Why does my compression seem low?

[Hubbell91Tsi]

I have Ross 9:1 pistons, with a stock head and comp 264/272 cam.

But im getting around 140-150 compression and the shortblock only has 3092 miles on it.

My brothers car has 8.5:1 compression, stock cam and get 180 across the board.

Maybe im wrong but i thought by putting a bigger cam in your compression would accually increase?

Any info would be nice .

Chase

 

[BigBeans]

Thats low.

I tested my Ross 8.3:1 motor, with 264/272 cams i got 192/192/190/188.

A leakdown test would be helpful.

Try putting a few drops of oil in the cylinders, if that helps, the air is getting by the rings. If not, it's either valve or head gasket problems.

 

[JEFFREYCASTGSX]

If you did this test cold, then it will read low too.

 

[95gst16g]

Warm it up a bit and make sure you hold the throttle wide open when cranking it over.

 

[Hubbell91Tsi]

it gets 150 cold... my bros car gets 180 cold.

 

[river]

do it while the engine is warm. you gotta let the pistons/rings expand.

 

[BigBeans]

do it while the engine is warm. you gotta let the pistons/rings expand.

True, but even cold he should see better numbers.

OP, this isn't an intentionally "loose" engine is it?

 

[dsm-onster]

Was the test done with the same compression tester? Different compression testers read different numbers for the same block. I know this for a fact. I have 3

 

[Eclipsethis90]

+1 For warmup before testing to allow for the expansion of the forged pistons, and retest

 

[shadohc]

you could try to adjust the valves

 

[viprez586]

you could try to adjust the valves

Ok.

Anyway OP, you should allow everything to warm up and expand, ring end gap, piston to clyinder wall clearance, heat transfer. All those things come into play when doing a compression test or CLT test.

Make sure:

1. Engine is at normal operating temperature.
2. Connect battery charger to keep crank speed the same throught test
3. Hold or prop throttle plate at WOT
4. Remove ALL spark plugs - less resistance.
5. Crank until pressure stabilizes or crank 7-8 revolutions.

 

[JEFFREYCASTGSX]

Forged pistons vs stock pistons will expand at different rates, the most important thing is have the numbers pretty even, warm your engine up and the numbers will change.

you could try to adjust the valves

This isnt no honda.

 

[1fast97gsx]

If you have forged internals do it on a warm engine. Also ... check timing, if that's off that might affect static compression readings. I'd look for everything to be about the same and check it again warm. IF it's still not how you think it should be then do a leak down. My Ross / Eagle 8.5:1 with 272 cams was around 165-170 on a failrly cold engine.

 

[BigBeans]

OP is right.

He isn't going to see this big of a variation because the car is cold.

I have the same type of pistons with lower compression, same cams, and my numbers when cold are way higher than the OP's (185+...i think my tester might read a little high).

Assuming both my tester and the OP's are accurate, he should be making more compression than i am, not 40lbs less. Either his engine was built very loose to take abuse (and therefore more subject to variations in temp), or he has a problem.

I'd try a different tester if possible, and if your bros car made 180 with lower compression on the same tester, you know you've got issues.

 

[Hubbell91Tsi]

ok well i blew another head gasket and the pistons are very lose! Like lots of play in these... Almost like the pistons are to small or the rings are just fried... doesnt really make any sense because this shortblock only has 3200 miles on it.

Finale compression before i pulled the head back off was

130 135 140 133

 

[viprez586]

The motor was probably nice bored+ honed to the specific pistons.

When I had my motor rebuilt, each cylinder corresponded to it's own piston and piston rings. So the piston-to-cylinder wall clearance is good, and ring end gap is good. Most good engine rebuilders do this practice.

Meant to say *not* instead of nice. I can't Effing edit my posts!

 

[Hubbell91Tsi]

I think the bore is to large, no way i should have 140 compression with my setup!

 

[georgiaboy]

Did you freshen the stock head before you put it back on the new short block? Do you know the valves and seats were in good condition before reinstalling the cylinder head? Did the engine show signs of power loss over time or was the problem there all at once? Did you use an MLS gasket? If you did, was the block decked and the head shaved? I wouldn't think that the head gasket would be your problem being that the numbers across the cylinders are consistently low, but ya never know. At first glance, I would suspect that your cam timing is off. Did you check and re-check cam timing?
As mentioned, I would definately try another guage just to be sure. Also, make sure the engine is at operating temperature before you test and when you do, yank the spark plugs and make sure you have a good battery (you can even jump your car off while doing the test to make sure that cranking revolutions are high enough and this keeps the compression numbers from falling off after the battery gets winded).
Also as mentioned, individual pistons and rings should be clearanced for individual bores. Looser is faster, but there are drawbacks as well to fitting a piston loosely in the bore. Did you assemble the engine? Did you check for proper piston-to-wall clearance on each piston? Were the piston-to-wall clearances between all cylinders the same? Could the rings not be seated, yet?
Since you popped the head gasket, a leakdown test at this point wouldn't do much good, but that is what I would have recommended before the trouble. I am sorry you gotta tear it back down again, but at least this way you can recheck for mistakes. I know you'd rather be out beatin' the hell outta your ride!

-Les

 

[Hubbell91Tsi]

I didnt build the engine some no name did. I was using a stock headgasket now i have a triple layer. The block and head were not shaved.

 

[Burnett03]

Thats low.

I tested my Ross 8.3:1 motor, with 264/272 cams i got 192/192/190/188.

A leakdown test would be helpful.

Try putting a few drops of oil in the cylinders, if that helps, the air is getting by the rings. If not, it's either valve or head gasket problems.

Putting oil into the cylinder WILL NOT tell you if your rings are worn on our motors. The reason is because our pistons are dished and the oil just falls into the middle of the piston. It wont run down along the walls. Now if you had a high compression motor such as a honda B18 then ya this would work. Your compression for your setup is exactly what i had on my 9:1 build with 272 cams. The elevation is what made mine that way. I wouldn't worry to much about compression as long as they are all within 15% of each other.

 

[95talonracer]

Who built the motor and were the right steps taken for grinding the rings? Or maybe a tooth off on the cams?