4HUNJYHQ

hi all, just wondering if anyone can make any sort of reasonable guesstimate of a comp ratio from cylinder pressure, accross the 8 on my 400 we got minimum of 200 and max of 230 psi if thats any help. Its got a internally balanced 400 bottom end, and very ported double humps, and im a bit unsure of what fuel to put thru it if the comp ratio is going to be fairly high. Also, the heads have had "m & m performance engines" stamped on the top of the intake face, if anyone could shed some light on the origin of this engine builder, it would be greatly appreciated

cheers, Andy

allports

A bit of a grey area this 1. A cam change or cam timing change would change the reading imensely. You cant guage compression off a compression test. If its fitted with a hi-torque or a gear reduction starter it will spin faster and therefore give you more cranking pressure. It does sound a little high though. Overlap effects this reading and if the cam is a pussy it will read high on cranking. Retard the cam and it will drop the pressure off. So the best way is to puu the heads and cc them and check the deck height at the same time, if its got dome pistons put them 1/2"-1" down the bore and cc them as well as calc your compression that way. Im summary 200-230 sounds high. A worn cam cam sometimes give you a high reading at cranking speed due to the short duration of valve events and a lack of overlap.

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Smitty2

I am with allports

but one hint when doing comp tests...
all cylinders should be within about 20psi of each other

hth

Smitty

4HUNJYHQ

thanks for the reponse allports, its got a stock 400 starter on it(bolt locations are different between my old 010 350 and this 400, couldnt use my hi torque). The cam isnt very lumpy at all, it revs pretty smooth and quick, tho it has a big flat spot down low. and it hasnt been tuned with a light yet, will try to get to that this arvo, we did have to advance a fair way to get it to fire, it wouldnt start on anywhere near the stock settings. Im not real keen on tearing the heads off straight away as i need it on the road for a wedding in a couple of weeks, thanks for the the response though, i didnt realise how much of an effect cam timing had, thanks for the advice

cheers, andy

4HUNJYHQ

yeah smitty, it was the last one that went 230, the rest were 200-220, we thought it might go a touch higher, will test again after its tuned properly