Curious compression test...
 

I just measured the compression on my 79SC, the numbers are as follows:

1-110psi
2-109
3-111
4-109
5-112
6-110psi

I rebuilt the engine 18 months ago, I changed the pistons to 9.3:1 and a 964 cam (the jugs are Alusil). I have about 6000 miles on the engine since the rebuild. The car was warm when I ran the test, I also had the throttle wide open.

These numbers seem a little low to me!!! Should I be concerned?

Thanks

Stu

Stu,
Those numbers are very consistent. I would be suspicious of the compression gauge. Either check another engine with that gauge or better get a second gauge to double check your engine.
Do you suspect a problem with your engine?
I just don't believe every cylinder low and so close.

Thanks for your comments.

I did check the gauge using the gauge on my compressor and it read correctly, however, I'd imagine the gauge on the compressor is not that accurate. I will get another gauge as you suggest.

The engine runs fine (it uses about 1/2 quart of oil every 1000mls). The reason i ran the test was to get a baseline, not because i thought i had a problem.

Thanks, Stu

slow cranking will keep the numbers down too.

You should also make sure to push full trottle while cranking; this will allow best flow of air into the cilinders on intake.
With throttle closed compression readings are lower.

Stu,
If the compression gauge checks out OK, I would say it is time for a leak-down test.
You did mention the throttle was open during the test.
Did you hone the Alusil cylinders?
Could be rings that haven't seated?
As John Walker mentioned "slow cranking".
If indeed the compression is that low "with the throttles open" and "cranking up to speed", a leak down should tell us where the air is going.
An answer to the low numbers would be nice.

Stu....is it a "screw in" gauge or the rubbertipped "push into the spark hole" type...the latter being unreliable on hard to reach 911 heads...& if the "screw" type is the seal good & the flex line solid ?????

my recent comp test on my 67 912.....all close but all under 80....still runs fine & doesn't get hot, like my 2.7 DOES !!!!

Gents,

Just to close out the thread:

I tested the compression with a different hose (same gauge) and recored the following (looks like I had a leak on the hose):
#1 125
#2 125
#3 130
#4 123
#5 130
#6 130

I also bought a leakdown tester from Harbor Freight (has anyone used one of these?). I recorded the following with 16psig on the inlet:

#1 25%
#2 28%
#3 22%
#4 25%
#5 25%
#6 28%

Not really sure what this means? Note, all audible leaks were past the rings.

Thanks

Stu

John, my car is at TRE while we try to figure out what is wrong with the engine (I didn't build it). It came back with compression numbers around 190 psi, which everyone thinks is very high, but I do have a high torque starter that spins the motor very fast. I was thinking that since slow starters indicate lower compression readings, then higher, faster starters would read higher? Everyone I have spoken with is not too sure about this one...

-Wayne

Ok, If 190 is high then I am curious about my compression #'s since mine range from 195 to 205 on an 88 Carrera with 9.5:1 JE's and a DC22 cam from Camgrinder. Starter was refreshed during rebuild. Have not done a leak down yet. Are the compression #'s high or does it sound about right for the combination of parts? I did double check several of the cylinders twice to make sure on the compression.

GBStu,
Well now you know where the air is going.
I'm not familiar with the Harbor Freight leak down tester.
16psig at the inlet air pressure seems very low.
Could you explain?

Doug,

Here's the link to the tester
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=94190
It was $39.99 at my local Habor Freight store.

Here's the link to the operating instructions:
http://www.harborfreight.com/manuals/94000-94999/94190.pdf

The inlet gauge is 0-100psi, the downstream gauge has a percentage scale and pegs out at about 20psig i.e. if i block the outlet then at 20psig inlet the 2nd gauge is maxed out.

Note: I took the tester apart and found that it had a .060 orifice between the gauges.

My test procedure is as follow:

1) Set the regulator pressure so the 2nd gauge read 0% (about 16psig), the tester is not flowing any gas at this time.
2) Connected the tester to the cylinder and read the % drop in pressure directly from the 2nd gauge.

So I guess what the test is telling me is that with 16 psig on the inlet I am able to maintain 12psig in the cylinder after flowing through a .060 orifce.

I hope this helps

Stu

Stu, what elevation are you at? Going from sea level to 5000 feet can change the reading by 30 lbs or so, supposedly.
Your readings sound a little low. I would expect around 140 to 145. Michaels reading sound a little high. I would expect 170 to 180 for his.
There are so many variables, the guage, the cranking system, throttle open or closed, camshaft timing, static compression ratio, altitude etc.

If I have 9.3:1 compression pistons then the sea level compression should be 9.3*14.7 = 136.7 psig

Since I live at 1200ft the compression should be 9.3*14.07 = 130.9psig

Is this correct?

Thanks, Stu

Not exactly. The intake valve closes on the compression stroke about 72 degrees after bottom dead center. The dynamic compression is lower than 9.3-1. A good dynamic compression number is in the 7.5-1 to 8-1 range. If you know all the details you can calculate it using this program:
http://kb-silvolite.com/calc.php?action=comp

The math for comparing cranking compression based on dynamic C/R, from sea level to 1200 feet I have on a spread sheet at the house. If I get a chance I will find it.

From what I calculate, you should be in the 150-160 range if you have 9.3-1 compression and 964 cams.
I used 28.6 absolute pressure.
My guess is the pistons are not 9.3-1. If you had 8.5-1 pistons a reading of 130 would be on the money.

GBStu & John,
You have already identified air passing through the rings. This is the purpose of the leak-down test.
There has been a great deal of discussion on this board about running this test.
Bottom line the L-D test and the Compression test are in agreement.
The classic use of a compression test is to spray some oil into the cylinder being tested. If the compression raises after the oil is sprayed in, the ring/cylinder fit become the primary suspect.
Stu, did you measure the cylinder bore? Did you check the ring end gap? And lastly did you run any kind of hone through the cylinders?
I suspect at least one of the above is responsible for the L-D and Comp. readings.
If you are somewhat satisfied with the engine performance, run the car as is.
You have a baseline now for any future checks.
I suspect new cylinders and rings are somewhere in your future.
If you had nikasils, I'd be more optimistic.
Important question!!
Did you get pistons for alusil cylinders? As you may already know they are different than nikasil pistons.
Good luck Man,