Practically Classics Forum

Helping out a pal with a vintage 1932 standard 9. It has some issues, ie it wont start. The handbook gives no figures for compression so before we do a test does anyone have an idea what compression value should be

I could do with knnowing psi or Bar, maybe not on this specific car but anything of a similar age should give me a ball park idea. it's a 9hp engine

cheers

My rule of thumb is to multiply the compression ration by atmospheric pressure, ie about 15 lb/sq.in In your case, I would expect the compression ratio to be around 6:1, so anything around 80-90lb/sq.in is probably ok, but in any case, I would only worry if there is an appreciable difference between cylinders.

thanks for getting back so quickly, I think we're closer to 50-60 psi. but before it stopped it ran very nicely, just won't start again

Oh the carb is tatered but that's a different issue

I'd agree - I've never know a non-start to be due to poor compression on all 4 cylinders, I'm much more interested in the difference between cylinders, which should ideally be within 10%.

You could do a "wet" test (i.e. put a small amount of engine oil down each plug hole before the test) and if the compression pressure shoots up then you've got worn bores, but again this is a problem that develops slowly over time, and is very unlikely to give a non-start situation suddenly IMHO.

Remember when looking at compression pressures that they will normally improve on a hot engine, so as you're presumably testing it cold, then you can make a little allowance for that too.

Tried easy-start? Not a good way to regularly start an engine of course, but can be useful for diagnosis, i.e. if it tries to fire you've probably got a spark OK and vice-versa. Not likely to do too much harm on such a low compression engine.

Cheers!