"The Purge"
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6.0 PPH of Nitrous Flow per cubic inch level
I have racers ask me often: What is the maximum amount of nitrous that an engine can handle?? So based on several of my customers with professional built nitrous engines that are set up for “big nitrous”, I came up with this formula. PPH of N20 Flow multiplied by the cubic inches of the engine is a good average answer. You really have to be on top of it...at the 5.373 PPH + per cubic inch level.
The formula above is close and based on real data from class championship winning cars!
- 4.4 to 4.6 PPH per cubic inch is real safe for a purpose built nitrous engine
- 4.8 PPH per cubic inch is safe
- 5.0 PPH per cubic inch is pushing it hard
- At 5.5 PPH per cubic inch the MPH might increase slightly but there won’t be much difference in E.T. You will just hurt parts often in small block applications.
- 6.0 PPH per cubic inch is possible for larger ci engines ran at a richer a/f ratio
The formula above is close and based on real data from class championship winning cars!
A few thoughts
Cold air has a tendency to drop molecules out of suspension. Look at your house windows in the winter time…they are wet. Imagine keeping fuel atomization high in nitrous oxide application. In an over fueled situation you will have some fuel plating out on the port walls and raw fuel has a tendency to pull more atomized fuel out of suspension. And run into the cylinder. Raw fuel doesn't burn until it gets in the pipes and the heat vaporizes it. So way over fueled only raises the dynamic compression ratio and can bring on detonation. The smaller the engine the worse this is on it. This is an extremely over fueled case such as the old NOS style tunes or multiple stages that are over fueled or bigger single stage systems that are over fueled. Combined that with too much ignition timing and cam timing that generates a large amount of low end cylinder pressure and you will be lifting ring lands on a regular basis. That being said timing is the number one cause of damage in nitrous applications. I would be more worried about timing than being over-fueled. They will live a long life even with too much fuel if the timing is right.
Keep in mind engines don't make maximum power at stoichiometric. We are always richer than that. The extra fuel is used as a combustion coolant. Bigger tunes richer air/fuel ratios are used to control detonation, and this is a strategy normally employed. 11.4- 12.8:1 A/F is where they typically end up. Engines accelerate quicker when slightly rich because they make more average torque through the usable RPM band. The air-fuel-ratio that yields maximum power is only a shade richer than the one that is most detonation-prone so always watch your data and keep notes for weather changes.
13.3:1 - Lean best torque
12 to 12.7:1 - Mean best torque
11.76:1 - Rich best torque
11.5:1 to prevent detonation for timing sensitive and big tune-ups
A ratio change from 13.1 down to 11.5 to one will NOT change your HP by 10-percent. Usually when you are close but not lean, you can be pretty far off and not make a ten percent change in power. Now if you’re too lean, that’s a different story. After you get the fuel close you don't have to worry about it too much. Ignition timing is the make or break in nitrous applications. I have seen 1° of timing make up to a 40 hp difference. Timing is much more important! As far as A/F: 11.7 to 12.8 seems to make more average torque through the RPM band and that's what accelerates the car. Smaller cubic inch, high rpm efficient engines might like 13.1 to 13.3 AFR but most of the time that's not the case. The more HP you make per cubic inch, the leaner this might become.
When you get into the 13.9 to 14.7 AFR range after the high gear shift is when they burn up from being “Lean”. That’s been my experience anyway. The heat from the lean condition causes Pre-Ignition. The lean mixture enters the combustion chamber as the piston reaches BDC for intake; the piston next reverses direction and starts to compress the charge. The spark voltage requirements to light the charge increase in proportion with the amount of compression; almost anything can ignite air/fuel mixture at Bottom Dead Center! BDC or before is the easiest time to light the mixture. It becomes progressively more difficult as the pressure builds.
