Hi, I've been lurking around this site for a while but just enjoying others posts but I may need to ask a few questions from the experts on my project so please be patient with me!

Many months ago I started a project installing a 964 engine into a 1985 car.
I bought a 1990 engine, loom and ECU from a garage over here in the U.K which seemed basically sound and was sold with leakdown figures of 7-10% across the board.

I spent the next couple of months stripping the intakes & fuel lines, shroud, alternator etc to give it all a good clean up as it was all covered in cack!

I had various bits powder coated, changed all the seals and rubbers including crank seals while it was all nicely accessible, replaced the inlet system, new clutch and mated it up to the 915 'box. My mate came round to help me adjust the tappets and get the engine back in the car when we discovered a broken head stud.

At this point I took the engine to my local Porsche specialist to re-do the leakdown where the figures came out at 13-20% on cyl 1-5 but 38% on cyl 6. Re-checked it with same result – air leaking past the rings through into the case.

So I’ve removed all the intakes, fuel lines and shroud again, put the engine on a stand and slowly, carefully broken down the top end. The rest of the head studs were not looking too clever with the coating chipped, scratched and peeling away so I’m glad that I made the call to change all the head studs, not just the broken one. I’m still in two minds whether to split the case and do the bottom end at the same time, at least then I’d know that the whole motor was solid.

I’ve given the cylinders, heads, pistons & cams to my local specialist who is going to check them all and see what needs machining and what needs replacing. What I’m trying to figure out is what mods I can do on a sensible budget while it’s all apart?

I was thinking Supertec or 993 head studs, ARP rod bolts, slightly stiffer valve springs & maybe IF the cams need regrinding possibly a slightly more aggresive profile (price dependant) but open to opinions or suggestions?

 

Now that it's apart, what shape are the P/C set and rings in? I've found doing a leakdown on engines that have been sitting for periods of time can end up being very random. (Not all have been 911 engines.)

 

I have heard that and the initial test was done within a week of the engine being run so still some residual oil in there. I guess over a few months the oil will drain out and could give a different reading.

All seemed pretty good to my very untrained eye. There was no obvious nasty surprises like broken rings or scoring in the bores. Trouble is as soon as that head stud broke it had to come apart anyway. The thing that surprised me most was how bad the condition of the head studs was. Considering they're not very exposed, the coating was chipped, scratched and flaking off so I suppose it's no wonder they corroded and one gave up.

 

I don,t see a head gasket Early 90 engines should be modified to include them .It is a common leak area. The bores look good from what I can see The crosshatch is still visable. Heads should recieve new guides and I would use larger RS size valves at the same time.They use a 8 mm guide instead of the stock 9mm and can be installed on the original valve seats. Resize the rods and use ARP or raceware bolts and new rod bearings. I wouldn,t bother to change the cams or springs unless the stock are scored/pitted.

 

Mine was rebuilt with new dialever head studs which I believe are the same as the 993. From what I can gather most independent P car specialists will use these on the rebuild (and replacing them on a top or full rebuild seems to be common practice).

I'm interested that you've used a 915 box. I didn't know that was possible and am interested why you've chosen that box instead of the G50?

 

I've heard two schools of thought on the head gasket, some people seem to put them back as before and some add the gaskets. I think I'd prefer the gaskets as they all seem to leak eventually without but I guess it depends how much it adds to the bill!

So if you go for the larger RS sized valves, do you have to replace the all of the valves?

915 box is the only option on a 1985 car, they changed to G50 in 1987. People have retrofitted G50s into earlier cars but it's a LOT of work as the G50 is longer so it involves cutting through the torsion bar tube etc. The only replacement part needed to fit the 915 box to a 964 or 993 engine is a different flywheel and the notch for the sensor needs to be enlarged.

 

You can get away with Porsche rod bolts, but i`d suggest putting in ARP head studs as oppossed to the 993 or any other ones. They are the same prices and miles better.

 

Thinking more about this I believe that you can modify the heads but I don't think it's possible to modify the cylinders to take the gasket so it would require a new set of cylinders or a set from a later car. It would be nice but I think cost prohibitive.

 

I,ve sure I,ve seen the tops of the cylinders cut for a sealing ring. I,ve never had to do it myself so I would search or post here about the" early 964 head gasket fix".The factory fixed lots of these under warrantee. They added a gasket or sealing ring to do it,not just reassembled the same as before. RS valves are about $45 each from Pelican. The matching aftermarket guides are about $6.50 . Original Porsche guides are known to be soft and aftermarket are longer lasting. . I would expect them to be worn out on at least the exhaust side by 60000 mi. Often the valve stems are worn down too. If your machine shop measures and recommends guides and a few new valves I would go for the larger RS ones instead.You could just replace 6 exhaust or intake or all 12. You should see a power increase without having to spin the engine any faster to get it. You mentioned you were considering new cams. New cams will push the power peak up where you might then be wanting better valve springs, stronger rods etc. which is much more expensive than new oversize valves.

