just right", and then leave it alone.
Back to consistency...once your fuel system is dialed in reasonably close, the biggest contributor to consistency is TEMPERATURE. From one pass to the next, your engine, cylinder head, motor oil, fuel, transmission, and rear end temperatures need to be as close as they can get to being the same every time.
As your cars goes laps, the motor and trans will become heat saturated. They'll cool down quick after the first lap as other items on the car soak up the heat. After two or three passes however, there's no where for the heat to go except off to the air. That is slow. You may see that as your car heats up and oils gets thinner in various components, you'll go quicker. Also as your motor gets hotter, it will make more power. These things are obviously not good for consistency! Consider changing the oil after every lap to get heat out of the motor. Run a trans cooler (off board if necessary). Keep track of your fuel temperatures and where your jugs are sitting (in the sun?). Put a cylinder head temperature gauge on the car and use the fuel shutoff to warm the car to a particular temperature before each pass. If you can manage temperatures well, you'll be extremely consistent running nitro.
Where can I get it?
How much does it cost?
In my area, it sells for $35+ per gallon when purchased in 5 gallon cans. If you can get a bunch of your buddies together and buy a big drum, it can be had for as little as $20 a gallon. To save some money, you might want to warm your motor up using straight methanol, then drain the tank and put the nitro in. Fire it up and run it just for a minute to get the nitro up to the nozzles and get the barrel valve adjusted for idle. An even cheaper way to do it is to figure out where your barrel valve is set optimally and use a leak down tester to measure the flow. Write it down, and the next time you decide to run some nitro, just set it with the leak down tester. No need to waste precious nitro during the adjustment process.
You could also leave the barrel valve set for nitro, put a big pill in it, and warm it up on the cheap stuff by leaning it with the shutoff valve to get it warm. Drain the tank, put in the nasty-juice, and fire it up again just to get the nitro through the system. Don't forget to change the pill back!
How much should I use?
How should I mix it?
You can mix it by volume and you'll nearly always be safe concerning temperature as long as you keep a couple of things in mind. According to the chart, if you carefully mix a batch of 20% (e.g. one gallon of nitro to 4 gallons of alky for a total of 5 gallons), it will only be truly 20% at 68°F. On an 80° day, your mix just became 18%. That's okay, because usually on a given day, the temperature gets warmer (and safer) as you go. If you took that 20% by volume batch and ran it with a fuel temperature of 50°, now it's a hotter batch and could hurt parts. To see how temperature effects your mix, check out the nitro test chart.
If I know I'll be racing on a roughly 80° day, I'll mix up a 22% batch by volume, knowing that it will really be about 20% in the tank.
It is best to mix up the batch at least the day before. When nitro is mixed with methanol, it creates an endothermic reaction that causes the fuel temperature to drop...sometimes as much as 15°. You'll go out and go like hell and then wonder why you start slowing a little later on. It may be because the fuel is warming up and specific gravity of the nitro has changed.
Best of all to use is a hydrometer kit for around $200. We sell the best kit available! You get a rugged padded case, big glass cylinder, thermometer, and four hydrometers, plus charts and instructions. After mixing a few batches and having some left over, you will not know exactly what you have left. Once mixed, left to sit and be forgotten, a hydrometer is the only way to know what you really have in that jug. Buy a hydrometer!
One other thing you must consider, is that even if you are mixing by volume, a certain percent mix by WEIGHT is what you are trying to achieve. A hydrometer is basically measuring the weight of the fuel. Since nitro is heavier than methanol, to get a true mix, you must compensate for the extra weight of the nitro. In other words, the only time mixing by volume is the same as mixing by weight is if the two components being mixed have the same specific gravity. This is not the case when mixing nitro and methanol, so you'll need to figure in a correction factor.