A lean AFR mixture causing glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. Like a spark plug ground strap ect, Igniting the charge while the piston is very early in the compression stoke. The result is for the entire compression stroke, the engine is trying to compress a hot mass of expanded gas. That puts tremendous load on the engine and tremendous heat into its parts. Substantial damage occurs very quickly. This all occurs before the spark plug fires. It causes a catastrophic failure very quickly because the heat and pressures are so intense. An engine can live with minor detonation occurring for considerable periods of time. There are no engines that will survive for any period of time when pre-ignition occurs. The typical pre-ignition indicator is a hole in the piston. This occurs because trying to compress the already burned mixture…. the parts soak up a tremendous amount of heat very quickly. The only ones that survive are the ones that have a high thermal inertia, like the cylinder head or cylinder wall. The piston being aluminum, has a low thermal inertia (aluminum soaks up the heat very rapidly). During pre-ignition the crown of the piston being relatively thin, it gets extremely hot, it can't dissipate the heat, it has tremendous pressure loads against it and the result is a hole in the middle of the piston where it is weakest. You can also have detonation and pre-ignition. Too much ignition advance will cause detonation which will break down the boundary layer that protects the piston from full combustion heat which will expand the piston, scuff the skirt and lift the ring land. Lean will over heat the stressed ring land area causing a hot spot and pre-ignition will melt a hole right down the side of the piston. Unlike detonation.. Most of the time…The only sign of pre-ignition is white smoke pouring out the pipes and the engine quits running.
“There’s definitely a point of leaning it out to where it melts it, or go way past it.. where nothing happens”.
ALWAYS remember to tune your car to run the quickest travel times. If you try to make your wide band sensor look good, you might be giving away some power in the process. I have people call me all the time who have just got a wide band to try to tune with. As they make the meter read a better AFR, their performance went down; you should always tune for best travel times and then look at the meter to see what it says. Your spark plugs will verify if your 02 sensor(s) are accurate. Start out a little rich, retarded and with cold plugs. Trust the plugs…they rarely tell a lie. But here is how the wrong heat range plugs can confuse you: If you keep adding ignition timing until the back split slows down and you still can’t get good color on the ground strap, yet the porcelain base has good color, you probably have too cold a heat range of sparkplug. If you have a lot of heat on the ground strap and the porcelain is very clean, but your wideband says 10.1-11.5 range then you have too hot of a heat range plug… just some things to keep in mind.
Note: Always examine all the data you can. You can never have too much data to base tuning decisions on.
Example of fuel plating out on port & manifold walls and intake pulses in the video below courtesy of Jon Kaase Racing Engines
Keep in mind engines don't make maximum power at stoichiometric. We are always richer than that. The extra fuel is used as a combustion coolant. Bigger tunes richer air/fuel ratios are used to control detonation, and this is a strategy normally employed. 11.4- 12.8:1 A/F is where they typically end up. Engines accelerate quicker when slightly rich because they make more average torque through the usable RPM band. The air-fuel-ratio that yields maximum power is only a shade richer than the one that is most detonation-prone so always watch your data and keep notes for weather changes.
13.3:1 - Lean best torque
12 to 12.7:1 - Mean best torque
11.76:1 - Rich best torque
11.5:1 to prevent detonation for timing sensitive and big tune-ups
A ratio change from 13.1 down to 11.5 to one will NOT change your HP by 10-percent. Usually when you are close but not lean, you can be pretty far off and not make a ten percent change in power. Now if you’re too lean, that’s a different story. After you get the fuel close you don't have to worry about it too much. Ignition timing is the make or break in nitrous applications. I have seen 1° of timing make up to a 40 hp difference. Timing is much more important! As far as A/F: 11.7 to 12.8 seems to make more average torque through the RPM band and that's what accelerates the car. Smaller cubic inch, high rpm efficient engines might like 13.1 to 13.3 AFR but most of the time that's not the case. The more HP you make per cubic inch, the leaner this might become.
When you get into the 13.9 to 14.7 AFR range after the high gear shift is when they burn up from being “Lean”. That’s been my experience anyway. The heat from the lean condition causes Pre-Ignition. The lean mixture enters the combustion chamber as the piston reaches BDC for intake; the piston next reverses direction and starts to compress the charge. The spark voltage requirements to light the charge increase in proportion with the amount of compression; almost anything can ignite air/fuel mixture at Bottom Dead Center! BDC or before is the easiest time to light the mixture. It becomes progressively more difficult as the pressure builds.
