Compression ratio confusion, discussions with dealer/servicing

Did you take bore size into consideration when comparing the S85 to the 2JZGTE?

2jz bore is 86mm..

So the compression pressure test, is not directly related to compression ratio?

I always thought that a 10:1 engine would yield a 147psi compression test.. Maybe I have been wrong all these years :|

Has anyone else got comp test results on an S85?

There hasn't been much data posted here with results from compression tests. Here is the only relevant post I've found, which suggests 165 is the right number (but don't take one post as gospel): http://www.m5board.com/vbulletin/e6...uring-discussion/204756-m5-potential-engine-damage-need-advice.html#post2319723

Measured compression pressure will vary car to car depending on valve timing and, of course, compression ratio.
The effects of valve timing vary with cam design and the RPM at which compression is measured.
The fact that the valves are opened early and closed late in order to optimize HP at high RPM's. lowers compression pressure at cranking speeds.

Cam Timing vs. Compression Analysis


Throttle position during the compression test also affects measured compression pressure.
I always test with the throttle WFO.

Thanks for the responses guys..

I was expecting 160-180psi.. and I told the service techs this, and both (bmw head and my local head mech) said that was impossible for the compression test for this motor, considering the MAX that is in their manual is 145psi.. It makes zero sense to me, and im so totally confused :|

I thought about the cam timing etc, and the fact the engine had just had an oil change it could have needed a while to run to get the oil thru it to ensure the vanos worked correctly.

The other annoying thing is they did two comp tests, one before they replaced the oil cooler and the results were very poor, and one after the oil cooler and the results were good. Now I pray they dont want to charge me for both comp tests at $400 each, considering that either they should have only done the one after teh oil cooler was installed, or at least left the engine apart so they didnt need to do two full remove/reassembly.. I know they did two full tests cos they broke my coil pack on the second test and told me I need to buy a new one.. they said it must have been damaged prior and cracked when they removed it.. Except the fact they didnt mention in two weeks ago on the first comp test, which means it must have been removed, reinserted, removed and broken..

And I have to pay for it..

Sigh.. Nightmare..

Cam timing is the answer.

Static compression (12 to 1) is only part of the equation.

My other american V-8 can vary cranking compression from 180-230 with the only change in the camshaft...small, easy idling cams will generate big psi...while large cams just bleed off the compression at low rpm (and during a test since the engine is only spinning ~400 rpm).

Based on the facts you provided, your engine is fine.

I found this on wikipedia, for anyone else interested in knowing:

It still doesnt explain why my supra got 165psi which is HIGHER than the static CR, but it helps explain in general.

Dynamic compression ratio

The calculated compression ratio, as given above, presumes that the cylinder is sealed at the bottom of the stroke, and that the volume compressed is the actual volume.
However: intake valve closure (sealing the cylinder) always takes place after BDC, which may cause some of the intake charge to be compressed backwards out of the cylinder by the rising piston at very low speeds; only the percentage of the stroke after intake valve closure is compressed. Intake port tuning and scavenging may allow a greater mass of charge (at a higher than atmospheric pressure) to be trapped in the cylinder than the static volume would suggest ( This "corrected" compression ratio is commonly called the "dynamic compression ratio".
This ratio is higher with more conservative (i.e., earlier, soon after BDC) intake cam timing, and lower with more radical (i.e., later, long after BDC) intake cam timing, but always lower than the static or "nominal" compression ratio.
The actual position of the piston can be determined by trigonometry, using the stroke length and the connecting rod length (measured between centers). The absolute cylinder pressure is the result of an exponent of the dynamic compression ratio. This exponent is a polytropic value for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. Under ideal (adiabatic) conditions, the exponent would be 1.4, but a lower value, generally between 1.2 and 1.3 is used, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison. For example, if the static compression ratio is 10:1, and the dynamic compression ratio is 7.5:1, a useful value for cylinder pressure would be (7.5)^1.3 × atmospheric pressure, or 13.7 bar. (× 14.7 psi at sea level = 201.8 psi. The pressure shown on a gauge would be the absolute pressure less atmospheric pressure, or 187.1 psi.)
The two corrections for dynamic compression ratio affect cylinder pressure in opposite directions, but not in equal strength. An engine with high static compression ratio and late intake valve closure will have a DCR similar to an engine with lower compression but earlier intake valve closure.
Additionally, the cylinder pressure developed when an engine is running will be higher than that shown in a compression test for several reasons.
The much higher velocity of a piston when an engine is running versus cranking allows less time for pressure to bleed past the piston rings into the crankcase.
a running engine is coating the cylinder walls with much more oil than an engine that is being cranked at low RPM, which helps the seal.
the higher temperature of the cylinder will create higher pressures when running vs. a static test, even a test performed with the engine near operating temperature.
A running engine does not stop taking air & fuel into the cylinder when the piston reaches BDC; The mixture that is rushing into the cylinder during the downstroke develops momentum and continues briefly after the vacuum ceases (in the same respect that rapidly opening a door will create a draft that continues after movement of the door ceases). This is called scavenging. Intake tuning, cylinder head design, valve timing and exhaust tuning determine how effectively an engine scavenges.

I cannot explain why someone else got a different cranking compression number than yours...seemingly similiar engines. Could be many things that could impact, throttle opening, air density, battery level for cranking speed, guage inaccuracies, engine temperature...all can impact what the end result is.

Not being entirely familiar with the VANOS on the S85...that might have a big impact too as it impacts the valve timing events...although I could not explain why an oil cooler would change the test results.

I would say you can not compare two different type of engines to each other (or two S85's that have been modified with hard parts-displacement/CR/cams/etc).

I also don't think you should try to connect the static compression ratio with the cranking compression numbers. I've seen where the same engine with differences in camshafts will produce drastically different cranking compressions...

I don't know if you engine is good other than the BMW instruction that says your numbers are within spec. Another telling sign is all the numbers are fairly consistent-so all cylinders are doing equal work and one isn't 'dead' or 'dying.'

For example: I've got two Chevy V-8's...both small blocks...both are 220 psi average...but one is 11.9 CR and the other 10.9 CR. Why differences...mainly the camshaft.

Cam Timing bang on!!