 

https://rennlist.com/forums/performance-modifications-forum/111791-ni-resist-rings.html
Steve Weiner also is reported to modify the cylinders to use a seal from a 993

 

Hi Alex,

I’m in the final stages of an engine rebuild (by a mechanic), so I’ll toss in a few comments FWIW.

I checked around and selected the ARP head studs. I agree with olly964T – better value than OEM repurchase. Bought mine from EBS Racing. Very nice product. http://www.ebsracing.com/item.wws?sk...r=ARP&weight=8

Wouldn’t you only need an upgrade on rod bolts if you are anticipating a big HP/rpm increase?

Also got the higher performance valve springs/retainers from EBS. Perhaps a little unnecessary, but…. http://www.ebsracing.com/item.wws?sk...pk=331&mfr=EBS

Since I had a little pitting on a few cam lobes, I opted to upgrade to the 993SS profile from Dougherty (after way too much research). Has one of the better low-mid boosts (as opposed to mostly pushing up the top end). I can send you the spec’s if you like since they are not on his website. Quite economical and presumably stays within the stock intake limitations. http://drcamshafts.com/

I guess I might humbly question the value of the valve size upgrade. Johnsjmc says the larger RS valves fit on the same seats – maybe I’m just showing my ignorance, but how does that work? So, what’s the cost to remachine the heads? Maybe get input from a 964 engine guru to be sure that the stock engine can take advantage of larger valves? I really do not know.

Did get new exhaust valves and guides since they were a bit toasty/sloppy.

If you cannot opt for a gasket, can you at least have the mating surfaces of the heads and cylinders flycut to maximize a nice flat fit?

Just my amateur 2 cents. Good luck!

 

If you are not going to use gaskets or rings then you can "lap" the head and cylinder.If you are going to spend to flycut them then that is most of the labour needed for a gasket ring. I wouldn,t be so far into a 90 without at least exploring the cost of the proper fixes.
I would use the supertech or raceware or ARP stud kit , they include nuts and washers and I think is around $500-600. The best OEM stud is the 993 but they are very expensive now. Something like $45 each plus the nut
The 96 3.6 has 1 mm larger valves and makes 10 more hp than the 95. The RS valves are only 2 1/2 mm larger so there is lots of meat on a standard valve seat to fit the new valves. The valve seats need to be cut/ground anyway if you are replacing guides or valves and it takes little extra time to cut the seats slightly larger. . They are actually cheaper than the standard valves so if valves and guides are needed anyway I would consider them.
The standard rod bolts are only safe to the stock rev limit and no higher. There is little or no safety margin .Frequent reving to the limit stretches them also causing eventual failure of the bolt or the rod bearing . On the other hand stock valve springs are usually OK until about 7500. HD valve springs only wear out cams earlier and lower fuel mileage. Lightweight valve spring retainers are perhaps a better investment.

 

John, I guess you're talking about 993 3.8RS valves since 964RS valves are exactly identical to 964 C4/C2 valves unless I'm very mistaken.

Cheers,

Harald

 

Great replies chaps, much appreciated.

Steve, would be interested to see the cam profiles. The ARP rod bolts are a bit more assurance in case of over-rev (easily done with a 915 box). I was toying with the idea of the AASCO springs & possibly retainers for similar reasons. Do you mind me asking what sort of money is involved with re-grinding cams as I haven't got a clue?
http://www.aascomotorsports.com/valv...e%20train.html

John, all the heads, P & C's etc are at my specialists for inspection before I get the good / bad news about what's good and what’s not. I guess it will make most decisions easier after that. I thought that all the mating faces needed cleaning up before re-assembling regardless.
Are the larger valves the 1995-96 Euro RS spec ones on the pelican link below?
http://www.pelicanparts.com/catalog/..._pg2.htm#item9

One other modification that was recommended to me was to moon or scallop the bottom of the cylinders as it's supposed to be a relatively simple mod to do that is simply material off, anybody heard anything good or bad about this?

 

I don't have the figure right in front of me, but I think it was $295 (per cam?), which was the same price as a stock regrind. No weld buildup required. I can send you my cam profile research spreadsheet if you want to PM me your email.