Old & New

...or the compression was lower in the last four cylinders due to oil leaving the rings as the engine is tested (as previously mentioned), if they were done in that order.

I would have a good look at #2, 5 & 8, and go from there. I wouldn't worry about compression as much as leakdown... Worn rings will give you lots of blowby and the issues that accompany that condition, and leaking valves would fail at an accelerated rate with forced induction.

Normy

Maybe its me...but why does one side of the engine have lower compression than the others?

I had mine checked today, after reading this thread. Results:

4 4%, 175 8 5%, 165

3 4%, 170 7 4%, 165

2 8%, 170 6 5%, 165

1 5%, 175 5 15%, 160

Hmm...the mechanic that did it [I don't have a tester or a decent compressor] told me that #5 either has a gouge in the cylinder wall or that the ring gaps were lining up, and that he figured #2 was probably just a bit of carbon. All the air was going into the crank case.

But that all doesn't explain the clear trend for the right/drivers side bank to have lower compression. He said it had something to do with the cam being slightly off on the right side....

Hmm....?

N!
'85 S2 5 Speed

drnick

normy,

supercharge that car and stop worrying about the stone chips!

Fastest928

Worst case: blow a head gasket at high speed, sieze a piston, put a rod through the block and dump oil just as you enter a nice long high speed sweeper.

I have three leak down gauges to do a leak down test......one is confirmed no good....it happens. And check the car on a rolling road dyno.

Marc
DEVEK

Jim bailey - 928 International

Marc....." And check the car on a rolling road dyno. " And where would one gain access to a rolling road dyno ?? and at what cost ? Besides what would you be checking ? Your scenario stops too soon "Worst case: blow a head gasket at high speed, seize a piston, put a rod through the block and dump oil just as you enter a nice long high speed sweeper. " you left off , " slamming into a school bus load of nuns and bursting into flames just as you realized you had the 30 million dollar winning lottery ticket in your glove box ....... " now that is BAD !
Is it time for another lively rendition of the Supercharger Jingle ?? ah 1 , anna 2 , anna 3 ...........

Tony

In the words of "Goose" from TopGun
"you crack me up...you really really do"

SteveM928

You forgot about the asteroid striking the earth at that same moment too.

drnick

i dont like nuns anyway..

GoRideSno

That happened to me once. It was before I supercharged the car though. It wasn't as bad as it sounds and it sure makes for interesting conversation.

Andy K

ViribusUnits

Jim, how much credit do you put into the leak down results.

I'm of the opinion that while it'll help you isolate the problem as "rings" "intake valves" "exauste valves", or "Headgasket", I don't belive it's an accuret measure of the health of the engine.

Jim Nowak

If you think there will be some blowby mixing with your fuel, use Castrol or Valvoline synthetic. I was using Mobile One on a supercharged car and was getting some detonation. I switched to Valvoline synthetic and it cured the problem.

Jim bailey - 928 International

leak down is obviously a static test of how the rings happen to be sealing to the cylinder at that one spot where you happen to be testing the leak down . There could be major wear or scratches only at that point on the cylinder wall and or at other points . It is just one indication of condition of piston rings cylinder bores and valve sealing . Compression tests the piston is moving so it can detect more problems including issues of camshaft timing cam lobe wear etc. .

ViribusUnits

The leak down seems to tell you where the problem might be, the compression seems to tell you if you've got a problem. Does that seem correct?

Lagavulin

It is good news that all cylinders within each bank are consistent.

Given the evidence, your mechanic is correct in his assessment. It appears that your driver side camshaft is 'retarded' slightly with respect to the passenger side. That means that the intake (..exhaust too, but not important is this context) is opening later, thus closing later which allows the piston to travel further up the bore with the intake valve off of it's seat.

As a result, two things are happening simultaneously. The piston is pushing the charge out through the open intake valve, and there will be less volume above the piston once the intake closes which impacts an engine's dynamic compression ratio, and is what the gauge is reporting.

Normy

Oh cool, thanx Lag!

-I changed [with the help of said mechanic....actually he did pretty much everything while I watched and learned] my timing belt last November. Well, recently, I finally managed to put the requisite 2000 miles on the car; the belt tension gauge said it was on the edge of being too tight....I thought that it might have had something to do with this.

N!