No matter how carefully you mix by volume, (right to the fraction of an ounce) if you check it with a hydrometer, it will be a hotter mix than you want. You should compensate for this since it can end up being several percent higher. As the temperature increases, this becomes less of an issue, but at 68°F, the following holds true:
Weight vs. Volume correction
By weight By volume Correction
10% 7% -3%
20% 15% -5%
30% 23% -7%
40% 32% -8%
50% 41% -9%
60% 51% -9%
If you are trying to mix up a true 20% batch, then subtract 5% from this and mix for that by volume. So, use 15% for the desired mix in the examples below. If mixing a true 50% batch, then shoot for 41% to mix by volume. If you check it with a hydrometer, and the temperature is close to 68°, it should be danged close to 50%.
If you don't have a hydrometer, the math on the mixing can be a little tricky at first. It is easiest to mix up 5 gallon batches. Just don't trust the crappy marks molded into the side of the jug. A one gallon milk jug works fine and won't melt from the alcohol/nitro mix. The half gallon milk jugs work great for making odd percentages, you can mark the jug every 2 ounces or so. Check your calculations twice! Here is an example:
Desired mix: 22% nitro (truly 28% by weight due to the 6% correction factor)
Total finished volume: 5 gallons (640 ounces)
.22 X 640 = 140.8 ounces of nitro (or 1 gallon & 12.8 ounces)
So...put one gallon of nitro in your empty 5 gallon jug. Use a pyrex measuring cup or other nitro-proof container stolen from the kitchen, and pour 12.8 more ounces of nitro in the one gallon milk jug. Fill the milk jug up to the mark with methanol and pour it in the fuel jug. Put 3 full gallons of straight methanol in the fuel jug. Shake well. That's it.
Partial batch conversions can be much trickier. For example, you have 3.5 gallons (448 oz.) of a 20% mix. You want to try 25%. How much nitro and alcohol do you add to the 3.5 gallons to make 5 gallons (640 oz.) of 25%? Are you sure? Here is the formula I've come up with:
X = G(P) - S(N)
X = Nitro to Add
G = Finished Quantity
P = Finished Percent Mix
S = Lesser leftover Quantity
N = Percent Mix of Lesser Quantity
Using the example above and solving for X:
X = 640 ounces(25%) - 448 ounces(20%)
X = 640(.25) - 448(.20)
X = 160 - 89.6
X = 70.4 ounces of nitro
The rest would be methanol...
448 oz. of 20% + 70.4 oz. of nitro = 518.4 oz.
640 oz. total - 518.4 oz. = 121.6 oz. of methanol to add (one gallon with 6.4 oz. poured out)
Remember, that this "25%" we just mixed up by volume, is REALLY a bit more than 30% because of the correction factor explained above.
Seem complicated? A pain in the ass? Then buy a hydrometer
If you do have a hydrometer, then you first have to start with checking the purity of the methanol you'll be mixing with your nitro. Methanol is hygroscopic, meaning that it absorbs moisture right out of the air. Since water has a higher specific gravity than methanol, you can check the purity and make sure it hasn't absorbed a lot of water by seeing if it is right around .792. If you keep it in air-tight jugs and rotate your fuel stock, it'll probably be right there. If you're seeing higher than .800 at 68° with what should be methanol, then it's time to pitch it - it has water in it.
I thought I had to run richer on nitro!
As I mentioned earlier, most experts recommend running a pill up to 5 steps richer when starting on 20%. That is just too much. Mainly, they don't want you to burn your stuff up and call them nasty names. I don't want that either, but you should know the truth here.
I did all the calculations to figure the proper mixtures at various lower percentages, and I thought I had screwed up. On paper, a 15% load actually requires one step LEANER! What?!?!? How can this be? I didn't understand what was going on, but the light bulb finally came on.
Remember that the air/fuel ratio (AFR) we commonly refer to is pounds of air to pounds of fuel: feathers to lead. Weight, not volume. The AFR for alcohol: ~5:1. The AFR for 100% nitro: ~1.3:1.
