What should engine compression psi be?

[wildone63]

On any given engine,or more precisely how do you calculate what it should be?
Ive never seen figures quoted in workshop manuals as to what an engines
compression psi reading should be with no leaks etc.
Is there a way to calculate what it should be?
Maybe using compression ratio or cylinder capacity etc?

[journeymanpro]

I have in my head approx 20 times the engines compression ratio.

[Boosted LS1]

150 to 200 depending on the engine or even less if it's prehistoric.

[PaulKemp]

I have definitely seen it in Haynes manuals.
For most situations consistency across all cylinders is more important.
Cranking with all plugs out and a wide open throttle, the needle will rise in steps usually 5 rotations and stop at the max pressure.

[Mroad]

I would imagine it's a combination of Compression Ratio and Boyles Law:

https://en.wikipedia.org/wiki/Compression_ratio

Bottom bit on Dynamic Compression ratio, there is a fudge factor applied to the compression ratio for thermodynamics which will vary on the engines design.



For a static CR of 10:1, 7.5:1 is used due to the losses which would give 13.7bar or 198.7psi when compared to standard atmospheric pressure.

As mentioned above 20 times the static CR sounds like a reasonable quick calculation for the ball park.

[tapkaJohnD]

wildone63 said:

On any given engine,or more precisely how do you calculate what it should be?
Ive never seen figures quoted in workshop manuals as to what an engines
compression psi reading should be with no leaks etc.
Is there a way to calculate what it should be?
Maybe using compression ratio or cylinder capacity etc?

You don't.
No compression gauge is a precision instrument, they are uncalibrated and can even vary with atmospheric presure. The only way you can compare readings is any done on two engines on the same day with the same guage.

What you CAN do is to compare the cylinders across the block. That is perfectly valid.

JOhn

[wildone63]

Thanks,certainly a mixed bag of replies here,that wikipedia page seems a bit hard to grasp.
so as a rough guide an engine with say an 8 to 1 compression ratio
should read around 160 psi ?

[Boosted LS1]

Thereabouts, maybe a tad less.

[spikeyhead]

wildone63 said:

Thanks,certainly a mixed bag of replies here,that wikipedia page seems a bit hard to grasp.
so as a rough guide an engine with say an 8 to 1 compression ratio
should read around 160 psi ?

the air goes in at atmospheric pressure, which is 15psi
It gets squeezed by the compression ratio, so if everything is perfect then multiply the 15 x 8
which gives 120 then take some off for imperfect squeezing.

[Evoluzione]

It can't be calculated exactly because the CR quoted is static, no engine created any compression (or power) whilst being static so as soon as it moves you can throw that bit out the window, or at least it's only part of a much bigger moving equation which involves cam spec and timing and cranking speed and more.
Hit Google with the term 'Dynamic compression ratio' to find out more, you'll find calculators that'll give you a rough idea.

Edited by Evoluzione on Thursday 9th April 20:17