AJK

I have 79 with about 70K miles, just acquired. I took some compression readings as follows: Cylinder 1-155 psi, 2-135, 3-135, 4-150, 5-140, 6-135, 7-145, 8-145. I did not do a leak down nor did I check with/without oil squirted in the cylinder, so not a complete test, but what is the consensus of the condition of this engine?

Thanks

Big Dave

Normally the lowest (135) should be at least 90% of the highest (155), but there are too many variables that influence compression tests for me to have an opinion. Did you hold the throttle wide open when pressurizing all the cylinders?

hinchcliffe

AJK where in Webster? I'm in Fairport.

PorKen

I don't think that's too bad.

I did my '80 recently. Warm engine, throttle wide open, all plugs out, battery charger connected.

1-4 160PSI

5-140
6-150
7-150
8-125

I'm hoping either it's a stuck ring (car was basically sitting for 10 years) or head gasket, not a broken ring. There is light white smoke 'puffing' in the oil filler at low rpms.

But it idles fine, and revs nicely, so for now I'm fine with it.

Imo000

Those numbers are not too bad. However, I think there might be some carbon build up in the combustion chambers or on the piston tops. If you can, get the combustion chambers cleaned, then do another compression test. As stated above, the engine should be warm and the throttle fully open during the test.

mark kibort

numbers look good. I had max 155 on my 5 liter euro with 82 cams, and close to 125-140 with my old 84 . the fact that your compression is strong and is not that bad as far as the range of compressions, tell me the engine is probably in good shape.

ive never done the full throttle as part of the compression test. I always seem to forget. I think ive done it once, and all it did was reach the max pressure in one less rotation if i remember correctly

MK

slate blue

I'm sorry but I will have to disagree, if you got down to 110 psi per cylinder you would be lucky if the car started. I have two euros, one with high comp, around 200 psi per cylinder the other has 9.3 comp ratio and each cylinder has about 175. My S4 has an average of 185 psi per cylinder and its comp ratio is 9.4. So you are definately on the low side. If you had a 8.5 to 1 ratio I would say 160 to 165 is perfect. I would also consider the engines power or lack of it and it's oil usage before condeming it.

Cheers and good luck.

mark kibort

scots euro 82 10:1 engine while it ran for only 3 hours , 2 track days, put out 190 psi and some down to 175 and one at 155,and was REAL fast! (just up to the point where it seized!)

my old 79 was very strong 200-210hp at the rears with 120psi per hole too!

my S4 with the 85 cams does and has done 185 and not a psi more but even in all cylinders (maybe one or two are 190ish). S4 compression ratio is 10:1 unless you have a Aussie

my 5 liter euro with 85 US cut pistons was 155psi , while my ole 84 was in the 125 to 135 psi range. (243 rear wheel hp)

anerson has some holes way down, 90-100psi and it still can start and run 1:42 at Fontana!!

dyno is the only way to check the real condition. you know what the numbers should look like

MK

AJK

Thank you all. I think I will go ahead and believe all is well, or at least not real bad.

For Curt, I live just off Woodhull about a mile south of Webster Park. If you belong to PCA, I have been a Niagara Region member for about 25 years, just not active for the last 10 or so.

checkmate1996

I'm about to do compression test on my 928. This may sound dumb, but can you explain how you keep the throttle wide-open during the test?

GlenL

Hold the gas pedal down while cranking the engine.

Remove the fuse for the fuel pump, first.

checkmate1996

cool. thx

hinchcliffe

AJK, if you get bored stop by Tully's the first wednesday of each month after 7PM. Niagara PCA has a monthly meeting going over events and drives coming up. Let me know if you're interested, it always nice to meet another 928-er. I thought i was the only one in the niagara region.

That wasn't you a few weeks ago I saw on Rt 250 at Main Street Webster? Looked like a early red car with S spoilers?

sublimate

Can anyone explain to me why the pressures are so high? Since atmospheric pressure is 14.7psi I thought that the max you would see from a 10:1 compression engine would be about 147psi (or 125psi with 8.5:1), and that would be assuming a 100% volumetric efficency. Is there really significant overpressurization (due to pressure pulses or intake momentum) at cranking speeds? Or is there something else I'm missing?

Imo000

Engines WILL run with low compression, especially if only a couple of cylinders are low. With a couple low cylinders the engine might shake a bit at idle but anything above idle would run just like a normal one would. Even if all cylinders are below 75psi the engine might have a hard time starting but it WILL run maybe a bit on the rough side but it will run.

Excessive compression is mostly due to carbon build up in the combustion chamber. This is not good, it will cause detonation (pinging), eventually casing the ring lands to break over time.