Background:

Forged 346
Forged Lunati Pistons with .0042 piston to cylinder wall clearance (ring gaps 1st .022 2nd .026)
Absolute Stage II heads 10.8:1 compression
Comp Cams 224/228 112 +2 (installed 2 degrees retarded)

I have about 450 miles on the motor now so I thought I would do a compression test. Here are the results:

1: 128 2: 125
3: 128 4: 125
5: 125 6: 130
7: 130 8: 128

These seem very low compared to my stock shortblock compressions results. I seem to remember them all being 190+. Anybody have some words of wisdom on this? I did build this motor for a large dose of nitrous but this is little discouraging.

Thanks
Josh

 

I did mine just last week. 59cc heads, stock pistons, TSP231/237 cam. They were all right at 180psi. Your numbers are lower than what I would have expected for cranking pressure, but I'm no expert in that area.

 

Was the motor warm? New engines take some time for the rings to seal. The fact that they are all very similar makes me think that there may have been some error in the testing. Can you do a leak down test? It's a more accurate measure of ring seal.

 

The motor sat for about 2 hours before the test was done. It was warm but probably not hot enough for the forged pistons to be fully expanded. I will do a leak down on my next plug/oil change(which is right around the corner). I wonder if I did not put the fitting in tight enough...I just did it as tight as I could by hand because it has rubber o-rings and I did not want to crush or damage them. Could advancing or retarding the cam effect cranking pressure? This was my first time degreeing a cam in and I am not 100% sure I did it right. The cam is either 2 degrees retarded(what I wanted) or 2 degrees advanced(the whole ICL thing was confusing me that day). That will be my next big project...when I get my new rockers.

BTW It has not used any oil and the plugs are clean(no oil) everytime I check them.

Thanks
Josh

 

I would think that the rings have not seated yet. Im sure they partially have seated, but not all the way. Are you using dino oil for the first few changes?

 

Off the shelf 10w-30 dino oil is all that I have been putting in it. Did my first oil change at 20 miles then I did another one around 200-250 miles.

I would have thought that after 450 miles the rings would be seated. I follow the break it in like your going to drive it theory of engine building so I have been getting on it pretty hard here and there. Of course I heat cycled the engine 5 times before any driving just to make sure all of my torque settings were up to par and there were no mechanical problems.

I guess I will do another oil change and give it some more miles and check it again.

Thanks
Josh

 

Might want to run a leak down to determine for sure if something is wrong. Cam duration, advance/retard tend to vary cranking compression quite a lot.

With retard the cranking compression will be less than advanced with a given cam.

 

That cam is going to bleed off a little of your compression, especially being on a 112 ICL. The higher the ICL, the higher decrease in overall dynamic compression....

W/ my old 230/230 110 ICL cam, my motor made a whopping 230 psi of cranking comp . My current cam on a 114 ICL, made a tad over 205 psi per cyl.

Your #'s are low. Drive the motor for about 800 miles, let everything seat. Try again...

josh

 

You really oughtta' do a leakdown. It's much better at diagnostic work. It's always something small. Have you contacted your engine builder?

 

I had about 1000 miles on my new motor with AS stage 2 heads, G5X2 cam and rings at .016, .018 and I had 220 PSI. Works for me

 

Country, 220 is pushing it w/ a stock piston....JMO

josh

 

Pssssst.... new motor The only thing stock in the motor is....... nothing

 

They look pretty even.So all cylinders are the same.I would let it break in more as said.
Is there a possibilty that you might have too much lifter preload holding the valves a little?