My buddy has a set of 540 ci blower motors that are not running right. The motors are 8.3 to 1 compression ratio with blower pistons. They are only making a 109 lbs of compression per cylinder. We think this sounds very low, the pistons and cylinders are in good shape, no signs of scuffing or detentation. We think that it is possibly a cam issue. How many pounds of compression should these motors have?

Thanks,

MM

A 8.5:1 502 should have around 150lbs
Being a blown motor the overlap on the cam may give you lower readings. Compare all the cylinders they should all be fairly close, if you have one that is low, that will be your problem, either rings or valves usually. Although cracked heads and blown head gaskets can give similiar results.

they should be within 10% of the other cylinders, if you have one thats low squirt some oil in it and re test if the reading goes up then the problem is with the rings seating if it doesnt go up its either a valve or a head gasket

If all the cylinder pressures are the same and it is actually a 8.3-1 motor I would expect to see 140-150 psi,My 8-1 540 blower motor cranks 140 psi,almost makes me think the cams are way retarded,super big with a ton of overlap or some engine parameter (combustion chamber,piston dish etc) got overlooked. A motor with less than 85 psi of cranking compression usually won't even start and your bud isn't too far from there,Smitty

Might want to try a different gauge or verify its correct too,Smitty

Slow cranking can cause lower psi readings too. Slower the engine turns, the more actual time psi can bleed off during overlap.

To make compression tests repeatable and more reliable I like to open the carbs or throttle bodies to full open.

Remove power to ignition coil of course.

Remove all spark plugs.

Crank engine for 8-10 seconds for each cylinder

Make sure batteries are charged 100% before doing above.

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That said, a typical Roots type supercharged marine/street motor will have very low compression and quite large camshaft. This leads to low cranking compression.

Why ? Two things directly effect cranking compression readings.

#1: Actual compression ratio

#2: Camshaft

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My suggestion would be to re-do the test in the fashion I have suggested, if you haven't done it this way.

Then, we'll go from there.

BTW: Make sure your compression guage connector is actually sealing off the cylinder. Not all cyl heads use same spark plug, thus guage spark plug adapter.

As for cam overlap, again you don’t want much with a blower. Quoted from the B&M blower manual: “High overlap camshafts are actually detrimental to a supercharged engine’s performance. The longer the exhaust and intake are open at the same time, the more usable air/fuel mixture is blown out the tailpipe. When choosing a cam profile to work well with a supercharger, moderate duration is desirable, but high overlap is undesirable. The best choice is a “torque” type camshaft.”

per magazone article

Thanks guys the motors are out of the boat at the engine builder getting new cams installed as I write this post.