A lean AFR mixture causing glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. Like a spark plug ground strap ect, Igniting the charge while the piston is very early in the compression stoke. The result is for the entire compression stroke, the engine is trying to compress a hot mass of expanded gas. That puts tremendous load on the engine and tremendous heat into its parts. Substantial damage occurs very quickly. This all occurs before the spark plug fires. It causes a catastrophic failure very quickly because the heat and pressures are so intense. An engine can live with minor detonation occurring for considerable periods of time. There are no engines that will survive for any period of time when pre-ignition occurs. The typical pre-ignition indicator is a hole in the piston. This occurs because trying to compress the already burned mixture…. the parts soak up a tremendous amount of heat very quickly. The only ones that survive are the ones that have a high thermal inertia, like the cylinder head or cylinder wall. The piston being aluminum, has a low thermal inertia (aluminum soaks up the heat very rapidly). During pre-ignition the crown of the piston being relatively thin, it gets extremely hot, it can't dissipate the heat, it has tremendous pressure loads against it and the result is a hole in the middle of the piston where it is weakest. You can also have detonation and pre-ignition. Too much ignition advance will cause detonation which will break down the boundary layer that protects the piston from full combustion heat which will expand the piston, scuff the skirt and lift the ring land. Lean will over heat the stressed ring land area causing a hot spot and pre-ignition will melt a hole right down the side of the piston. Unlike detonation.. Most of the time…The only sign of pre-ignition is white smoke pouring out the pipes and the engine quits running.
“There’s definitely a point of leaning it out to where it melts it, or go way past it.. where nothing happens”.
ALWAYS remember to tune your car to run the quickest travel times. If you try to make your wide band sensor look good, you might be giving away some power in the process. I have people call me all the time who have just got a wide band to try to tune with. As they make the meter read a better AFR, their performance went down; you should always tune for best travel times and then look at the meter to see what it says. Your spark plugs will verify if your 02 sensor(s) are accurate. Start out a little rich, retarded and with cold plugs. Trust the plugs…they rarely tell a lie. But here is how the wrong heat range plugs can confuse you: If you keep adding ignition timing until the back split slows down and you still can’t get good color on the ground strap, yet the porcelain base has good color, you probably have too cold a heat range of sparkplug. If you have a lot of heat on the ground strap and the porcelain is very clean, but your wideband says 10.1-11.5 range then you have too hot of a heat range plug… just some things to keep in mind.
Note: Always examine all the data you can. You can never have too much data to base tuning decisions on.
Example of fuel plating out on port & manifold walls and intake pulses in the video below courtesy of Jon Kaase Racing Engines
Boundary Layer and Detonation
When an engine is detonating, the sparkplug will initiate the flame front, but on the opposite side you have low octane end gases cooling in the quench area, too much heat caused by too much timing in this particular case causes these low octane end gases too light off and the two flame fronts collided, this causes a big pressure spike. The big spike in pressure breaks down the protective Boundary layer and exposes the aluminum to full combustion heat and bad things happen after that. Same principals apply whether it's Naturally Aspirated, Boosted or N/A with Nitrous.