If you saw a graph of the specific gravity change of the fuel (weight) for 0 through 100% nitro and also the AFR required for 0-100%, the lines would cross at about 33%.
From 0% to 33%, the change in the specific gravity of the fuel rises quicker than the change in the required AFR. The fuel is getting heavier QUICKLY at low percentages so you get more pounds per minute for the same air flow. The required AFR is not much different than straight alcohol. You actually have to go leaner to hit the optimal AFR you need.
After 33%, the AFR is changing quicker than the density of the fuel - more fuel is required to make up the difference. If you calculated it all out for 15%, you will see you have to lean the mixture from the optimal alcohol jet. This is probably why many people who run 20% say they don't see much of a gain in performance. They are way way way too rich and cancelling much of the benefit.
However, you NEVER want to calculate your optimal mixture on paper, put that in your car and try to run it. Always be richer a couple of steps to start. The one variable in the equations that you will be guessing on is the volumetric efficiency (VE) of your motor. This is a measure of how well your cylinders are filling. A typical tunnel ram/hat injection setup can be better than 100%. On my lower RPM, tall stack setup (optimal length for tuned pulses) I see something around 112%. That is overfilling the cylinders by 12%! That means I need more fuel going in there to mix with that extra air. I figured the approximate VE out by finding the optimal pill on the track and then back-figuring what my VE is.
Other reasons for keeping it rich to start is because depending on a lot of factors, an optimal mixture may rattle your bearings out from detonation. You may need to use colder plugs, thicker head gaskets, less timing, on and on. You'll learn these things by slowly getting closer to "optimal" while watching the spark plugs, the time slips (detonation robs power), and leaking down the cylinders and inspecting the oil filter to watch for trouble.
So...know where optimal is, and then start rich and sneak up on it.
Everybody's motor is different, so making a prediction on the amount of fuel it needs is an educated/calculated guess. There are some factors that really matter when running nitro and their impact varies. When someone asks me how much nitro they can safely run, these are the things that I ask. Most of these effect either the volumetric efficiency of your motor, or the dynamic compression ratio. Some of the biggies and how they effect the situation...
- Static compression - Since most who want to try nitro are currently running alcohol, it isn't uncommon for someone to have a 14:1 or 15:1 compression ratio. This is a bad place to be running 20% or more because if coupled with the wrong cam (see the next item), it'll detonate itself to pieces. The fix is not always putting a thicker head gasket in place (see "squish" below). There are some easy ways to lose compression. Grinding the valves or seats (or both) to drop the valve faces .020" deeper in the head can add several CC's alone to chamber volume. Milling the piston domes can make a huge difference. Most piston manufacturers allow up to .030" to be milled from dome faces. Changing the head gasket .010" can mean up to half a point (.5) of compression. Rounding the edges and polishing the casting texture out of your chambers can be worth a CC in itself.
- When does the intake valve close? - If the intake closes really late, the dynamic compression ratio will be lower. If the intake closes fairly early as is common in a small duration, tight overlap cam, the effective compression will be much higher. Someone with a lot of static compression can get away with it if the intake closes late. This and static compression really go together. For example, a combination of 14.5:1 compression and a cam with 260° duration @ .050" and an intake valve that closes at 58° will be asking for severe detonation with nitro. The same motor with a cam in the range of 285° @ .050" and an intake that closes at 70° will be much safer. A lot of cylinder pressure is lost with that valve hanging open longer and it helps the situation.
- Stack length & RPM - There is a formula for calculating the optimal stack length for a particular runner cross section and engine RPM range. The higher the RPM you rev, the shorter the stack needs to be to reinforce the pressure pulses in the intake runner. If someone pulling 7500 RPM is running the super-short Jr. Fuel stacks, they aren't filling the cylinders very well and dynamic compression will be quite a bit lower. Someone pulling to 9,500 with 12" of stack on their injector is going to suffer as well. Basically, when calculating a tune-up, I take into account the stack length by changing the volumetric efficiency in the formula. A high VE means more fuel is needed. A properly tuned stack is worth a 20% VE gain over a badly mismatched setup. That doesn't mean 20% more performance, but it could...more fuel burned means more horsepower.
