Compression test - 944 S2

[Mac944]

Hi All,

i've had a search but was unable to find all the info I was looking for so apologies if I've missed it.

I'm trying to verify whether the results of a compression test on a year old S2 race engine is OK or not.

According the the workshop manual it appears a fresh (stock) S2 engine has a compression of 8 bar (116psi) and I spoke with a well known Porsche specialist very familiar with 944s and race prep and they seem to think around 120psi is about correct but after looking around this forum and others I've seen results from stock engines with over 160k miles with a compression of around 160-170psi with suggestions that a new S2 engine should produce around 200+PSI.

Can anyone confirm what the correct compression should be for a fresh S2 engine?

Compression test procedure I followed:

Ran engine up to temp.
Removed plugs.
Opened throttle body to full.
Fitted compression gauge and cranked 5 times, measured reading and repeated for remaining cylinders.

Many thanks in advance!

 

[Neil Haughey]

There is no way the S2 lump could produce 200 psi, that is a duff reading for sure as stock compression is only something like 10.5:1. With atmospheric air at very roughly 15 psi one can see that it would be theoretically possible to read 160 psi or so but this assumes no leakage anywhere and 100% cylinder filling when cranking over, for this reason I have also found such reported values a bit hard to believe. If the workshop manual says 116 psi then there is your answer right there surely.

 

[bmnelsc]

While searching myself I came across a formula for calculating cranking pressure as follows: Atmospheric pressure X compression ratio X Volumetric Efficiency Constant (1.15). Atmospheric pressure needs to be adjusted for your elevation. In Edmonton, Alberta, Canada that works out to 13.5 X 10.5 X 1.15 = 163.0125. I'm not sure about this Volumetric Efficiency Constant but I'm not an engineer. I would take the VEC to represent an engine in very good condition. Also remember that any carbon buildup on the heads or the ring around the top of the cylinder would affect the compression ratio as it reduces the volume of the cylinder and thus changes the ratio a wee bit as both minimum volume (piston at TDC) and maximum volume (piston at BDC) would be altered. Not sure this is really significant.

Based on this info I would take the manual spec of 116 psi to represent the minimum cranking pressure - e.g. below 116 means time for a rebuild.

 

[Mac944]

Thank you for your replies.

I know where where you're both coming from, I'd calculated the equation as the same. Reason for confusion is because within the manual I couldn't see any mention of the compression test procedure used to come to the figure of 8 bar, whether it was wet or dry etc. It just states fresh is 8 bar (116psi) and up (but presumably close to 8) and min compression 6.5 bar (95 psi).

Then I'm seeing reports on this forum and others where folks are recording values anywhere between 160 - 200+psi on a well used engine along with a second equation of 18.5 x 10.9 / 1 = 201.65psi.

My compression test results are 160, 170, 168, 170, so there either much higher than what Porsche state in the manual, or a bit more based on the first equation or low based on the second equation and based on reports of many other S2 results.

Hopefully you can see my confusion!

 

[zcacogp]

My compression test results are 160, 170, 168, 170, so there either much higher than what Porsche state in the manual, or a bit more based on the first equation or low based on the second equation and based on reports of many other S2 results.

.... or your compression text gauge isn't terribly accurate! I'm often surprised at how inaccurate a lot of garage tools are - tyre pressure testers, torque wrenches etc, when tested in a lab.

Those figures are very consistent so personally I'd not worry. How much power does the engine produce?


Oli.

 

[chrisg]

IMHO / limited experience, doing a compression test is for diagnosing:
1. Overall state of the engine (rings, carbon build up etc) vs a known figure

2. Cylinder to cylinder comparison - are they all at the same state of play, or is one noticeably lower indicating valve or ring problems.

Looking at the figures above produced I would say thats a pretty good/close spread with only one cylinder down at 8 out of 168 = 4.8% not too bad !

 

[Mac944]

It's a brand new compression test kit which has a label on that mentions it was calibrated at time of manufacture (which doesn't necessarily guarantee it's accurate but it shouldn't be far off).

Engine produced 267bhp when mapped last year. Car has competed in approx 8 events since.

Steel liners with forged internal and has an oil change (Millers 10w60) every 4-5 hours of racing.

 

[vitesse]

I have always understood that you put a drop of oil in each cylinder when you test them to improve the seal to effect a better comparison with a running engine-but could be wrong.

 

[zcacogp]

Antony,

If the tester is new (and not old, dropped, bashed and generally abused!) and the power output is good then I'd go with Chris's comments that the variation across cylinders is tiny and it sounds like a healthy engine. Don't get yourself worked up about figures that someone on the internet has posted! []


Oli.

