compression check on a never run engine

[Truckedup]

Going with a friend to look at a newly rebuilt engine on a stand.. It has never been run but the oil system is primed and it has a flwheel and starter...He want to turn it over to check the compression.. It has solid lifters, a street cam and 10-1 pistons...I would expect the compression to be lower than normal on a cold never run engine..Many variables here but would you expect the compression to be maybe 100 psi or lower? Obviously all the cylinders the same would be nice..

[Tom68]

Oiled bores, should crank nice and high.
If the lifters are hydraulic and haven't all been bled you'll get variations. Priming alone won't bleed all the lifters.

[GerryP]

Biggest influence will come to cam's intake closing point and engine cranking speed. For consistency cylinder-to-cylinder, you need a good starter and a strong battery being kept on a high charge. If this is a street solid cam, it would expect to see a good engine deliver at least 160PSI even if it has never been run.

[rebelrouser]

Do a cylinder leak test, not a cranking compression test. I leak test all my engines on the engine stand, before the dyno. Then again after they are run a while, I look for the leak to be slightly better. I leak all used engines I want to purchase, really saved me some money over the years. A cylinder leak down test is not affected by cranking speed, camshaft grind, etc. On a new engine with just regular rings that have not been fired I usually see 20% to 10% leak. If it has more than that, I tear it down to find out why.

[Truckedup]

I recenty started my vintage land speed race bike after sitting in mt shop for four years....A pre lube , fresh fuel, it fired right and sounded just fine..A warm compression check showed only 75 psi....Then did a leak down test....I normally use 90 psi on the supply...

[PackardV8]

Good advice, all. Both leak-down and compression on a fresh build will tell much about cylinder finish and piston fit. A precision engine honed with a torque plate will ususally show much better on a leakdown test than will a shadetree or production house rebuild.

Today's hypereutectics fitting tightly with thin metric rings will definitely test higher than old school loose fit forgings with thick rings.

[superpursuit]

What engine is it? Assuming it is a V8 of some description? If so and if it has flat tappets you may not want to do a compression test as this will wipe all the lube off the cam & lifters & it will probably fail later if it is not re - lubed before start up.

Allan.

[SupStk]

What engine is it? Assuming it is a V8 of some description? If so and if it has flat tappets you may not want to do a compression test as this will wipe all the lube off the cam & lifters & it will probably fail later if it is not re - lubed before start up.

Allan.

This is my thought too. If some one wanted to compression test one of my flat tappet engines before the cam was broke-in he'd be told to pound sand

[Truckedup]

Chevy engine, I am not the buyer, so .....I have seen fresh engines not tuned well enough to fire quickly so someone just grinds away on the starter ....

[SupStk]

I have seen fresh engines not tuned well enough to fire quickly so someone just grinds away on the starter ....

Guess that is a pet peave, grinding away until the battery is dead and doing it again with the charger. Almost bad as the dude proudly exclaiming "I cranked it until the oil pressure came up, didn't want to fire it without pressure ya know!"