- "Squish" or quench - This refers to the open space above the flat area of the piston when it's at TDC. This is deck height + head gasket thickness. Common belief is that this area should be as tight as possible to prevent two flame fronts from propagating and promoting detonation when they collide. If you run aluminum rods, you'll need some extra space for rod growth at high RPM. I like to keep my squish less than .060" but more than .030". If you have a .015" deck height already and decide to throw a .060" gasket on your motor to shed some compression, your squish is big and this could go against you later. This is why if you need to lose compression, sinking the valves or going with a longer cam may be a better answer.
- Timing - Nitro burns slow and theoretically you should need more advance. However, if you're setup is not optimal in some of these other areas, you may never be able to get your timing up where it should be without aggravating a detonation problem. Since timing is easy to adjust, always whack about 10 degrees out of what you normally run with alcohol and sneak it up a couple of degrees at a time. If you quit seeing any top end MPH gain (and assuming you don't run a top end retard mechanism) then stop! No need to run any more timing and hurt things. Detonation at the top end will hurt performance and you'll see a drop in MPH over a lower timing setting. Also, the parasitic loss of power from more timing can hurt top end performance. The earlier the spark, the more resistance the piston must overcome on it's way up to TDC on the power stroke. This is why many run a retard mechanism. They run lots of timing for the bottom end, and then roll it off on the top to help eliminate this slight loss.
- Head flow - If someone tells me their heads are "ported to the max", they have huge intake valves, and they flow amazing numbers, I naturally assume their VE will be higher and they will need more fuel. Poor flowing heads will benefit a great deal from a high-speed bypass. When the motor quits breathing (at a lower RPM with low flowing heads), you must take away fuel to match the diminished air flow.
Take these things into account when making your decisions about how much to run and how to set up your motor to run with nitro.
Running your car on nitro
As mentioned above, your biggest enemy is running the car lean and promoting detonation. Assuming you are setup close, that can still happen: Dirty or clogged nozzle(s); clogged fuel filter; undersized fuel tank vents; collapsed hose on the suction side of the pump; cavitating pump (air leak); worn/failing pump; loose linkage on the barrel valve; or severe weather change (more oxygen) caused by lower temperature, lower humidity, or higher barometric pressure.
Use a big fuel filter and blow it out regularly. Some paper-type fuel filters are not nitro safe! The best units use a stainless screen mesh. If you use hose and not tubing on the inlet side, use a spring inside to keep it from collapsing. You can buy the spring material by the foot from Summit for different size braided stainless hose. Flow your nozzles using a leak down tester. They should all be the same within 1%. For example, a clean set of nozzles may leak 18% individually with a tester at 100psi. Check them often and if they leak less, remove it and clean it out. If you buy a used pump or have an old one, send it in and get it reconditioned. We usually only charges $95 to rebuild one, and can turn them around in a couple of days. Don't buy an old pump from someone on Ebay and then expect to bolt it on and run it. That is a great way to pick up a cheap core, but then send it in to someone and get it flowed so it is a known quantity and you can trust it. If you know the exact flow, you can calculate the tune-up before you ever get to the track.
The old timers say to adjust the barrel valve as follows: While idling, lean it until the engine speed begins increasing. Now, slowly richen it and continue richening while the idle speed drops. You will reach a point where continuing to richen the mixture will not cause the RPMs to drop anymore. Do this several times until you sneak up on that exact spot where the RPMs quit dropping while richening it. Now turn the link in the rich direction two more flats. Not very precise, but it will get you in the ball park. I found a setting that produces good throttle response, lets the motor get warm without letting it get too hot (I check the heads with a temperature gun), and doesn't put too much fuel in the oil. Then I check the barrel valve with the leakdown tester and write down the magic number. The best results for my 362 CID setup running 20% nitro was 18% leakage. While running 50% nitro, my barrel valve liked to be at 31% leakage. Leakdown testers vary, so of course your number will be different. Also, if your idle check valve opening pressure is different, your leakdown percentage will be different.
