LOW COMPRESSION on nitrous motors

[alpinesupra]

Been seeing a growing trend among the nitrous bikes here lately and im wonder why are you guys doing this, is there a secret i dont know? but more and more nitrous motor bikes are using low compression pistons that you would use in a turbo set up.

Every nitrous motor ive ever help build or witnessed built for cars mainly, but has always used HIGH comp pistons and head combo and they have absolutely loved it.... last motor i helped build was a 408 stroker ls1 with a 300 shot combo on it, made around 875 iirc.

maybe someone can shed some light on this for me...

[BROTHER-N-LAW]

You are using a bad comparison. Car heads differ by brand for things like chamber size. That will give you different compression. So you say high compression pistons on a car motor, but if you have a large chamber, the you have less compression. Next you say 300 shot on a 408 motor made 875. 575 HP is a street motor, and 300 shot on a car motor is really not much. So you can use alot of compression with it and won't effect it much.

[Turboguy]

You gotta look at this too. These bikes are already bordering 13:1 stock so when they lower it it's usually a couple points 10-11 myself. Now these cars are not that high on street motors and gas cause your just askin for it. It also allows more room for the nitrous in the combustion chamber much the same as boost. It's also easier to tune with lower compression.

[Turboguy]

Agreed lol. I live by the modo if it don't fit force it lol

[Moody]

lots of different opinions on this, imo i like the compression with my 150 shot of course im a big dude and need the torque to get me rolling. last winter i dropped my compression down with the intent to spray more but we didnt and i can tell a huge difference between 2 points of compression with the same shot

[alpinesupra]

I understand a car and bike will differ but I figured yall would get the point behind my question.
So using my bike as a example, 05 zx10 has 12:5-1 stock comp, if I shave head and thin out the hg that'll put me guessing about 13:7-1 ish give or take and id spray as much as it will take before boom then build it, so instead of thinning and shaving stuff to make more comp people are using turbo pistons and a thicker head gasket and spraying and I'm wondering why??

[Mr.Grey]

They aren't necessarily using turbo pistons. You can get low compression nos pistons too. I've seen this done though, only problem is its hard to get a good 60 without spraying 100 out the hole and ramping it in smooth can be a huge problem.

I'd rather make for example 230 on motor and spray 100 to get the power I'm looking for then have 140 on motor and have to spray 200 to get there.

[alpinesupra]

Well how ever you work your motor over for low compression why would u want low compression on nitrous, that's all I'm trying to get at.

[Turboguy]

It's not low compression it's just lower. Put a 300 shot on a 12.5t:1 motor and see what happens. Also tuners have more trouble tuning with that kinda stuff. It's just safer and more reliable. True u r stripping the motor of its raw power down low but u can get it rite back by adding more spray. Look at it like this lets say with stock squish it gives u a 1/8 of a inch to pack all you spray in. Now say u open that area up to 1/4 of inch now u have more room for more boom. Same as boost. Your forcing in power not sucking it in.

[Turboguy]

I meant forcing as a power added not actually forcing. Just sayin it's not naturally aspirated. True with good fuel and proper tuning u can make things work but it's harder on the motor. That's the Beauty of a power added. If it ain't enough give it more. Really it all comes down to the builder. Some guys like it some don't. I have done it both ways and haven't had a bad result ...yet! But I prefer to lower it a lil bit for safety reasons. But ultimately it's your money if someone 15:1 and big spray ill give it to em but it's at their own risk. This is y went turbo years ago and left nitrous in my wake nitrous is cheaper but I prefer turbo.

[FtCampbellBusa]

I have wondered this myself having a car background!

[flyinphill]

Been seeing a growing trend among the nitrous bikes here lately and im wonder why are you guys doing this, is there a secret i dont know? but more and more nitrous motor bikes are using low compression pistons that you would use in a turbo set up.

Every nitrous motor ive ever help build or witnessed built for cars mainly, but has always used HIGH comp pistons and head combo and they have absolutely loved it.... last motor i helped build was a 408 stroker ls1 with a 300 shot combo on it, made around 875 iirc.

maybe someone can shed some light on this for me...

You have asked a tricky question because it involves a bunch of pretty complex engine theory concepts. And the complexity of the situation is intensified because so many different and often conflicting principles must be considered at the same time. I can explain it to you in good detail, but it will take a long time, far more than can be contained in a single post. If you really want to go down that road, we can do it here a little bit at a time.

