Engine rebuild advice needed
01-26-2010, 10:00 AM
#16
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is it cooking oil or burning it? if the oil turns verry verry black evean after a few days after the oil change then id go with the engine vent setup. also are you shure that there are no oil leaks before you go ripping the whole thing apart?
01-26-2010, 10:15 AM
#17
Nordschleife Master
DON'T PANIC
Get the engine apart and see what you've got. Could be broken rings. Those can produce lots of smoke while not messing up the block. Yes, BTDT and I'm not the only one.
Be prepared to see minor scratches in a mirror surface. Those don't matter unless they're deep.
Is this a race car? If not then look for a stock re-build. How much money and time are you willing to into the engine? If you do the work yourself it'll cost $2000 for the rings, bearings, gaskets and so on. And you'll want to get the heads re-built ($500-$1000). Figure another $2000 to go with bigger pistons.
Pistons come in tolerance groups and a new block will most likely be different than what you've got. If it's the same model then between the old and new blocks you'll likely have the pistons you'll need with bin-picking for pretty ones and hand-fitting for size.
Get the engine apart and see what you've got. Could be broken rings. Those can produce lots of smoke while not messing up the block. Yes, BTDT and I'm not the only one.
Be prepared to see minor scratches in a mirror surface. Those don't matter unless they're deep.
Is this a race car? If not then look for a stock re-build. How much money and time are you willing to into the engine? If you do the work yourself it'll cost $2000 for the rings, bearings, gaskets and so on. And you'll want to get the heads re-built ($500-$1000). Figure another $2000 to go with bigger pistons.
Pistons come in tolerance groups and a new block will most likely be different than what you've got. If it's the same model then between the old and new blocks you'll likely have the pistons you'll need with bin-picking for pretty ones and hand-fitting for size.
01-26-2010, 04:26 PM
#18
Nordschleife Master
A new block with good bores will solve this problem for sure.
Going to 6L will require the 100mm bore and the aftermarket stroker crank. Which then leaves you needing different rods etc.
Boring to 104mm (do not go to 104.5mm the rings jump in price) and using 968 will get you to around 5.4L iirc.
it is possible that you do just have broken rings. But I think it is more likely to be a scratched wall from the pistons coating coming/wearing off. You can however go up to a first overbore sized piston. But you will need to find a machine shop to deal with the alusil, and then you need to find an oversize piston.
Either way the bottom end needs to come apart IMHO.
I have also seen engines with very very bad valve guides and seals and they did not consume nearly as much oil as the OP has said it is consuming. This is simply a matter of size, in order to consume as much as is being consumed it really will be rings, or bores.
Checking the plugs is a good idea though.
Going to 6L will require the 100mm bore and the aftermarket stroker crank. Which then leaves you needing different rods etc.
Boring to 104mm (do not go to 104.5mm the rings jump in price) and using 968 will get you to around 5.4L iirc.
it is possible that you do just have broken rings. But I think it is more likely to be a scratched wall from the pistons coating coming/wearing off. You can however go up to a first overbore sized piston. But you will need to find a machine shop to deal with the alusil, and then you need to find an oversize piston.
Either way the bottom end needs to come apart IMHO.
I have also seen engines with very very bad valve guides and seals and they did not consume nearly as much oil as the OP has said it is consuming. This is simply a matter of size, in order to consume as much as is being consumed it really will be rings, or bores.
Checking the plugs is a good idea though.
01-26-2010, 05:26 PM
#19
Nordschleife Master
I'm probably wrong, but my understanding of the 928 pistons is that the oversize ones are balanced to weigh the same as the originals, which would mean that boring only the scratched (if any are) cylinders and refinishing correctly for alusil is an option?
01-26-2010, 07:45 PM
#20
Burning Brakes
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Ahh, I didn't know about the oversize being balanced to original spec, I thought it was an all-or-nothing job. I am definitely pulling the motor, at this point it is coming down to which is more economical, buying a "new" engine or rebuilding the one I have. All things being equal, I'd rather get the car back on the road ASAP, so dropping in a new engine would be the better option. It seems that rebuilding the existing one is going to cost just as much as a new engine, if not more, so the trick would be to simply find an engine that has a decent service history and go with that one. I would then have a spare engine to play around with which I could rebuild slowly as funding accumulates. Does anyone know where I might find such an engine? Evidently 928 international warantees their used engines, although I haven't looked into what the warantee actually covers. I guess I should pull the engine and strip her down and see what I've got, then decide which way to go. If the block needs doing, it's probably cheaper to find a used engine, otherwise if it is just valves, that would be the better option. Thanks all for the advice!
01-26-2010, 09:19 PM
#21
Nordschleife Master
A boroscope on the bedroom nightstand also sets the tone just right when your date comes up for a nightcap.