Nitro won't burn well unless the motor is warm. When the front of the cylinder head is just hot enough that you can't hold your hand on it, it's ready. Speaking of warm, some folks put covers over their fuel tank to help keep their fuel temperature stable (and low). I set my jugs in the sun in the morning to get them warm. I'm not concerned with the power loss I'll see with getting the fuel up to 80°F, but I am VERY concerned with consistency. I figure if I get my fuel warm to start with, things won't be changing much over the course of the day. If you are in a dial-in situation, that can win rounds for you. Of course, if you are racing into the late evening or night, your fuel cooling off can speed you up and you might break-out.
You are probably running a high compression ratio with your methanol setup. This is okay with less than a 50% load. If you normally run 40° of timing, you'll probably want to back off to 34° or so to start out, and then sneak it up while carefully watching the plugs and the time slips. If the MPH drop off, you've past the point of optimal ignition timing. I've run 25% with a 14.3:1 static ratio and had no problems, and 50% worked great with a 13.2:1 setup. I ran 67% with an 11.5:1 setup and bumped it up to 85% with 11.2:1 with no problem. I was running a very long overlap cam that hung the intake valve open a long time and this lowered my dynamic compression ratio. If you get nervous, you can always throw a thicker head gasket on it. I lose .4 off the compression ratio for each .010" I add to the deck on my motor. As you probably know, copper head gaskets come in a wide variety of thicknesses. If you get too thick on the gaskets though, you'll kill the quench effect of your combustion chamber and may actually induce detonation.
Those rubber parts...
Your fuel system is likely full of o-rings to seal it here and there...check valves, filters, port fittings, etc. Depending on what those o-rings are made from, you may spring some leaks over time!
There are three really common materials used for o-rings. Here they are along with some other names you may see for them and their compatibility with various fuels.
As you can see, EPDM o-rings are the only kind to have if you plan to run nitro. Good thing they are fairly easy to find. Viton o-rings (usually brown) will be toast in a hurry, so don't even try it. If you are running low percentages of nitro, you can get away with using Buna-N for a while, especially if you don't leave the fuel in it for more than a day or two. If your Buna-N o-rings do swell up, you can dry them off and lay them out for a few days and they will return to normal size. Viton goes to hell in a hurry, and doesn't recover very well. Gasoline will cause EPDM to shrink beyond recognition and nitro or alky will cause Viton to soften and swell up.
Also, don't forget that you will get plenty of fuel in your oil after a few passes, and rubber seals and gaskets in your motor will suffer. The o-ring behind your fuel pump, rear main seal, valve cover gaskets, etc. may freakout over time. Keep an eye on them and stay compatible where you can.
There's a catch! It turns out that while EPDM o-rings are compatible with nitro, they are NOT totally compatible with WD-40, Tri-Flow, or any of the other petroleum based preservatives. If you blow out your fuel system after use and chase it with a corrosion resisting fluid, you may find that your EPDM o-rings have swelled and become larger later on. Rinsing them off and drying them won't bring them back to normal size either. O-rings are cheap, so I bought a bag of each type that I use and I just replace any that won't easily go back where they came from. You don't want a ten cent o-ring to keep you from making the call for the final round, so keep extras on hand.
When the fun is over
Nitromethane isn't corrosive, but methanol is. You will NOT want to leave the stuff in your tank, pump, and lines very long. Things can start to get fuzzy in there after 24 hours.
