PowerLabs

So, as you know, my "9.8:1" engine compression tests out as 145PSI... Theoretically the number should be 172PSI. I am wondering if the pistons that are in there are not really -11CC chambers... Does anyone know?

The number on the piston reads "6444X05"

tjwong

According to their parts catalog its a -11cc dish. The only way to know for sure was to CC them. But from what your pictures show I would say its close comparing to a Arias piston with a -12cc dish. It looks very close as both have a very similar design.

1.8t

Contact Brian Nutter on ls1tech. He is a piston guru and a weisco rep.

TheGovernorZ06

sam,

u tore down the motor?

black06vette

Sorry Sam,

I have no input with this, I only use Diamond pistons and I've never had a problem.

Ed

PowerLabs

No, I did not. I ran the leakdown test and it came out fine, then I ran the compression test again and it came out as 145PSI again, so I am trying to figure out how the engine can have low compression when it is not leaking any pressure...

TheGovernorZ06

Oh ok I thought u did by the pics

excessive81

i assume the builder took the pics as it was going together
you got pics of the heads?

Old Timer

Cam duration, overlap, and phasing strongly affect (reduces) actual cranking compression pressure readings. Have you factored these into your theoretical compression PSI reading?



Barry

PowerLabs

Yes...

ajg1915

What heads are using and what's the chamber size ?

Also Head gasket used and thickness ?

PowerLabs

Stock LS2 heads (64cc chambers), stock GM MLS head gaskets (0.050")

ajg1915

Is the throttle plate open during cranking and how many times do you crank the engine over on each cyclinder ?

145 PSI is pretty low. Something seems amiss to me. Have you thought about pulling one head to confirm that the correct pistons went into your engine.

Not sure if you access to a boroscope and can verify the correct pistons are in there without pulling the head.

Hope you get it figured out soon.

PowerLabs

I crank it until the pressure gauge stops climbing. Probably takes 6 to 10 revolutions?
I had the head boroscoped, the piston I see is the one pictured here.

May pull a head later on, I'm too busy now though.

ajg1915

Have yor verified push rod length to see that they are the correct length ? maybe they are too short and you're not taking in enough air ?

1.8t

Pushrod length only affects preload on the lifter, not lift.

ajg1915

Up to certain point that is true, use to long of a push rod and you could have some nasty problems.

If the pushrods are too short, it could explain the loss of compression.

1.8t

If one used a pushrod long enough or short enough to cause a loss in compression, it would be painfully obvious upon installation. Either way, its not the case in this scenario.

0Arun@ChampionMotors

Where does the piston sit-in or out? This is stock stroke correct? LS6 heads can be anywhere from 63-65cc. But based on 64 I get a compression of 9.5 to 1.

ajg1915

I don't disagree with you but I've always learned to start with the basics and begin the process of elimination in diagnosing any problem.