Need Only An Estimate on Compression Ratio When changing Heads

[Larry E.]

Alan Colvin's book indicates that the original head(3782461) on the 66 300Hp
had a chamber volume of 62.076 CC's and the 3884520 Power Pack head had a volume of 60.53 CC's. ALL THINGS BEING EQUAL (No other changes)
EXCEPT FOR THE CLY. HEAD CHANGE WHAT WOULD BE THE DIFFERENCE IN THE INCREASE OF COMPRESSION RATIO?? I DO NOT NEED AN
EXACT FIGURE>JUST AN ESTIMATE. eg. Increase of .2 or increase of .5/ CC Decrease??? Thanks in Advance. Larry

Again Only as estimate is needed. Perhaps there is a formula for C/R vs. CC Head Volume that apply to the
S/B/C. ?? Thanks In Advance>Larry

[Duke W.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

[QUOTE=Larry Evoskis (16324 Perhaps there is a formula for C/R vs. CC Head Volume that apply to the
S/B/C. ?? Thanks In Advance>Larry[/QUOTE]


... not that I'm aware of. In order to compute a new CR due to a change of any value that effects CR you need to have a baseline using accurate or at least reasonable estimates for the various parameters. If the heads are off you can measure deck clearance and if you don't have an accurate value for piston volume it's possible to make a reasonable estimate based on crown geometry. Head gasket thickness is the easiest parameter to adjust CR to your target range.

Look at the restoration documents thread at the bottom of the sticky section, download and read the compression ratio.pdf link. In addition to explaining the required measurements, it has a link to an online CR calculator.

Duke

[Larry E.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

Duke: Thanks for the response. NOW>What you are asking me to do> I am not going to do. ALL I WANTED WAS A
ESTIMATED BALLPARK NUMBER; SURELY SOMEONE OUT THERE CAN GIVE ME A EDUCATED GUESS
ON WHAT THE INCREASE OF C/R WOULD BE WITH A DROP IN THE VOLUME OF THE CC IN THE THE HEAD.
EVERYTHING ELSE WOULD BE IDENTICAL. Again: I AM NOT AFTER THE PRECISE COMPRESSION RATIO
FIGURE. Thanks again: Larry Let me repeat>
the original head(3782461) on the 66 300Hp
had a chamber volume of 62.076 CC's and the 3884520 Power Pack head had a volume of 60.53 CC's. ALL OTHER THINGS BEING EQUAL

[Larry E.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

Hard to believe no one has an estimate for me so I will give you my ESTIMATE.(BTW>The setup above is for
my friend's car) I believe for every decrease of one CC would equal .2 increase in Compression Ratio. That being
said if his L75 is true to the specs. his factory C/R of 10-1 would increase to close to 10.4 C/R. Either way this
needs the use of pure 100LL gas TO BE SAFE AND TO KEEP THE GAS FROM GUMMING UP. JMHO>LARRY

[Mark F.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

Hard to believe no one has an estimate for me so I will give you my ESTIMATE.(BTW>The setup above is for my friend's car) I believe for every decrease of one CC would equal .2 increase in Compression Ratio. That being
said if his L75 is true to the specs. his factory C/R of 10-1 would increase to close to 10.4 C/R. Either way this
needs the use of pure 100LL gas TO BE SAFE AND TO KEEP THE GAS FROM GUMMING UP. JMHO>LARRY

[Larry E.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

Thanks Mark:What you said makes sense. That is what I was looking for. Larry

[Patrick B.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

Larry- To support Mark's analysis of the order of magnitude of the change in CR with the head change, here is a direct CR computation:

The baseline compression ratio of 10:1 with the 62.08 cc head = (670 + 62.08 + V) / (62.08 + V) where 670 is the volume of one cylinder of a 327 engine in cc's and V is the net volume of the deck clearance + head gasket volume + piston dome or recess volume.

Solving for V gives the result: V = 12.36 cc's

Computing the CR with the 60.53 cc head: CR = (670 + 60.53 + 12.36) / (60.53 + 12.36)
Therefore, CR= 10.19 with all but the head held constant, which is slightly less than Mark's estimate.

The rough rule of thumb for a 327 or even a 350 is that a CR change of 1 point results from a change in combustion chamber volume of about 8cc's which includes changes in deck clearance and head gasket volume or piston dome volume.

[Larry E.]

Re: Need Only An Estimate on Compression Ratio When changing Heads

Patrick: Thanks so much for this info; you guys are right on. Larry