The best bet is to drain the tank, blow all the lines dry, and blow some WD-40 or Tri-Flow through them. Blow air and Tri-Flow through your pump with the engine cranking. Drain the fuel filter and blow it dry. I remove the hoses from the injectors, wrap them in a rag and blow air and Tri-Flow through the barrel valve while jacking the throttle to get all the fuel out. Blow air back through your tank vents with the cap off to get any fuel out of them. Spray a 2 second blast of AMSOil "Fogging Oil" down each injector tube while cranking the motor over with the throttle wide open. Disassemble, blow out, and spray your bypass canisters.
Another solution is to drain the tank, pour in a little racing gasoline, lean the barrel valve a few turns, put your hugest pill in, and fire it up. It'll still be really rich, but this will chase out all the nitro and leave a friendly, preservative film of gas in everything.
The High-Speed Bypass
Lots of folks are shy of the old high-speed bypass, and quite a number of racers don't even run one. Don't be fooled into thinking that a high-speed is going to make a HUGE difference for you unless you run an Enderle style barrel valve that accepts the screw-in main jets. They usually create a rich condition on the top end. Hilborn and Kinsler pills in the main check valve can still benefit from a high-speed, but it isn't as drastic. If you run poor flowing heads, a high-speed will be a big benefit to you. A properly tuned high-speed with the correct pill might get you a tenth and a few MPH. You will not gain anything by simply adding a high-speed bypass, there is more to it than that. Opinions vary of course, and this is not specific to running nitro, but you won't want to mess with a high-speed bypass while running nitro until you have this figured out.
The pumps we run on injected setups are positive displacement and have a very linear output. If you graph it, just draw a line up and to the right at a pretty steep angle. The more RPM, the more they pump, and it increases proportionally. This is a good thing because as your motor's RPM increases, it also needs more fuel. Unfortunately, the air flow curve on even the best motor is not simply a straight line up and to the right. It has humps and bumps in the graph as the RPM climbs, and then at some point, you exceed that RPM where your induction system efficiency has peaked. Now you've reached the "knee" in the air flow curve, or that place where that line up and to the right suddenly makes a break and gets much flatter. But...your fuel pump is happily pumping more fuel in and the fuel curve continues up and to the right. The engine is now way too rich at the top end and making less power than it should.
The Poor Man's method: This method assumes you don't have any onboard data acquisition and little or no dyno time with your motor. All you have are time slips, your ears, and 5 or 6 test passes.
Those that do not run a high-speed have probably found a pill that averages the pump output with the fuel curve. At some RPMs, the motor is a little bit lean, and at others, it is a little bit rich. To make the top end really perform, they have to use a pill that is much leaner than the bottom end and midrange would really like. They are sacrificing a bit of performance down low so that the top will charge. Let's call this their "no-bypass, best performance pill". At this stage, there is really nothing for the high-speed to do. There is no excess fuel at the top end that needs to be bypassed, so incorporating a high-speed at that point would be risky, and almost certainly make performance DROP, not increase! Before running a high-speed, you first have to find a specific sweet spot, so disable the high-speed to start with. Just turn it around (swap ends) in the return line, and it will be ineffective for now.
With the high-speed bypass disabled, you will now need to richen things up a step at a time until you get the best numbers to half-track. Ignore the poor top-end MPH for now. At your highest RPM, there will now be more fuel than your motor can burn, and it may begin popping at the very top-end or quit pulling. Until you reach that point, the high-speed bypass will do almost NOTHING for you. Usually, it will only take a couple of richer pills to finally achieve the best low-end numbers. You will see your ET and top speed fall off, but do not despair! The high-speed will fix this for you. Your motor is now making peak torque on the bottom and mid RPM range, but falling down on the top end from all that excess fuel. NOW you need the high-speed bypass.
