Compression Ratio

Richard in Anaheim CA

Richard in Anaheim CA

Well-known member
I ran across a head in my shed from long ago.

It has the typical high compression configuration with no identifiable marks on it. I had written 182 cc on it.

Can anyone out there translate that to compression ratio?

Thanks,

Richard
Anaheim CA
 
Guten Tag Richard.

if you divide the displacement of the cylinder by the volume of the combustion chamber, the result is the compression ratio.
 
Ignoring the volume of the piston above the block deck, that is 4.5:1. So it is actually higher than stock, depending on the pistons.
 
Werner said:
Guten Tag Richard.

if you divide the displacement of the cylinder by the volume of the combustion chamber, the result is the compression ratio.
Click to expand...

I am unsure where that mindset comes from, but I don't believe your advice is accurate based on my experiences in the machine shop. Piston protrusion, piston top ring location, compressed head gasket thickness, etc. typically must be accounted for when doing the calculation.

But for the sake of discussion, let's take a stock 201 cubic inch displacement, ...and a 0.125" overbore for comparison using your formula. With a stock bore and a 182cc combustion chamber in the head, your formula would net a compression ratio of around 3.62 to 1, ...and a dismal 3.42 to 1 on the 4.000" bore engine. That is even less than what the Model-T had!! :eek:
 
BRENT in 10-uh-C said:
I am unsure where that mindset comes from, but I don't believe your advice is accurate based on my experiences in the machine shop. Piston protrusion, piston top ring location, compressed head gasket thickness, etc. typically must be accounted for when doing the calculation.

But for the sake of discussion, let's take a stock 201 cubic inch displacement, ...and a 0.125" overbore for comparison using your formula. With a stock bore and a 182cc combustion chamber in the head, your formula would net a compression ratio of around 3.62 to 1, ...and a dismal 3.42 to 1 on the 4.000" bore engine. That is even less than what the Model-T had!! :eek:
Click to expand...

Well then, if Werner's is incorrect. what is the equation you use?
 
You can look it up on the web, many examples.
Werner is partially correct.
The formula is swept volume + chamber volume + gasket volume / chamber volume + gasket volume.
Piston protrusion above/below deck, piston dome/reliefs, piston land area (dia and ring placement) and valves volume will effect the true CR.

OP's 182cc inquiry is ~5.2:1 not withstanding the above true criteria which is unknown.

There are devices available to make an actual CR measurement on an assembled engine.

Best, J
 
Jim Brierley has written a fun book about hopping up Model A engines that includes a graph for compression ratio that accounts for overbore and chamber size. This book has a lot of good information for stock builds all the way to racing builds. I think it is called "4 Bangers and Me" or something like that. He posts on here regularly.
 
Ruth said:
Well then, if Werner's is incorrect. what is the equation you use?
Click to expand...

Again, dividing displacement of the cylinder by the combustion chamber volume does not give an accurate number for calculating the compression ratio. The things I mention above affect the number outcome.
 
BRENT in 10-uh-C said:
Again, dividing displacement of the cylinder by the combustion chamber volume does not give an accurate number for calculating the compression ratio. The things I mention above affect the number outcome.
Click to expand...

I see what you're saying. So the 5.5 head I have may not really be 5.5? Oh, No!! :eek::eek:

I think Richard in Anaheim CA may have been looking for a ball park figure? :p
 
Thanks everyone.

The combustion shape has the v shape instead of the U shape stock head.

The estimates in the 3 and 4 range seem unlikely.

The 5.2 estimate was about what I had guessed BUT, I need to start over with checking the cc.
 
Richard, put a 5.5 or 6,2 head on ,that will make it pull good,I remember how good your sedan went when we toured LA in it, specially when we went to John Waynes driveway and on the 405
Lawrie from Aus.
 
johnneilson said:
There are devices available to make an actual CR measurement on an assembled engine.

Best, J
Click to expand...
You can easily do it yourself.

Very thick 1500 oil is tipped at the top dead center of the piston through the spark plug bore to its lower edge. This gives the volume of the combustion chamber.

At bottom dead center, the additional oil volume gives the cylinder volume.
 
I will second what Werner said. Nothing like data to answer a theoretical question. If you do the test make sure the piston is about to descend on the power stroke.
 
I would like to note that the valves of a four stroke engine are not closed at Bottom Dead Center. Because on the A engine are mounted standing valves, the cylinder volume can only be measured by a separate volume measurement up to the visible edge of the cylinder.
 
Werner said:
You can easily do it yourself.

Very thick 1500 oil is tipped at the top dead center of the piston through the spark plug bore to its lower edge. This gives the volume of the combustion chamber.

At bottom dead center, the additional oil volume gives the cylinder volume.
Click to expand...

That's a way, much easier with a whistler and cleaner.

John
 
Guten Morgen John.


Do you mean brake cleaner? That's very thin and seeps past the piston rings, I think.

But, please tell me, what is "a whistler" in this case?
 
I guess one could do the test with the cam gear off so that the exhaust valve did not open at the bottom of the stroke.
 
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