zin
10-31-2011, 08:44 PM
I'm not sure if I should add/restore to this OP or just put it in-line at the end, but since this was really meant to become a KC article, I decided to just keep adding to this one post while answering questions in-line.
(to make some sense of this first line, please read below.)
I'll start with some of what nitrous is and what it isn't.
Nitrous IS 14% richer in oxygen (by weight) than normal atmospheric air.
Nitrous IS NOT flammable.
Nitrous is two parts Nitrogen and one part Oxygen. Its molecule looks like a water molecule (a Mickey Mouse head).
Nitrous IS considered an oxidizer and for our use it is. Though technically, the oxygen atom in nitrous is bound to the nitrogen atoms, and so is not available for combustion, until the molecule breaks up, at which point it isn't nitrous anymore, just two nitrogen atoms and one oxygen atom. This is also why nitrous is safe to use and straight oxygen is not. The oxygen is restrained until it is heated over 575*(approx), at which point the atoms break apart in an exothermic reaction, which simply means it gives off heat energy in the process. Straight oxygen will make anything it can go off, go off anywhere and as soon as possible, like in your intake or in your face! The nitrogen not only "escorts" the "rowdy" oxygen into the chamber, but hangs around as a combustion buffer to make sure things don't get too out of control.
Nitrous will NOT blow-up your engine... Only you can do that!;) (have I mentioned my dislike of the new smilies?..) Anyway, a reasonable amount of nitrous properly tuned will do no harm, but even easier than boost, you can over do things. Even a base nitrous kit will have hardware capable of adding at least 125HP to any engine, and all you have to do is switch a couple of $5.00 jets to do it! No turbo swap required, though I suspect pistons and rods would be shortly thereafter!
Tuning parameters required to ensure safe use of nitrous:
1. DO NOT install "The BIG JETS" "just this one time". The "big jets" are usually the ones you KNOW you shouldn't run, but since you had such a blast on the small ones you just gotta try them out. Don't. You might get away with it, but it is only common sense that if you try to make more power than the parts in the engine will handle you'll run over the crankshaft! If in doubt as to how much the engine can handle, stick to the 40-50% rule: 40-50% more power than what the engine made from the factory. The reason for this is the factory knows we'll hammer their products and generally over-build them by this margin, at least. Some not so much, but most. And yes, some are built like a brick sh!thouse and can take more than that, but like I said, "if you are in doubt".
2. Timing. More than a lack of fuel, too much timing will kill any engine faster than all but the highest explosives. A quick run-down on timing and why we need to retard it when we run nitrous.
Engines will take best advantage of cylinder pressure if the peak pressure is developed at about 15-20* ATDC (After Top Dead Center), because an engine can't do much to change the speed at which the fuel it is running burns, a certain amount of advance or lead/head start must be given to the burning mixture. Most 2-valve engines will need about 34-38* of advance (BTDC) in order to develop peak pressure at the optimum point ATDC. Most 4-valve and some high efficiency 2-valve heads (Yates, etc) won't need as much lead.
Nitrous will add oxygen to the mix and WILL increase the rate at which a given fuel will burn, so if we do nothing, it is the same as ADDing a bunch of timing, not only do we develop peak pressure too early, killing HP, but it will also bring on detonation, and all the nastiness that comes with it.
3. Nitrous Fuel Ratios: IF the engine is properly tuned for fuel and/or compensates properly for air flow changes, the proper range of nitrous/fuel ratios will be in the 5-6:1 area. Some maximum effort engines will run leaner in the 6+:1 range, but that really isn't on our radar at this level. In this vein I should mention that non-mass air metered engines will over-fuel to an extent when nitrous is added, this is simply because the added volume of nitrous and fuel will off-set some of the air that normally would be there. This isn't always a problem, in fact it is often tuned for without even being realized. Most often the blame is placed on a "rich" tune-up, even if the nitrous fuel ratio was technically lean.
4. Nitrous Bottle Pressure: Pretty much all commercial nitrous systems are tuned using 900-1000PSI bottle pressure, with the most common being 925PSI. The reason is simple, to get 900+PSI you have to have a pretty warm bottle, as would be common on a summer day. This helps to ensure that if you don't mind the pressure you'll be more likely to run rich for lack of bottle pressure than lean for lack of bottle cooling. Bottle pressure is directly related to temperature; as a rule (again), 70*F will get you 750PSI, 80*F will get you 850PSI, and so on, until you hit 1050PSI, at which point you've reached the "critical" point where the bottle is hot enough that there is no more liquid in the bottle, more like a very dense fog or whipped cream. This is also the point at which you can get both phases (liquid and gas) swapping back and forth, which can really do a number on your tune-up. I'll also mention that the higher the pressure you start at the more severe the pressure drop will be during the run... In fact some older (ok ancient) systems would have you try to maintain 600PSI to help regulate the pressure drop, but that requires icing the bottle and an entirely alien jetting combo...
