Compression Testing - The Results Are In

[robwright]

OK after chasing a few boost and vacuum leaks and still not being happy with my AFR readings being too low on full boost I decided it was finally time to carry out compression testing on my engine today. I have to say whilst not completely disastrous the results were what I expected them to be and they showed consistently low across all four cylinders. The dry readings were as follows:

1. 115 psi 2. 105 psi 3. 115 psi 4. 115 psi

With CR of 8:1 and not even accounting for temperature rise due to compression the straight sum works out at 120 psi so I believe the reading should be a minimum of 130 psi and probably arouind 140 psi. The wet readings with the cylinders oiled up were as foillows:

1. 140 psi 2. 120 psi 3. 132 psi 4. 130 psi

More like the readings I should be seeing so it looks like new rings are required [] This would also explain the not massive but more than averrage oil consumption. So has anyone out there re-ringed before? How much work is involved?

 

[colin944]

Not that hard to do once you have the engine striped down .Probs worth doing head gasket at the same time also easier to do with the engine on a stand.BTW porsche wanted £ 100 per cylinder for rings for my S2.

 

[robwright]

I will have to do it with the engine in mate but will definitely upgrade to a widefire HG while I am in there. Promax do the rings at £40 per cylinder and do a widefire HG gasket kit for £182, which I happen to think is quite good value. Was just wondering if anyone who had done it before had any hints or tips. I guess that's my rear wheel arch paint bubble repair fund gone then []

 

[MarkK]

Promax did the rings and head on my ex-racecar turbo,they did it all with the engine in if i remember,great job,they made a high milage engine do a whole season,plus extra races here and there.They also tuned it just a little bit,went lovely[]

 

[JamesO]

ORIGINAL: robwright

OK after chasing a few boost and vacuum leaks and still not being happy with my AFR readings being too low on full boost I decided it was finally time to carry out compression testing on my engine today. I have to say whilst not completely disastrous the results were what I expected them to be and they showed consistently low across all four cylinders. The dry readings were as follows:

1. 115 psi 2. 105 psi 3. 115 psi 4. 115 psi

With CR of 8:1 and not even accounting for temperature rise due to compression the straight sum works out at 120 psi so I believe the reading should be a minimum of 130 psi and probably arouind 140 psi. The wet readings with the cylinders oiled up were as foillows:

1. 140 psi 2. 120 psi 3. 132 psi 4. 130 psi

More like the readings I should be seeing so it looks like new rings are required [] This would also explain the not massive but more than averrage oil consumption. So has anyone out there re-ringed before? How much work is involved?

Its odd I've done compression tests on 4 different 944 engines and number 2 always seems to be the lower one.
You could do the rings with the engine in the car but it will be a ball ache, might be tricky to support the engine with no head or crossmember which you will have to drop to take the sump off.

 

[robwright]

James thanks for that mate. That's exactly the sort of thing I was looking for. There is nothing on Clarke's about doing this and I wasn't aware you had to drop the cross member to remove the sump. Will be interseting to hear more from others who have done this or not as the case may be and what solutions they embodied. I am in no rush as like I said the readings aren't disastrous it just means I can't exploit the car's full potential until the work is done []

 

[colin944]

I am in the middle of doing this on my engine at the mo.I can't see how you would get the sump off without dropping the cross-member.If you go that far you may as well pull the engine out.I know you are a good mechanic Rob so you shouldn't have to many problems.Basically you are going to be doing a mini rebiuld you may as well do the belts,valve stem oil seals etc while you are at it. By the way £182 for a widefire seem good to me.I payed £150 for a standard one from Porsche but I did get all the top end seals at the same time in the kit.

 

[robwright]

That kit from Promax at £182 includes HG, valve stem oil seals, all the cam cover gaskets, inlet and exhaust manifold gaskets. Basically all you need to remove and reinstall the cylinder head.

