Old school 2.8's what do you guys think?
 

2.8 thoughts?

Well, you have 200 views. There are probably more people with a similar engine in the Autocross and Racing section. You are right, it is an expensive build so not very common.

Otherwise, I'd just call Steve at Rennsport and/or Henry Schmidt, or Jeff Gamroth from Rothsport. Aase Motors is local for you (I think).

Eric at PMB Brakes did a 2.8 RSR recently for his 914-6. That was highlighted in Pano a couple of months ago.

You could call him, he is very helpful.

John

Personally I like the idea of a 2.5 short stroke better, like what he has in the 914-6 he drove to, during, and from PCA Parade Monterey.

did it many yrs ago (circa 1977) starting with a 2.4 - and raced it for about 10 yrs before a serious over-rev at Laguna

- at which time I traded all the 2.8 pieces for stock 2.7 stuff & a 7R case

check out the many documented issues in old Pano articles & Bruce Anderson books; I got lots of advice at the time from Jerry Woods, John Truman, John Thomas, Rennsport, etc

heavily depends on what heads you start with - the first time I did a test bolt-up I discovered 12:1 CR - which i reduced to 10.5 - which in turn required "barrel shims" which in turn required "adjusting" the chain housing gasket thickness - a new set of PMO Webers, tall manifolds, lots & lots of final tuning, etc etc -- ........ well you get the picture

- but it sure did have a lot of torque for a vintage engine -

(today my stock '89 3.2 runs circles around any of the benefits i got from that build - but there were no factory 3.0 or 3.2 motors to buy back when i did it)

would I do it again ? - no way!

JMHO,.....

A properly built 2.8 in a light car is a VERY potent package, especially with close-ratio gears.

Just like cooking,...there are many different recipes and some are better than others.

I sure wish Grady was here to ring in on this as he too, was a fan. :)

I have a 2.8 long stroke, and a 2.8 short stroke.

The long stroke was built with an early aluminum case and 2.7 heads. Ported some, but the normal valve sizes. Doesn't make the HP numbers (308? flywheel) Porsche advertised (110 hp/liter). About 250 at the rear wheels. I've been wondering how much I can increase the valve sizes on those heads and keep within the 92mm cylinder diameter, and if that might get closer to the factory numbers.

I don't think a guy can get to the real 2.8 valve sizes, as that called for changing the valve guide angles, which meant a whole different head casting I think. I long ago wrote off trying to purchase an original set of RSR heads. Or having the guy in Florida make them. Huge $. That may be a reason it is hard with stock valve sizes to get closer to factory HP numbers at the rather modest CR they used (10.3/1)

The short stroke 2.8, using an early '76/7 Euro SC/Turbo case, has 3.2 heads. Ports plenty large (per Steve) as they are, and easy to go to larger intake valves too. I was working to find where I wasn't making the HP I had hoped when a J&E piston ripped in half on the chassis dyno, but by all reports that is a great 2.8 if you have a reason (like class rules) to run that instead of a bigger motor.

Some of the Club Racing guys with really well professionally built 2.8s (made me feel a bit slow the one time I raced a bunch of them) are contemplating running heavier cars with bigger motors - like the 3.6. They say those cars can motor by them on long straights (theory - you need horsepower to push air, and more hp to push more air, especially as drag is exponential), even if the lighter cars have somewhat of an advantage in the corners.

How do the two 2.8's compare to each other in terms of driveability and feel in the car?

The short stroke blew up before I got to drive it, so I can't say.

It is a race motor, so many of the features of street car drivability are unimportant. On the track you can expect to be shifting at or above 8,000 RPM to make best use of the torque curve. Henry Schmidt calls it the best motor Porsche never made, or something like that. He has a high opinion of the configuration. Peter Dawe has built some also, and his son Trevor says they always produced better dyno numbers than the long stroke version, though I don't know if that was a comparison with the factory motor (906 cams, titanium rods, really big intake and exhaust ports and those special heads, and who knows what all else to get the last ounce of performance from it) or one built with a more compromised 2.7 type head.

Guys are also building short stroke 3.0s - 66x98 - for racing. They seem to do well. It may be that the increased cylinder area - what the combustion pressure presses down upon to create torque - may outweigh (up to some point) the reduced crank lever arm length.

