Compression - What should it be?

Phil Robson

Phil Robson

Well-Known Member
Hi All,

My V8 is running a bit rough again. Between 1000 & 1500 rpm it has a rough (unbalanced) note to it, above that it is OK & it idles well.

I've run the car (10.5:1 engine) on standard unleaded for 30,000 miles of (hardish) driving and the only similar problem I've had is with the carbs which were sorted about a year ago.

I did a compression test yesterday. The range was 138 to 150 psi (6 pots were between 138 & 142). The only test that was done 30,000 miles ago was a leakage test & the range was 26-40%. Does anyone know a correlation between the two?

The book says 175 +/- 15psi for a high compression engine.

I still think it may be a fuel problem as the car still goes like the proverbial above 1500 rpm. Any ideas?

Thanks,

Phil (East Yorks RO)
 
It's been written somewhere that it's not the compression pressures that are important but that they are all equal(ish)
Your's sound OK for a mature engine
Maybe it's the requirement to have a bit of freeplay in the carb linkage that's the cause -one carb opens a bit before the other. Maybe yours is a little too much
 
Thanks for those ideas, gents.

I'll have a good look at the balancing of the carbs & report back, (Crossed-fingers!)

Phil (East Yorks RO)
 
Usual rule of thumb is that they should all be within 10%, so if you take 150 as top, 10% is 15, which puts your minimun 138 within spec.

I've also read somewhere that things like starter motor, battery, and having all the plugs removed can have an effect on the actual values, as long as its making good compression and the values are close for each cylinder, your ok.

As the others have said, I would be looking more at problems with fuel and sparks.

Richard
 
Good news!

Try the simple things first! I took the airbox off to balance the carbs & saw the problem straight away.

The throttle connection between the carbs was so loose at the near-side that the threaded mounting moved when the accelerator was pressed. Hence the off-side carb was lagging behind the action of the near-side. More noticable at the lower revs, but amazing how much better it goes in full flight aswell.

Time to enjoy!

Thanks again,

Phil. :D
 
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