DirkSx1

I have a 89 951 Turbo. Compression is down to 140 lbs per piston and is starting to burn quite a bit of oil. Car only has 52K miles on it. The head was recently redone so should be OK. I was thinking of taking the head off and dropping the pan and replacing the rods and pistons with new high performance ones.

I know this may seem like a stupid question but assuming the cylinder walls are in good shape, e.g. clean with no scrapes etc, can I simply buy and install the new balanced rods/pistons with suitable sized rings onto the crank or does the engine have to come off brought to an engine shop for matching / machining the new rods/pistons..

BTW I had the pan off last winter to replace the pan gasket which was leaking and checked the bearings at the time and they were all good. So unless unless something drastic happened in the last 1500 miles which is I drove the car in the last 12 months, i would think the crank should still be good to go.

V2Rocket

If the head was recently re-done (properly), and the engine has 52k miles on the lower end, and you are losing compression and using oil..sounds like a ring problem, which likely means the cylinders are scratched up...

If needed, new oversize Wossners are available at a very good price, around US$640/set including pins and rings from Racer's Edge. I don't know if you've got any alusil-capable shops in Canadialand though.

Paulyy

Well the pistons need to go in from the top. so you'd need the head off and the sump off.
Easier to remove block.

George D

Mahle is now making pistons for our motors. If your bores are fine, just replace the rings. Unless your motor was molested, you have "high performance" rods and pistons.

blade7

You'll need the head and the sump off to check the bores for damage in situ, if you pull the pistons, and the bores and pistons are serviceable I'd consider lapping the bores lightly and fitting new gapped rings. Be aware aftermarket pistons run bigger clearances than the originals.

DirkSx1

Thanks for the feedback everyone.

Just a bit more info. head was rebuilt by local machine shop that does all engine machining for local Porsche dealer, so should have been done correctly. When I replaced the oil pan gasket last winter, I took the opportunity to install the Lindsey three piece cross over member just in case I need to ever do it again in the future. Looks like the future got here sooner than expected.

I just head the head off to replace a blown head gasket, # 4 cylinder. Cylinder walls all looked clean, perfectly smooth and compression was taken after reinstall of need gasket. As previously mentioned, compression 140 on all cylinders. I can only surmise that there was also ring damage as a result of the blown head gasket . I have since driven 140 miles which is when I noticed the heavy oil consumption (1 liter). There is however no exhaust smoke at idle, and nor while driving. I have to presume it occurs during deceleration.

So based on the feedback I should to the following:

1- Assuming cylinder walls are still OK, lap wall and replace rings.
2- I can also replace the piston with better one at the same time.

No one commented on whether I could simply change the rods at the same time without the need for any machining of the rods / cranks. Can i simply swap them out??

There reason i am considering changing the rod and pistons for higher performance ones is deal with the higher HP of the engine due to my mods, it was dyno'd at 340 HP at the rear wheels and i have since changed the turbo. I was told that the stock ones can safely handle up to 350-400 HP at the flywheel and it would be prudent to changed to higher performance ones given what has been done to the engine so far. Is this a correct assessment/decision.

blade7

I think 89 is around the time Porsche began fitting the cast rods, though if you have the forged rods they are probably good for 350+ bhp. Have you checked the intercooler pipes for oil ?

DirkSx1

Re intercooler pipe, I did not check it for oil since I replaced the head gasket. Will do tonight but it should not be turbo since it only has approx 2500 miles on it since new.

dizzyj

I would not have an issue with 140 compression across the board. Certainly would not cause a rebuild

JustinL

Agreed. What was your target for the 951 engine compression numbers? 140 is fine IMHO. I would look elsewhere for the oil burning like the turbo seals.

blade7

I'd check if any of the valve stem seals had dislodged too.

DirkSx1

Interesting inputs. Based on comments, the 140lbs compression is more than acceptable and suggests there are no broken rings. BTW what is the factory compression standard supposed to be?

This leaves either a turbo oil seal leak or loose valve stem seal. I would not have thought the latter could not account for the loss of 1 liter of oil in 100K miles. I will check for oil in the turbo pipe and if nothing, i will start looking at the valve stem seal as a possible source.

The confusing part is that there is no noticeable smoke coming from exhaust at idle and none that I can readily notice when I am driving. There is however some black soot on the rear bumper which i have so far attributed to running a bit rich.

I know the car drips a bit of oil when parked but nothing major that would point to a significant oil leak, am pretty sure it is the rear oil seal. The oil pan gasket is new as is the gasket for the thermostat housing which has the oil filter/oil cooler outlets, so presumably not the source of the leak. Could a rear oil seal leak on the crank cause this much oil to escape?

All a bit baffling!!

JustinL

Too high standards man! And change your oil more frequently.