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Post 3.2 rebuild - What would you do?

I recently had my 3.2 top-end rebuilt with 3.8 Supercup Cams, race valve springs, Ti retainers, rod bolts, new rings, SSIs and MB911 muffler in addition to everything else you might expect. Its now done 1900miles and had its first service and oil change at 1650 miles. It burnt a truckload of oil for the first 1500 miles (circa 7 litres) but it was running in with cheap mineral oil. My mechanic says it will stop using oil now that it is on Castrol RS, and the oil consumption has certainly decreased from about 1100miles onwards. I am currently monitoring the consumption but I am not sure how it will burn less when the RS oil is thinner.

Perhaps more worrying, is that it puffs a little on lift-off / overrun. This was most obvious on a dyno run on Saturday. There is certainly nothing you can see from inside the car - its not that bad. My feeling is that the top-end is not 100% following the rebuild. Maybe a pinched valve stem seal? What else is possible? FYI, this was a 75,000mile engine that never burnt a drop before rebuild.

Interestingly, it made 243.6fwhp on the dyno. We couldn't get any AFRs with the MB911 twin out muffler (I don't entirely understand why) but another car with the same chip was very rich so assuming the same condition there is clearly some horsepower still on the table.

I know what I should do. I have to talk to my engine builder, but any thoughts would be appreciated - at least so I sound like I know what I am talking about.

RB
Old 07-04-2005, 05:09 AM
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pinched valve stem oil seal sounds possible. maybe you couldn't get any afr's coz the duel out is a duel out and not a single? most permanent o2 sensors are mounted before any silencer can potentially split the exhaust output. maybe thats why? not really sure to be honest. anyway, even if thats all helpless ive bumped the thread!


Andy
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Old 07-04-2005, 06:21 AM
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My opinion as a auto tech and experienced engine builder. With the technology that exists today concerning piston / ring /cylinder metalurgy combined with modern machining technics and equipment; there is no reason that an engine of that size would be passing that much oil into the combustion chamber, period.

On a fresh rebuild, after the initial start-up / run-in (first heat cycle) the rings should be almost completely seated. To the tune of say 95% seated.

Also I'm not sure the point of running mineral oil. Just use a petroleum based engine oil of the correct grade and viscosity recommended from Porsche until the 500 mile oil change. Then you could go to full synthetic.

Your problem I believe is at least the wrong oil type and I would assume viscosity as well. I have never heard of anyone using straight mineral oil in a internal combustion engine for break-in purposes.
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Old 07-04-2005, 08:21 AM
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My bad, by mineral oil, I meant simply a cheap multi-grade non-synthetic oil - like 10W40. I thought everyone used this type of oil for running in?
Old 07-04-2005, 10:09 AM
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Sorry, I understand now! -Yes that should be OK. Some people prefer to use non-detergent on the first run making sure the viscosity is correct for that application.

I think you have a mechanical issue.
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Old 07-04-2005, 11:22 AM
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richard,

i have also read here that the SSI's (1 and 3/8" ID primaries) may be a little on the small side for a 3.2. Perhaps this is restricting the power output given the extra breathing capability you should get with the big cams

see, for example: SSI heat exchangers for 3.4?
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Old 07-04-2005, 06:37 PM
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Quote:
Originally posted by obrut
richard,

i have also read here that the SSI's (1 and 3/8" ID primaries) may be a little on the small side for a 3.2. Perhaps this is restricting the power output given the extra breathing capability you should get with the big cams

see, for example: SSI heat exchangers for 3.4?
I'll second that - I was told be two very reputable sources NOT to run SSI's or run out of breathing room. I am going with 964 B & B headers (1 3/4") for exactly that reason.

I'll also agree with asphaltgambler - after an initial run in your oil consumption should go way down!
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Old 07-05-2005, 12:05 PM
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Be careful with 1.75 inch primaries, that may be too big. I believe B+B make a set with 1 5/8 and that may be a better match. On Ralph's 3.5 I believe he used 1 5/8 headers, it makes some good power

There are lots of folks making good power with SSI's on 3.4/3.5/3.6 motors........ Never seed a direct dyno comparison between the SSI's and other larger stuff to see what difference they make.

