Looking at a set of heads with 58CC combustion chambers. They would be going on a 2004 Z06.

What would my effective compression ratio be with these heads, assuming a .051 head gasket?

Thanks a ton for the help.

 

Well..............just to give you something to think about.........

The factory head gasket is about .051" compressed. And "most" LS motors have a negative deck clearance, meaning the piston protrudes out of the block, typically in the .005"-.008" range.

So.....example....we will use .006" piston out of the hole as a standard, as well as 3.898 bore, 3.622 stroke and 6.089 for rods....all which are stock numbers.

When using these as "standards"....we will manipulate the static compression ratio (SCR) using the combustion chamber and head gaskets thickness and dynamic compression ratio (DCR) is a function of SCR and the intake closing event.

***Using a 228 XER lobe on a 112 center line your ABDC (intake close point) at the base of the ramp becomes 70.5* and I will use that as a standard in these examples.*** I just used this profile to show what the DCR would be.

Example 1.....a stock head gasket thickness (.051") with a stock 243 chamber (64.5cc chamber) will yield roughly a 10.60 SRC and a DRC of 8.10. And a quench of .045".

Example 2.....a stock 243 chamber with a .045" gasket will yield a SCR of 10.80 and a DRC of 8.20. And a quench of .039"

Example 3.....a milled head to 62cc and a stock gasket yields....11.00 SCR and a 8.30 DCR. And a quench of .045"

Example 4.....a milled head (62cc) with a ..045" gasket would yield.....11.10 SCR and a DRC of 8.40. And a quench of .039"

Example 5......a milled head (58cc) with a .051" (stock) gasket would yield.....11.6 SCR and 8.8 DCR.....with a quench of .045".

Example 6.....a milled head (58cc) with a .045" gasket would yield.........11.8 SCR and 8.9 DCR.....with a quench of .039".

And remember that the gasket and chamber=SCR which will not change once the engine is together.....but the DRC will be dependent on your cam choice.

And typically anything above 8.7 DCR is considered border line on pump gas without pulling timing. And the SCR really does not mean squat....its the DCR that makes the power. But I will say it takes a decent SRC to accomplish a adequate DRC.

Here is a little insight on quench..........
Tighter "quench" (and I hate that terminology) will result in more squish turbulence (better terminology). Any increase in charge turbulence (squish, tumble, or swirl) will increase flame speed. Any increase in flame speed is a good thing, both for BMEP (less time and heat losses), and for detonation tolerance (if the normal flame front can consume the charge end gases during the fuel's autoignition delay period, no detonation will occur). So yes, you will not need as much timing for MBT, but it's not due to detonation limits, it's because the engine NEEDS less timing for MBT (a smaller burn angle results with a faster flame speed).

Now having said that, there is a limit for clearance, too tight and the squish is suppressed. How tight you can go and still have effective squish depends on the depth of the quench area of the heads/pistons (ie. the radial length).

Thanks...................Dont hesitate to ask if there is something I missed......

 

Wow, as usual you're a wealth of information. I can't thank you enough.

So the basic question is can I safely run an 11.6 CR on the street with 93 octane fuel? I know I'll have to have the engine re-tuned with a new timing curve.

Also, do any of the octane boosters really work?

 

The static compression really means nothing......just a mechanical measurement without the cam being inputted into the equation. And we all know the engine will not run without the cam. Keep in mind the DCR will dictate the RON/MON/PON sensitivity of the engine. Do you want to build an topend that must run on 93 ??
But in saying that....your static comp has to be decided with your cam selection in mind. They work together............Which opens up a whole other can of worms....cam selection.............LOL

Example 5.....from above..........was calc'ed with a 228 on a 112CL XER lobe cam.....but if we manipulate the ABDC we can see what effect it will have on the DCR. If we close it just 2* sooner your DCR goes down to 8.6, from the 8.8 thus increasing your "tuning window".
Also keep in mind.....the better the combustion chamber design, the better it will ward of detonation.

So....in a nut shell, the higher the DCR, the more "power" the engine will make....until diminishing returns......which you will not come close to in this build. Its commonly believed that 8.5-ish DCR with an LS engine is the border line with "pump" gas. But like I said chamber efficiency and design will alter the "norm".
Also....remember if you tighten up the quench, it will tend to help the chambers efficiency.
When ever you decide your path....get with guys who know "that" build. Example....if you do TEA 243's, have Brian T. spec the cam because he understands the combustion dynamics and flame propagation of that cnced chamber.
This is where most "builds" fall short....a mix & match of parts. Remember its a system, and its "all" related, every part is "talking" to the part it "works" with

Here is some DCR reading material.


Thanks..........and remember, the higher the VE...........the engine will be less tolerant of high DCR's. And for the record...I'm running 8.7 DCR on a 91 tune.