Nitrous and Compression ratio!!??

[Allforspeed]

I am porting my stock heads and doing some work and was wondering if shaving some off the heads to up the compression ratio would have any negative effects on using nitrous. I plan to run a 100 shot with a zex kit. I know that lowering the compression ratio is good with forced induction but not sure what the deal is with nitrous as this is my first car on the bottle i plan to polish the combustion chamber, unshroud the valves and smooth out and sharp edges to help with detonation, run colder plugs, etc... all that... what do you guys think? compression up=less nitrous can be used or more danger?

 

[mackey]

How much compression are you looking at? A 8.5-9.5 compression car would be good for nitrous. I don't think you can go any higher than that by just milling the heads without changing the piston dishes..

 

[04sleeper]

If you are just shaving the stock heads then it will not affect the nitrous use. You will be just fine still.

 

[Allforspeed]

i was under the impression that 87-93 5.0 was around 9:1 stock depending on the head gasket. was told i could shave a little to get 10:1 and be safe with pistons, i do have some sort of random cam that i dont know what is in there, was told its an e303 or the most mild of the alphabet cams, though the guy didn't know **** who i got this car from. maybe i'm totally off, 9:1 you think is best with nitrous? i will shave the heads just a tad either way to get a fresh surface for some race style headgaskets for the nitrous with the 351W ARP head studs, should be pretty good, leaving the stock bottom end alone for now, got 130K, but should probably be OK as long as i'm not greedy or stupid

 

[Funanin]

greed kills us all.

 

[90mustangGT]

greed kills us all.

and stupidity takes out the people around you while you are at it.

I thought you could run lots of compression with nitrous? I've talked to people who are running real high compression with their sprayed engine.

 

[Allforspeed]

i guess my next question then is does anybody know if i can just mill the head surface down enough to get to 10:1 compression without intake manifold probs and other stuff?

 

[66P51GT]

There are a lot of variables, but if you are starting off with a 9.0:1 ratio, you will have to reduce the combustion chamber volume by about 8cc to reach 10.0:1.

Considering a 4" bore, that is about .040" you will have to remove. Depending on your heads, the combustion chamber may be smaller than the bore so you would have to remove more to accomplish the same reduction.

I was told .040" is a lot to remove from the heads but there are probably many opinions on this. It also depends on the heads themselves.

I recently had .015 removed from my RPM heads and had the block decked .015 as well. The net is .030 and is good for about a 6cc reduction. With my notched flat-top pistons I'm looking at about 10.2:1 static compression.

You may be able to get away with not having to machine the intake but you will have to mock up the motor after the machining and check the fitement of the intake manifold. If it sits too high, you can try a different (thinner) type of intake manifold gasket. You will also need to purchase another set of pushrods since milling changes the valve geometry.

I haven't had a chance to mock up my motor yet, I'm still waiting on the machine work on the crank. It has actually been done for weeks but I've been so damn busy lately, I haven't had a chance to get to the shop during business hours.

 

[04sleeper]

Just remember. The more you remove from your heads, the smaller the deck thickness on the head is. Not good for head gasket retention. Also if you take off more than I'd say .030 then you may run into intake alignment problems.

 

[Michael Yount]

Increased compression and nitrous get along just fine. However, you may have to pull more timing -- keep your eye on your tune.

Others have provided solid counsel on the geometry issues. You start cutting and you have other things to consider. How will lower intake ports line up with the heads? How will the lower intake bolt holes line up with the holes in the heads? Will you need shorter pushrods, or to shim the rockers up? If you cut small enough to avoid those issues, I think you'll only get maybe .3 of a point increase. If you're looking for a serious raise, different pistons or a smaller chamber head are the way to get it. In any event, why not leave it alone, and add another 50 shot on the nitrous hit? That'll get you more than the compression increase anyhow.