It can be pretty hard to know where to have one come in and what pill to run without having your setup run on a flow test machine. Before you turn that high-speed bypass around and go wild with it, hook it up to a pressure regulator with a gauge to record the opening pressures with different shims, and make yourself a reference chart. Air and fuel will bypass a bit different, but it will get you close. Don't just take someone's word that 4 small shims will make it crack open at 46 PSI. CHECK IT! Springs vary, and the depth of various bypass canisters are different. Assuming you called an injection expert and they provided a "cracking pressure" as part of the tune-up they gave you, you should have an idea of how many shims to start with. For example, if the fuel system guy told you that you should start bypassing at 48 PSI, and 1 big shim and 3 small ones makes your bypass start leaking at 50 PSI, then that's a great place to start. Go make a pass and study your time slips. Remove one small shim. Keep removing one small shim at a time until you see the MPH level off and quit improving. You should now see a very strong bottom end, and your top end charge should be back. You might want to lean things one step now on the pill and see if you speed up or slow down. The fuel curve for your motor may be exceptionally straight.
Other Ideas: You could figure out all of this on a dyno, too, by making many test pulls with various settings. Dyno time is expensive though, and you can get really close while armed with just an airflow chart that shows how much air is passing into your motor through a pull. Assuming that the optimal air/fuel mixture remains constant (it does), you can quickly find the "knee" in your fuel curve and the place where your high-speed would be the most effective. If the airflow/RPM graph starts to flatten out at 7250 RPM, that is where the high-speed should crack open. "What pressure is that then?" you ask. You can calculate this on paper fairly easily if you get help with the math (buy a manual or call the experts).
If you have an EGT probe(s) on your motor and can record the data during a pass, you will get an idea of where the "knee" is in your curve too. The temperature graph through the RPM range should somewhat resemble the shape of your fuel curve.
The high-speed is the best way to torch your motor with nitro. If it ever pops out the injector tubes, GET OUT OF IT, it's too lean! Throw a small shim in it and reduce the high-speed pill size. Do not intentionally keep leaning it until it pops out the intake...start rich and quit removing shims when performance improvement stops.
Once you have found the optimal pill and high-speed setting, you will eventually need to change things a bit for the weather. The recommendation is that for every main pill step (.005) decrease, (richer), also add one small shim to the high-speed to raise the bypass pressure. This is really guess work. If you spend a couple of hundred bucks getting your system flowed, your high-speed will be right on the money and your shims can be custom made for each different tune-up. The bigger the main pill, the less system pressure is present and the lighter the spring in the high-speed needs to be in order to open at the same RPM. I can't stress enough that you (or someone) needs to do the math to see where the pill and high-speed should be set.
Scary Story: I was using a Hilborn check valve, and purchased a new Kinsler quick-disconnect style unit. The problem was, the Kinsler unit is .125" longer inside than the Hilborn. If I had not checked it with a regulator and instead simply popped the same number of shims in it as before, I would have began bypassing at 24 PSI and probably would have burned up my motor for sure!
Remember that a spring and poppet style high-speed bypass canister does not pop open suddenly and dump a huge amount of fuel. It cracks open slightly and gradually opens to regulate the top pressure achieved. Electronic lean-out valves coupled with an RPM activated switch are more predictable and less trouble to setup.
I have done some ET/MPH/weight/horsepower calculations, and it looks like 50% nitro gives me about 180 HP at the wheels over straight methanol. Sounds great, huh? It is, but when you start increasing the power output by that much, things change. Suddenly, my converter was stalling WAY too high. The 5800 RPM stall speed increased to 6800 RPM. Not a great thing...lots of torque is thrown away and lost to heat in the converter. I had the converter re-stalled to suit, but now if I plan to go back to straight methanol, things are mismatched again. Speaking of heat, the tranny gets much hotter than it did before, so plan to deal with that. I built an external cooler unit that plugs into the car and circulates the trans fluid through a cooler in the pits. Works great, and no extra weight!
