I'm trying to figure out how much additional cyl. pressure nitrous is making.
Example: If I'm running 12.5:1 compression allready, what would a 150 HP shot of nitrous bump that up too? How about a 250 shot? What determines how high the cylinder pressure is going to go when you hit the big button? :confused:

Hope I'm asking this right.

Steve,

Nitrous does not change the compression ratio of the engine. Those are mechanical features that are set when the engine is built. As you mention, nitrous does increase cylinder pressure. Cam and ignition timing are other variables which affect cylinder pressure as well.

To account for the additional cylinder pressure due to the nitrous reduce ignition advance by about 1.5-2 degrees for every 50 HP of N2O added. Also, with 12.5:1 and N2O you NEED to be running some quality high octane race gas. Something along the lines of VP C16. Be sure to use plugs that are a couple of steps colder than what you are as well.

Hope this Helps,

The increase in cylinder pressure is determined by:

1) amount injected
2) rpm at injection.

Rough example - 400 hit introduced at 6800 rpm makes the same increase in cylinder pressure as a 200 hit at 3400 rpm.

When do you know cylinder pressure is too high? Detonation signs on plugs / bearings - if the fuel octane is good enough to suppress detonation - folding down the top ring land / piston pin deformation / piston pin web deformation / head gasket retention problems / crank cracking / excessive cam chain wear / slack in harmonic balancer key etc etc.

Detonation and popped head gaskets usually show their face in smaller systems on "routine buildup" engines.

IMO

MAT

Try reading FAQ advice from nitrous kit manufacturers. I think 12:1 is at least 2 points over. Preasure equals heat and completion of burn. If you can get away with 12:1 the retarding the timing might be more than needed for an 8:1 or 9:1 engine. But read the advice of the manufacturers. I know turbo's require the retarded timing and can't go with already high compression engines from what the manufacturers say. You don't want to cut loose a fire in the extra preasured cylinder that's also going to be generating more heat than normal.
Check the cams available for nitrous also. Going with a longer duration is good for nitrous.

Thanks, that clears some things up. I've got a friend running 14:1 and squezzing the crap out of it with a 400HP shot and I'm surprised he hasn't sent the carb through the hood. LOL
I do know the side effects of playing with this stuff from looking at the remains of some unfortunate big block internals.
I've heard some conflicting stories on excactly what's going on in the cylinder with compression & cylinder pressure.
For example, I was told that a 11 or 12:1 motor can reach as high as 15-16:1 with a squeeze.
But how much squeeze? How much jetting? What kind of cam?
You can see my confusion. LOL
I do understand that there are cam grinds more suited for a nitrous application as there are some for blowers etc. Overlap, duration characteristics playing a key role for the application. I do understand it, though I can't describe it, it's more of a mental picture in my head. LOL
I understand a cam alone plays a role in cylinder pressure.

I guess my question now is, what's the difference between cylinder compression & pressure?