Super 90 rebuild

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james boyter
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Super 90 rebuild

#1 Post by james boyter »

I have case no. 805887 that I acquired many years ago that I'm going to start the rebuild process. What I would like is information or suggestions and what members have experienced, good or bad. The motor is complete except for P/C and cam. The crank is non counter balanced, rods seem to be from 912. I'm getting stuff together to ship to the machinist (?) Any comments would be useful.

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Martin Benade
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Re: Super 90 rebuild

#2 Post by Martin Benade »

Do you know which crank it is, an actual S90, a C, or an earlier B one? All three of those come un counterweighted, and two different flywheels between them. Also different main bearing diam.
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james boyter
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Re: Super 90 rebuild

#3 Post by james boyter »

Not being an expert, crank has 55 mm mains. Is there any other tell, tell signs for a s-90 crank. They measured 54.96-54.82-54.99

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Vic Skirmants
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Re: Super 90 rebuild

#4 Post by Vic Skirmants »

Early S-90; they were not counter-balanced.

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Harlan Halsey
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Re: Super 90 rebuild

#5 Post by Harlan Halsey »

According to the Porsche Spec Book 805887 is a late 1962 case. Judging by the next engines, the C and SC, the S90 had a lot wrong with it. But I have always been drawn to what, in my opinion, they got right: The S90 had black heads for better heat transfer, iron sprayed aluminum cylinders, larger intake valves, larger crank main journals, higher compression, and Solex 40PII carburetors. And they painted the fan shroud silver Altogether a very neat package.
However some of it turned out to have been a mistake: The larger intake valves were only used on that engine. (and they are expensive to come by these days.) In the later engines they went back to the 40mm intake valves and increased the exhaust valve diameter instead. (SCCA race cars used both the large intake valves and the large exhaust valves, but the cylinder heads tended to crack between the valve seats.) The 55mm #1 main bearing necessitated a different flywheel, bearing and shims, unique to this engine. According to legend, if you rode the clutch, the thrust bearing wore out prematurely. Hence Porsche reduced the #1 journal diameter back to 50mm, widening the thrust bearing surface, on the later engines.

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Re: Super 90 rebuild

#6 Post by ray nelson »

James
Another malady unique to the S90 was soft intake valves. I had the misfortune of experiencing that problem first hand. This was also mentioned in one of Pellow's books. Hopefully the replacements I put in from 1967 were not a problem.
I always wondered why it was hard to keep the valves adjusted to spec until I saw those valves with a knife edge perimeter. the seats looked good though.
Now I think I need to check that TO bearing.
Ray

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Re: Super 90 rebuild

#7 Post by Vic Skirmants »

I have never seen a race head with 40mm intakes and 34mm exhausts crack between the seats.
However, I do leave the original 38mm seat in place and grind it to fit the 40mm valve. Leaving the base of the seat the same size it was for the 38mm valve gives a venturii effect for good cylinder filling. That's the problem with S-90 heads; the seat is too big at the base.

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Re: Super 90 rebuild

#8 Post by james boyter »

I dug out the flywheel and in the process of cleaning I found a scribed notation that in 69 40 gr. I assume 40 thousands was taken off in 1969. Does anyone have any thoughts on the flywheel.

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Re: Super 90 rebuild

#9 Post by Mike Horton »

Vic Skirmants wrote:Early S-90; they were not counter-balanced.
Vic, did anyone ever machine, or manufacture a replacement venturi intake seat, to fit the S90 intake seat holes?
Back in the late '70s, and early '80s, Lycoming had intake valve seating issues in high altitude turbo'd F/I , and NA "high performance" IO-540 cu in angle valve engines, and came out with a replacement true venturi seat, for the later smaller head dia. intake valves. This completely eliminated the seating issue, and I suspect the increased flow velocity, more than made up for any performance issue... Good question, and idea.

James, my just younger '62 S90, now has an SC/912 counterweighted crank, late bearing set, the AA 22° head angle cast iron P/cylinder set, and I'm going to match weight a set of the 616...01 late rods. Due to the ... er...uh "PC" 10% ethanol, a just under 9:1 C/R, a refreshed stock 022, and split shaft 40 PII Solex, and stock "big can" muffler. I also have one of Ron's centered full flow oil filters. This is to be a temporary travel engine, while I go through my original '68 912 engine.
Mike

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Re: Super 90 rebuild

#10 Post by Vic Skirmants »

"Vic, did anyone ever machine, or manufacture a replacement venturi intake seat, to fit the S90 intake seat holes?"
Not that I know of. Good idea.

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Harlan Halsey
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Re: Super 90 rebuild

#11 Post by Harlan Halsey »

I'm not sure what a "venturi" valve seat is, but I replace valve seats whenever necessary, cleaning up the head on my milling machine, turning aluminum bronze seats on my lathe to .004" interference and shrinking them in. Sometimes this is necessary just to get the chamber cc s equal.
Machining valve seats is trivial, cleaning up the head a little less so and the LN 2 shrink into the 300 F deg head maybe more than most shops care to attempt. But I think that's the way it was done originally. (Beating seats and guides into the head at RT has seemed a sub-optimal approach to me since Chuck Forge and I worked the LN 2 shrink procedure out in the 1970's. At that time I recall that Jurgen Barth who went to school with Chuck at Stanford and then worked at Porsche for a couple of years said that Porsche had LN 2 readily available.)