The skirt side of the piston is thinner and more flexible, soaks heat easier and the piston crown is a smaller diameter by design. Detonation breaks down the boundary layer in the cylinder that protects it from soaking up too much heat. Detonation spikes break down the boundary layer and the piston over heats, expands and drives the top land and skirt into the cylinder wall. That's how you get the scuff marks, pinched rings and also lifted ring lands . In every internal combustion engine there's a boundary layer about 2mm thick on the cylinder walls and 4-6 mm thickness on the piston top and cylinder head. This is a unburned boundary layer of air/fuel mixture insulating the metal components of the combustion chamber, piston and cylinder wall from the flame front. The boundary layer is in thermal contact with the metal & cools it. In reality, combustion is never ever complete and this creates a boundary layer of air/fuel mixture insulating the metal components of the combustion chamber from the flame front. The boundary layer is in thermal contact and cools the metal. Keep in mind that pouring temperature for aluminum is approximately 1380 degrees. Combustion temperatures inside your engine exceed 1800+ degrees. If you subject a piston to 1800 degrees, it will just melt. The reason it doesn't melt is due to the thermal inertia created boundary layer. Without it, melt down occurs. The thin boundary layer isolates the flame and it is quenched as the flame approaches. This combination of internal combustion actions protects the piston and chamber from absorbing too much heat. A detonation spike will cause the boundary layer to breakdown which in turn lets extreme heat transfer into those surfaces when the spike breaks down the boundary. Engines that are detonating will overheat the piston because the boundary layer of gas gets interrupted against the piston. The piston expands into the cylinder wall causing it to scuff, collapse skirts, ring lands will lift and some will collapse due to the hot piston and high pressure spikes and expanded piston moving up & down the cylinder. It isn't piston rock or not enough piston to wall clearance causing it. The problem is detonation. This is a dead giveaway sign of it.
Detonation is often times caused by too much timing but can be caused by hundreds of different things. Extremely rich can raise the dynamic compression ratio and cause it, fuel with a motor octane that's too low for your combination will cause it, a fuel with too low of a distillation curve for your application will cause it. Engine coolant too hot on the starting line will cause detonation, the coolant heat and thus combustion chamber heat on the starting line only goes up as you go down the track, the cylinder can't dissipate enough heat and it will detonate, too tight of a converter will cause it to detonate. Tight converter will lug the engine down, build heat early in the run and detonate when you click into high and lose mechanical advantage of the transmission's low gear. Too tight of a converter and too high rear gear will detonate every time when you shift into high regardless of the tune or timing. I have had them detonate with only 2 degrees total timing with a single stage Direct Port nitrous jet in the 40's. You can't take enough out to fix it. (Changed the converter at the track and raced all weekend with the same tune-up with no detonation),. That's why you get those random lifted ring lands sometimes with the same old reliable tune you have ran for years. If the converter man sends you one that is too tight...it will detonate the engine. You can leave the tune the same and put the correct converter in it and it will stop hurting itself and the plugs will look totally different, the timing can be run back in the normal range and all you changed was the converter. I have seen this many times. Ever notice?.... in a older daily driver that when you’re going up a hill and you press down on the accelerator pedal a little without making it downshift and you hear it start detonating/pinging. Lugging one down will detonate every time. That's just a few things that can cause it, just to give you an idea. I have seen many different causes. When you tear up stuff through the years, you learn/see what caused it and how to prevent it. Data is your friend, gather as much data on your car as you can. If you guys are serious about going faster put a Data-logger on your car and check it every pass, compare previous runs ect. You will learn all kinds of new stuff and learn how to prevent some damage from happening before it's too late. Study all the data you can if your the guy or gal making the tuning decisions. You can never have enough data. Then do things consistently and watch for changes and how things reacted.
Thanks for reading and I hope this helps,
Robert
The skirt side of the piston is thinner and more flexible, soaks heat easier and the piston crown is a smaller diameter by design. Detonation breaks down the boundary layer in the cylinder that protects it from soaking up too much heat. Detonation spikes break down the boundary layer and the piston over heats, expands and drives the top land and skirt into the cylinder wall. That's how you get the scuff marks, pinched rings and also lifted ring lands . In every internal combustion engine there's a boundary layer about 2mm thick on the cylinder walls and 4-6 mm thickness on the piston top and cylinder head. This is a unburned boundary layer of air/fuel mixture insulating the metal components of the combustion chamber, piston and cylinder wall from the flame front. The boundary layer is in thermal contact with the metal & cools it. In reality, combustion is never ever complete and this creates a boundary layer of air/fuel mixture insulating the metal components of the combustion chamber from the flame front. The boundary layer is in thermal contact and cools the metal. Keep in mind that pouring temperature for aluminum is approximately 1380 degrees. Combustion temperatures inside your engine exceed 1800+ degrees. If you subject a piston to 1800 degrees, it will just melt. The reason it doesn't melt is due to the thermal inertia created boundary layer. Without it, melt down occurs. The thin boundary layer isolates the flame and it is quenched as the flame approaches. This combination of internal combustion actions protects the piston and chamber from absorbing too much heat. A detonation spike will cause the boundary layer to breakdown which in turn lets extreme heat transfer into those surfaces when the spike breaks down the boundary. Engines that are detonating will overheat the piston because the boundary layer of gas gets interrupted against the piston. The piston expands into the cylinder wall causing it to scuff, collapse skirts, ring lands will lift and some will collapse due to the hot piston and high pressure spikes and expanded piston moving up & down the cylinder. It isn't piston rock or not enough piston to wall clearance causing it. The problem is detonation. This is a dead giveaway sign of it.