Just some food for thought......

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Old 07-05-2005, 12:18 PM
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You bet - It's all like following a recipe - a little here and a little there - and you can have a something that tastes great. Add or subtract something out of porportion and you have something that tastes like crap.

Ralph is running a pretty mild cam (in the whole scheme of things) for emission purposes. The more agressive the cam, the more you can take advantage of a freer breathing exhaust system.
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Old 07-05-2005, 12:31 PM
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new pistons and cylinders, or a re-ring job? white teflon stem seals tend to come off and act like little oil pumps as they run up and down on the valve stem. 964 seals are better.
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Old 07-05-2005, 01:09 PM
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Rering - but its only on lift-off that it puffs oil so this leads me to suspect something in the top-end. It has loosened up incredibly since I collected the car and feels a lot faster which in my totally non-expert opinion, suggests it did take a while to run-in.

Re SSIs, I realise they are perhaps a little undersize, but after reading seemingly thousands of posts, it seemed to me that the sacrifice in topend was minimal and that you probably won a bit back in torque. For a street/track day car, this is a compromise I would make every day of the week. This is also the line that SSI have consistently run for many years when answering why they have not made a HE with bigger primaries.

Anyhow, the hp number was affected by IMHO the car running very rich - which is really a separate issue to the oil burning. If it still makes "only" 245fwhp after its remapped, I will really scratch my head. If it makes anything over 250fwhp post remap then I will be happy with the hp. Honestly, the car feels fast - it very, very nearly hangs on the bumper of a 285hp 964RS in a straight line (which is heavier than my car) up to 100mph.

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Old 07-06-2005, 04:43 AM
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Just to close this off - my engine builder has agreed to pull it apart and take a look at why it is burning so much oil. He agrees its far too much. Unfortunately I have to wait for second week in August.
Old 07-14-2005, 01:23 AM
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Sorry to hear it 911-32 = but I know you will feel much better knowing it is taken care of. Please report back what you find.
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Old 07-14-2005, 07:15 AM
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thats a bugger richard - at least you can be happy that the builder is standing behind his work. always a good sign
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Old 07-14-2005, 03:56 PM
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Good luck!
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Old 07-14-2005, 04:44 PM
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Its the right result. I got video of it on the dyno and you can see the puffs of blue smoke on overrun clear as day - something I could never see in the mirrors. It looks like it needs a top end rebuild !!! Obviously an error somewhere be it pinched seal or otherwise. Fingers crossed and will keep you posted.
Old 07-15-2005, 08:48 AM
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Well the engine is now apart again. The good/bad news is that my mechanic has found what the problem was. More than half of the valve stem seals are wrecked as they have been pinched between the valve guide and the collar/retainer. It was like running a motor with half the valve stem seals missing - hence the massive oil use. They have learnt that they have to use a different - shorter - guide with the high lift cams. For this one the heads have to come apart and the guides will be machined by 2mm to give ample clearance for the extra 1.5m of lift.

So let this be an FYI for anyone thinking of using the 993 Supercup cams.

Richard
Old 08-17-2005, 10:06 AM
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Well I have had the engine back in the car since the beginning of August and it pulls like a train now. No oil usage and it revs much more freely. On the same dyno today it made 260hp, so I am pretty pleased. Peak power is at about 6300rpm and power is over 250hp right up to 6700rpm. The engine is rich over 4600rpm but not by too much - maybe 5+hp left on the table.

Old 10-13-2005, 12:32 PM
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nice
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Old 10-13-2005, 12:49 PM
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so, 911-32, do I understand this right: the big cams push the retainers into the stem seals and muck them up?

Also, I assume you are still using the RoW stock 10.3:1 pistons. did these pistons require any machining to clear the 3.8 supercup cams?

I also assume you are using the stock carrera 3.2 intake. does the engine stumble at all at lower rpm part-throttle conditions?

Also, is that dyno chart rear wheel horsepower?
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Old 10-13-2005, 02:12 PM
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