5. Fuel quality: For the most part, if you have a fuel that is good for the compression ratio you are running, it should be fine with a reasonable amount of nitrous, BUT, better fuel won't hurt you IF your engine has been tuned for it. Fuels that are good for high boost applications are generally good for nitrous too, this is because they tend to be high specific gravity(SG) and high octane. The high SG generally means a slower burning fuel and a denser fuel (more mass/lb per cubic inch or CC). Where you can get bit is changing to a race fuel without tuning for it. Since the race fuel is slower burning, it has the effect of lowering ignition timing, it is also denser so the same volume of fuel injected actually richens up the mixture. None of this is a problem if you engine actually wants these things or you've tuned for it, and even if you haven't, it would still be better to have the tune-up off a bit than detonate the engine. Just know that this is often the reason why you may see people go slower with race gas than pump gas.
Now for some commonly asked questions:
When can you hit the nitrous? The simple answer is as soon as you think it will actually hook-up! Adding nitrous at very low RPMs can be problematic, but is also hard to do. At very low RPMs the air flow is low but the nitrous flow is steady, just as it would be at 8000+RPM, so the cylinder pressures can be quite high and the concentration of oxygen MUCH higher than the amount of ignition retard programed. In the real world, if you are running an automatic, you really can't hit to too low, or over-rev it, but if you run a stick, you might be able to bog it and if you are running just a WOT (wide open throttle) switch, you may be injecting nitrous at very low RPM. Backfires are also more likely in this situation.
What causes nitrous backfires? Nitrous backfires happen the same way any other backfire does, but with more "gusto" due to the added energy in the fuel and nitrous in the intake. In the end, a source of ignition sneaks past the intake valve, and sets off the air fuel charge in the intake. This can be a bouncing intake valve, too much timing, incandescents (glowing hot spots) in the chamber, etc.
How much can I run? Ah, the $64K question. The safe way is to limit the engine to a total HP less than the weakest part's rating, or "pill it up till you sweep it up"... then replace the part that broke and try to break it again! Rinse, repeat as needed. :eyebrows:
How much will it add? This is particularly difficult on turbo engines... The HP rating of a jetting combo is typically rated for a N/A engines. What this means is that the potential for added HP due to added intercooling not to mention added boost (creep) that can show up. As a rule (lots of those here!) it could be expected to see a 50HP jet yield 50+HP and 75ft/lb of torque.
Why does only 50HP feel like so much more? This is primarily due to the why nitrous is injected, 100% at minimum RPM, which give very high power at a point the engine typically doesn't make much power. This is also why it feels stronger the lower you hit it and less so the higher you hit the system. As an aside, the higher the RPM, the less time there is not only to burn the fuel and make power, but also less time for the air fuel to "heat soak" and become unstable, making detonation and pre-ignition less likely, not that you should try to rev beyond the normal range of your engine, in fact short shifting often yields quicker ETs
Ok, at this point, it's late and I'm getting tired... If I'm missing something you'd like to know about, post a question and I'll address it as best I can.
As an aside to this subject, as it happens our engines run a higher base fuel pressure than most our contemporaries, and as such the jetting is generally set to be "safe"/rich at just 35 PSI, so expect to have to modify "standard" jetting for less fuel, but if in doubt, start with "safe" and make small changes to tune out the excess fuel.
Below is a cut and paste, some redundant, but I'll incorporate whatever isn't
Other useful info:
1. Flow change due to pressure:
New pressure / Old pressure, square rooted, x original flow rate = new flow rate.
Example: 40psi/ 20psi = 2. Square root of 2 = 1.4142136 x 19lb/hr = 26.87lb/hr.
2. Lb/hr to CC/min = lb x 10.5
Example:19lb/hr x10.5=199.5cc/min
3. Grams per lb = 453.79
Ignition Timing.
The amount of ignition retard varies with the amount of nitrous added:
A conservative baseline is 2 degrees for every 50 HP worth of nitrous added. An example would be retarding the timing by 8 degrees when 200 HP of nitrous is added. (200/50 = 4; 4x2 degrees=8).
Upon experimentation, many find that as little as 1 to 1.5 degrees per 50 HP is optimum for their particular engine.
