948cc compression

[flying]

what sort of compression psi should a 948cc be around...i noticed in the book it mentions low compression ..........

this what psi pressure i got when tested

port 1 8.5 psi
port 2 7.5 psi
port 3 7.5 psi
port 4 8.5 psi

these seem a bit low.....shouldn't it be between 10-14psi per port ?

the car is not running correctly on low revs and back firing...checked everything and im suspecting there is a head gasket leak or the valves are chocked up and not closing properly

any suggestions
thanks

[MoggyTech]

Jees I hate BAR readings, so used to PSI

Readings I would expect 9 to 10 bar, if you ever get 14 BAR I'd be amazed.

1 and 4 look ballpark, 2 3 a little low, but nothing major. Try a wet test with a little oil squirted into each cylinder. Big improvement = worn bores/rings. Little to no improvement sticking or leaking valve.

Could also be head gasket getting ready to blow between 2 & 3
Fit Payen type copper gasket only.

Also do smoke test from oil filler cap at fast idle, smoke = piston blowby.

[bmcecosse]

The minimum pressure should be ~ 150 psi (180 psi would be excellent), although an engine will run (poorly) down to about 120 psi. There is obviously something wrong with your measurements - but they are all in the same 'range' - so assuming what you have measured is actually BAR (approx 1 atmosphere = 14.7psi) then your pressures are really quite low - average of ~ 120 psi. Best way to take the pressures is with a nice hot engine, good fully charged battery, take out all the spark plugs and prop the throttle open to let some air in the engine. Then crank it round with the gauge in each hole in turn - until you get steady readings. To check if the worst of the leakage is down the bores - squirt a really good dollop of engine oil into one bore - and then immediately take a new reading on that bore - then move on to the next and so on. If the readings come up significantly (and they likely will) then that's where the pressure is going - down past the rings and pistons.
You then need to decide if it's worthwhile popping the head and sump off - and pulling the pistons out for inspection and possible new rings.