Also the 2.5 was done I read for both the SCCA and FIA
 

The Trans Am series started an under 2.5 liter. Porsche with the 2 liters were beating most every else, but then raised their displacemnt to the class maximum and the FIA basically settled on that size also. The 911S wasn't homologated till 1968 and lightweight packages could be added, which was kind of an end around the rules.

Reason I like it is it has the 66mm short stroke crank that can rev safely to 8000rpm+ without the torsional vibration problems of the 70.4mm stroke crank. It also uses the RS clyinder size so you are not boring the case out to the ragged edge (assuming you use an early case) like with a 2.8 or 3.0 short stroke. Makes it more durable while giving plenty of power at the top end.

Well, from what I can see from my 2.7 and 2.8 70.4 cranked engines, careful application of red Loctite and torqueing the flywheel bolts to 150 lbs/ft have solved the problem of flywheel shedding. It seemed Porsche didn't figure that out back in the day with the 2.8 RSRs, although I've not gone over 8,200 in any routine way (rev limit set there) with either. The RSR's advertised peak HP was at 8,000, so I have no idea where Porsche had its drivers shift, though the efficient shift RPM would certainly be well above 8,000.

My cranks 70.4 cranks haven't broken either, though I'd love to have one with RSR fillets if I could afford it.

I am kind of working on a little casual project.
 

First I don't know who is cutting custom billet cranks these days, but I would endorse fillets, having it made with 9 bolt retention and knife edging, etc. and balancing the assemblies especially if operating in the 8000rpm ranges or higher. The other direction would be to lightened everything attached etc. (Titanium rods, custom pistons, custom wrist pins etc.) is the other way to do it. I know some Porsche rods were built to get into this level of rpms.

My casual project has been going through articles and charting out the various HP/RPM levels that people are getting with the various components from Carbs/Injectors down to the cranks etc. being ran.

Thus example would be that I have found PMO 50 sized carbs powering engines to about the 328hp range, and 60 Series cam running to 324hp naturally with the proper ports and valve sizes, compression ratios.

I am a firm believer of marrying the parts you use to target an operating range of rpm. Thus if you have something that is designed to operate at a higher level or lower level, then you limit basically the power band.

Example just for conversation:
Manifolds 3000-7000
Cam 2500-7500
Heads 3200-7200
Carbs 1800- 6500

When you think about things based on just the example, your power band for the four known components would be 3200-6500.

If everything was married together
Manifolds 3000-7000
Cam 3000-7000
Heads 3000-7000
Carbs 3000-7000

Then the engine would run good from 3000-7000!

To answer the question on where Porsche pilots would shift. Reading they have always stacked their races with potent driver teams with varying team orders. In endurance events some I am sure were sent out to short shift, or to conserve tires etc.. And then you had the rabbits. But in the modern, these cars are so well engineered that they are tending to run the crap out of them for the entire event. Some engines seemed pretty indestructible and they would be hammered right up to redline, time after time-- lap after lap. I have studied all of the 917 drivers in depth and all of the 935 pilots to see who I think was the fastest. My personal favorites were Stommelen and then maybe Redman (the crazy one) next.

I am sure that shifting orders also were given in consideration of the dyno curve! Where the peak fell off. Thus to not waste, you would throw the shift right at peak hp would be the most efficent and you would want to not fall off or below peak torque! But I will have to bounce this thought around in my own head a little!

I really like this engine.
 

I think you could spin it up to 8k all day, because of the lesser rod angularity. I think you could run a production rod, but I would go custom pistons completely. If street driven, I would port the heads, but then to go a little under cammed for the real world. Thus I assume a mag case. I think the limit on a magnesium case is the 2.9 big bore set up that uses the 2.7 sized case bores (which is thinner barrels, but do-able), but would I do the thinner barrels, I generally won't personally!

For a Mag case, idealy I actually like 2.6 size using a 66 stoke on 92 bores, but only if you are planning to rework the RS heads to flow and in consideration of the compression ratios.

If late aluminum case 3L, I like that short stoke build if you are on a budget for a 2.8 l with the 66 stroke and 95 pistons, but I would cam that up, and do a lesser porting.

Example of rpm capability!
 

Remember discussing Old School but done with modern aftermarket components was the engine build goal.

