Hi all,

Just wondering if there is a rough way of telling from what engine cylinder compression you have to what the compression ratio would be.

I know that this will not be an exact science on working out compression ratio, but it must give you an approximate ratio idea.

My way of thinking is, the higher the compression the higher the compression ratio eg 10:1 and if you have a lower compression (excluding any faults) that the ratio must be lower eg 8:1.

Is anyone able to tell me factually there compression ratio and also there results from a compression test.

Would be good to get a list of results of all differing ratios and they compression result.

Thanks in advance, Kev.

Kev has to be pulled apart and measure using fluid to get the combustion chamber exact size

No other way

Yea low comp = low comp ratio. (ratio of initial volume inside combustion chamber to final volume at full stroke, as its simply a measure of how much the air is 'compressed' compared to outside. 8:1 suggests the air is compressed to one 8th the volume it originally was)

You can get a pretty good approximate of compression ratio to a few decimal points by measuring the ratios of pressure with a compression gauge. Measure the compression, and with the gauge remember its not going to be 'absolute' pressure as it doesn't take atmospheric pressure into account.

So! with your reading from the gauge, ad atmospheric pressure to this value, if ur at sea level, its 101.3kPa, 1.013bar, 1 atm... depends what units your using. This will be P2, let P1 will be the value of your atmospheric pressure. Might as well assume the pressures at sea level being yours, it doesn't change much unless your at top of some huge mountain.

Now, the relationship between CR (comp ratio) and PR(pressure ratio is the following)



Where gamma (the upper case power symbol) is the ratio of specific heats, and for air it is usually around 1.4. Note that the ratio of specific heats changes with temperature, but for say ur avg day, this will do for an estimate.

So plug in your values for gamma, P1, and P2. and you will then have a value for (V1/V2). Go 1 / (v1/v2) to obtain your compression ratio of (v2/v1)...

and wholla!


more info here: http://en.wikipedia.org/wiki/Compression_ratio#Compression_ratio_versus_overa...

Dave, I understand to get the exact ratio this is required thanks.

Chris, you bamboozled the shit out of me with that! Once I know what compression I have, can I ask you to work out an approximate ratio please.

Thanks for the replies, Kev.

haha. ur fine mate, tell me what ur getting, and whether you live on a massive hill or not....

and i will throw ya an estimate :P

Thanks Chris, I also been wondering about this for some time, now it's clear I'll go out and clean up my shed.

Hi Kev,

I think you are thinking of two different things.

Compression Ratio - this is the ratio of the total swept volume of the cylinder and combustion chamber, at bottom dead centre, compared to at top dead centre. For an engine with 8.0:1 compression, the total volume at BDC is 8 times bigger than at TDC. The only way to determine it is to measure it during the assembly process. It depends on the bore and stroke, the combustion chamber volume, any deck height volume, and any volume added by dished pistons, or removed by dome pistons, etc etc.

Compression - this is the air pressure the rising piston generates in the combustion chamber. You measure it by taking out a spark plug and inserting a pressure gauge.

They are related in a general way - a higher compression ratio is likely to give you a high compression reading on the gauge. BUT they are not exactly related. You might have a high-compression ratio engine with, say, stuffed piston rings or burned valves. It still has the same Compression Ratio but it would give a low reading on the gauge. At the other extreme you might have a fresh 36hp engine with a low CR (7:1 or less), yet it might give a good reading on the gauge because you have good rings and valves.

yep as per above theres no comparision it must be measured when apart

the above formulation i added is only a estimate, but is published and can be used for a 'rough' figure.

its certainly not as 'certain' and exact as it is by physically measuring. thats for sure , but will certainly do the job if its a curiosity thing that might be out by +/- a decimal-ish...

Using compression, i can get the values for pressure, and in turn change it to a compression ratio. Compression itself of course is NOT the ratio

And just to make it more complicated you then also factor in the valve timing.
If you have a wild cam the inlet valve will still be open for a significant part of the "compression" stroke.
That means a high performance engine can have, on paper at least, a higher compression ratio than would normally be considered safe for the fuel used, if the cam is chosen to suit.