FredR

Doc,

I doubt anyone sat down from scratch and calculated the actual compression test pressure from fundamental basics, rather someone analysed real world data and came up with a mathematical algorithm to explain the results measured. Thus the equation is a modified form of the gas law that considers the static compression ratio [gas law] and dynamic compression behaviour of the valve train. Most probably someone somewhere will likely be able to assign coefficient values based on tabulated data and thus predict what we can expect to see in real world testing. I just found it interesting that the article pulled numbers from nowhere and came up with exactly what I measured.

I see no logical reason why repeatable results cannot be obtained but quality control is everything and the human element is probably the biggest variable in play. My tire inflator pressure gauge recently took a dump. The clue was it seemed to take a long time to come up to pressure and at an indicated 30 psig it seemed to be going nowhere further. Checked the pressure with my standalone pressure gauge and the real world pressure was 60 psig! Bottom line it should be possible to agree a definitive value we can expect to see for a given motor configuration.

SwayBar

Fred, correct, I'm running GT cams.

From what I understand, overlap (..which is tuned/adusted by LSA) has no effect on cylinder-filling pressure. Overlap occurs when both the intake/exhaust valves are simultaneously open during the transition from the upward exhaust-stroke to the downward intake stroke.

On the following compression stroke, the exhaust has closed a long time ago, while the intake remains open while the piston rises in the bore and cylinder volume decreases.

When the intake finally closes, it is at that position of the piston within the bore, and the resultant volume above the piston, determines the dynamic compression ratio, and what is actually measured by the compression gauge.

Therefore, a GT intake valve closes later than an S4's which means the GT's piston is higher in the bore, thus there's less volume available for compression versus the S4 intake, and why a GT-cammed engine compression will read lower than an S4's.

From that understanding, I don't see how LSA affects the compression readings of an engine - unless I'm really missing something here.

davek9

Hi FredR, On S4's typically as you all stated, 170, to 185, some as low as 155 on non recent head work.
When one or two get in to the 30's, that's' when the engine comes out

The gauge I use has a screw on metal tube to goes down inside the spark plug wells and it uses o rings for head and extension connections.
I's not HF but a decent Auto store automotive type, I think there is a Digital one at our shared space, but never tried it (needed a battery
Also I always try to do this on a warmed over engine when possible, and I crank until the reading stops climbing (about 5/6 cranks)

Dave K

SwayBar

Okay, after writing all that, let's say we take a cam and make a second one identical to it except for increasing its LSA.

Will that affect the resultant dynamic compression ratio?

It depends...

If the LSA was increased by moving the exhaust valve out, no.

But if the LSA is increased by opening/closing the intake sooner, then yes, the dynamic compression ratio will be increased.

Fred, maybe you already said this...

FredR

The Lobe Separation Angle has nothing to do with when the valves open and/or close - it is a measure of the angle between the exhaust and inlet valve lobe peaks. Increasing the LSA reduces the overlap and as I understand causes the values generated in a compression test to increase for a common cam profile but ironically, the larger LSA results in a reduction of the maximum cylinder pressure generated [at 14 degrees after TDC] thus lower max power but not something I am fully familiar with.

I studied the concept when contemplating fitting a set of S3 cams to my S4 motor- to date I have not been sufficiently motivated to fit them.

FredR

Hi Dave,

Thanks for sharing that- a constant human element is the best comparison and given your experience I have confidence in your numbers that align with those of myself and Swaybar. My numbers were taken just after fitting my revamped S4 motor with the GTS inlet cam and 32 freshly lapped valves. I also met your "engine out" criteria- I had zero pressure on No 7 due to two bent inlet valves caused as the engine experienced some damage to cam sprocket and cam cover on the 1/4 bank when my late S4 was totalled some 12 years ago..

GregBBRD

928 engines that have a cylinder or cylinders that are up to 40% lower than the best cylinder can fool people.

Therefore, I'm very conservative when reacting to compression readings. I never trust my first compression test on a 928 engine I've not worked on, before. I need to see compression test "problems" more than once, on the same cylinder.

928 engines have very low valve spring pressure and a tiny bit of carbon between the valve and the seat can result in a low cylinder or several low cylinders.

If I have an engine with a low cylinder on an engine, I'll always make sure the pieces involved in making spark and injecting fuel are perfect and then send the car back put to be driven. Sometimes I'll even do a carbon cleaning proceedure and run a carbon cleaning additive in the fuel.

I'll then retest in a couple of weeks.

A huge majority of the time, there won't be a problem. The "bit" of carbon will be pounded/blown out and the compression will return to normal.


Of course, if there is a test that results in one (or more than one) cylinder with 0 to 30 pounds of compression, that's a completely different story.
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BIMMERMIKE

"..........run a carbon cleaning additive in the fuel."
What do you use for carbon cleaning?

docmirror

Yes. The head on the 968 has a solenoid to change the LSA between the exh cam which is the driven cam from the crank, and the intake cam where the position of the solenoid changes the LSA. I have tested the comp on a 968 with the solenoid in both positions and gotten somewhat modestly different readings. Lets remember that modern VVT engines are designed to adjust the LSA(and overall cam to crank timing) constantly. Lets not forget why there is a LSA designed into an engine from the start. HP, torque, and different engine speeds all conspire to change how the engine runs, and the peak of power. fixed cam timing engines are always a compromise where the cam(s) operates best within the range of RPM selected.

I've always considered that the overlap on the 928 was a bit too high. As I see reversion causing plenty of carbon build up in the intake, it's apparent that they had quite a bit of 'compromise' built in to the LSA overlap so that the engine would breath correctly way up at 6200, or 6600 for the GT. I'm sure that the reversion into the intake would be much less apparent if our engines spent more time at 3700-4400 RPM, rather than at a sedate 2200-2700. The 928 engine could have done well with a VVT setup for low speed operation, and then higher speed operation where more overlap is useful.

YMMV, objects in mirror, contents have settled, and may cause **** leakage.

UKKid35

My day was going so well, and then I did the compression test

Seven cylinders give 185-190 psi as predicted by everyone on here

But one is half that

How can my car still do just short of the stated max speed, with one cylinder seriously compromised?

hacker-pschorr

Read Greg's posts again.

UKKid35

Thanks, I'll just pretend he said 50% rather than 40%

What I'll actually do is run the car for another month or two and test it again

docmirror

Get this:


Know what you know before going further. It will evaluate where the leak is going and then you can make a plan. It may be able to clean up with cleaner. There's also something called the "cylinder rope trick" which might work. The rope is fed down into the cyl, and the piston is moved up to press the rope into the valve faces.

edit: running it as is for another month isn't going to fix it. Might also order a digital endoscope that fits on a laptop and have a good look at the valves in that cyl.

Speedtoys

Continental did a test with airplane engines, where they caused so much leakage that the cylnders barely held anything in a leakdown test.

But still made 95% from new HP at RPM.

Leakdown is about health, but things move so fast, theres not a lot of time for leaking to happen..you still made good power.

GregBBRD

Liqui Moly Ventil Saber. May be relabeled as Valve Clean in the US.