Compression ratio question

[Caleb]

i have stock CR on my 350sbc now but im about to swap heads on my truck. so last night i was playing with some numbers and came out to 9.6:1 with a .041 head gasket and 10.3:1 with .015 gasket. They are aluminum heads im putting on and ive heard they had leeway for more compression on lower octane. so if you were me would you push higher compression or lower? and could i get away with 87 octane or would i have to go with 89 or 93? i get i should probably test a listen for pinging, but im worried that if it pings even during testing that it will break something. whats the best way to go about this?

[SkinnyG]

I am running iron heads at 10.9:1, with as little squish as I can get away with. I have a cam with a lot of overlap, and it seems to be working. It does not seem to ping on 91, and I think max timing is 34 or 35 at the moment.

As I understand it, one of the the reasons this is working for me, is the 0.015" head gasket I am using - it minimizes squish, which (I think) increases turbulence in the combustion chamber, making it less prone to detonation.

But I'm no professional engine builder....

[Keith Seymore]

I wouldn't go much higher than the 10.3:1.

I had an 11:1 aluminum headed big block on the street. It was exactly the wrong compression ratio, because I couldn't run pump gas and if I ran race gas it wasn't enough compression to fully take advantage of the better fuel.

K

[franken]

Seem like a .7 difference is a lot since CR is the swept volume of the cylinder vs the combustion chamber volume. I'd say a decimal place was lost somewhere and its more like .07 or .007 in CR.

Of course I'm leaving dish, valve clearances in the piston, and the star, volume created from the gasket, which can simply be added to the stroke, right?

I would think that w/ a 3.48" stroke, adding .015" or .041" won't change things more than a few hundredths of CR.

Area=Pi*D=3.1414*4=12.56 in^2
V=A*thickness
.015*12.5656=0.188484 in^3
.041*12.5656=0.5151896 in^3
3.48*12.5656=43.728288 in^3
See how the gaskets disappear numerically and even in relation to the combustion chamber volume aren't significant.

I googled converting from cc to cuin and got 0.0610237441 so we multiply combustion chamber cc by that and guessing 64cc, we get 3.90cuin.

Go w/ the thin gaskets but you'll never notice the difference.

This is just basic math, realizing the ratios are out of bounds, and putting a few numbers in a calculator. All we need to know is how to calculate the area of a circle, and the volume of a cylinder.

[imjeff]

A lot depends on which cam you use.

[franken]

I think the OP is talking static compression.
Jeff, I think I like that list of vehicles...

[greg64]

Caleb, I'd say the answer depends on a lot of factors.
First, how hard are you going to work the engine? If you're just going to cruise around town, the higher CR will probably be fine with the aluminum heads. But, if you're going to tow, and really work the engine up big hills (I live in the mountains) then I'd go with the lower CR.
Second, the cam has a lot of effect. The more radical the cam, the more CR you can get away with (obviously within reason). If the cam is stock/mild, I'd lean towards the lower CR.
Third, how close to the top of the deck are the pistons? The further down in the hole they are, the lesser the squish, which reduces the turbulence, which reduces the mixing, which makes it more likely detonation will occur.

[Caleb]

By squish are we talking about quench height? I understand that I want between .015 and .045 or something like that. And I assumed my pistons were .025 in the hole which is pretty common for stock. I wanted a good quench height but when I saw my CR jumped up I kinda backed away. And for those wondering I used summit racing website for this. My engine shifts at 4800 never gets above 5k. Only towing I will do is a max of 2 ATVs. And I did a little research on my cam and it says it NEEDS 9.1:1 CR. cam isn't too radical but it's not stock. Max lift with the new 1.6 roller rockers will be a little over .500's.

[greg64]

Caleb, I just checked the HT383 engine here http://www.chevrolet.com/performance...ck-ht-383.html and it's 9.1 but that's using vortec iron heads. This seemed like a good baseline because it's a truck engine, and gm says it's okay to run it on 87 octane fuel.