Detonation is often times caused by too much timing but can be caused by hundreds of different things. Extremely rich can raise the dynamic compression ratio and cause it, fuel with a motor octane that's too low for your combination will cause it, a fuel with too low of a distillation curve for your application will cause it. Engine coolant too hot on the starting line will cause detonation, the coolant heat and thus combustion chamber heat on the starting line only goes up as you go down the track, the cylinder can't dissipate enough heat and it will detonate, too tight of a converter will cause it to detonate. Tight converter will lug the engine down, build heat early in the run and detonate when you click into high and lose mechanical advantage of the transmission's low gear. Too tight of a converter and too high rear gear will detonate every time when you shift into high regardless of the tune or timing. I have had them detonate with only 2 degrees total timing with a single stage Direct Port nitrous jet in the 40's. You can't take enough out to fix it. (Changed the converter at the track and raced all weekend with the same tune-up with no detonation),. That's why you get those random lifted ring lands sometimes with the same old reliable tune you have ran for years. If the converter man sends you one that is too tight...it will detonate the engine. You can leave the tune the same and put the correct converter in it and it will stop hurting itself and the plugs will look totally different, the timing can be run back in the normal range and all you changed was the converter. I have seen this many times. Ever notice?.... in a older daily driver that when you’re going up a hill and you press down on the accelerator pedal a little without making it downshift and you hear it start detonating/pinging. Lugging one down will detonate every time. That's just a few things that can cause it, just to give you an idea. I have seen many different causes. When you tear up stuff through the years, you learn/see what caused it and how to prevent it. Data is your friend, gather as much data on your car as you can. If you guys are serious about going faster put a Data-logger on your car and check it every pass, compare previous runs ect. You will learn all kinds of new stuff and learn how to prevent some damage from happening before it's too late. Study all the data you can if your the guy or gal making the tuning decisions. You can never have enough data. Then do things consistently and watch for changes and how things reacted.
Thanks for reading and I hope this helps,
Robert
Don't chase the leanest possible AFR to make power!
There's a widespread belief that to make maximum power, you have to run engines at the absolute lean limit. This is a very dangerous way to tune a nitrous engine there’s no need to chase the leanest possible AFR to make the most power! That's a really important point! Fuel is not the enemy and doesn't hurt parts. Detonation and Pre ignition is what hurts parts. Nitrous engine power and torque isn't massively influenced negatively over a wide range of air/fuel ratios. The amount of power an engine can produce is defined by the amount of air/oxygen the engine can consume on the intake stroke. What you need to do is supply enough fuel to make sure you combust all the available oxygen. To do this under full load without detonation we have to run the AFR rich enough to control combustion chamber temperature, it is especially important when injecting large amounts of nitrous. We use the extra fuel to quench/cool/control the combustion temperature. This is critical to the reliability of any nitrous injected engine. When an engine is very sensitive to knock, a richer AFR allows you to use a little more ignition timing and/or allows you to use more PPH of nitrous per engine cubic inch than you can with a leaner AFR. The Air/Fuel ratio directly affects the combustion chamber temperature. The leaner the AFR, the less timing your engine will tolerate before it detonates.
