I cannot find any mention in the forums of what good compression should be on a
2006 Cayman S
engine. Anyone know what it should be?
Porsche
's compression test doesn't a pressure created during cranking like old school testing methods. They recommend a tool that provides compressed air into the cylinder and then measures loss of that air.
I did one recently, but I live at 6,100 ft...
I did one recently, but I live at 6,100 ft and those numbers will be low compared to a sea level check.
Its always nice to see even numbers across the board. That's probably more important that whether they are all low or high, especially with the mileage that I expect your car has... much less than 100k.
I don't have the numbers on this computer, but I can post later if you are interested.
Hello Gator,
It seems that you are describing a leak down test
A
Porsche
dealer did a leak down for me but after forty five years of interpreting compression tests that is how my mind works. Leak down tests can vary shop to shop depending upon the equipment used. I think there is less variance between compression test equipment. There are already enough variant factors such as altitude, oil temperature and so on. PCormier I would like your results as I live at 5500 ft.
There will likely be some variation even in compression tests depending on how carefully they set up. The testers that screw into the
plug
hole tend to read a little different from the push and hold variety also. I don't know if there's a reason
Porsche
specifies leak down test or not. They tend to have their reasons.
Assuming perfect sealing I'd expect to see something like 120 psig at that altitude with the 11:1 compression ratio ((11 * 12.0 psia ambient)-12.0 psia= 120 psig). At least assuming the old dog hasn't lost his memory completely. I also approximated ambient assuming an one inch mercury lapse per 1000 feet.
Guesstimating leakage probably more like 105 to 110 psi. Metal rings/valves never seal perfectly especially at cranking speed.
I'm with pcormier that uniformity is the most important factor unless you really have an odd ball reading.
Greywolf,
I believe that uniformity on the leak rate is important too, ideally being between 3-5% for an engine that has it's internal organs in good health (rings, valves, etc)
.
The Compression numbers were, counter clockwise from the left-front most jug:
192, 192, 188, 195, 197, 192 psi.
I am at 6,100 ft here in Colorado Springs. I had all the plugs removed, the engine was somewhat warm since it took a while to get this far. I did NOT hold the throttle plate open. I DID remove the fuel-injection fuse. I DID crank until the psi number stopped climbing, usually 3-5 revolutions. My compression tester wasn't a cheapie, but certainly wasn't super expensive either. It is the screw into the hole type.
The greatest difference was between 197 and 188. So, 9 divided by the average of all the numbers, 192.66, or .046 , or 5% variation. This is probably crappy math so if anyone know the correct way to calculate the variance, I'm all 'ears.'
Either way, I'm happy with these numbers. I'm sure that there is that much slop in my testing methodology.
Pretty obvious I dropped a decimal in there somewhere. Sorry about that. Probably one of those pesky constants, gamma or whatnot, to account for heating from compression. The mind must be going.
FWIW that looks like pretty good uniformity to me also.
