1970 Drake Offenhauser 2.6-litre four
1970 Drake Offenhauser 2.6-litre four: The Offy might have died in 1933 with the bankruptcy of Harry Miller, the great visionary who created it. Design techniques proven in the great Miller straight-eight-cylinder racing engine of the 1920’s had been used by Miller and his team to make a 2.5-litre four cylinder unit for boat racing which also showed, by chance, excellent performance in a racing car. At first limited by racing rules to 2-valves per cylinder, the engine was given 4-valves by engineer Leo Goossen when this was allowed in 1931. Progressively scaled up in size year by year, the engine was a 3.6-litre four in 1933 when Fred Offenhauser bought the tools, drawings and patterns to make it from a bankrupt Harry Miller.
He set up his own one-man Offenhauser engineering company, which became two-man when Leo Goossen, the designer who had always interpreted Miller’s vision, joined Offenhauser, whose engine soon became known as Offs, thus when the four first won Indianapolis in 1934 it was already officially an Offenhauser engine, of 4.2-litre size. The 1934 win was the first of no less than 30 victories the Offy scored in the 500-mile race until 1976. In many years, especially in the 1950’s the Offy totally dominated Indianapolis, in 1954, on the first of several such occasions, all 33 of the starting cars at the speedway were powered by the four-cylinder Offy.
The Offy used in US championship racing had four valves per cylinder operated by gear-driven twin overhead camshafts raising the valves through inverted-cup tappets, the head and cylinders were a single casting, originally of iron and, since 1969 of aluminum with inserted dry liners. The separate crankcase was aluminum, of tunnel-type construction into which the crankshaft, with its 5-main bearings bulkheads, was inserted from one end. To suit US racing rules the Offy was enlarged to 4.5-litres in 1938, in 1946 the rights to the engine were bought from Offenhauser by Lou Meyer, three time Indy winner, and Dale Drake, their Meyer and Drake engineering continued to produce the Offenhauser engine, rule changes brought it down to 4.2-liters again in 1957, by then fed its alcohol fuel through Hilborn constant-flow port-type fuel injection. Offy customers led the way toward a new short-block configuration for the reduced-size engine.
With Lotus as its ally, Ford mounted a concerted attack on the Offy, beginning in 1963, it nearly won a controversial 1964 race and finally, in 1965 Jim Clark’s Lotus-Ford beat the Offy-powered roadsters, but unwilling as he was to give up on the great engine, Dale Drake built a new factory to produce the Offy in California’s Costa Mesa, near the Orange county Airport. He was joined by his son John, by Leo Goossen and by Walt Sobraske, master machinist who first worked for Harry Miller in 1921.
While Ford was making unsupercharged 4.2-litre eights, Drake decided to make the supercharged 2.7-litre fours that the rules allowed. Drake had a sound basis on which to do this, because the Offy was well-suited to supercharging with its four valves per cylinder and integrated head/block design, avoiding gasket problems. Also supercharged predecessors had been built and raced. A 3-litre Offy of the 1950’s was based on the 220 block and crankcase, a direct descendant of the original marine engine of 1931. In 1950 with mechanically-driven centrifugal blower, 3 qualified for the Indy 500 but none finished. One qualified and failed to finish the next year, and in 1952 one blown qualifier retired during the race. A blown Offy also qualified in 1957 when the supercharged-engine displacement limit was cut to 2.8-litres. It too, failed to finish.
Supercharging’s second wind was in large part the inspiration of Dick Jones, operator of Champion’s West Coast dynamometer facility and a man eager to see the Autolite-sparked Fords beaten. Jones carried out some tests with a Roots-type blower which yielded a much fuller torque curve than the centrifugal units. In the late 1965 Leo Goossen designed a proper engine to suit the basis of the robust crankcase of the 4.2-litre Offy four and using the shorter gear tower of the 220 variant of which had served for the experimental engine build by Jones. This new Offy had 144 mm tubular-shank con-rods and a suitable altered cylinder block.
Well oversquare at 104.8*79.5mm for 2.739cc, it weighed 216kg. It had its 4-vlaves equally inclined at a 72 degrees included angle in a pent-roof combustion chamber, with a single central spark-plug ignited by a Scintilla magneto prepared by Joe Hunt. Valve diameters were 39.7mm for the inlets and 34.9mm for the exhausts. Lift by the direct-acting radiused top tappets held in proper alignment by a keyway was 10.2mm, and duration was 290 degrees for the inlets and 270 degrees for the exhausts cams. Valve spring pressure was 73kg static and 163kg with the valve fully open.
