RezinTexas

9k miles on 383 H/C/I/E stroker motor. No problems at all, engine runs like a champ. I decided to check the compression on each cylinder for the record. Here is what I got:

1: 225
2: 250
3: 240
4: 245
5: 240
6: 250
7: 210
8: 215

The lowest number is 16% less than the highest, and the lowest value is 10% less than the average value of 234 psi.

Is this normal? any thoughts?

RezinTexas

I looked through the record of my build measurements and I could not find any correlation from the piston to wall clearance and the piston ring gaps.

Cyl..Comp..PTW......Ring Gaps

1....225...0.0040"...0.020-0.024"
2....250...0.0046"...0.019-0.025"
3....240...0.0039"...0.018-0.024"
4....245...0.0031"...0.019-0.026"
5....240...0.0040"...0.020-0.024"
6....250...0.0034"...0.018-0.024"
7....210...0.0041"...0.019-0.023"
8....215...0.0048"...0.019-0.022"

Note: the cylinder bore measurements were taken without torque plates.

A.R. Shale Targa

iTrader: (2)

Those numbers are still really high for a pump gas engine. It doesn't surprise me that the rear two are the lowest. The rings if removed and measured for force against the bore would no doubt read the lowest in those cylinders. As the coolant enters the front of the block it is at it's lowest temperature after running through the radiator. As it draws BTUs of heat from each cylinder doing it's thing, the temperature continues to elevate. Obviously it's ability to cool is diminished by the time it reaches the back of the block.
Whenever a circle track customer of mine overheats his engine the ring seal goes to complete ****. It just takes all the tension out of them and upon tear down I can literally remove the compression rings and twist them 90 degrees without them breaking and they'll stay that way.....TOAST !!!!

RezinTexas

anything I should be concerned about with compression that high?

A.R. Shale Targa

iTrader: (2)

Detonation perhaps with a vehicle weighing 3 tons

RezinTexas

Patrick G did the tuning and he said everything was spot on, so I'm inclined to think its OK, but I don't know for sure.

Anyone know of a reference for "normal" cylinder pressures on high performance engines? My SCR is 11.35 and DCR is 8.5.

Thanks

S10xGN

iTrader: (11)

My old stock 280ZX showed 220 cranking PSI and ran OK on regular, but it was a 5 speed with 3.90 gears...

A.R. Shale Targa

iTrader: (2)

Pat G could tune a bologna sandwich to burn rubber..
There are so many factors that can change psi numbers. Hot; Cold; Oil in the cylinders; Fuel on the cylinder walls; Rings rotated and lined up....etc.
I'm sure the tune is spot on and I know fuel injection and coil on plug ignition are top notch for preventing detonation but old school mouse motors run into issues with octane at between 160-180 psi and knock sensors should work but it's difficult to hear or see the effects untill teardown.

A lighter vehicle with decent gears does help and so does old school non-reformulated gas. 220 is still making pretty stout POP in the holes !!!!