Another compression problem

[The Wave]

While trying to troubleshoot another problem I did a compression test on my new engine and got rather disturbing numbers. The setup is wiseco 8.5:1 pistons on a 0.040 bored block that I bought from Car Shop Inc. This is my first engine build, and when I got the short block I noticed there was about 1mm between the side of the piston and the piston wall. I figured that they must know what they were doing and that it was normal. Now Im not so sure. I get about 120-123psi across the board. I am running HKS 272/272 with crower springs and ferra SS valves. There is no weird colored smoke coming from the exhaust. It does seem to be low on oil. I have a small oil leak, and the oil seems to be leaving much quicker then the leak is. Do you guys think they put in pistons that were for an unbored block and there is massive blowby?

 

[bastarddsm]

that is normal... forged pistons require much more clearance than the cast stockers, also your gauge compression is good as well, gauge compression is effected by many things, cam profile being one of them, and big cams are known for having very low cranking compression because the valves are open more time than smaller cams, meaning that the piston uses less of it's stroke to compress the air. the actual numbers are irrelevant, because of this matter.. what you are looking for is that the difference between the highest and the lowest is not more than 10% of the highest compression of course if the compression is ridiculously low, something like 25psi, then you have a problem, but generally as long as your over 100psi, everything is ok. also you should look for drastic changes in the compression, for example if you check it and get 120's across the board, then 2000 miles later you check it and get 110 across the board, and 2000 miles after that get 100 across the board, you most likely have a problem.

another example is my car..last time i looked i had 140 120 142 143 and it haas been like this for about 12,000 miles since i put the motor in, it get run very hard often, seeing 15-16psi on a non turbo shortblock, and loves every minute of it.

anyway good luck, and as long as your motor isn't knocking or smoking blue or white, all is probably good!

 

[The Wave]

Thanks!! Is there a way to make sure that the piston-ring/cylinder wall seal is good?

 

[turborebel]

How much oil are you adding per 500 miles?

 

[Maglin]

The top of the piston expands the most and thus their will be what looks to be excessive piston to wall clearance but if it doesn't "rattle" after it's warmed up I wouldn't worry about it. I would probably get some adjustable cam gears and you can adjust your valve overlap to bring you compression numbers up. If you are not running anything over a 20G or 50 trim you don't need so much overlap. Also your cruise mpg suffers because of it due to the lower compression.

If this is an all out drag car then you don't worry what the compression numbers are but the variance between cylinders. Most street cars 15% is ok. On high output drag motors you really don't want to see over 5% and 10% would be enough to warrant a tear down. On said drag motor you will likely be pushing 45+ PSI and running a decent amount of overlap to help scavenge the cylinders at those 8-11K RPMs.

 

[dsm-onster]

Jeremiah, your mentioning of the overlap and top end is interesting. I've run a small amount of overlap on my turbo and noticed a drop in performance up top. But, HUGE increases in spool speed / transient response! It can make almost any turbo "streetable" if you can deal w/ the seductive lope. I suppose you're discussing a drag motor where the intake manifold pressure is often times higher than the exhaust manifold pressure, correct?

 

[steel_3d]

Thanks!! Is there a way to make sure that the piston-ring/cylinder wall seal is good?

Pour a capful of oil in the spark plug hole before you test each cylinder. If your compression numbers go up drastically you might have ring problems.

 

[Dream On]

How many miles are on the motor? It takes some time for the compression numbers to increase due to the rings seating.

 

[heppm01]

This is my first engine build, and when I got the short block I noticed there was about 1mm between the side of the piston and the piston wall. I figured that they must know what they were doing and that it was normal. Now Im not so sure.

Did you measure the piston to wall clearance, or just eyeball it?

Wiseco's should be installed with about 0.006" clearance; that is about 0.15mm. If yours are 1mm, then you are in seriously deep shit.

 

[steel_3d]

.006" is pretty loose for Wisecos. Maybe if you measure at wrist pin height. At the bottom of the skirt, where they specify to measure, it should be more like .003-.004" for anything but very high horsepower engines.

I'm guessing what he's talking about is just that the piston was rocking in the bore, which is due to the fact that the piston crown is almost a millimeter smaller in diameter than the skirt, to allow for heat expansion. All pistons will rock when cold, even stockers.

Just for shits, here's the measurements of my 40-over Wisecos:

3.3835 - at the bottom of the skirt
3.3820 - 90 degrees to the wrist pin
3.3785 - just below the oil ring land
3.3540 - at the crown

 

[GVR4592]

Try a different compression tester.

 

[Maglin]

Wiseco says to run a .0025 piston to wall clearance. I have 2 sets in 2 motors and both are .020 over and both came with the same spec sheet.

One thing you also have to do is file your rings to fit. I'm going a little loose and running .0018 and .0020 for top and middle ring gap. They where all around .0013ish give or take .0001 which probably wouldn't be a problem with someone running 15 lbs of boost. Over 30+ it would be a hugh issue and cost a lot of money.

 

[GVR4592]

Wiseco says to run a .0025 piston to wall clearance. I have 2 sets in 2 motors and both are .020 over and both came with the same spec sheet.

That's what the spec sheet says, but that's also for low horsepower low boost applications. Call them and tell them you're running 20+psi and that number jumps to .0035-.0040. Just something to keep in mind, always call the manufacturer and tell them the specifics of your setup before you set any of the clearances.