Machined from a billet of SAE 4340 steel, the crankshaft was fully counterbalanced and turned in thin-wall copper-lead bearings 60.3mm in diameter. Rod journals were 54mm. Gudgeon pin was 27mm diameter, pistons, carrying 2-compression rings and 1-oil ring, were configured to provide an 8.5:1 compression ratio. Rob Debisschop of AiResearch provided an exhaust-driven turbo-supercharger and Stuart Hilborn of the eponymous fuel-injection company helped him work out the piping and fuel delivery to adapt it to the new Offy. Mechanic Herb Porter was an important ally of the project.
On the champion dynamometer Jones found that the turbo-blown engine produced 626bhp@8500rpm against 530bhp for the roots version at the same speed. In fact, the turbo-blown engine’s power curve was almost a direct extension upwards of the output of the blown Offy of 1954. Both produced 500bhp at 6500rpm but that was where the old one stopped. The late-1960 version could rev safely to 9500rpm. The turbo-Offy’s staggered the first drivers to try them “It’s the first engine I can’t drive full-bore off the corners”. Both versions of the engine appeared at Indy in 1966, 3-of each kind making the field, and all had problems, especially with cooling. The highest placed at the finish was Bobby Unser’s 8th, a turbo-blown model. The next year 7 of the 8 qualifying Offy’s were turbo-blown units, with 2 of them placing 7th and 8th at the finish.
No small amount of development was needed to get the engine to the point where it could win for Bobby Unser in 1968. At first it overheated severelly, water passage size increases helped and later the block was made asymmetrical for the first time to add more water capacity around its exhaust ports, which were divided into 2-ports with a small water passage between them. Internal pipes directed cool incoming water into this passage and onto the inner surface of the exhaust valve seat area.
Crankcase design was changed to gain the needed strength with the higher outputs. With the high heat inputs of the turbocharged engine, block quality was also a problem. The 1966 engine had aluminum blocks, which were unpredictably porous in spite of Drake’s best attempts to seal them. For the new 1967 engines a switch back to iron was made. In 1969 aluminum blocks were back. Thanks to a new foundry, Turner in Bell, California. To shrink the turbo-Offys to 159.4cu-inch to meet the 1969 capacity rules Drake made sets of sleeves which were pressed-in, with a 0.006inch interference fit, to bring the bore down to 102.4mm and the displacement to 2616cc. when these were installed in an old block they created a slight combustion chamber overhang around the edges, the new 1969 blocks had a smaller combustion chamber diameter to eliminate this.
New permanent-mould-cast pistons were provided. Some Offy buyers preferred impact-forged TRW pistons that seemed to give more consistent performance and reliability above 8500rpm. Careful development smoothed out the engine’s torque curve, filling in a former low point at 6000rpm. Joe Hunt’s magnetos were replaced by flywheel-triggered Mallory ignition system. Max torque was 510lb ft at 7000rpm and peak power was 727bhp at 8250rpm with an boost of 24psi.
The engine was further strengthened to stand up to the crew chiefs who liked to add 5-10 percent nitromethane to the methanol fuel. These improvements underpinned the engine’s subsequent success, which included 5-straight victories in the Indy 500 from 1972 to 1976. Not until 1981 did the new engines such as the Cosworth V8 eliminate Offys from the Indy starting field. In 1973, the year of peak supercharged development before limits on boost and fuel consumption were imposed, some engines were being boosted as high as 42psi for Indy qualifying, producing four-figure horsepower levels. On the McLaren engine dynamometer, on 37 psi of boost, the 2.6-litre Offy generated peak torque of 650lb ft at 7600rpm, it produced a max of 959 bhp at 8000rpm and was still delivering in excess of 950bhp at 9200rpm. This represented 369bhp per litre, the highest specific output produced up to that time.
Stroke/bore ratio 0.78:1.
Compression ratio 8.5:1.
Con-rod length 144 mm.
Rod/crank radius ratio 3.6:1.
Main bearing journal 60.3mm.
Rod journal 54mm.
Inlet valve 39.7mm.
Exhaust valve 34.9mm.
Inlet pressure 2.66Atm.
Engine weight 216kg.
Peak power 727bhp@8250rpm.
Piston speed corrected 24.4m/s.
Peak torque 692NM@7000RPM.
Peak bmep 483psi.
Engine bhp per litre 277.9bhp/litre.
Engine weight per bhp 0.30kg/bhp.