what is a safe compression?

[SRVFAN7]

I'm building a 350 motor and I have all my parts picked out... I'm wondering what my compression ratio is going to be. I've tried calculators, but they all give me different numbers from 9.1 all the way to 12.5. Anyone have a dependable formula? Can someone figure what it will be form this info... 4.030 bore 3.48" stroke 6" rods 11cc domes pistons 66cc heads using a pretty lopey cam we'll go with .039 gasket thickness if any other numbers are needed just use a typical number for it. What ratio would be too high for a street strip car? I've read the archives and I'm fairly confused on what is good and what isn't. haha Any info would help! Thanks Tony

[Ramanstud]

we'll also want to know iron vs aluminum.

From what I understand (I'm in the same situation) you can get away with 10:1 safely on a smooth-idle cam that doesnt bleed off a lot of compression, with aluminum heads. If you're all iron, drop that by one point (9:1). Some people will go higher depending on cam and experimentation (that last .5 ratio is variable I think, depending on gas quality and the specific characteristics of your motor), or they just like adding octane booster....

However, given the minimal HP gained by a half-point or so of compression on a daily driver... it's probably not worth the risk when you consider engine knock dangers and gas prices.

my $.02

Everyone else: feel free to correct me!
:cheers:

[SRVFAN7]

the heads are aluminum... anyone know approx. what compression ill have with these specs?

[vader86]

looks to be in the low 12s my figuring, too high for pump gas

[belair55]

Assuming zero deck height and .25 ring land height compression would be 12.407:1 way to high for pump gas.

with .020 deck height comp. would be 11.705:1 better but still high.

[Taijutsu]

Why would you want a domed piston? A dome creates a slower burning combustion chamber. A flat piston would give you a better burn and lower CR. I run 10.25 w/o problems. I have a 218-224 cam, 6" rods, TFS heads, air gap and solid cooling system. I had zero problems even on 112 days w/air. I agree that conservative it best when in doubt.

[SRVFAN7]

Alright...I'm finally understanding some of these calculators and the numbers they are asking for... I also leanred some things about how neg and pos numbers work for a dome on a piston. I figure w/...

4.030 bore, 3.48 stroke, 6" rods, 67cc head, .02 gasket thickness

Ill have about a 10.0:1 The reason I was looking at domed pistons at 11cc was b/c it was in this package i was looking at and I didn't know...HA I did a lot of reading today and learned a lot..I'm a quick one! What you guys think of this combo anyways? Any suggestions on gasket thickness? Thanks!

Tony

[vader86]

If you would just change the pistons to something like dished or flat-top, you could do all of that. But 11cc domed pistons raises the CR too much.

[SRVFAN7]

yea i forgot tomention that the 10:1 compression was with -5.4 dome pistons its all good i figured it out thanks guys