I'm kind of at a crossroads right now with a mild 355 that i'm building and could use a little insight on useable compression ratio for the engine.The basics of the package so far are a 94' 350 block ,tapped for roller cam spider and thrust plate,stock crank + rods,Speed pro NP345H Pistons,.025 in the hole(1.56 pin height),ZZ4 roller cam and 906 Vortec heads.This motor is going to be carbed also.
From what i can figure,i'll have about 9.6 to 1 compression and am a little concerned about detonation.I haven't bought head gaskets yet,so i'm wondering if i should get thick ones to reduce compression alittle or use thin ones to get the quench as good as possible?I read that motors with good quench are less prone to detonate,but also that too high compression will detonate.I've also read that vortecs are less detonation prone because of they're efficiency as well as the ZZ4 cam might help with it's descent exhaust duration and lift.Alot of factors to consider,could really use some personal experience,help!

 

i was running an 11.7:1 cr with no problems quench has also been shown to help fight detonation. Id go with a .015 compressed thickness gasket to get you in a good area. 9.6:1 is a mild build running mid grade you should be fine.

 

is your 9.6:1 CR based off a .041" thick head gasket? .015" thick is going to bump you up a few more tenths in CR.

Example, my CR is based on:
64cc heads
4.030 bore
3.75 stroket
5.7 rod
9.025 deckheight
4.060 head gasket bore
.041 thick head gasket
-16cc dished pistons

9.86:1 CR for my 383

I use this sometimes as a reality check: http://www.race-cars.net/calculators...alculator.html

 

I agree 100% with the recommendation to go for tight quench. Use a .015 gasket, EVEN IF you intend to run 87 octane fuel.
My Vortec motor with 9.4 CR and the stock cam was tuned to peak hp with 87 octane. Adding more timing than optimum not only resulted in no hp gain, also no detonation. Now with the ZZ4 cam in the crate Vortec (quench measured at .049), I've added 5 degrees more timing below 3000 rpm tapering to the same (30 deg) at high rpm WOT. I run a 180 deg tstat. Never any ping with 87 in summer weather.

 

I didn't read your post that thoroughly, but one thing came to mind;
speed pro H345's have a 1.54" compression height, NOT the 1.56" that they advertise. Be wary!

 

Thanks so much for the responses,i'm feeling alot better hearing you guy's running as much or more compression successfully with your Vortec combinations.Thats just what i was looking for.
Sonix, apparently the H345's came in two versions,the 1.54 pin heighth and the 1.56 pin heighth, i actually thought i bought the 1.54,but when i assembled the short block ,the pistons were higher than i thought they'd be.I measured one of the pistons and got 1.56 pin heighth.Did some digging and found some reference on line somewhere about two versions.Go figure.

 

I'll be damned... learn something new every day.

 

10:1 static compression ratio on premium fuel with Vortec heads and a Comp XE276HR cam ( installed straight up ).
Remember though that it's the dynamic compression ratio that you should be concerned about. The static ratio must be combined with the camshaft duration specs to determine how much cr you can effectively use.

 

That's not true, either. They came both ways. I am a professional automotive machinist, and I have measured several sets of these. I have a set of H345NCP30 pistons on my desk that are 1.560". You really have to check. If your set is already in an assembled engine, then check how far down they are at top dead center. But either way, using the FelPro steel shims at 0.015" is fine, assuming the pistons are at least 0.020" in the holes. If you got a set that is 1.540", then it's that much more important to use the thin gaskets.

 

wow 1.540 comp height on the H345NCP30 you dont say atilla.... and you managed to get in the automotive machinist line also good for you

 

I figure a stock vortec ran 9.4:1 with not much quench and a small cam. 9.6:1 with good quench and a performance cam sounds like it would be no problem.

 

so let me learn something today, too - what is quench? i've been reading some of these head gasket posts because my heads are soon to come off, so help me out here... never heard of that before when i rebuilt the engine a few years ago and used felpro xxx gaskets - can't remember which ones - so i'd like to make sure it's right this time. (not pulling the heads cuz of the gaskets, tho)

 

See this thread for a little more info on quench...
https://www.thirdgen.org/forums/tech...questions.html