I often get asked does ignition timing affect the Air/Fuel Ratio. The answer is no. The A/F ratio is fixed at the point the intake valve closes. At that point no additional air and fuel can be added. The point the intake valve closes at is a long way ahead of where the ignition event occurs. Regardless of what you do with the ignition timing it does not affect the actual A/F ratio in the chamber.
I often get asked does ignition timing affect the Air/Fuel Ratio. The answer is no. The A/F ratio is fixed at the point the intake valve closes. At that point no additional air and fuel can be added. The point the intake valve closes at is a long way ahead of where the ignition event occurs. Regardless of what you do with the ignition timing it does not affect the actual A/F ratio in the chamber.
Flowing Fuel Pressure..why a .073 jet?
"Flowing pressure" and "System pressure" are two different numbers. Most Nitrous tuners use either a Holley # 70 jet or a .073 flare jet for the fuel pressure settings. Up to around a .032 fuel jet in a Direct-Port System the fuel pressure will match what the system pressure actually is an/or what your data logger shows the pressure is. A Data Logger Sensor mounted before the fuel solenoids is showing system pressure. Above a .032 fuel jet in a Direct Port System the .073 jet flow tool will be a little different by a few tenths or so in comparison to the actual "System Pressure". But don't let this confuse you. You need to flow your fuel pressure on all settings with either a .073 flare jet or a Holley 70 jet. Don't change it! The reason we use a .073 in a Flow Tool is it provides the closest PSI average on all settings in comparison to the actual system pressure. It is our point of reference and an industry standard.
Again...Don't let this confuse you. Always flow your fuel pressure on every horsepower setting with a .073 jet in your flow tool. Just to reiterate: Let's say I am flowing a customers system with a 24 fuel jet at 5.5 psi or 30 fuel jet @5.5 psi in the fuel side of their Direct-Port system . I set that 5.5 PSI on my Flow bench through a .073-inch orifice, I don't activate the fuel side of the Fogger and adjust the regulator to set the "system pressure to 5.5 PSI". Many times they will be the same pressure until the bigger settings and then there maybe a .2 to .35 difference or so. But that doesn't matter.. The flow tool pressure through a .073 or Holley jet is what matters;)
Fuel jet x 8 .073 Flow Tool PSI Actual System Fuel Pressure
.019 5.0 5.10
.024 5.0 5.0
.026 5.0 5.0
.028 5.0 4.90
.030 5.0 4.86
.032 5.0 4.85
.040 5.0 4.70
Fuel jet x 8 .073 Flow Tool PSI Actual System Fuel Pressure
.019 6.5 6.70
.024 6.5 6.5
.026 6.5 6.5
.028 6.5 6.40
.030 6.5 6.38
.032 6.5 6.35
.040 6.5 5.65
Again...Don't let this confuse you. Always flow your fuel pressure on every horsepower setting with a .073 jet in your flow tool. Just to reiterate: Let's say I am flowing a customers system with a 24 fuel jet at 5.5 psi or 30 fuel jet @5.5 psi in the fuel side of their Direct-Port system . I set that 5.5 PSI on my Flow bench through a .073-inch orifice, I don't activate the fuel side of the Fogger and adjust the regulator to set the "system pressure to 5.5 PSI". Many times they will be the same pressure until the bigger settings and then there maybe a .2 to .35 difference or so. But that doesn't matter.. The flow tool pressure through a .073 or Holley jet is what matters;)
Fuel jet x 8 .073 Flow Tool PSI Actual System Fuel Pressure
.019 5.0 5.10
.024 5.0 5.0
.026 5.0 5.0
.028 5.0 4.90
.030 5.0 4.86
.032 5.0 4.85
.040 5.0 4.70
Fuel jet x 8 .073 Flow Tool PSI Actual System Fuel Pressure
.019 6.5 6.70
.024 6.5 6.5
.026 6.5 6.5
.028 6.5 6.40
.030 6.5 6.38
.032 6.5 6.35
.040 6.5 5.65
NITROUS PRESSURE vs NITROUS TEMPERATURE CHART
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