compression ?

[maximo]

What is the normal and bad compression of the cylinders of 930 ?

Regards
Max

[x98boardwell]

7:1 or 7.1:1. I may be slightly off but that is close

[JohnJL]

If you mean for compression testing the figures are in psi...I'm note sure on a 930 but others will chime in.

[maximo]

Originally Posted by JohnJL

If you mean for compression testing the figures are in psi...I'm note sure on a 930 but others will chime in.

yes psi compression testing

[Noah930]

Funny. We all look for these numbers in a PPI, but I've never seen anyone really publish what the factory would consider "good" numbers.

I wonder if that's partially due to the fact that not all mechanics really know how to do a proper compression check (motor should be warm, for example). Temp will alter the values.

I've heard mechanics/shops say that numbers in the 90s-120s are OK. But, again, this seems to be more hearsay than actual figures someone looked up in a shop manual. Obviously, the values that you would get in a normally-aspirated motor (somewhere in the 160-180 range will be much higher than what you would expect in a turbo motor, with it's lower static compression. Everyone also seems to agree that the clustering of the numbers is important (most important?). As long as all 6 cylinders are within a few psi from each other, most likely the motor will be healthy.

If someone actually has seen officially published compression values for a 930 motor, I'd like to see them.

[jwasbury]

I checked mine (1979 3.3 liter) and found all cylinders were around 135, except #6 which came in at about 120. Still a good number, but I'm keeping an eye on that one. This was with a warm engine, throttle plate open.

[DonE]

Compression tests are really meant to look at compression across cylinders, not compared to other motors. For example, I had a tired OEM starter when I ran a test just after a rebuild. The numbers were in the low 120's. Then the starter went out and I replaced it with a high torque unit. Being paranoid about my engine, I ran another compression test about 3000 miles later and found I had numbers in the high 130's, and consistent across all cylinders. Nothing but the starter had been changed. If you want to get an idea of the true state of the cylinder you have to perform a leak down test.

[fredmeister]

I figure shouldn't the values be roughly compression ratio times 14.7psi plus 1 times 14.7psi.
For example at 7 to 1 compression 7x14.7 = 102.9 plus 14.7 or 117.6psi.
This assume 100% VE or Volumetric efficiency. In reality it must be less because 100%VE is not attainable at low rpm speeds especially with our manifold design.
This is alot lower number than 120 or 130psi that most guys are quoting. Just theoretically what I suspect to be the case.
Someone else chime in please.....sanity check needed.

[maximo]

Originally Posted by fredmeister

I figure shouldn't the values be roughly compression ratio times 14.7psi plus 1 times 14.7psi.
For example at 7 to 1 compression 7x14.7 = 102.9 plus 14.7 or 117.6psi.
This assume 100% VE or Volumetric efficiency. In reality it must be less because 100%VE is not attainable at low rpm speeds especially with our manifold design.
This is alot lower number than 120 or 130psi that most guys are quoting. Just theoretically what I suspect to be the case.
Someone else chime in please.....sanity check needed.

Thanks ! .... good information !

[JMA-RUF]

I was told around 115psi is around the norm for a 930 motor.

[maximo]

Originally Posted by JMA-RUF

I was told around 115psi is around the norm for a 930 motor.

Thanks JMA-RUF

[timc]

Just as critical, if not more, that the numbers are all even, say within 10% of each other.

[bcoats]

I just had a leak down performed on my 3.3l motor, warm with 6K on rebuild, OEM P&C's and leak down results were 2%-3% and compression 115 across all cylinders (one was 120) if that helps. . .

[fredmeister]

Sounds pretty close to my theoretical calculation. Well within gage error.
As long as all are within 10-15% of each other is more important than the actual number.
Leak down is a better judge of engine condition too. Those numbers look really good.

[930turbo]

10% or less is the key. Every tester will deliver a different number depending on the internal volume of the gauge & line, cranking speed, etc. 7:1 is a static measurement. Dynamic compression takes valve opening into consideration. Your actually measuring trapped stroke compression which can vary considerably based on valve timing. The achilles heel of these motors is the exhaust guides. They will wear to catastrophic failure while a compression - or even leak-down test will pass with flying colors. Listen to your valve train. If it still goes clickity clack after an accurate adjustment, beware. That's you exhaust valves rocking side-to-side in the void as it opens and closes. There's no where for the heat to go and they eventually snap off.

[JFairman]

The duration and lift of cams installed and where they are timed changes where the intake valve closes at the beginning of the compression stroke so that affects compression at starter motor speeds too.

Mine has 964 cams timed around 1.26mm and compression is around 115-120psi with a cold motor.
If you test the motor hot and like burning fingers while doing it than by the time you get to the last cylinder it will have cooled alot compared to the first cylinder you did and that will throw the comparison off to some degree.

You should also hold the throttle wide open while cranking the starter motor for about 10 seconds on each cylinder.