You can get boroscopes on ebay for pretty cheap, but I think it may be hard to see what is going on. I got one to look inside my current motor and could not see squat. Seems like you need to pull the heads whatever the case so why not just do that and see what's up. Try not to worry too much about problems you don't yet know you have.
If you have a scratched wall you can just take the one hole out to the first oversize. The pistons in all oversizes weigh exactly the same so you would just need to do the scratched one. Not sure what it would cost to do one hole. I was quoted $300 to do all 8. I would guess 1 would be about half that since most of the work is getting it setup on the machine.
When I pulled the heads off mine I did see some scratching, and I decided to do the whole motor. But once I got the pistons out I noticed that the old oil rings were really brittle. Most of them were either broken or broke when I pulled the pistons. The new oil rings were nothing like that. Not sure if that normally happens or if this points to an overheating or what. In retrospect, I think I could have just changed the rings and been fine. Many have said (Greg Brown included, I think) that you can have what seems like a lot of scratching and be fine.
I'm not sure why everyone seems to jumping from your oil consumption to a scratched block. These cars are pretty hard on valve guides and seals. It might be that a valve grind and guides and seals is all you need.
I think people are pointing towards pulling the motor becase, in general, it will take less time to pull the motor, do the work and put that motor back in, than to do the work with the motor in the car. But if circumstances point you towards wanting to do it with the motor in there is absolutly nothing wrong with that. You can do a fine job with the motor in. It will just take a little more time.
If you have a scratched wall you can just take the one hole out to the first oversize. The pistons in all oversizes weigh exactly the same so you would just need to do the scratched one. Not sure what it would cost to do one hole. I was quoted $300 to do all 8. I would guess 1 would be about half that since most of the work is getting it setup on the machine.
When I pulled the heads off mine I did see some scratching, and I decided to do the whole motor. But once I got the pistons out I noticed that the old oil rings were really brittle. Most of them were either broken or broke when I pulled the pistons. The new oil rings were nothing like that. Not sure if that normally happens or if this points to an overheating or what. In retrospect, I think I could have just changed the rings and been fine. Many have said (Greg Brown included, I think) that you can have what seems like a lot of scratching and be fine.
I'm not sure why everyone seems to jumping from your oil consumption to a scratched block. These cars are pretty hard on valve guides and seals. It might be that a valve grind and guides and seals is all you need.
I think people are pointing towards pulling the motor becase, in general, it will take less time to pull the motor, do the work and put that motor back in, than to do the work with the motor in the car. But if circumstances point you towards wanting to do it with the motor in there is absolutly nothing wrong with that. You can do a fine job with the motor in. It will just take a little more time.
01-26-2010, 11:52 PM
#22
Burning Brakes
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I was under the impression that you needed the stroker crank to build to 6.5 L, but could get away with just boring out to 104 mm and keep the original parts for 6.0L, which is what I was suggesting.
A new block with good bores will solve this problem for sure.
Going to 6L will require the 100mm bore and the aftermarket stroker crank. Which then leaves you needing different rods etc.
Boring to 104mm (do not go to 104.5mm the rings jump in price) and using 968 will get you to around 5.4L iirc.
it is possible that you do just have broken rings. But I think it is more likely to be a scratched wall from the pistons coating coming/wearing off. You can however go up to a first overbore sized piston. But you will need to find a machine shop to deal with the alusil, and then you need to find an oversize piston.
Either way the bottom end needs to come apart IMHO.
I have also seen engines with very very bad valve guides and seals and they did not consume nearly as much oil as the OP has said it is consuming. This is simply a matter of size, in order to consume as much as is being consumed it really will be rings, or bores.
Checking the plugs is a good idea though.
Going to 6L will require the 100mm bore and the aftermarket stroker crank. Which then leaves you needing different rods etc.
Boring to 104mm (do not go to 104.5mm the rings jump in price) and using 968 will get you to around 5.4L iirc.
it is possible that you do just have broken rings. But I think it is more likely to be a scratched wall from the pistons coating coming/wearing off. You can however go up to a first overbore sized piston. But you will need to find a machine shop to deal with the alusil, and then you need to find an oversize piston.
Either way the bottom end needs to come apart IMHO.
I have also seen engines with very very bad valve guides and seals and they did not consume nearly as much oil as the OP has said it is consuming. This is simply a matter of size, in order to consume as much as is being consumed it really will be rings, or bores.
Checking the plugs is a good idea though.
01-27-2010, 12:02 AM
#23
Burning Brakes
Rebuilding a motor and doing it right is very expensive . If you are not after a performance build , a used motor ( fully checked out ) is a better plan IMO . Terry
got 20-30K+ extra? Check out all the stroker/ after market threads Terry
01-27-2010, 01:51 AM
#27
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I would send it to Performance Auto Works in WV. They are a trackside shop at Summit Point and specialize in Porsche rebuilds. They did all of my work, top notch.
www.performanceautoworkswv.com
Ask for Dave
www.performanceautoworkswv.com
Ask for Dave
