Compression

[JEFF]

Can compression ratio be figured out from psi?

Because berimetric pressure is 14.7:1 ---- is

147 psi 10:1 comp ratio?

Does anyone know the stock psi and comp ratio

in a '72 240Z ?

- diccussions at work.... I say 147psi does not make a 10:1

engine .... my friend says it does ! who is right?

- Jeff

[MikeW]

I'd say the compression ratio could be determined from the pressure but only theoretically. Besides having the barometric pressure vary according to weather and altitude, the biggest problem would be the condition of the engine itself. How could you tell the difference between a low compression ratio and an engine with a high compression ratio with pressure leaking past the rings or valves?

[SteveK]

Yes you can but just based on atmospheric and compressed readings only--- it wont be accurate and will just give you a range, figure on dividing the PSI by 17-20 for a ball park.

If you have more data such as cam timing etc then the picture gets more accurate

[halz]

Can compression ratio be figured out from psi?

- Jeff

No! Compression ratio is a theoretical, absolute and unitless ratio. Measurements of psi gives a real number which will vary from cylinder to cylinder despite the calculated compression ratio.

For the record, my engine has cylinder pressure readings of 185-195psi for a 9.15:1 compression ratio.

[Phred]

Imagine, if you will.......

An engine with a measured, cc'd, computed compression ratio of 10.5:1.

This engine has a stock cam with the intake valve closeing at about 32 degrees ABDC. Now you decide to put in a bigger cam without changing any other engine components. This cam has a longer duration and the intake valve closes at 48 degrees ABDC. Now your cranking pressure is less than before, but you still have 10.5:1 compression ratio.

So the answer has to be "no". Cranking Pressure (PSI) can not be used to determin compression ratio.

Phred

[SteveK]

Listen to the Phred what he doesn’t know about engine building could be written on a matchbox in poster paint.

Hey Phred if your around are Nismo exhaust headers any good ?

I have an MSA jobbie that is of average build quality and trying to match the ports up is difficult at best and in some cases overlapping my exhaust ports by quiet a few mm I wondered whether to persevere or cut my losses and start with a better quality unit.

[JEFF]

Well said by all - Thanks for the replys.

Now ... What is the stock comp ratio & stock psi in a 72 240Z ?

I'm just thinking about it..

- Jeff

[MikeW]

Well said by all - Thanks for the replys.

Now ... What is the stock comp ratio & stock psi in a 72 240Z ?

I'm just thinking about it..

- Jeff

Have a look at this engine calculator.

http://www.classiczcars.com/forums/showthread.php?s=&threadid=1292&highlight=L6

If your 72 has the stock E88 head the calculator shows 8.76:1

The Factory service manual for my 1972 shows 8.8:1 and a PSI between 171 and 185.

[JEFF]

Thanks mike -

I have seen that engine calculator before ... that's one cool toy!!

I do not see flat top pistions for an L24 - my engine is .030" over with

flat tops.

- Jeff

[MikeW]

Thanks mike -

I have seen that engine calculator before ... that's one cool toy!!

I do not see flat top pistions for an L24 - my engine is .030" over with

flat tops.

- Jeff

Flat top pistons are the default for an L24.