minimum compression needed to ignite

You dont need any compression at all to ignite a fuel air mix, unless its deisel.

or even diesel

Compression is irrelevant for purposes of ignition- you dont need to compress your bonfire for it to light do you?!

Generally, however the more compression, the more efficient will be combustion.

On something like yout 1923 Renault, I guess it will have something in the order of a 6:1 CR, so look for about 90-100psi of pressure on your compression tester- with each cylinder being within about 10-15% of the others.

A leakdown tester would be more useful in judging engine condition though.

Tim..

I have found that below 70 psi won't run (on more modern vehicles 1960 on)

mrcheerful

Or even Diesel.

;-)

mrcheerful ( snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :

Tell that to my ol' 2cv van.

Guys, i checked compression yesterday, i came to 60psi. All 4 cilinders measured the same, so that is rather oke. A leak down is probably the best way, though. This egine has got a carburator underneath it, so it has to pull up the mixture from below. That is offcourse a lot of work. The more comp here the better the suckion seems to me. Do you agree? Pls elaborate a little more on the no comp needed to ignite. I think i follow, but i don't think it will do in a engine. No comp, no raise of comp, no torque etc.