worm2500

i need everones opinion,

i live in Indiana elevation is about 350to 400 feet.I am wanting to order pistons for a nitrous motor i am building. what compression would be safe to run on 93 octain with about a 150 shot.

my77stang

aluminum heads is 10.5-11.0:1 with a proper air/fuel mixture and correct timing.
cast iron heads is 10.0-10.5:1 with a proper air/fuel mixture and correct timing.

definately buy forged since you are spraying, TRW / Speed-Pro's are the best bang for the buck in a forges piston. If you can spend more you can get into JE's or something of the like, they are usually lighter than the TRW's which translates into faster rev's and less stress on the wrist pin and rotating assembly as a whole.

Hamutoff

A complex question at bestā€¦. I have looked deeply into this, and to answer you or to raise the proper questions, I have to ask you:
Are you going to drive this car around? Commute? Are you going to be sitting at traffic lights then slowly going up a slight incline ever? This seems to be the time the engine would pre-ignite the a/f mixture if it was going to. What cam are you using. This def. will help dictate what gas you can use.

If you are only drag racing its easier. However if not you should also figure out your a)DCR (dynamic compression ratio) and b)quench otherwise you could either 1)give up plenty of power(loose power) in your combo and/or 2)ping, use expensive gas/octane booster or destroy your engine

You absolutely should get forged pistons, if you are close to the edge they will surviveā€¦better. More on dcr: http://www.empirenet.com/pkelley2/DynamicCR.html my engine buildā€¦ depending on the heads I choose: 58 or 61 cc my combo would yield 9.74 or 9.38 with a DCR of 7.72 or 7.45 ā€¦ unless I change my cam, cam change could yield DCR of 7.9ā€¦its all about cylinder pressure you want as much as you can get for your intended applicationā€¦
{I hear that a DCR of 7.5 *should* work with the cheapest gas out there (with aluminum heads) } More on quench: http://kb-silvolite.com/article.php?...ad&A_id=39 its actually counterintuitive: in a very general sense, you make a SMALLER area in your CC to reduce chances of pre ignition with same octane value gasoline Having that tight area really stirs up the A/F mixture and tosses it about the cc. This promotes a fast and more complete burn. If you have a large quench the a/f mix will tend to more slowly burn or burn different in your various cylinders and you could have unburnt deposits on the piston and between the gasket and cylinder bore which leads to: For one use head gaskets that match the overbore you are using, this may seem a *DUH* but most SBF gaskets out there esp in auto parts stores are 4.100"ā€¦. uhh yeah I could only find one out there at summitracing.com for my .030" overbore (very common btw)ā€¦ You could imagine unburnt fuel or carbon build up in this .070" exposed area and when it heats up it will pre-ignite your air/fuel mix causing pinging and possible destruction. The worst thing you could do is double up head gasketsā€¦ some folks build an engine that WONT run on pump gas so they try to lower their compression ratio by raising the head away from the blockā€¦ this is a good way to kill your quench... you'd be better off in this situation changing your cam to try to "band-aid" the problem

Hamutoff

Here are some sweet forged pistons (for stock 302 rod length): http://kb-silvolite.com/forged.php?a...mp;RodLen=5.09

I used the kb747 for 5.4" rods

67mustang302

Yeah, quench and cam timing will play a huge role in cylinder pressure and detonation resistance. You could have a high dcr engine with only 10:1 that detonates like mad, but a low dcr engine with 11.5:1 that doesn't detonate. Mine's at 9.44:1 static and it's setup for nitrous, dcr is around 8.31:1(small cam). But, I also tried to get the quench height smaller(it's at .045)and it uses a long rod setup so it has more dwell, which changes ignition timing requirements and detonation resistance(among other things). It's not as simple as just a static CR, but as a rule of thumb, aluminum headed engines in most builds can easily tolerate 10-10.5:1 static with a good tune, since the majority of those combos have decent dcr's. Advancing and retarding cam timing can change the dcr.

Hamutoff

I agree you could have a high static compression ratio 10:1 but live easy on cheap pump gas... with a low DCR however you still might not pass smog!!! (esp CA)