It is important to note that the purpose of retarding the ignition timing is to simply correct the peak cylinder pressure to the “sweet spot” of 10 to 15 degrees after top dead center (ATDC). Typically a naturally aspirated engine will make best power with 36-38 degrees of advance (BTDC). This amount of advance is what is required to produce peak pressure at the 10-15 ATDC “sweet spot”. Due to the fact that nitrous has approximately 14% more oxygen than normal air, the burn rate is accelerated, as a result, if the timing is not retarded, the peak pressure will happen closer and closer to top dead center (TDC), this not only reduces HP by taking peak pressure away from the “sweet spot”, but adds a huge strain on engine parts. Just imagine if peak pressure happened at TDC! All that pressure/energy is trying to push the crank straight out of the bottom of the block! NOT GOOD. If too much timing is removed, the only negative is there is lost power due to not being in the “sweet spot”, almost “chasing” the piston down the bore. This is why it is better to start with a conservative 2 degrees per 50 HP of nitrous than to try and hit the bulls eye the 1st time out. Once you find what you engine likes, you can run with that, but start out safe.
Flowing Nitrous and Fuel to optimize your tune.
Much has been said about “flowing your system”, but not a lot has been said as to why. There are two main reasons to flow a system. The first is to establish an overall nitrous to fuel ratio. These will vary from a lean 6:1 to a rich 5:1. It’s not to say that some engines or people don’t prefer something higher or lower (unlikely), but these are typically the ranges used for most applications. The second is to simulate an activated system to ensure the fuel system is up to snuff and that the pressure desired is what will be produced while making a pass. In more involved forms of flowing direct port type systems, the individual nozzles can be flowed into graduated cylinders in order to establish and correct the cylinder to cylinder balance of fuel. Nitrous is not balanced this way as there is no reliable/practical method of measuring nitrous flow from an individual jet/nozzle. Thankfully, the high pressure of the nitrous tends to help even out small flow variances, provided the nitrous jet’s orifice is reasonably consistent jet to jet.
A final note: There is an old saying: “There is more than one way to skin a cat”, ask 10 nitrous “gurus” how to tune your engine, you’ll likely get 10 somewhat different answers, and none of them are necessarily wrong! There are many ways of getting to the same place, and everyone has their “pet” formula, none are guaranteed to work every time on every engine, but flowing your fuel system is a big step toward ensuring your learning cure is an easy one!
Let me know what you think and if there is something I haven't covered, but need to.
Mike
(to make some sense of this first line, please read below.)
I'll start with some of what nitrous is and what it isn't.
Nitrous IS 14% richer in oxygen (by weight) than normal atmospheric air.
Nitrous IS NOT flammable.
Nitrous is two parts Nitrogen and one part Oxygen. Its molecule looks like a water molecule (a Mickey Mouse head).
Nitrous IS considered an oxidizer and for our use it is. Though technically, the oxygen atom in nitrous is bound to the nitrogen atoms, and so is not available for combustion, until the molecule breaks up, at which point it isn't nitrous anymore, just two nitrogen atoms and one oxygen atom. This is also why nitrous is safe to use and straight oxygen is not. The oxygen is restrained until it is heated over 575*(approx), at which point the atoms break apart in an exothermic reaction, which simply means it gives off heat energy in the process. Straight oxygen will make anything it can go off, go off anywhere and as soon as possible, like in your intake or in your face! The nitrogen not only "escorts" the "rowdy" oxygen into the chamber, but hangs around as a combustion buffer to make sure things don't get too out of control.
Nitrous will NOT blow-up your engine... Only you can do that!;) (have I mentioned my dislike of the new smilies?..) Anyway, a reasonable amount of nitrous properly tuned will do no harm, but even easier than boost, you can over do things. Even a base nitrous kit will have hardware capable of adding at least 125HP to any engine, and all you have to do is switch a couple of $5.00 jets to do it! No turbo swap required, though I suspect pistons and rods would be shortly thereafter!
Tuning parameters required to ensure safe use of nitrous:
1. DO NOT install "The BIG JETS" "just this one time". The "big jets" are usually the ones you KNOW you shouldn't run, but since you had such a blast on the small ones you just gotta try them out. Don't. You might get away with it, but it is only common sense that if you try to make more power than the parts in the engine will handle you'll run over the crankshaft! If in doubt as to how much the engine can handle, stick to the 40-50% rule: 40-50% more power than what the engine made from the factory. The reason for this is the factory knows we'll hammer their products and generally over-build them by this margin, at least. Some not so much, but most. And yes, some are built like a brick sh!thouse and can take more than that, but like I said, "if you are in doubt".