 

[colin944]

That's a good price! only £30 more than a standard gasket. Andy is a top bloke though did me a fantastic deal on a floor when porsche couldn't get me one.He was also good for advice when I was in the early stages of my restoration, always spot on.

 

[zcacogp]

Those look like pretty borderline readings to me Rob; it's a lot of work tearing down the engine, and there isn't that much difference cylinder-to-cylinder (which more critical than outright results, IMHO.)

If you are doing rings you may as well have the head lightly skimmed and the valves re-lapped. For the additional work, it would give you a nearly-new engine.

Have you done a leakdown test, out of curiosity?


Oli.

 

[robwright]

Oli haven't done a leak down test but I think the wet results clearly display that the rings are worn. The only other option is that the liners are worn, which I very much doubt. It is a lot of work yes I know but I am committed to this car!!! The money isn't an option. Well actually it is but the time isn't so it kind os negates the money argument. As well as upgrading to a widefire HG I will relap all the valves.

 

[DavidL]

Might be worth an engine flush and redo the CT.
If you have carbon build up around the rings then you may get spurious readings on your CT which "improve" if the carbon is preventing the rings sealing properly.

JM did it on mine and it did make a difference (to the measurements anyway, not to the drive)

 

[robwright]

At 163k up on the clock I am thinking it is probably worth changing them. Obviously with the amount of work involved I am thinking of deferring this until better weather and funds permit. It may well be that the best option is to take the lump out, which will obviously be a longer term project. The sump and cylinder head will have to come off anyway and the big ends would have to be split so it is a no brainer to at least change the bearing sheels and upgrade to a widefire HG and do the valve stem oil seals etc. If the lump is out then it would also be a no brainer not to do the belts, crankshaft seals, balance belt seals and whatever else neds attention i.e. the water pump, rollers and tensioners. I am now of a mind that this is no small undertaking so if I commit to doing it then I may as well commit to doing the job properly. Essentially by the time it is finished I will practically have a new engine!! There is obviously other infrastructure to think about as well. An engine hoist and stand etc, all of which most people don't just have lying around for one time use. In the meantime the car drives absolutely fine and as I have mad a pretty long term commitment to the car I may as well wait until I have all the funds and equipment secured to do her justice. After all if I hadn't have done the CT the I wouldn't have even started this thread []

 

[TTM]

How does the engine perform as a whole?

Is oil consumption any bad?
If not then it may be interesting to measure the vacuum in the sump, particularly under boost, before taking the engine apart.

 

[robwright]

It does use some oil yes. More than I would expect. Difficult to gauge though as I don't drive the car all the time anymore. Just most weekends. I would say it is still within Porsche specs of up to 1.5 litres/1000km but I think we all agree that is a pretty broad safety margin for them. It does tend to have a nice blue puff out the back when accelerating hard after changing up under high boost. Like I said though the car still performs very well and if I hadn't have had an AFR gauge installed then I probably wouldn't be doing anything about it. Not sure of the practicalities of measuring the sump vacuum under boost. I suppose one could adapt a gauge onto the oil dipstick tube. That is one of the methods I can use to measure crankcase vacuum on my diesel locomotive engines.

 

[colin944]

I agree with you Rob. If you are comitted to keeping the car for a long time,then you might as well run it and start buying the bits to do an engine rebuild.It sounds more like something you would like to do rather than have to do. That way you can still run the car until you have all the bits and bobs that you need and then do the build over the winter.I'm sure you would get plenty of offers of help to do the heavy bits.

 

[zcacogp]

Rob,

I'm with you all the way on doing the job properly; if you do anything significant then it's certainly worth doing the lot. But ... and I'll say this carefully ... what is the problem you are trying to solve?

ORIGINAL: robwright
... if I hadn't have done the CT the I wouldn't have even started this thread []

It sounds like an awful lot of work for some fractionally low CT readings and a slightly higher oil consumption than you'd like given that things are otherwise fine.


Oli.