 

[blade7]

Engine produced 267bhp when mapped last year. Car has competed in approx 8 events since.

Steel liners with forged internal and has an oil change (Millers 10w60) every 4-5 hours of racing.

Why not try another dyno run to check the output now. What material pistons were used with the iron liners and what clearance are they running. I know you say you ran the engine up to temp but by the time you pulled the plugs etc the pistons may have contracted a fair amount. The original pistons running in Alusil have similar expansion rates and run very small clearances compared to aftermarket pistons/iron liners.

 

[Mac944]

Hi Oli,

I'm not overly concerned by the figures in isolation, however it does feel as though I've lost a bit of power which is reflected in my recent lap times, we're talking by 2-3 seconds per lap. Comparing footage of the car even over the past 4 or 5 races it seems to be lacking slightly and I'm struggling to get near the pack I'm usually very competitive against.

There have been a number of changes to the car this season (not the engine) and the tyres will be past their prime at this point so that may explain part of the change but that's why I'm looking at the engine. Even if the figures are consistent, if they are lower than they should be that may explain why the car doesn't feel as quick. I also want to prevent further damage if it is indeed starting to wear out.

Thanks

 

[Mac944]

Hi Blade,

Unfortunately I don't know the tolerances off the top of my head but these were all set to correct specifications for the pistons and liners at the time. Liners were installed by a Porsche specialist race shop and rings were fitted by a very good local machine shop. I had a list of the tolerances at the time and these were checked and verified by the various shops and builders at the time. I'll try and dig them out.

Doing a dyno run is certainly an option, unfortunately the original dyno is a 4 hour round trip and I would need to hire transport to get the car there. All doable but I wanted to conduct some initial tests beforehand.

Thanks

 

[blade7]

Was it EMC, if so what do they suggest ?

 

[Mac944]

EMC lined the block and supplied some of the internals. I asked them what pressure figures I should see and they seemed to think I should be running around 120psi. On the very first turn of the engine I see 120psi but I believe the process for measuring compression means cranking the engine until the needle on the gauge stops, on that basis If I crank the engine 5-6 times a dry result gets me approx 170psi across all 4 cylinders. The head hasn't been skimmed. It's not running a low profile gasket so 170 instead of 120 seems a bit of a stretch.

 

[blade7]

Doesn't sound like a compression issue. I think a there's been some post on here about spongy cam followers on S2's, maybe whip the cam cover off and check them ?

 

[Neil Haughey]

Are you running a data logger? If the car is down on the straights from what it used to do you can look at your forward acceleration directly in the data. Even if your comparing different tracks just look for two sections that are on a similar gradient or better flat and pick two speeds, say 80 to 110 on your traces, then look at how long it is taking to get between those speeds. This is important as if the car is genuinely underperforming it could be all sorts of stuff like the fuel pump not keeping the pressure up etc. etc. an hour on the computer will resolve it one way or another.

 

[Mac944]

Thanks Blade, I will check the followers.

Thanks Neil, unfortunately I don't have a data logger at the mo, it's on my Christmas list.

I completed a full dry and wet compression test tonight, results:

Dry - 170, 172, 168, 174
Wet - 180, 190, 182, 190

Also noticed a chatty tappet, engine has always been a bit chatty but this sounds to be coming from intake side between 3 and 4, here's a vid https://youtu.be/cDuBGqDs4cs

 

[Neil Haughey]

I get a chatty tappet sometimes but it always settles down to a purr. I put it down to these types of car just not being run often enough, mine in particular only run on track once this year so far.

Are you burning much oil on track? I thought I was down on power and asked NineX to leak down test the engine last time they had the car, but as it purred and ran really sweetly they confirmed with me that it wasn't burning any more oil than at any other time other the past 5 years so didn't do the test. I am glad they didn't as I was seeing over 130 on the dash at Combe the other week (real speed around 123-124 mph).

 

[Mac944]

Hi Neil,

I don't believe we're burning any more oil than usual. It only normally requires a small top up after each session (my old engine that was very tired and worn was burning between half and a full litre).

Sounds like it's probably OK then!

Thanks for your help!

 

[Neil Haughey]

You would be amazed how seemingly odd or innocuous things can make a car feel really sluggish. On mine the air filter had gone manky, plugs a tiny bit worn and CO a bit to high but the main thing was the geo was running way to much toe in on the rear which then causes scrub slowing the car down. Ben Demetriou described my car back then as feeling really gutless. The same car now though just seems to sing up top and keep pulling, OK its only a couple of mph faster but you know how this game is, on the race track 2 mph more is everything, get that 2 mph in the corners as well and you have suddenly gone from the back to the front (well that is my hope and I am lot more than 2 mph faster in the corners).