Built Motor Compression Tests

[Mike 99GSX]

I just finished installing a built 6 bolt and built head with 272 cams into the car. Everything is running(idling) fine motor wise but I've been having some clutch(drivetrain)problems so the car hasn't really been in motion to break things in. Its only been around the block a few times otherwise just idling in the garage. I decided to run a compression test on the motor before trailoring it up to my tuner to get the clutch stuff straightened out. The motor had between 170-175 PSI in all 4 cylinders. The pistons are JE 9.0-1. My question is: is it safe to say that my rings are not seated yet because the numbers are a little low? What is the general time for seating piston rings? Also, I was told that bigger cams will "skew" the compression results somewhat. I was wondering if people thought this was true and could explain to me why this would be? I was thinking that the bigger profiles could keep the valves open a little longer than stock so that would let some compression escape during a normal test but I don't know if this is right?

 

[QuickerDSM]

Originally posted by Mike 99GSX
My question is: is it safe to say that my rings are not seated yet because the numbers are a little low? What is the general time for seating piston rings? Also, I was told that bigger cams will "skew" the compression results somewhat. I was wondering if people thought this was true and could explain to me why this would be? I was thinking that the bigger profiles could keep the valves open a little longer than stock so that would let some compression escape during a normal test but I don't know if this is right?

I'd let it break in a couple hundred miles of normal driving and re-test the compression. It doesn't sound too low for those cams IMO. If you find later that they are not sealing properly, Total Seal makes a quick seat powder (required pulling the head) that you apply to the cylinder bores. It helps seat rings that have light pressures, but could also be beneficial if you have a ring sealing problem.

Without writing a book on this subject, larger cams have more overlap between the intake and exhaust timing events. This larger 'gap' bleeds off more compression during low rpms in exchange for larger top end gains. This gap, measured in degrees, is basically the time the intake and exhaust valves are open at the same time. Smaller gap (like stockers) will produce more compression at low speeds and vice-versa.

Hope that helps some.

Mike

 

[allwheelTSI]

was this engine hot or cold when you did the test? did you get the cylinders torque plate bored/honed?

 

[Mike 99GSX]

Yes the motor was hot when I did the test. I don't know what a torque plate bore means. The block was bored .020 over if that's what your asking?

 

[Ryan H]

Mike, a torque plate bore is when they use a torque plate when boring out the cylinders. Basically they bolt on a plate on top of the block to simulate the torque/pressures of having the head on the car. They do this cause when the head studs are clamped down at 90in/lbs that torque actually distorts the top of the cylinder bore very slighty. but i doubt not having it torque plate bored is the cause of this....I've read many times that babying the car when breaking it in will not let the rings seat, you need to beat the car a bit to get some decent cylinder pressures so the rings seat well. Accel from 30-60 at moderate throttle (less than 10psi) and decel with the engine breaking back down to 30 and do that a couple time, after that the rings should be broken in. If your too carefull with the engine the hash (sp?) marks will wear off before the rings are seated...hope this helps.

 

[Mike 99GSX]

Thanks for the reply. I do not what method Buschur uses to have the cylinders honed. Its a Buschur Stage 3 block and stage 3 head. Believe me, I'd love to be breaking in the motor with your method if it wasn't for my current tranny problems Oh well, I'm sure a foot of snow will fall the day I get it all straightened out