Compression Testing

[JamesHigdon]

Ok, to my understanding a Hyperutectic piston is between cast and forged in quality and is basically a cast piston with a higher silicon content for better wear. A side-effect of the silicon is that the pistons expand more with heat so they are installed loser in the bores and with loser ring gaps. This seams to mean that you cant do a cold compression test on a hyperutectic motor because the gaps would be to big, but to try and do an accurate compression test on a hot motor seems impossible, so how do you do it? For reference the short block started out as an 89 blazer peice and was rebuilt using the stock hyperutectics with the cylinders receiving a hone for clean up and some new cast rings. Thank for any help

 

James Higdon

[Truck99]

James, hopefully someone will correct me if I'm wrong. We don't build our own motors, but here's the way I understand it... Hypereutectic pistons are cast pistons with, as you stated, a different chemical composition (more silicon.) Ring gaps shrink in all pistons with heat. The thing with hyper. pistons is that they are not strong enough to take an improper ring size (too small gap). If the ring gap completely closes, the tension will often pop the top ring land off the piston.

 

My understanding differs from yours about expansion, however. I believe that one of the benefits to hyper. pistons is that they can be run with slightly tighter piston-to-wall clearance because they actually expand less than forged pistons. The ring issue is actually a separate issue.

 

You can do a compression test on a motor with hyper. pistons. People do it all the time. The clearance differences are not enough to invalidate a compression test.

 

Disclaimer - this is the way I understand it. Someone please correct me if I'm wrong.

 

cs

[poorboy]

If you want an accurate way to check the condition of your motor do a cylinder leakdown test. This will let you know exactly how much leakage you have through rings and valves.

[JamesHigdon]

ss99, i have assumed and been told the reason hyper. rings needed more gap was because the pistons expanded more, but from what ive learned over the past few days i believe you are right. The reason i ask this questions is because ive got a motor that seems to run well but the last compression test i did on it came out very poorly and i guess i was looking for some other explinations. poorboy, ive run dry compression test before where you simply hook the gauge up and turn the motor over until you stop gaining pressure and repeat for all cylinders to see what the variation is, what exactly is a leakdown test and if anyone knows what does a wet compression test tell you that a dry one wont ? Thank you both for the help

 

James Higdon

[txtom]

James,

Go to this link for answers about leak down testing.

Link

[MIKE65]

James all a leak down test is your checking the cylinders abilitiy to hold pressure and if not what percentage is it leaking. if it is leaking you can then determine where its going, out the exhaust, out the intake valve or past the piston rings into the crankcase.

[supertx]

FS

 

One thing you can do when it is cold is look for is consistency between the clyinders if the pressure on all clyinders are pretty close the same you probally don't have a problem but if you find a clyinder that way off you need to check it

[1-Crew]

And last but not least.. The wet test (done by putting a tablespoon or so of oil in the hole before testing) differentiates between ring or cylinder problems and valve problems. The oil will temporarily seal a loose ring so if the compression results are better than a dry test, the rings are not sealing well (for whatever reason, worn bore, worn or broken rings, scored cylinder wall, etc.). If the numbers are close to the same and still low, suspect valves (bent or burned) or weak battery.