Compression test results- Aggressive Camshafts
 

Hi Folks, wanted to ask the collective about this.

I'm looking at a 79 SC that, per the seller, has been rebuilt about 8 years ago with Mahle 'RSR' pistons, carillo rods and has Dougherty 594/595 camshafts.

The car had a compression test performed and the numbers were really low (at least from reading posts on here regarding compression results).
Cyl 1- 118 psi
Cyl 2- 119 psi
Cyl 3- 120 psi
Cyl 4- 118 psi
Cyl 5- 110 psi (there is oil seepage at the middle driver side cylinder base gasket)
Cyl 6- 115 psi

From reading up on a post that Wayne Dempsey posted, engines running with very aggressive camshafts have a tendency to give low compression readings.
Here is the post that I'm referencing.

The question is- What are acceptable low compression readings? Are these above numbers considered normal for a car that runs these aggressive cams?

I'm also getting the leak down tests today as well.

Would love to hear folk's opinion about these compression test results.

Thanks in advance!

Lots of variables, including the camshaft overlap. More important that the numbers be consistent accross the board, which unfortunately they aren't. Further investigation might be required....

Jonny, out of interest, is it only cylinder 5 that concerns you (maybe 6)? Others are all <2% spread.

A leak down test on #5 will tell you what the difference is. As for the other five, they all seem fine. Personally, I would not strip the motor down just for that one cylinder being 10 psi lower.

Compression readings
 

There are several variables here.Type of gauge is one.A Snap On Gauge usually reads less than Mac Tool one by 10%.Also if you start at #1 then #5 is last to be tested and battery might be a little lower hence lower number.Proper test is open throttle full and 5 revolutions.Leakdown is what I prefer as you can listen for leak at intake or exhaust valve and rings by removing the oil cap.2 to 5% is a healthy motor.When you get to 10% the ring gaps are approaching 1mm and time to re-ring or replace.

#5 jumps out at me, yes....

Great feedback folks, thanks!
What concerned me is that most folks have reports 180+- psi when they posted their compression test results, these numbers are a long way from 180.

My 993 compression tests out at 190-200 across all cylinders (stock 97 engine).

I'll get the leakdown tests and post them here.

That cam is 100 deg LC. This means there is a lot of overlap timing. And that overlap leads to low dynamic compression. 115-120psi range is about the best you will get.

As others have said, Uniformity of reading is key. Follow-up with Leakdown.

A borescope image will tell you how much carbon buildup there is. Lots of carbon increases the possibility of bits trapped in the valves lower compression and leakdown.

Following


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Jamie, thank you! Thank you for clarifying why these numbers are so low, this is very helpful! Agreed on uniformity. I'll share the results of the leakdown once i get it.

Jakg104, here’s another data point on an SC. Fresh top end with 20/21 cams (not much overlap at all but more than stock). Compression was all in the 145-150 range. One cylinder with 160. I wasn’t worried about lowness of numbers but the 160 seemed too varied. Followed with a leakdown which returned 2-5% across the board. The 160psi cylinder returned 2%.

Good luck with your follow ups. Hopefully engine is a goer for you.

@Glenfield- Great info and datapoint for sure, thank you for sharing. Seems like the compression goes down as the overlap increases, it seems? I'm still green regarding cams..

Yes exactly. jpnovak took the words out of my mouth above. With long duration cams like 594/595 (I think they're Webcam actually), you'll lose compression at low RPM. Part of why cars with aggressive cams have lumpy idle running. Leakdown will tell you more about the condition of your top end.

aha! That's why they sound lumpy. Good stuff, thanks!

Compression readings
 

I always get a good belly laugh when a car is posted for sale with 150,000 miles and the compression test is 180 lbs.That means the carbon build up from oil consumption is so bad it has increased CR by 2 points.That is a rare model that Porsche sold with 12.24 to 1 CR.Buy that right away.Leakdown test only.A compression test only shows the valves are moving up & down.Kind of like breasts on a bull.Useless.Ciao Fred

Leakdown is the only relevant test

FWIW, compression and leak down tests on a ~30k mile 3.2 with Web 20/21 cams. Snap-On testers...

https://amoroso.smugmug.com/1988-Porsche-911-M491-M470-Poi/Compression--Leak-Down-tests

http://forums.pelicanparts.com/uploa...1614091944.jpg

Real RSR pistons require RSR heads or the engine will be a low CR pig.
My 79 3.0 has mahle RSR style pistons 10.5 CR on stock cc heads, we're making an educated guess but we think my pistons were made for a race team to make faux RSR engines for practice/back-up.

One thing about compression tests is that the gauges vary. Actually, the Shraeder valve in the end of the hose causes the variation from one gauge to another because of varying spring rates. Using a tire valve stem Shraeder valve will really screw up the readings.

Aircraft engines use a leak down tester to measure "compression." i.e., 78/80. The first being the raw psi on the gauge at the cylinder and the 80 being the regulated input air pressure. Automotive, on the other hand, uses input regulated to 100 psi and measures leak down in a percentage of that, i.e., 95psi would be "5% leak down."

Look for the uniformity across all six, not the absolute number. In that case, yours looks fine.

Try another compression gauge or two and see if they read higher.

And yes, a big cam could lower your readings significantly and still be "good."

Famoroso, thanks for sharing, i appreciate it. That's a healthy engine! Interesting that the compression is this high with 20/21 cams..