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I've started learning more about cars, and I have a question. How do I know what compression pressure I should expect during a compression test?

With a naturally aspirated engine, I think I can use Boyle's Law. Assuming an naturally aspirated engine, I would expect the calculation to work like this.

Ptdc*Vtdc=Pbdc*Vbdc

Where P is pressure and V is volume. Tdc and bdc are top dead center and bottom dead center respectively.

Rearranging by dividing both sides by Vtdc:

Ptdc=Pbdc*Vbdc/Vtdc

I think the compression ratio is equivalent to Vbdc/Vtdc, so I can rewrite the expression as:

Ptdc=Pbdc*c

Where c is compression ratio.

It seems like Pbdc should be the same as atmospheric pressure in a naturally aspirated engine.

Okay, so let's make it concrete and assume that compression ratio is 8.8:1 and the atmospheric pressure is 1007mBar.

Ptdc=1007mBar * 8.8

Ptdc=8861mBar or 129psi

Assuming that a good compression test should measure at least 75% of that, I would expect 129psi*.75=97psi is the minimum to consider a cylinder in this engine to pass a compression test.

Is my thinking correct?

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The initial pressure is lower than atmospheric due to the pressure drops caused by filters and pipework.

The reason why volumetric efficiency is calculated.

Also why older car had vacuum gauges fitted , and some mechanics still use them to indicate valve issues etc

Then you will also need to consider that the act of compression will cause an increase in the temperature of the gas in the cylinder, so that needs to be accounted for to get the final pressure due to the compression process. So check out Charles Law.

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  • $\begingroup$ Thanks. Can VE be calculated in some straightforward way, or does that require lab equipment for measurements. The reason why I was asking this is the I haven't seen an expected compression pressure value in my shop manual for my car. Also, for a cold engine when doing a compression test, is the temp rise significant? I would have thought that compression happens fast enough that maybe only pressure needs to be accounted for, but it makes sense that temp could be a factor as well considering the ideal gas law. $\endgroup$
    – Wren T.
    Jan 23 at 7:40
  • $\begingroup$ @WrenT. I found every car workshop manual that I used had the compression pressure values stated - needed as one of the parameters for tuning / diagnosis. $\endgroup$
    – Solar Mike
    Jan 23 at 7:57
  • $\begingroup$ Here's my manual (img1.wsimg.com/blobby/go/b55bc70e-923b-4ddc-9f6f-90e4df0ddf7f/…). Having said that, I've looked through a few times, and I haven't found it. Would you mind taking a look and see if I just missed it? $\endgroup$
    – Wren T.
    Feb 25 at 21:49

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