Builder

Can the C/R be tested after an engine is built? I receltly purchased an assembled SBC, but the seller didn't know what the C/R was. How is that test performed?

Thanks!

Brandon Boomhauer

iTrader: (10)

(as posted in ls1 internal section)
yes, JZ'ta has a link in his signature where you enter your combustion chamber volume, deck height, pistion dish/dome/flat - cc's, head gasket thickness, bore, etc..

edit. link.
http://www.rbracing-rsr.com/compstaticcalc.html

Brandon Boomhauer

iTrader: (10)

From http://www.rbracing-rsr.com/compstaticcalc.html

Builder

Oh, yes. That part I understand. Guess I wasn't too clear with my question. I was hoping there's a way to test without taking one of the heads off. I know the engine was rebuild and I can assume 0.030 over. Is there a way to measure C/R through the spark plug hole? No exact dimensions are available...except stock SBC info with the overbore.

raejenls1

just bolt a starter to the block a set of jumper cables and stick compression gauge in it.

Brandon Boomhauer

iTrader: (10)

compression gauge to measure such as 10.8:1? please link... I was under the impression they were showing PSI of air that they hold..

JZ'sTA

iTrader: (23)

I haven't heard that one before. Maybe your talking about a different kind of compression check.
Unless you are running a iron 6.0 block or have had your aluminum block resleaved that .030 number you think is way wrong.
The max bore anyone takes a LS1 block or LS6 block is .010
Tell me your combo and I will do my best to get you as close as possible.
What style casting heads?
What block?
What pistons?
What size head gaskets?
Were the heads milled, if so do you know how much?
What size crank? Stock crank and a 4" crank are the most common.
If you can answer these question I can get you pretty close.

Builder

This is a complete, assembled, 1976 iron SBC. I have no idea what's in it (got it for $100), but it's really clean...a recent rebuild and no apparent oil leaks. I presume it's 4.000" x 3.48" SBC stock and 0.030" over for the rebuild with 5.7" rods.

Holly carb, Edlbrock intake, Hooker headers (nothing to do with C/R). Heads look like aftermarket, but no visible logo or casting numbers. I don't know if it's been decked, heads milled, rods short or long, etc.

I thought there would be a way to tell from checking through the spark plug hole with a compression gauge and a mic to check stroke.

If the stroke and bore are known, maybe the PSI translates into C/R? Is there a formula for that?

71CamaroLS1

There are two variables which prevent direct conversion of cranking compression to static compression ratio. One is valve timing - the later the intake valve closes, the more of the compression stroke is "bled off" back through the intake valve, and the lower the cranking compression will be. The other variable is simple leak down, through ring gaps, past valves, etc. which also bleeds off some cranking pressure.

The good news is, cranking pressure is still a very useful measure, if you're trying to see how much more compression you can tolerate, or if you're trying to see what fuel to use to stay out of detonation. Basic rule of thumb on an iron motor seems to be about 180 to 185 psi max on 92 octane - a bit more with modern combustion chambers.

Here are two articles for reference:

http://chevyhiperformance.com/techarticles/81679/

http://www.popularhotrodding.com/tec...eze/index.html

71CamaroLS1

Also, just remembered this online calculator:

http://www.wallaceracing.com/dynamic-cr.php

It's intended to help calculate dynamic compression based on static compression and valve timing, but it does give cranking compression as one of it's outputs (uncorrected for leak down, presumably). If you know your valve timing and cranking compression, you could keep plugging in various static compressions until you got close to your cranking pressure number, then fudge it up a couple of tenths to account for leak down.

Builder

Thanks for those links. That's sort of what I am looking for. But, it's looking like I should probably pull the heads and check everything out before making an actual determination. There's no way to tell what's been done in the rebuild, otherwise.

Thanks folks... I appreciate all of the responses.

Brandon Boomhauer

iTrader: (10)

thanks for the link on the dymanic... been curious on that..