The 1500 SUPER - Porsche's first racing engine.
In the last issue I gave a detailed outline of the restoration of America Roadster #12322. At the onset of the restoration, owner Gerald Barnes was very clear about his intended use for this car – it would be entered in the 2002 Porsche Parade Concourse in Boise, ID…but when the show was over, it WOULD be driven (maybe even raced or rallied)! With that in mind, there would be no corners cut or exceptions made in the mechanical or functional aspects of the car. The engine had to perform as a daily driver and, if need be, have the durability and performance to function as a "gentleman's racer." First, let's look at a brief history of the America Roadster engine.
The America Roadster was the first dual-purpose vehicle built by Porsche. Intended for both street and racing use, it was powered by the new 1500 Super, which was the most powerful engine the factory had to offer. According to factory records, the concept of a 1500 racing engine first began in mid 1950. With a bore of 80mm and a stroke of 74mm it would be the largest and most powerful engine based on the original magnesium VW 36hp crankcase. Carrying internal design number 502, the first 1500 engine had a Hirth crankshaft (with roller bearings on the rods only), 32 PBIC Solex carburetors, and a 7:1 compression ratio and produced 55 hp at 4500 rpm. This design was further developed (Type 527 and 528) with the larger Solex 40 PBICs and a new camshaft and by the end of 1951 produced 70 hp at 5000 rpm with a compression ration of 8.2:1.
At only 1581 pounds it wouldn't take much horsepower to make this America Roadster a fun car to drive, but to be competitive, it needed an edge. Because an original appearance was mandatory, the original carbs, exhaust and distributor had to me retained. Nothing could visually deviate from stock or the judges at the Parade would surely notice. Performance gains would be limited because of the small, single throat carburetors and restrictive exhaust but some serious internal component changes, as well as cam, valve and combustion chamber optimizing, could bring the needed increase in durability and hopefully some additional torque and horsepower.
After a complete disassembly and cleaning the parts were inspected. The original magnesium crankcase was in excellent condition with virtually none of the usual corrosion damage. The original Hirth crankshaft was intact but badly worn and had been subjected to an amateur repair on one rod journal. The original chrome plated aluminum cylinders were worn but in otherwise good condition. The heads had no cracks, still had the original, early valves with 8mm stems and had never been flycut. The early 40PBIC carbs and steel intake manifolds were in excellent condition. All other engine components were intact and easily restorable.
The first change was the crank and connecting rods. The Hirth roller bearing crank was the design of choice for performance engines of the early 50's but advances in plain bearing technology have rendered it obsolete. A lightened C crank was used with Carrillo rods. The early case had to be bored oversize to the standard late 356 line bore dimensions and oil grooves and bearing tang slots for the late bearing shells had to be added. After machining and cleaning the case was stripped of all aluminum and steel parts and was sent out for Dow 1 coating. Dow 1 is a silicone conformal coating (corrosion protection) that is goldish in color and was used on all early engine and transaxle cases. While the case was out for coating all the studs were blasted, tumbled and recoated with the original black phosphate and the oil galleys were flushed and new plugs installed.
The next item to be addressed was the camshaft. The stock 1500 Super used a cam very similar to the later Super cam but ground on the early two-piece case billet. The problem in simply picking a sport profile for grinding the cam was that the early engines use a radius lifter that requires a different lobe profile than the flat lifters of the later engines. A call was made to Dema Elgin and he had two options, a 270 and 280 degree. The 270 was chosen because of the carb and exhaust limitations.
The heads in the early 1500 used valve sizes similar to the later engines but the very early engines still used the smaller VW 8mm stems with double springs. Later in production the exhaust valves were changed to 10mm for better heat transfer away from the valve head and eventually the intake was also changed. Because durability was a concern, both the intake and exhaust valves and guides were changed to the later 10mm type and later single valve springs were installed. The intake valves were unshrouded and the intake and exhaust ports were cleaned-up and matched to the manifolds and exhaust. Unshrouding the intakes substantially improves the flow but also increases the volume of the combustion chamber thus lowering the compression ratio. A compression ratio of 9:1 was needed to match the new Elgin cam profile so new high dome pistons would be required. The original bore of 80mm was increased to 80.5mm by boring and Nikasil coating the original aluminum cylinders. New, lightweight, forged pistons were made to order by JE Pistons with low-tension cast iron rings compatible with the new Nikasil cylinders. The cylinder bases had to be cut to clearance the rod bolts and, because the Nikasil coating required machining the sealing surface of the cylinder tops, special 1.5mm cylinder shims had to be made to get the correct deck height.