2. Timing. More than a lack of fuel, too much timing will kill any engine faster than all but the highest explosives. A quick run-down on timing and why we need to retard it when we run nitrous.
Engines will take best advantage of cylinder pressure if the peak pressure is developed at about 15-20* ATDC (After Top Dead Center), because an engine can't do much to change the speed at which the fuel it is running burns, a certain amount of advance or lead/head start must be given to the burning mixture. Most 2-valve engines will need about 34-38* of advance (BTDC) in order to develop peak pressure at the optimum point ATDC. Most 4-valve and some high efficiency 2-valve heads (Yates, etc) won't need as much lead.
Nitrous will add oxygen to the mix and WILL increase the rate at which a given fuel will burn, so if we do nothing, it is the same as ADDing a bunch of timing, not only do we develop peak pressure too early, killing HP, but it will also bring on detonation, and all the nastiness that comes with it.
3. Nitrous Fuel Ratios: IF the engine is properly tuned for fuel and/or compensates properly for air flow changes, the proper range of nitrous/fuel ratios will be in the 5-6:1 area. Some maximum effort engines will run leaner in the 6+:1 range, but that really isn't on our radar at this level. In this vein I should mention that non-mass air metered engines will over-fuel to an extent when nitrous is added, this is simply because the added volume of nitrous and fuel will off-set some of the air that normally would be there. This isn't always a problem, in fact it is often tuned for without even being realized. Most often the blame is placed on a "rich" tune-up, even if the nitrous fuel ratio was technically lean.
4. Nitrous Bottle Pressure: Pretty much all commercial nitrous systems are tuned using 900-1000PSI bottle pressure, with the most common being 925PSI. The reason is simple, to get 900+PSI you have to have a pretty warm bottle, as would be common on a summer day. This helps to ensure that if you don't mind the pressure you'll be more likely to run rich for lack of bottle pressure than lean for lack of bottle cooling. Bottle pressure is directly related to temperature; as a rule (again), 70*F will get you 750PSI, 80*F will get you 850PSI, and so on, until you hit 1050PSI, at which point you've reached the "critical" point where the bottle is hot enough that there is no more liquid in the bottle, more like a very dense fog or whipped cream. This is also the point at which you can get both phases (liquid and gas) swapping back and forth, which can really do a number on your tune-up. I'll also mention that the higher the pressure you start at the more severe the pressure drop will be during the run... In fact some older (ok ancient) systems would have you try to maintain 600PSI to help regulate the pressure drop, but that requires icing the bottle and an entirely alien jetting combo...
5. Fuel quality: For the most part, if you have a fuel that is good for the compression ratio you are running, it should be fine with a reasonable amount of nitrous, BUT, better fuel won't hurt you IF your engine has been tuned for it. Fuels that are good for high boost applications are generally good for nitrous too, this is because they tend to be high specific gravity(SG) and high octane. The high SG generally means a slower burning fuel and a denser fuel (more mass/lb per cubic inch or CC). Where you can get bit is changing to a race fuel without tuning for it. Since the race fuel is slower burning, it has the effect of lowering ignition timing, it is also denser so the same volume of fuel injected actually richens up the mixture. None of this is a problem if you engine actually wants these things or you've tuned for it, and even if you haven't, it would still be better to have the tune-up off a bit than detonate the engine. Just know that this is often the reason why you may see people go slower with race gas than pump gas.
Now for some commonly asked questions:
When can you hit the nitrous? The simple answer is as soon as you think it will actually hook-up! Adding nitrous at very low RPMs can be problematic, but is also hard to do. At very low RPMs the air flow is low but the nitrous flow is steady, just as it would be at 8000+RPM, so the cylinder pressures can be quite high and the concentration of oxygen MUCH higher than the amount of ignition retard programed. In the real world, if you are running an automatic, you really can't hit to too low, or over-rev it, but if you run a stick, you might be able to bog it and if you are running just a WOT (wide open throttle) switch, you may be injecting nitrous at very low RPM. Backfires are also more likely in this situation.
What causes nitrous backfires? Nitrous backfires happen the same way any other backfire does, but with more "gusto" due to the added energy in the fuel and nitrous in the intake. In the end, a source of ignition sneaks past the intake valve, and sets off the air fuel charge in the intake. This can be a bouncing intake valve, too much timing, incandescents (glowing hot spots) in the chamber, etc.
