Compression Test

[pegasus45]

HI All

I want to do a compression test on my car,

can anybody give me an idea of what sort of pressure i should be getting from each cylinder.

Its a 1970 MK 3 1300 cc .

The reason is that the rev counter is not constant and it travels from between 800 and 700 rpm.

its been suggested that i should check the compression. at this could be the reason if one is low

thanks

phill

[Nick Jones]

Wandering/hunting idle is probably not compression related.  More likely worn distributor causing ignition timing to vary or mixture slightly off.

Compression test is always interesting though.  Note that the variation between cylinders is more useful than the absolute readings.  Usual rule of thumb is no more than 10% between them.

Typical readings for your engine would be somewhere between 130 and 160 psi I would think.

Compression test should be done with the engine fully warm, all plugs removed (disconnect the coil LT wire to disable the ignition), and throttle held wide open whilst cranking.

I normally allow turn the engine over for 5 compression strokes per cylinder.  All being well you should see the gauge max-out after 3 or 4.

Nick