rebuild 3.0 or install used 3.2 (long)
 

I have an 1981 911SC with approximately 170,000 miles. There is one broken headstud on exhaust side of the #6 cylinder. Recently, that cylinder has developed an oil leak and is losing compression. I believe the head gasket is blown. Otherwise, the engine runs well (4 bar+ oil pressure and no smoke). It is definetely down on power and seems winded by 5K RPM. The idea of a 3.2 swap seems appealing with little down time and increased power, especially at the top end.

My fear with rebuilding is that when the engine is actually disassembled, I will need, at the very least, one new cylinder/piston/head (if I'm lucky!)

1) If I decide to swap, what is my 3.0 worth in this condition?

2) I'm capable of a DIY rebuild, but I like the ease in tuning and upgrading of the motronic 3.2.

3) Other than rod stretching and worn valve guides, what problems do 3.2's have? Are broken headstuds as common as with 3.0's?

I'm on an extremely limited budget. What can I expect to pay for a used 3.2 in good condition?

Your knowledge is greatly appreciated.

TODD

Your P&C are probably still within spec. If you run a 911 long enough with broken studs the continued head lift off/slam down action causes the sealing surfaces to get damaged or deformed. You can have the case decked and the heads surface machined to fix that. If the cylinder surface is damage though, it might be cheaper to get another P&C because you'd need to machine all 6 to make them even. I know, I know, you can just do one and use a different gasket but that is just plain creepy IMO.
I think a good 3.2 with DME and harness goes for around $5000 these days.
-Chris

Re: rebuild 3.0 or install used 3.2 (long)
 

rod bolts should be replaced with ARP or similar _if_ you're tracking the car or if you have the rods off of the crank. Survey says that they are unlikely to break unless the engine is revved to/over 7000 consistently, and/or overrevved, for example due to a missed shift. However, if you have an unknown engine, you don't know if it has been revved high or overreved in the past, and the bolts could already be stretched/fatigued.

valve guides should definitely be replaced if you have anything apart.

there is a school of thought that says that the only reason 3.0s are breaking head studs more than 3.2s is age. in any case, if you have the engine apart, new heads studs are a good idea.

That's about all I can think of.

Re: rebuild 3.0 or install used 3.2 (long)
 

Todd:

1) If I decide to swap, what is my 3.0 worth in this condition?

I think $1500 should move it quickly. 2k may work, but the miles are quite high, so everyone will have the same fear (p&c out of spec?) than you do. Also the head and cylinder that are affected may have some damage by now.

2) I'm capable of a DIY rebuild, but I like the ease in tuning and upgrading of the motronic 3.2.

When the CIS works well, it runs very nicely. I would not call the motronic easy to tune. Chips aren't trivial and it seems to me that you have to get one custom fit to your specific 3.2. MAF upgrades are very pricey and the 3.2 doesn't seem to react as nicely to exhaust mods than the 3.0.

If you spend some time on the 3.0, richen the mix, put headers or SSI, put 20/21 or 964 cams, you'll have a very good running engine that will be very comparable to a stock 3.2, if not eat it for lunch. The motronic sytem can have DME problems, and it still isn't talking to every injector individually, like a true modern EFI. It's still firing in banks. I think it is still somewhat of a crutch.

3) Other than rod stretching and worn valve guides, what problems do 3.2's have? Are broken headstuds as common as with 3.0's?

I think yes. Not as common yet, because they are newer. I think the material is the same, so there is no reason it should be less frequent.

The other problem the 3.2 sometimes has are prematurely worn valve guides. If you buy an engine on a palette that you can't see run, with good compression numbers, you may have found one with bad valve guides (something the compression numbers won't really show).

4) I'm on an extremely limited budget. What can I expect to pay for a used 3.2 in good condition?

One from a dismantler with a 90 day warranty? Try 6500, EXCHANGE. ;) There goes your core! Of course you can do better from a private party, but you will be had pressed to find a decent and well documented 3.2 for much less than 6k. And there is no warranties. If the sucker blows up on the first test drive, you will eat it.

I would rebuild the 3.0. You will have a fresh engine. A used engine always is a pig in a poke. If you are determined to go 3.2 and motronic, I'd buy a core 3.2 and go through it. Rod bolts, head studs and valve guides. Then put it back together and be happy.

You are on a limited budget, but you also want to make sure you don't pull your enigne every 6 moths. That is time consuming and will cost more in the long run too. I'd do it right and rebuild the 3.0 or a core 3.2.

Hope this helps.

George

I agree with George; If you're on a limited budget, I think you should rebuild the 3.0. I've replaced a broken stud or 2 on 3.2's so it does happen. If you intend to track the car, I wouldn't do the 964 cams without stiffer valve springs. The valves are just that much closer to the pistons with the 964 cams - one missed shift...
-Chris

I agree, replace the valve springs if you track. However, you don't have to for the street. Here a discussion we had earlier. Go to the very end of it.

http://forums.pelicanparts.com/porsche-911-technical-forum/153689-piston-valve-clearance-964-cam-3-0-sc.html

Looks like the difference in actual lift between the stock SC and the 964 cam is not signifciant at all (0.015"). Remember, the 964 is still a smog cam, nothing like a wild S cam etc. I guess it is the duration giving you more low end. If the cam were for a V8, you'd advertise it as an RV grind. ;)

George

I think the duration is the culprit; I think the valve lingers open longer before "running away" from the approaching piston.
-Chris

Chis:

Yes, you are correct. I was wrong. The max lift isn't necessarily the point in time where the valve gets closest to the piston!

George

I've decided to keep the 3.0 and rebuild. The 3.2 seemed like an easy "bolt-on horsepower" idea, but there are just too many "what ifs?"

If I just do a top end rebuild, what can I expect to spend in parts and machining? I've already accepted the fact that I may need to replace one P & C. If the others are in spec, I will only re-ring.

I'd say $2000-$2500 should give you a decent top end, including reground cams and rockers.

George

Thanks, George.