And BNL is correct (although some of the other comments have been questionable), a 300HP shot on a 575HP engine is a good size shot, but it is not in the realm of what I would call REALLY big. It is the same as taking a 240HP Busa and putting a 125HP shot on it. It is a good amount of nitrous, but if the tuner watches the setup and timing closely, just like with your car motor, it should be achievable with just about any compression ratio. But now take that same 240HP Busa engine and put a 250Hp shot on it, that is what I consider to be a really big shot of nitrous on a Busa motor. Now the game has changed. That would be like taking your car engine and putting a 600HP shot on it. Do you think that car engine you spoke of will handle a 600HP shot with sky-high compression? If you don't know, then try it and let me know how it works out.

So if you or anyone else on here want a really in-depth answer to your question, (there is no such thing as a simple answer to this question despite many attempts to do so), then let me know and I can take it one step at a time. But before asking you need to understand why increased compression will, in theory, give more power in a naturally aspirated engine. If don't fully understand this then look up the Otto cycle, a thermodynamic principle that dictates the operation and efficiency of sparked piston engines. There is plenty of web info on both this and the efficiency of an engine based on compression ratio. If you look it up and still can't follow, then I will attempt to do a better job of explaining it.

[flyinphill]

nothing will happen.. it will go fast. ive seen a 15:5:1 running 260. its all in the tune and quench area. thats one of the best things about a stroker is that you can get the same compression with more squish and you have more room to pack in nitrous. (but not worth the negatives of a stroker in my opinion) realistically though, not many people are spraying more than the engine can take as it is. and ur saying "forcing it in" that doesnt really happen with nitrous unless your ram air is supercharging quite a bit and the VE on your head is over 100%, and then you have to worry about exhaust cam timing so you dont get reversion. supercharging a NA engine is out of reach for most "normal" engine builders and tuners.


op: put you a good light weight 13.5 or 14:1 piston in, run good fuel (16, import or e85) and spray that piss out of it.

This post made me wonder about the setup you are talking about, so I am curious if you would share some more details:

1) 15.1:1 Compression-How do you know it had this much? 15.5:1 is not impossible, but it does take some work to achieve, especially without any stroke. So did someone that knew what they were doing actually cc everything and come up with this number? Most "engine builders" I have ever pressed for details have not only never cc'ed an engine to support their compression ratio claims, but they don't even really know how to do it. You would be surprised at some of the big-name builders that have NEVER cc'ed an engine in their lives, but are nevertheless claiming big compression numbers. We have had many customers come in with engines believing they had a certain compression ratio, but when we measured them, they were way short of what they thought.

2) Quench Area- This would be a good statement except for the fact that every big-nitrous Pro Mod-style motorcycle engine I have ever seen had the pistons way in the hole. They all had at least .100-140" piston-to-head, which effectively kills the entire concept of squish and quench. It makes no sense to me why they do it, or why it works. But it does, so they keep dong it.

3)260Hp of Nitrous- How did you get this number? Is it based on theoretical flow rates by jet size, or is it a number that was verified on a dyno? If it was on a dyno, then what were the numbers? How much did it make on the dyno at its best timing setting without nitrous, and how much did it make with 100% nitrous?

I am just curious because I have never seen this set of conditions be true and produce the best power for the combo. If it has been done, I would like to know what it took to make it work.

[STREETDRAG]

nothing will happen.. it will go fast. ive seen a 15:5:1 running 260. its all in the tune and quench area. thats one of the best things about a stroker is that you can get the same compression with more squish and you have more room to pack in nitrous. (but not worth the negatives of a stroker in my opinion) realistically though, not many people are spraying more than the engine can take as it is. and ur saying "forcing it in" that doesnt really happen with nitrous unless your ram air is supercharging quite a bit and the VE on your head is over 100%, and then you have to worry about exhaust cam timing so you dont get reversion. supercharging a NA engine is out of reach for most "normal" engine builders and tuners.


op: put you a good light weight 13.5 or 14:1 piston in, run good fuel (16, import or e85) and spray that piss out of it.

