Compression Test Expectation
 

Engine history

239k miles original bottom end
GT heads (not cams) fitted 80k miles ago
No head skim or valve seat work, valves not lapped
Original S4 cams reused
WSM Spec cam timing used
Car achieved 164mph with lights up and roof open a few weeks ago (SatNav verified)

Test will be as follows

Warm engine
All plugs out
LH Relay removed
Fully charged battery
Full throttle
Starter for approx 5 compression cycles


Any guesses on what the average compression figure will be?

I saw values in the range 185 to 189 psig.

Same as Fred, but I'll bump it out 1 psi:

185 to 190

The numbers will vary slightly based on the gauge and fittings you use. For casual measurement I use a gauge on the end of a ~10" long hose, with a quick-connect between hose and gauge. The volume of the hose, fitting, and the tubing in the gauge itself conspire to give a lower reading. For serious stuff I have a smaller-ID tube instead of the hose, connected directly to the gauge. Especially in small-displacement engines, there's a difference. Meanwhile, you are looking as much for consistent readings (all within 5% or better). Do a little leak-down test to listen for any leaks if the readings you get are at all suspect. After all that, remember that cam timing affects cylinder filling, etc.

Ring seal is usually temperature sensitive, less so with al-al pistons and block/liners.

Make sure you have a battery charger connected while you test, as slower cranking later in the test cycle will affect the readings.

I would pull the EZK fuse (or relay) instead of the LH. That will isolate the ignition as you crank with no plugs installed. The EZK normally reads the CPS and passes that to the LH for fuel pump and pulse duration purposes, so disabling the ignition also disables the fuel injection. The coils really don't like to fire with no plugs connected, so EZK off can save the coils from possible internal flashover damage.

Thank you - that is exactly what I needed to know

As it happens I have two compression testers, one is on a relatively short hose, but the other which I have just bought is as you describe with a quick connect in the middle

The short hose tester has given some strangely high readings in my other car - 270psi, whereas I am expecting 240psi max

I want to use the 928 to compare compression testers and find an adjustment factor, as the other engine is now in pieces and I need to know what the readings should have been

I'm also quite curious to see what the 928 measures as I've never done a compression test on it before, seems remiss of me after nearly 17 years

On a fresh Valve / Head job, I've seen 220 psi, however that is on an 85/86 32v, and the compression ratios changed depending on what MY 928 you are testing and as said the gauge and methods used.

Dave K

A quote from wikipedia:

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.)

Unquote

I do not know what the dynamic compression ratio for the S4 motor should be but interesting how those numbers dropped out compared to my measured values.

Dave,

I suspect those numbers are driven by the wider LSA of the S3 cams.

Ten different compression gauges will get you 11 different results....completely dependent on calibration and how many times they have been dropped.

Forget the actual numbers you get. Look for consistency.

Fred, it will have nothing to do with LSA, and it's all about when the intake finally closes.

For well running S4 engines....anything above 185psi is golden.....Sharky put down very high dyno numbers and compression too around 200psi

PV=NRT. Not reading from Wiki so my apology. Adiabatic heating of compression will have a significant affect on the final reading. Also, if it is a mechanical gauge, there is the hysteresis of the bourdon tube works.

Continental engine works has done quite a bit of research on compression(static and dynamic) for aircraft engines, as well as tractor engines. They have found that readings as low as 75% of the 'book' value of a static comp test will still make rated power. It's not completely understood, but it is the way it works in real life.

While static comp tests are somewhat useful, compound compression tests are more revealing in finding issues with the upper cylinder. Air leaks can be heard going through the rings from the oil filler, or through the valves from the intake or exhaust. A benchmark is usually set when an engine is new and broken in for aircraft and that becomes the standard for losses along the way. If 80PSI is put in at TDC under a calibrated fixed orifice, the resulting losses should be evaluated as the engine ages.

Well I can say going from sea level to mile high has hurt my 32v noticeably, about 15-20% down on power. So a critical bit is the elevation at which your measurements are taken. See the ideal gas law above! Just one of many variables to consider. I would be most concerned with an appreciable drop across cylinders.

You are almost exactly correct around Denver. The standard pressure at sea level is 29.92 In/Hg. At 5500' elevation the pressure is 24.43 In/Hg. That presents an 18.35% decrease in molecular density around Denver, and to the west.

Sway,

You run with GT cams if my memory serves me correctly and the S3 cams have the same profile but wider LSA [110 of the GT versus 114 for the S3] and thus by definition the S3 has less [or maybe no?] overlap so logically more compression pressure in this type of test unless of course I am missing something [distinctly possible]. That our motors see similar compression pressures should be no surprise. A static compression ratio increase from 10:1 to 10.5:1 accounts for 10 psi so where does the extra 25 psi come from in the case of the S3 values Dave experienced - gauge error? Perhaps Dave can advise what compression pressure he typically sees when testing an S4 motor with the same rig to give a better reference for comparison.

If you remember Marti's recent compression test he registered 150 psig with Colin's cam- I suspected those cams may have had a narrower LSA but Colin advised they had same as the S3 cams but with a much different profile, notably a greater duration.

Instrument error is what it is but doing a calibration check from time to time ought to be common sense. Lost count of the number of times operators told me there was something wrong with their oilfield equipment when it was the instrument that was reading incorrectly!

Rgds

Fred