compression numbers

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alanmcg

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after searching, i didnt find a dedicated thread to this, so thought would start one.

Q: what should the compression be on a 3.5 with stock cam and pistons? The bluebook says (for a 3.0 obviously) 142-156 psi - another book i have on '88 535 says same - 142-156.

Sven and I did (dry) compression tests on our cars today, mine were 148-152, his were 175-185. we thougt our motors were basically the same (although mine has more miles) but guess not.

what should the numbers be?
 
per Don L

The magic number is 20

so if you have 9:1 compression, you multiply 20 x 9 = 180 compression
for a m30b34 8:1 compression you would have 20 x 8 = 160 compression
 
There are lots of things that will affect the results of a compression test. Altitude, cranking speed, engine temperature, valve lash, timing chain slop, the wrong compression tester, etc.
If your static compression is 10:1 there is 10 times the volume in the cylinder at BDC than at TDC. However, actual compression doesn't start until the intake valve closes, so under test conditions it only may be 9:1.
If you are at about 3000' elevation, you will lose about 10% of your compression.
Your compression tester represents part of the chamber volume, so a properly designed and valved compression tester will give you a more accurate result.
One of the main things you are looking for in a compression test is uniformity from cylinder to cylinder. A fresh motor will have uniform numbers while a worn motor will have varied numbers since all cylinders don't degrade at the same rate. You should also observe the rate at which the pressure builds, it should be uniform from cylinder to cylinder.
 
You have confused displacement with compression.
There are 3.5 engines with 8, 9, 9.3 and 10 to 1 compression.
Which engine do you have?
 
mine motor came out of '89 735, what comp pistons would that have? and is stan's math correct that just mult piston comp by 20?
 
I have been reading up a bit on this topic since our testing. It seems that the camshaft valve timing can affect the dynamic compression greatly, in addition to some of those other things Mike mentioned. So when the intake valve closes at the beginning of the compression stoke will vary depending on whether you have the stock 262ish or a more aggressive Shrick type profile cam.

I am wondering if there is a unique approximate constant (like Stan mentions) for every type of cam, all other things being equal?

My cam was reground. Perhaps it was not a stock cam to begin with?
 
20 is way too high of an expectation. Here are some specs on a M30B35 that I found:

M30B35 Kat
Bore: 92mm
Stroke: 86mm
Displacement: 3430cc
Stroke/Bore ratio: 0.93
Piston speed: 16.3 m/s
Valve Timing: IO/IC 20o BTDC/64o ABDC , EO/EC 64o BBDC/20o ATDC
At valve lift: 0.5mm
Valve lift I/E: 10.5mm
Cylinder centre distance: 100mm
Connecting rod length: 135mm
Connectin rod ratio r/l: 0.32
Compression ratio: 9.0:1
Camshaft bearings: 4
Camshaft drive: simplex chain
Crankshaft bearings: 7

9:1 compression, the geometry of the rotating assembly and the intake valve closing @ 64° ABDC gives a dynamic compression of 7.28:1 and an expected cranking pressure of 140#. It will actually be a little lower because the intake valve duration is specified @ 0.5mm lift, so the intake doesn't really close until some point after 64° ABDC.

http://www.wallaceracing.com/dynamic-cr.php

If you run the same calculation and close your intake valve @ BDC you'll get the 20 number, but that doesn't happen in the real world.
 
Last edited:
I would put the boroscope in a bore on Alan's engine.
What date on the head and block?
What shape is the intake port?
 
b34

after talking to peter, now think my engine is a b34. came out of a 84-5 us 635, which would have b34 head, and low compression pistons (8:1) right? so does that mean my 150psi compression numbers are about right?
 
alanmcg said:
after talking to peter, now think my engine is a b34. came out of a 84-5 us 635, which would have b34 head, and low compression pistons (8:1) right? so does that mean my 150psi compression numbers are about right?
Click to expand...

Three readings over a period of a couple of weeks is the best method. Drive the car for at least an hour the day before, and make sure battery is fully charged and stone cold. Many times, a single test can give somewhat faulty results for a myriad of reasons, less than 10% loss in an old motor is nothing! and make sure valves are adjusted before the test. The good news is that these 6 bangers are hard to kill. I also use a bit of Marvel Mystery Oil in every tank of gas, have been using it in my antique motorcycles forever, regards, Rob
 
compression

Mike,
I have another car with an inline 6. I don't know how the cam timing compares with our M30's, but it reads 180psi for compression and it is setup to have a nominal slightly over 9.5:1 compression ratio. Before this motor was rebuilt, it was a nominal 8:1 ratio and compression figures were consistently in the 140-150psi range. My previous E9 was setup as a "9:1" compression ratio M30 with a Schrick 282 cam and it's compression numbers were around 170-175psi.

Based on this empirical data and my lack of technical understanding on how to compute dynamic compression ratio, Stan's rule of thumb doesn't seem too far off.
 
I think that too much weight is put on this absolute 20 number. Again, there are a whole lot of factors influencing cranking compression.

Some calculations:

P2/P1 = (V1/V2)^1.41
P2 = (V1/V2)^1.41 * P1
You use absolute pressure for these calculations, so P1 = 14.7, V1/V2 is the compression ratio, and 1.4 is the adiabatic index.
If you had a perfectly sealed cylinder with a true 10:1 compression ratio and lost no heat in the cylinder during the compression, your gauge would read over 360#. If you have a cold metal cylinder with a cold piston, some leaking around the rings, valves not closing until 65° ABDC, slow cranking speed, etc, your number will be much less. I'd be more interested in the cylinder to cylinder variations.

An interesting test would be to hook up a compression tester to a running engine. Remove one spark plug and disable the corresponding injector then measure the pressure at 1500 rpm, about 10x the cranking speed of the battery, or even higher.

Here's an interesting treatise on cam timing and compression:

http://victorylibrary.com/mopar/cam-tech-c.htm
 
M30 b35 engine will test at 180
M30 b34 euro 10:1 engine will test at 200
M30 b34 8:1 engine will test at 155
M 90 9.3: 1 engine will test at 185

There are only a few cams for us
K euro cam 264
M us smog cam 260
3 forked cam 272
The ones like the b,h and others are simply variations.
I have done dozens of tests on dozens of big six engines ( stock) own 3 testers.
There are no early heads on late engines:) you can't have a b35 with a 1986 casting date.
Alan there is a b35 engine at the yard 36 miles from Sven.
It is 1/2 off this week. 150 bucks.........
 
Just got results of a test in a US M30B34 from a E28.

Compression:
1: 155psi 4: 155psi
2: 155psi 5: 150psi
3: 150psi 6: 150psi

Leak Down
1: 2% 4: 12%
2: 9% 5: 13%
3: 12% 6: 12%
 
Stan said:
per Don L

The magic number is 20

so if you have 9:1 compression, you multiply 20 x 9 = 180 compression
for a m30b34 8:1 compression you would have 20 x 8 = 160 compression
Click to expand...

Stan
I have another data point for youl. I get slightly lower compression numbers on my all out race motor (12.5:1) as my jag street motor (9.5:1). This discrepancy between static compression and the measurement must be due to cam timing
 
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