BIG SHAFE

I am interested to know what type of effect a compression ratio has on a nitrous setup and which way should the compression ratio change (increase or decrease). I would assume the compression ratio should decrease due to the extra or more mass filling the cylinder.

Also would anyone have some rough numbers of how much mass or volume, for some specified shot, is pulled into the cylinder during operation, possibly using line pressure, nozzle charactheristics, bore & stroke, and engine speed.

Thanks.

Hot Rod Hawk

Theres more than one way to skin this cat [topic]. I like to keep the compression or add more in my case. Then control the burn with the proper fuel octane and ignition lead for the nitrous.

jonaddis84

"High compression ratios can be used with nitrous, but shifting the maximum pressure after top dead center becomes more and more difficult. I prefer to use street compresion ratios and then just work with adding more nitrous to get desired horsepower levels." Jon Erb, chief engineer KB pistons

Mindgame

I hope Mr. Erb isn't talking about race engines, cause he wouldn't compete in any major league racing classes with that approach.

The static compression ratio can be a little deceiving when you look at what goes on in class race engines because some of the really large plants are head-limited and don't achieve VE numbers like the smaller ones. So, stands to reason that they can tolerate more compression.

In the mid 90's I was driving a car that had the potential to turn 8 second ET's on a 10.5" tire, running a best of 8.94 at 156 mph. For what it's worth, the BBC in that car was built with a static CR of ~14.3:1 making right at 1000 hp before the 500hp shot of nitrous. Of course we were using 116 octane race gas and the awesome Dart big-chief cylinder head, so that helps things a bit. This all-steel 69 Camaro also weighed in at close to 3900 lbs and that plays its part too. A little 3200 Nova may have done well with a tad more compression.

Most race engine builders I know tend to drop compression by about half a point for real heavy nitrous use (300+ hp). However, I would not be surprised to see a monster small block or mountain BBC with 15:1+ and 300+hp of nitrous for the reason I gave in the 2nd paragraph. Lots of factors there but I wouldn't knock any compression out for a street build. Tuning plays a big part... but doesn't it always?

-Mindgame

Hot Rod Hawk

Well said Mindgame

sheppard00

I have been told that the cold charge will allow you to still spray an engine that is at the edge on needing to run high octane fuel na with out raising the octane level. you know like spraying a 250hp shot on a iron headed 10.1 engine on 92. does that make sence?

Injuneer

I don't think so. Detonation is influenced by a number factors, and intake charge temp is one of them. But the overriding consideration when running nitrous is the O2 content of the charge. The higher the percentage of O2, the faster the combustion takes place. At some point you *will* lose control. That's why you can't just spray some LOX. I believe that's the basis for the "rule of thumb" that says you should only spray about 50% of the NA HP level.... e.g. 250 on a 500HP engine, 500 on a 1,000HP engine. I'm spraying 300 on a 500HP engine (10.8 static CR, 230/242 114LSA), and even with C16, it appears to be near the "edge".

OneFlyn95z28

Thank you My fingers hurt LOL

New motor is 12.2 static and I have a 250 shot planned

LameRandomName

I always wondered where that rule came from.

I had been thinking it was something to do with there still being mechanical limits to how much nitrous will fit past a certain size valve.

sheppard00

that is quite a different ball game that is almost a point more comp. than 10.1 and 50 more hp. eventualy with a dence enough charge the compression will overide the cooling affect of the n20 because fuel and O2 can only be smashed together so hard before they mix. I just think you can push a little harder the cooler the charge.