This post made me wonder about the setup you are talking about, so I am curious if you would share some more details:

1) 15.1:1 Compression-How do you know it had this much? 15.5:1 is not impossible, but it does take some work to achieve, especially without any stroke. So did someone that knew what they were doing actually cc everything and come up with this number? Most "engine builders" I have ever pressed for details have not only never cc'ed an engine to support their compression ratio claims, but they don't even really know how to do it. You would be surprised at some of the big-name builders that have NEVER cc'ed an engine in their lives, but are nevertheless claiming big compression numbers. We have had many customers come in with engines believing they had a certain compression ratio, but when we measured them, they were way short of what they thought.

2) Quench Area- This would be a good statement except for the fact that every big-nitrous Pro Mod-style motorcycle engine I have ever seen had the pistons way in the hole. They all had at least .100-140" piston-to-head, which effectively kills the entire concept of squish and quench. It makes no sense to me why they do it, or why it works. But it does, so they keep dong it.

3)260Hp of Nitrous- How did you get this number? Is it based on theoretical flow rates by jet size, or is it a number that was verified on a dyno? If it was on a dyno, then what were the numbers? How much did it make on the dyno at its best timing setting without nitrous, and how much did it make with 100% nitrous?

I am just curious because I have never seen this set of conditions be true and produce the best power for the combo. If it has been done, I would like to know what it took to make it work.

I'm all in for this response... Come On

[flyinphill]

1. the bike was a stroker. and a big bore as well. so it wasnt to hard to get to that compression. i was just implying in my post that i dont really like stroker engines. and the engine was cc'd and completely blue printed.

With a stroke it is much easier to obtain, I didn't exactly follow what you were saying on the stroke issue. I am curious though who built the engine in question.

2.i cant comment on pro mod style engines because im not that familiar with them, but i think you would also have to take into account that those engine at that level probably have a substantial amount of dynamic compression, on top of a reasonable static compression, as well as a positive VE. and in addition they are supplying a boat load of dope, so if they sit that far down in the hole, you can beleive they are using the space.

I have never done any actual calculations to find VE, maybe I will do that if I am bored this weekend. But in any case I am not sure how much that really matters with that much nitrous going into the engine. With that much nitrous, the ports are attaining an unsteady state of super-atmospheric pressure, so that somewhat negates the value of VE. The Pro Mod bikes are not limited by how much nitrous they can get in the engine, they are limited by how much the pistons will handle. And that goes right to my point about the quench. Adding a bunch of deck height is, at least theoretically, the wrong way to add volume to the combustion volume.

3) jet chart derived, never dyno'd at that level to my knowlege. but everyone knows how correct nx jet charts are

That explains a lot. I just looked at the recommended jet sizes on a couple of websites for a 260HP 4-cylinder, and they stated #38 jets. These are misleading recommendations, because this size jet might make 65HP per cylinder in a common-manifold car engine, but it is nowhere near that amount of power in an open-stack individual-throttled small displacement engine. I can tell you from MUCH experience if that engine has #38 jets, it is actually probably making more like 160-180HP extra on the nitrous. To make 260HP, it would take at least mid-40's on jet size. That is about where a Pro Mod bike is jetted, and that is about what they make from nitrous. And there is a BIG differnce in how much compression can be tolerated between 160HP and 260HP.

i think you have to remember that there are several other factors that can influence how efficient the engine uses the fuel/nitrous. simple shape of the piston can have a huge influence on burn speed, and dialing in your cam duration can have a great affect on cylinder filling. rod length has a huge affect on hp and tq output, throw in some math, the chance that you got the porting and cam timing just right, and the right fuel/timing to prevent detonation and you have a combination for some serious power. i put it up there with the engines sherman built for the engine builders challenge, it detonated the entire way from 0-4000 but once it got in its own, it made huge hp.. it as a 16:1 engine, with ve estimated to be in the supercharging range. so now your talking a 19:1 engine, and it was on a fuel that was NOT super high octane race fuel. i almost want to say it was california high test since it was a street engine challenge.. now, explain to me how that thing didnt come apart!!

I really can't comment on this. When you say Sherman I am assuming you are talking about the car engine builder. I don't know specifically what engine you are talking about, or even how much that has to do with motorcycle engines. But I am always interested in reading and learning about anything racing from any type or racing. If you give me a source on what you are talking about, I would be happy to read it. Then I could have a discussion with you about it.