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!