More power and quicker ETs may mean that your top end RPM may be as much as 800-1000 RPM higher. Can your motor stay together putting out more horsepower at higher RPM? Your shift point will likely change as well. On 50%, my car liked to be shifted 1000 RPM lower than it did on just methanol (according to ET). I moved from 4.10 rear gears to 3.73 and shed almost a tenth off my ET from that alone. Nitro likes to be lugged! Gears, taller tires, short shifting...it all works.
You will probably need a bunch more nose ballast now with the added power. Traction may now be an issue, and tire pressures might need to be tweaked...you may now experience tire shake. Also, consider that if you pick up 10 MPH, it will take more distance or more aggressive braking to get stopped. Adding more power will place higher demands on your oil. If you love your motor, feed it a 50W. Get an oil filter that is easily inspected like an Oberg or a System One. Use a coarse filter screen if you run heavy weight oil, and check it frequently for flakes and signs of a spun bearing. They happen when shock loads increase and detonation visits. If you catch a failing bearing as soon as it starts to happen, you can sometimes pull the pan and roll in a new bearing. When I started running 50% and experienced bearing failures, I started keeping a spare set of rod and main bearings in the trailer. Consider running more main bearing clearance than you normally would. After two major bearing failures that cost me crankshafts, I now run .004-.005" on the mains, straight SAE 50 oil, and a Titan oil pump adjusted to 80 PSI. More oil through the bearing means less heat. Note that more clearance is not good if you aren't going to run a heavier oil and a real oil pump. I also went to King bearings. They make a wide variety of oversized bearings to let you run whatever clearance you need. Their bearings are softer, have much thicker layer of material, and even come in a fully grooved version. As long as you run a big pump, fully grooved bearings can be good. Most folks say that the groove removes surface area from the load side of the bearing. That's true, but it helps keep oil in the bottom of the bearing and helps get more oil to the rod bearings. The theory is, you want to move A LOT of oil though the system, and that will help. Sure, you'll lose some power to the parasitic drag of all that oil flying around. Big deal...personally, I'm tired of buying crankshafts and this has worked perfectly for me for several seasons. The bearings all look perfect now after every season of racing.
Since the SAE50 oil is very thick and you are likely running a bronze gear on your mag, you will probably want to preheat the oil somehow. Heat the oil first and pour it in or use a pan warmer. I built a 7 quart steel vessel with a heater probe in it and a heat blanket around it. When the gauge says 180°, I hook up an air hose to the top and connect a hose out the bottom to the oil gallery on the side of the block. I open a valve and it pre-lubes the motor with hot oil. I stop before all the oil is gone from the can so I don't fill the galleries with air (you can hear it). Since I check the filter and change oil after every pass, this helps chase the air out of the filter and gets the new, warm oil into the motor in about 60 seconds.
I started having problems getting head gaskets to seal with 50%. Consider wire o-rings in the block and copper head gaskets a must, along with head studs. I even blew head gaskets with this setup until I put a receiver groove into the head to accept the o-ring. Simply squishing the copper gasket thin where the o-ring is does not work well. The receiver groove in the head makes a superior seal. The stock torque recommendation for the heads is not enough...try 70 ft/lbs, but remember that if your block was honed with a torque plate, your cylinders won't be as round if you start running more head torque later on. A small price to pay in my opinion. Have them hone the block with the increased torque value next time it is in the machine shop. I've been told that using a value much higher than 70 ft/lbs. may cause a stock block to crack or the threads to pull, even while using studs, so be careful.
If there is something weak and ready to break on your car (tranny, axles, crankshaft, etc.), throwing another 100 HP or more at it may just find it. Be ready!
I hope reading the above hasn't scared you out of trying to run a little nitro. I believe that folks just starting out are the least likely to hurt parts because they are so careful. When you've been doing it a while and complacency starts to set in, you are likely to get zapped. If you make a list of things to do before every event (check the nozzles, check the fuel mix, etc.), and stick to it religiously, you'll probably be ok and have a lot of fun.
Spud Miller - Fuel Injection Enterprises