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Re: Super 90 rebuild

#12 Post by james boyter »

Mike, Thanks for the information, very helpful. I'm gathering stuff to send to the machinist. What cam do you plan on using and is there any particular reason to use the cast iron cylinders vs the biral. I have the 01 rods also, so its more seeing what the machine shop thinks about the guts. Your information was certainly appreciated. Jim

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Re: Super 90 rebuild

#13 Post by Dick Weiss »

James,

The Biral cylinders are just C/I w/cast on aluminum fins having sharp edges; NOT like the chromed or Ferral aluminum cylinders
w/the machined fins having flat edges.

Dick

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Re: Super 90 rebuild

#14 Post by Dick Weiss »

Ray,

That was the problem w/the 40mm S90 intake valves having thin/sharp edges being distorted thinner and needing more ofton adjustments. I just trim down the 30 degree angle to get the valve seating a little lower which gives a slightly wider seat contact.
Also, I trim down the 75 degree angle for a venture effect.
BTW, I don't use a seat grinder, but a Neway cutter set--by hand for better control.

Dick

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Re: Super 90 rebuild

#15 Post by Mike Horton »

james boyter wrote:Mike, Thanks for the information, very helpful. I'm gathering stuff to send to the machinist. What cam do you plan on using and is there any particular reason to use the cast iron cylinders vs the biral. I have the 01 rods also, so its more seeing what the machine shop thinks about the guts. Your information was certainly appreciated. Jim
James, I had in my eclectic collection of "Porsche stuff", a near perfect narrow lobe stock "-102" cam, I removed from an old Normal, badly converted to "S", without the thin head " S" tappets, which had due to a lack of mechanical clearance, damaged the case. For my uses, I really like the factory super camshaft, paired with freshly CNC resurfaced steel, thin head tappets, with a spherical radius, lubed with a good Moly make up lube, as I used in aircraft engine builds.

The AA P/C set, is as far as I am aware, what they put together for use with the through "B" 22° combustion angle heads. I actually prefer the cast iron cylinders, to the Birals, due to my in shop warranty experience with the Mahle Biral cylinders, in the early '70s TCM Continental "Tiara" aircraft engines, a 2:1 geared 406 cu in, 285 HP @4,000 rpm, clean sheet of paper engine in its day, chosen for use in a likewise new Piper Aircraft agriculture spray plane. These engines, were flown in the southern hot weather summer crop growing season, at full wide open throttle power, to lift the weight of the chemical in the hopper, to spray on the various crops, king cotton, in our TX South Plains area. My local dealer sold 28 of these A/C, before Piper replaced these new engines, with the more tried and proven Lycoming IO 540 cu in, 300 HP @ 2700 rpm, or IO 720 cu in, direct drive engines, 375 HP, @2600 rpm, some of the ones equipped with the true "venturi" intake seats.

When these Biral cylinder engines, were overheated, the cylinders would go out of round specs, in as little as 1 hour of admittedly hard use. When this occurred, the needed ring to cylinder seal, was compromised. Needless to say, the engine power suffered. On the factory warranty program, my shop, a Piper factory Service Center, "top overhauled" all 28 engines at least one time, with new piston/cylinders, some twice, as the Piper, and Continental factories, tried to solve the thermal issue, in a poor choice of engine and airframe. This was likely a 200 mph ram air cooling engine, installed in a 100 mph over the ground airframe. I learned a lot, during this program, over about 6 years.

My own '68 912 original engine, has since 58,000 miles, when I bought it from 23 years in AZ storage, never had more than around 100 psi on cranking compression tests, with corresponding leak down tests, for confirmation. My 1st '57 T1 Normal, had the original aluminum cylinders, and from 56,000 miles, had low compression readings, and poor oil consumption, as well. I ran two engines with the old "rock catcher" air filters, a poor choice in dusty West TX. My late '60 B Normal, which i converted to super specs, with cast iron Nural cylinders, always ran well, except for the too much Racer Brown cam, and got excellent oil mileage, in comparison. I have since, preferred, in my humble opinion, the far more stable cast iron cylinder structure, and have had good experience with the life of the engines, long term compression readings, and good ring to cylinder sealing, and oil control. Think VW beetle, and 911T engines. I now only build driver engines, and compromise the C/R, due to the sorry ethanol fuels found on the road, these days. I'm setting my S90 C/R at just under 9:1. Sorry, Ron. I don't trust the cross country fuel quality, these days.

I'd like to publicly thank David Jones, for sharing his knowledge on today's fuels, through the years, and Ron LaDow, for this excellent full flow filter kits, and new machined sump cover, with the redesigned drain plug, in a true low location, for better oil drain. Thanks, guys! And Vic, for his ever great hard earned, under fire, racing learned expertise. I also read every word Paul Christensen writes, thanks!

You asked, so there it is. I wrote a tech article for the 912 Registry BBS, on this issue, if you need more detail, contact me off forum, and I'll be glad to discuss it further. These thoughts are mine, determined by my experience, and offered as such. Others may differ in their experience. The Biral cylinders, if not overheated might serve well, and I don't have any history on my '68 engine prior to my ownership, but I have driven it over 16,000 miles, in this tired state. It is time for a better engine, and I'll overhaul this '68 original, with cast iron cylinders, as well, my S90 serving in the interim. No, I have no experience with the high end LN cylinders, so no help there,
Mike

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