 

[Lowtimer]

Oli, you express an entirely rational view. But it depends on one's mindset. There are people who take a perfectly reasonable view that a car is a lump of metal to do a job, and that old cars are never perfect, you fix what absolutely has to be fixed and when a car is used up you move on. There are other people who are never really happy unless everything on a car is exactly as good as it can. Providing a car is running safely, both points of view work for the people concerned.

If Rob will forgive me a bit of amateur analysis (and I hope this doesn't come across as being an interfering busybody, Rob) his real problem isn't actually his piston rings: it is that he is aware that his car is not quite as fit as it could be, and this is something he cannot be relaxed about. He has to have a plan to make it right, even if it is not immediately implemented. And because like me he feels an open-ended attachment and commitment to his car, he automatically focuses on taking any visible need for maintenance or repair and re-framing it into part of an ongoing, indefinite life extension project - i.e. "given situation X, how can I best proceed to fit it in with the vision of having this car fully fit for another 10 or 20 years?".

I am a fellow sufferer. For example, the biggest remaining thing on my car that is not how I want it is that the air-con doesn't work. Might just need a re-gas (unlikely) - might need a lot of work e.g. new condensor, a new receiver / dryer, a compressor rebuild and so on. The moment I get someone who knows about air-con to tinker with it, that will crystalise my knowledge of what's wrong with it and at that point I'll feel morally obliged to get it all properly restored. I can't afford that at the moment, the 2010 mechanical restoration and 2011 bodywork restoration having cleaned me out, so I am having to avoid opening the air-con can of worms for now. I have to be able to tell myself that there will soon be a point where I can afford it and get it done.

Ever since he noticed his slightly rich mixtures, Rob's can of worms has been open. It will be very hard for him to rest until things are where he wants them to be.

 

[zcacogp]

Simon,

That's a well written post, and you have put your finger very squarely on a relevant point - that of how much of anything automotive-related is strictly necessary, and how much is psychological. (I touched on a related point in my thread here:

http://www.porscheclubgbforum.com/tm.asp?m=606798&mpage=1&key=improved&#606798

... and you were one of the few who were decent enough not to laugh at me!)

I guess what I am trying to do is suggest an alternative point of view to Rob. Having been in a similar situation before, and having spent a LOT of money 'curing' things which were only fairly tenuously wrong, I have looked back and regretted the time and money spent - maybe even wasted, although that word is subjective and I certainly wouldn't have used it at the time. I'm offering (and in a dangerous environment as well - that being a forum of automotive enthusiasts) Rob a different point of view on his particular problem - an equally valid one, but different.

However, as any sufferer looking back, I can fully understand that it may not be a point of view that Rob wants to hear ... []

Best of luck with whatever you choose to do about the CT readings, Rob. I know it will run better as a result of any work you do on it.


Oli.

 

[chrisg]

Interesting points Oli, there is a very fine line between 'wasted' money and investment/ prophylaxis for the future. I do know very well exactly where Rob is coming from though.

I had a similar 'feeling' about the fuel/ brake lines on my car and decided to take the plunge last winter - very glad I did. Huge time consuming job, but can now drive it (particularly too and from France/Germany) in the knowledge neither the fuel nor brake lines are corroded.

Bottom line - all of our cars are min 20yrs old and the inevitable rust, corrosion (caliper lift vs iron oxide), degradation of anything rubbr etc has set in. For any 944 of this ilk the following list of recurring jobs/problems seems to crop up:

• Rear fuel or brake lines particularly over the axle
• Head gasket going porous/soggy - more so on Turbo's but also an issue for all 944's
• Caliper plate lift
• Belts (a regular service item though)
• Sills and replacing them
• Turbo & S2 front fog bulbs going - can't remove the light and fails MOT...
• Fuel pump - most noticeable on Turbo's on WOT
• PAS pump and/or lines - usually the cooling loop rottting
• ABS - any of a number of items
• Clutch cylinders giving the pedal on the floor scenario

I'm sure there must be other likely candidates I can't think of too !

Chris