Above the 5500 rpm redline, the valve gear of the early engines was heavy and caused valve float. Adding to this weight were the early pushrods, which were circlipped into the lifters to make lifter installation easier. These pushrods were made from steel tubing and a piece of wood was inserted into the steel before the ends were pressed on in order to reduce the internal dimension of the steel tube and cause the oil to pump to the valve gear faster on start up. The problem with these is that over time the wood swells, or gets stuck and blocks the flow to the rockers. Later engines used a solid aluminum or aluminum and steel combination that could have a much smaller internal hole and still maintain a lightweight. Super 90 pushrods were chosen for this engine because they were the lightest. They couldn't be circlipped into the lifter, which would make lifter installation more of a challenge, and they were slightly longer (as are all later pushrods) and the rocker stands had to be shimmed to match.
The Solex 40 PCIB carburetors, according to factory specs, used two venturi sizes, 26mm for normal driving and 29mm for racing. These carbs had the 29mm size and an additional set, modified to 32mm, were fashioned for the final testing. The distributor was rebuilt to stock specifications and standard breaker points were used. A 10.5-pound 200mm flywheel that was modified to accept a smooth acting VW bus diaphragm pressure plate replaced the original. The original VW style oil cooler was replaced with one of the new aluminum coolers from NLA. The oil pump was replaced with an early 356A assembly that was 30% larger. After balancing the engine was assembled with all of the original sheet metal, exhaust and air cleaners and was ready for final testing.
The initial break-in and testing was done at Carobu Engineering in Costa Mest, CA. They have a $100K DTS dyno that is used for racing engine development for everything from Nascar to vintage aircraft. After set up it took only a few cranks and the 1500 Super sprang to life. The rings sealed almost immediately in the new Nikasil coated cylinders. Maximum timing advance was set to 36 degrees and the engine was run through a break-in cycle for about 20 minutes. The initial full throttle runs would be done with the 32mm carburetor venturis. The DTS is a fully computer controlled dyno and gives readings for torque, horsepower and fuel mixture at 100 RPM intervals. With Weber carburetors, this precision makes it possible to optimize the fuel curve by balancing the emulsion tube, main and air correction jet and to see torque vs. peak horsepower advantages of larger or smaller venturis. With the early Solexes, we would have to be happy with just main and air correction jet changes because no optional emulsion tubes were available.
After several runs to optimize the jetting and timing advance (38 degrees max) the best power was recorded at 78 hp at 5300RPM with 88.5 ft/lbs of torque at 4000 rpm with the 32mm venturis. The factory specs for a stock 1500 Super were 70 hp at 5000 with 79 ft/lbs or torque. Considering that a stock 1600S made only 86 ft/lbs at 3600 rpm and a Super 90 made 89 ft/lbs at 4300, I was more than satisfied.
After dyno testing was complete the engine was installed in the car for final adjustments and fine-tuning. The maximum timing advance of 38 degrees flys somewhat in the face of popular (but untested) opinion that the 356 engine needs less timing because of today's fuel. Actually, maximum timing is more a function of combustion chamber design. The timing curve, on the other hand, can be affected by the fuel and even more by the cam design and overlap which reduces the dynamic compression ratio of the engine at low rpm requiring more initial advance at idle. The early single throat carburetors had a distinct disadvantage, especially with an aggressive camshaft. The firing order of 1-4-3-2 didn't give a steady signal (a situation that was later solved with the dual throat Zenith) and even the Normal engines had somewhat loapy idles. With this particular 1500 Super an initial timing of 18 degrees was necessary to maintain a good idle because of the 270-degree cam. Modifying the stock 383 distributor to limit overall advance to 20 degrees and adjusting the internal spring cam to bring the additional timing in by 3000 rpm easily achieved this. The result was a decent idle with very good throttle response at the low end.
Driving the America Roadster is a blast (after getting over the initial idea that the car I'm driving is probably worth more than my house!). The engine spins up freely to 5500 rpm and, if you can get used to the cashbox transmission with all the necessary double clutching and noise (first and second gear are straight cut), it's easy to see why this little car was such a hit with the weekend racing crowd. Plans are to display it at the 2004 Speedster Fest in Pebble Beach, along with most of the other America Roadsters. It will probably be trailered to the event but you can bet that after the show it WILL be driven.