350 block I think from a 1991 vette
flat top pistons with 4 reliefs(not sure of the volume of the reliefs)
pistons are .030 below deck at TDC
72cc heads

I was talking to the guy at the machine shop about zero clearancing this deck and using a .040 head gasket.
Im trying to setup that .040 quench area.
He seems to think that this block was running about 8.8:1, and with the .030 cut off to get to the zero deck height, he is estimating 10.5 or 11.0:1.

I know .030 is a lot to chop off but that still seems like a drastic jump. I dont have that much experience with compression ratio math but that just seems strange. Seems to me that you would have to try harder than zero deck height to achieve 11:1 with 72cc heads and a .040 gasket. Any experienced engine builders care to take a ballpark quess at this one?

 

At worst you'd be about 10:1. That's assuming the block is bored .030 over, the head gasket bore is 4.03", and the pistons are flat tops with no valve reliefs.

 

0.030 may sound like a lot to deck the block, but rebuilder pistons can be shy on the compression height so you may have to remove that much. I know my 0.060"-over 283 rebuilder slugs left a 0.045" deck clearance.

I haven't crunched the numbers but you won't get 11:1 with a zero deck clearance flat-top shortblock using 72cc heads and 0.040" thick composite gasket. A flat top piston like you describe with the thick gasket, zero deck clearance and a 64cc chamber is in the vicinity of 10.2 - 10.5:1.

The four valve reliefs are worth approx 3 cc depending on style, your 0.040" gasket about 9cc, and if your deck clearance is 0.030" that is another 8.4cc.

Do some searching and you will find compression ratio calculators all over. Eliminating that 'dead space' of 0.030" at the top of the bore and having zero deck clearance will reduce the tendency to detonate as well as increase compression - but you probably already knew that!

 

If you want a quick-and-dirty engine math aid, download this file for MS Excel or Corel QuattroPro:

Engine Calculator.xls

 

thanks

 

Vader,

First off, what program do I have to have to get that calculator to work?

Second, is that you at NastyZ28.com?

 

Your cr would be 9.48:1

Bore 4.030" stroke 3.48 Gasket bore size 4.166 gasket thickness .039" Deck clearance 0 piston valve relief volume 5cc.

Here is a link to a compression ratio calc
Go nuts

Pete's Calc Page

 

Download this calculator is the best on that I have seen on the web to this date, with this you will know your static and dynamic compression. you will input all the variable of your engine. This is a great tool to use not only for the static compression but the dynamic compression which will tell you what the best fuel/octane rating for your engine and how to avoid detonation which can KILL a engine faster than O.J....

http://cochise.uia.net/pkelley2/