Porsche Enthusiasts Club Forum

Hi All,

i've had a search but I'm a bit confused so apologies if this has already been covered elsewhere.

I believe it mentions the the workshop manual that a fresh (stock) S2 engine has a compression of 8 bar (116psi) and I called a well known Porsche specialist earlier today and was told 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 over 180psi 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?

Dry 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!

My S engine had about 190psi across the cylinders, on research I found that was fine. What were your results?

Between 160-170psi eek!

It's been steel lined and has forged internals and was kicking out substantially more bhp than stock so I'm guessing it should really be over 200psi then.

As there's no 1 cylinder that's substantially less than another, are there any ideas what may cause such a loss of pressure consistently across all 4 cylinders in a relatively short period of time?

Thanks

My s2 last set of compression readings were between 187 and 192 psi if this help in anyway.

regards

Mark

Combustion chamber volume,piston compression height and cam profile will all affect cranking pressure's on a modified engine.
The pressure's seem consistent across the cylinder's,are you trying to track a problem?

I'm a bit concerned I've lost performance over the last few races. Straight line speed doesn't seem what it was just a few races ago.

I wanted to check compression initially but if it's down then presumably that would explain why some of the performance may have dropped off but as you've spotted, cylinders are pretty consistent hence checking what compression should be and if it is low what I need to check to rectify it.

Thanks

You could do a wet test by oiling the bores to discount any bore wear/valve problems.

True, I should have done that at the same time. If all is good presumably the figures should go up slightly.

If there was bore or valve wear/issues these should be picked up in a dry test anyway shouldn't they? Unless all bores or all valves have started to go bad at the same time and at equal rates, which is unlikely isn't it?

Hi

I am an ex race mechanic... i think you are misunderstanding what a wet test does....

IF the wet test shows NO increase (or very little) it means your bottom end is fine.....

So you therefore have a top end (head or valve or camshaft) problem....

IF the readings go up drastically on a wet test, you have a BOTTOM END problem... (pistons or bores or very unusually crankshaft)



By the way you SHOULD do both tests together , FIRST DRY then WET afterwards >>>> with the engine at NORMAL OPERATING TEMPERATURE....NOT cold....

I hope this helps...

All the best Brett

Hi Brett,

Many thanks, to be honest this is the first time I've done a compression test as you could probably tell and it's a bit of a learning curve.

Given the dry results I have do you think the compression is lower than it should be? Engine was generating 267bhp.

I will retest in the next couple of days.

Thanks again

Hi

I would need to know if engine was hot or cold when test was done ??

Your results may be low because you did it with the engine COLD....

You MUST be at full temp (and i dont mean run up to temp on gauge...I mean after a good blast for 10 or 15 mins) and do BOTH wet and dry tests....

All the best Brett

Brett on a healthy engine with no issues how would wet test results differ from dry?

New to this compression stuff too

Hi

Apologies to you and others , as an ex race crew chief I sometimes forget others dont have my ancient knowledge...lol

Ok, let me explain Compression tests....

First temperature...

Both tests SHOULD ALWAYS be done with engine up to FULL temperature after a 15 minute run...

The reason for this is that all metals expand with heat...now this means in this case...pistons/piston rings/bores and even the crankshaft and both rod and main bearing on the crank....plus all top end parts... camshaft, head/valves/springs etc....

Now when you are testing the compression you are basically testing whether the pistons or more specifically the piston RINGS are making a good seal with the bores...if the piston rings are worn...or the bores are worn then the test WILL read LOW...

So if you do a DRY test and the readings are LOW, you then do a WET test (i.e you add a few squirts of oil down the plug holes which then TEMPORARILY seals the ring to bore gaps) ...

IF the readings then go UP a lot...say from 120 psi to 200 psi, then you KNOW the the problem is at the BOTTOM end of the block i.e worn bores or worn rings or extremely rarely and usually only on race cars running huge hp it CAN be bearings or crank wear...

That last one is extremely rare on a road car...which is good as it usually entails a new crankshaft plus rod and main bearings as a minimum...

NOW... if the reading do not go up a lot...maybe only 5 or 10 psi when doing the wet test, then its the top end of the engine....one or several of the head parts....so the good thing is you know you only have to pull the head, and you can leave the engine alone and in the car...

If its a head problem its amost always valves either not seating properly or never having been seated properly....

What i mean by that is on the race engines i used to work on (Hemi 7 ltr engines) there are 8 valves per head and 2 heads (similar to my 928 V8)

Thats 16 valves that I used to HAND LAP into the heads and on average thats around 1 hour per valve to do properly.... to get A PERFECT SEAL...

The low reading at the top end (head) CAN also be a worn camshaft or incorrect valve springs or worn springs.... but both those last two are rare....particularly if the engine was running ok before the problem occured,,,,and also rare on a road car....

The 2 MOST important things here are... engine FULLY HOT and do BOTH TESTS ..wet AND dry....

Phew.... Anyway I hope that helps.... any questions ask away.....

All the best Brett

Hi Brett,

Thanks for the useful info.

Did the another compression test tonight, unfortunately the compression test hose went bang during the wet test on the first cylinder. It was a brand new hose rated to 300psi, anyway perhaps I used slightly too much oil or it was just a duff hose.

In any case I manage to get some partial results. The dry tests were largely the same as before, the one wet test on cylinder 1 seemed to jump quite a bit.

Dry - first turn of engine was 120psi across all 4 and by 5th turn around 165psi.
Wet (only on cylinder 1) - first turn 150psi and by 5th turn 190psi.

I'll get another hose and retest but it seems from what you mentioned in your previous post that there's an issue with the bottom end. Rings is one thing but really hoping it's not the liners, was a fresh build just over a year ago.

Although I don't have the full wet test values if they are also consistent at around 190 whilst dry is consistent around 160 is there one thing it's likely to be?

Thanks

IMHO unless you spend a small fortune on a compression gauge the values given are an indication, nothing else.

To put things into perspective here is what my Gunsuns gauge read on the following cars:

Ford Mondeo 2.0 Mk3 petrol with almost 400k on the clock -250PSi - Other Mondeos on "good" cylinders read a similar figure
Nissan 180SX S12 (CA18ET engine) - 180PSi on a turbocharged engine!
MGB on an 15k Ivor Searle unit - 180PSi
Triumpg Stag - 175PSi
Pug 205 GTi -180PSi.

If it were me and you really wished to have a picture of true engine health I would find someone with a leakdown tester. That will give you a far more conclusive picture as to what is actually wrong with the engine. If you have a compressor you can buy a leakdown tester for around £75 IIRC.

Putting oil down the bores may not give you a conclusive result either. While piston rings account for blow by due to their very nature the oil will affect the compression ratios unless you use barely a teaspoon, and in a slanted engine that may not work too great either.