The 917 with the 70.4 cranks (5.0 liters) all broke! The 917 with the 66mm crank 4.5 won the race! The peaks hp rates were at 8400rpm and redline was 8700rpm. Thus this can be transferred over to the discussion. And then they broke parts at 9000rpms (mainly valve springs). If you calculate piston speeds and the capability of the bottom end taking into consideration of bearing size etc. I would put the ultimate rpm capability of the engine at 8900rpms if built with the best of the best! But the math would make the 66mm capable to 9200rpms theoretically. The limit on the 70.4 would be about 8700rpms which is dead on the overall redline that the factory imposed on the racers back in the 1970's. Thus maybe the Bosses told the 4.5 liter guys to push theirs a hair more! Don't know!

I also know that Porsche would increase duration, but lift stayed limited to .450 max for street engines. To mod the street engines into race engines, they added duration and thus lost low end driveability. The engine I am playing with a toy for track and fun and not for street. I think that the head config and the piston shapes don't take to compression increases well and therefore Porsche tended to keep things in the 10.5 range max.

Thus I am trying to figure out if some good ole fashion hot rodding would work! I think at lifts over .600 you would definitely be having dimishing results. So I would put the capability of the heads, ports, valves at .500 lift. I am even looking at the negative of a wedged piston being a negative in a Porsche twin cam combustion chamber is why the pistons were custom ordered!

PS just to share, I think collectively my family has about 5 old school engines laying around! Yes it would be nice to get some modern big bore 3.2/3.6's and such but we kind of got stuck in a time warp!!!!! And because old school parts are getting scare or expensive, looking more toward the aftermarket.

The mag case is not really a good idea for a 2.8. It has to be long stroke w/92mm bbls. Early aluminum is the way to go - you can't really overstress that. Stock rods for the 70.4, with extra strength rod bolts, are stout enough. But there are lighter rods available.

Optimum shifting (ignoring fuel saving or other strategies) does not happen at the horsepower peak, but beyond it. The idea is to match the delivered torque or thrust (torque at rpm multiplied by gear and ring and pinion) at the upshift rpm of the lower gear with the thrust after the rpm drop in the new, higher gear. The sweet spot, maximizing the area under the curve, is where thrust in old gear = thrust in higher gear. Porsche knew all about this, and in any Porsche motor of interest, this sweet spot, although it can vary with each gear depending on how the gears are spaced, is always above max HP by maybe 600-800 rpm. It depends not on the HP curve, but on the torque curve (and of course is way higher than max torque).

Reply
 

Personally I think a magnesium case is not a good idea period, unless low hp! But I do think it can be built to stand up to 2.8 (with the tricks!), but Porsche themselves at the 2.8 point, switched again to the aluminum going to the 3.0.

On the shifting, I have always set the shift light at Peak hp, because by the time the slow human being completes the motion, you are at least the better part of 600-800 rpms past that with a quick revving engine. My rule on setting up the gearing is so as to not fall out of the power band on a shift, but if racing you would want to minimize the drop off of rpms! But I am taking in what was said!:D

Also Porsches racing 2.5's were done with the 70.4 crank and was closer to the class maximum is what I have read! Thus there are too ways to build a 2.5!

They started with the long stroke, then once Nikasil cylinders became available (from the 917 project) they went back to the short stroke, big bore since it was more reliable. But they couldn't get the 90mm bore with the Biral construction. The Ni-Si-C coating allowed thin aluminum walls to be hard enough to run piston rings on.

After nearly 20 years, it has been fun to refresh my Porsche knowledge!
 

My interest in builds was renewed by one of my brothers who has been a constant Porsche Owner going on 30 years or more and his return to California with his California car from out of state!

Thus basically the old school Porsche engines are pretty easy to figure out! And I will just summarize basically and quickly, but I do have more precise build notes on cams, head porting, valve sizes, everything that I did mainly back in the 1980's and then worked on it again in the early 90's!

Thus to go bigger than 2.4 (2341- quite a stretch by Porsche- 2.3L is my opinion!) you can take a Biral cylinder and punch out to about 85.5, but to go bigger with Biral, you are running the risk of too thin! Therefore you could go 2425cc with a 70.4 x 85.5 which is a truer 2.4- my opinion!. But then I have seen bores of the Biral cylinders maxed out to 87.5 (which is a 2380 with a 66 crank and a 2540 with a 70.4!!! Therefore you can see why Porsche made the stroke jump, because of the case restrictions with the Biral cylinders! But it seems like at power levels of 270-275bhp with upwards of 86.7 cylinders, the 70.4 non fillet cranks were breaking. Thus that was the initial 2.5 (2493cc 70.4 x 86.7). Therefore for the 70.4 crank (non fillet, I would call a 240-250 bhp the max) and for that, I would knife edge and balance, etc.!

