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Alan L's Avatar
 
Join Date: Apr 2006
Location: New Zealand
Posts: 3,248
a bit of diagnosis help

I have slight issue with #s 1&2 on my track machine. felt slightly down on grunt so did some diagnosis.
The CRs are down on 1&2 - 100 -105 compared to 120 on the rest.
Leakdown is 'OK' on all - 96-98% - but on a 040" or 1mm orifice.
Made a smaller orifice (030") and was down to around 85-90% on these two cylinders. A bit of this leak is clearly thru the exhaust valves. The rest is thru the crankcase.
My problem is if there is a serious issue, I need to tear it down and get in back in within a month. And my parts delivery via pelican is about 10 days. And that is about as quick as it gets with no local stuff on the shelf. So, I'm trying to second guess with these numbers what I might expect.
Would broken rings show up more? The valves are easy fixed hopefully. But anticipating the parts is harder. If it is not broken rings, would you get uneven ring wear on 2 cylinders? Can I just slot new rings in if that is the case - ie would it be worth ordering a set of rings before tear down.
So, any ideas on the likely diagnosis? I have a spare set of p&Cs - but bound to be the wrong group weight to deal with 2 cylinders. My guess is either broken rings, or worn rings - but I suspect the leakdown numbers are too good for broken rings?? - based on the standard 040 orifice. particularly when a decent amount of it is in the exhaust valves - these are the only two cylinders with E leak and any noticeable blowby - but it is noticeable via the crankcase.
One last Q - no head gaskets? What do I seal them up with?
Anyone had any experience with numbers like this?
Thanks
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 06-14-2011, 12:50 AM
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I had the same symptoms on cylinders 5 and 6 it turned out to be broken rings due to a faulty wastegate. Are you using oil? There will be some smoke under acceleration if you have broken rings. Any indication of detonation (pinging)? That's what breaks them. All my parts came in quickly, after I knew what to order. You can pre-order if time is more important to you than the budget. If you want a list of parts and tools for a one half engine top-end overhaul, let me know.

Mark
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1991 964 Polar Silver Metallic Turbo Coupe
Old 06-14-2011, 03:33 PM
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Hi Mark,
Thanks. So a few Qs. I have all the tools, have stripped and rebuilt a few 911s.
Did you reuse the cylinders and get away with re ringing the pistons - or did you have to get new pistons. If the ring lands are OK I guess I can just re ring it?
Not using any oil that I can tell. Not heard any pinging (running at 0.8 bar). But with a zork it is hard to hear anything but zork (love it).
What seals the heads if they dont have head gaskets. Would like to make a best guess and pre order bits, hoping for the best when I pull it down. Hopefully I will have the necessary bits on hand. (Maybe your 964 is a bit different, but I dont think the 930 has head gaskets?)
regards
Alan
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 06-14-2011, 04:58 PM
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How many miles on your car? That has something to do with the condition of the valve seats and the rings. Broken exhaust studs will slow you down and a broken cylinder stud will be a pain (mine were all good).

Reused the cylinders, they looked perfect after 62,500 miles. Reused the pistons after cleaning up the ring lands because they had some burrs from the end pieces of the broken rings. Mine has the stainless ring head gaskets but some of the earlier cars had no gaskets. It shows the accuracy of the original machining that the heads seal to the cylinders because they are dead flat. Some folks add the sealing rings and it lowers the compression a tiny bit. I say if you have no evidence of leaking, put it back together the way it came apart. Just be careful on the torque and take it a bit at a time following the sequence shown in the books.

If the rings are good you will have to send your heads out to be refurbished and that will take a week or two, you can clean up the engine while you are waiting and make sure you have all the parts for the reassembly, there are many o-rings that you will need.

You have the Bentley and Bruce Anderson books, right?

Mark
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1991 964 Polar Silver Metallic Turbo Coupe

Last edited by lucittm; 06-14-2011 at 06:34 PM..
Old 06-14-2011, 06:29 PM
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Ha, noticed you on line while I did some searching for parts/threads.
I have Waynes book - brilliant.
That explains the mystery why I had read somewhere the 930 did not have head gaskets, then I find them listed in the Pelican parts. Mine is a type 68 engine - so that makes it around 86/87 I think. History unknown other than I imported the car from japan just after a 'rebuild'. The term is a bit of an exageration - I think it was a cheap patch up job for a blown #4 probably from o/boost. Never did a compression on it then, just a leakdown - the numbers were similar to now (3 yrs on). But I think the 040" leakdown orifice lets a bit much air thru to show up early faults. So dont know if this has just happened.
Suspect I will need to order head gaskets and hopefully just rings. looks like I need to tear it down in a bit of hurry and allow 10 days hopefully for some basic parts.
Looks like if you have a damaged piston you end up having to buy 6?
regards
Alan
PS I was just logged on to your thread of your broken engine.
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83 SC, 82 930 (track) - Stock except for RarlyL8 race headers, RarlyL8 Zork, K27-7006, 22/28 T bars, 007 Fuel head, short 3&4 gears, NGK AFR, Greddy EBC (on the slippery slope), Wevo engine mounts, ERP rear camber adjust and mono balls, Tarret front monoball camber adjust, Elgin cams, 38mm ported heads, 964 IC. 380rwhp @ 0.8bar Apart from above, bone stock:-)
Old 06-14-2011, 07:00 PM
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