How much can I run? Ah, the $64K question. The safe way is to limit the engine to a total HP less than the weakest part's rating, or "pill it up till you sweep it up"... then replace the part that broke and try to break it again! Rinse, repeat as needed. :eyebrows:
How much will it add? This is particularly difficult on turbo engines... The HP rating of a jetting combo is typically rated for a N/A engines. What this means is that the potential for added HP due to added intercooling not to mention added boost (creep) that can show up. As a rule (lots of those here!) it could be expected to see a 50HP jet yield 50+HP and 75ft/lb of torque.
Why does only 50HP feel like so much more? This is primarily due to the why nitrous is injected, 100% at minimum RPM, which give very high power at a point the engine typically doesn't make much power. This is also why it feels stronger the lower you hit it and less so the higher you hit the system. As an aside, the higher the RPM, the less time there is not only to burn the fuel and make power, but also less time for the air fuel to "heat soak" and become unstable, making detonation and pre-ignition less likely, not that you should try to rev beyond the normal range of your engine, in fact short shifting often yields quicker ETs
Ok, at this point, it's late and I'm getting tired... If I'm missing something you'd like to know about, post a question and I'll address it as best I can.
As an aside to this subject, as it happens our engines run a higher base fuel pressure than most our contemporaries, and as such the jetting is generally set to be "safe"/rich at just 35 PSI, so expect to have to modify "standard" jetting for less fuel, but if in doubt, start with "safe" and make small changes to tune out the excess fuel.
Below is a cut and paste, some redundant, but I'll incorporate whatever isn't
Other useful info:
1. Flow change due to pressure:
New pressure / Old pressure, square rooted, x original flow rate = new flow rate.
Example: 40psi/ 20psi = 2. Square root of 2 = 1.4142136 x 19lb/hr = 26.87lb/hr.
2. Lb/hr to CC/min = lb x 10.5
Example:19lb/hr x10.5=199.5cc/min
3. Grams per lb = 453.79
Ignition Timing.
The amount of ignition retard varies with the amount of nitrous added:
A conservative baseline is 2 degrees for every 50 HP worth of nitrous added. An example would be retarding the timing by 8 degrees when 200 HP of nitrous is added. (200/50 = 4; 4x2 degrees=8).
Upon experimentation, many find that as little as 1 to 1.5 degrees per 50 HP is optimum for their particular engine.
It is important to note that the purpose of retarding the ignition timing is to simply correct the peak cylinder pressure to the “sweet spot” of 10 to 15 degrees after top dead center (ATDC). Typically a naturally aspirated engine will make best power with 36-38 degrees of advance (BTDC). This amount of advance is what is required to produce peak pressure at the 10-15 ATDC “sweet spot”. Due to the fact that nitrous has approximately 14% more oxygen than normal air, the burn rate is accelerated, as a result, if the timing is not retarded, the peak pressure will happen closer and closer to top dead center (TDC), this not only reduces HP by taking peak pressure away from the “sweet spot”, but adds a huge strain on engine parts. Just imagine if peak pressure happened at TDC! All that pressure/energy is trying to push the crank straight out of the bottom of the block! NOT GOOD. If too much timing is removed, the only negative is there is lost power due to not being in the “sweet spot”, almost “chasing” the piston down the bore. This is why it is better to start with a conservative 2 degrees per 50 HP of nitrous than to try and hit the bulls eye the 1st time out. Once you find what you engine likes, you can run with that, but start out safe.
Flowing Nitrous and Fuel to optimize your tune.
Much has been said about “flowing your system”, but not a lot has been said as to why. There are two main reasons to flow a system. The first is to establish an overall nitrous to fuel ratio. These will vary from a lean 6:1 to a rich 5:1. It’s not to say that some engines or people don’t prefer something higher or lower (unlikely), but these are typically the ranges used for most applications. The second is to simulate an activated system to ensure the fuel system is up to snuff and that the pressure desired is what will be produced while making a pass. In more involved forms of flowing direct port type systems, the individual nozzles can be flowed into graduated cylinders in order to establish and correct the cylinder to cylinder balance of fuel. Nitrous is not balanced this way as there is no reliable/practical method of measuring nitrous flow from an individual jet/nozzle. Thankfully, the high pressure of the nitrous tends to help even out small flow variances, provided the nitrous jet’s orifice is reasonably consistent jet to jet.
A final note: There is an old saying: “There is more than one way to skin a cat”, ask 10 nitrous “gurus” how to tune your engine, you’ll likely get 10 somewhat different answers, and none of them are necessarily wrong! There are many ways of getting to the same place, and everyone has their “pet” formula, none are guaranteed to work every time on every engine, but flowing your fuel system is a big step toward ensuring your learning cure is an easy one!
Let me know what you think and if there is something I haven't covered, but need to.
Mike