Then Porsche with the advent of Nikasil's and the initial 90mm (2.7 units), the bigger displacements were possible and along came 89, 90, 92 and more later with the Nikasil so with 66mm stroke you get displacements of (2464, 2518, or 2631cc, but 2518 and 2631 fell outside of the 2.5 rules for racing, but would be cool street builds). But then because of the cylinder stud spacing, spigot sizes (92mm became pretty close to the limit) on the Mag cases! And even at 92mm those took valuable material away that could compromise the reliability! The nikasil barrels are basically the reason why engine sizes have grown to 3.8 liters in the more modern!

Therefore the ideal builds in a mag case (using Porsches own racing standard of 110bhp per liter) would be a 66 x 89 for 2464 (2.5), but bigger would be 66 x 90 or 2518cc or a fillet crank 70.4 x 90's or 2687 which was 2.7! But with precision machine work and high quality building, that could go to the 92 sizes easily or 2808cc with the 70.4 crank or the 2631 with the 66mm crank (2.6). But staying within Racing rules for the 2.5, for a 2493cc build- 70.4 x 86.7 is still a good engine if you run a custom crank! They also were having problems with the flywheels working loose, but with thread lockers and dowel pinning etc., this is easily fixed! Therefore when you think about a perfect stock engine, the 2.2 in a biral with the 66 x 84 is a real pussy cat sweat engine, but don't plan on going fast if a stock build! And then the 70.4 mag cases up to 2.7 are really also good, but with stock everything, I wouldn't call any of them fast either! They are just nice running engine sizes or family of engines!

I have alot of Porsche literature in my garage that I have been digging out from the 1970's, 80's and 90's! So still brushing up on things. I do believe and I have to research further, but I think a few Companies came out with steel cut cylinder sleeves. One I recall was I think Empi who had a nice 88mm barrel set. Thus no iron sleeve and no coatings! I think the biggest drop-in barrels that I think I remember for the mag cases was a 92.8, but the wall were getting pretty thin in that size! I think those steel barrel put heat into the spigot area and with the mag expansion, you were getting cracking around the spigot, so I think they tried things with the fins, being that these were not aluminum. This part is all off of a 20+ year memory, so I do need to check on this! Add, I believe these were a failure from the machining process to the inability to cool them. Thus I think others and Empi and such went back to the same processes as Porsche. But some racing operations have and still use steel.

The heads I was spec'ing measured out at 47mm/42mm on the valves with a port of 43/41 with alot of work on the floor and at the valves. I remember I was playing with a set of 130mm Carrillo rods and had JE custom cut the top of one of their stock Porsche cut pistons for clearance. I still need to spec out that chamber size and the ultimate compression because of the 3mm push upward. Also pushing the ring pack up 3mm put it right at the top of the bore eliminating quench area, but then had to crown the entire unit for clearance, so there was alot of work on that setup that I sold! I probably should have kept those rods and pistons, but someone if it ever got assembled has an interesting custom configuration! Also the valves I had found weren't the valves for those heads, but were the stock ones in a box (46/40 not 41).

I think without cutting the pistons custom, then for a long rod 2.8, I would have had to add thicker cylinder base spacers, chain box spacers, a longer chain or smaller gears, custom cylinder studs for proper thread engagement, and then even the carb actuation would have had to be adjusted to get it all perfect again. Thus maybe better to stay with the 127.8mm rods. Thus with Porsches, getting the wrong pistons can mess up your day and so a larger bore on an earlier engine, you need pistons for your stoke and rod length and also particularly for any build period!

Also Porsches don't like it when you make the engine wider or skinnier!

PS if someone wants to use this as a forward in their next book, give me Derek some credit, but you can picture one of my other better looking family members!:D:D;)

I have a 92mm bore 2.8 with carbs, hot cams, headers, JE pistons etc... It's plenty fast for the street. Great revving engine and a blast to drive.