Dave Dunstan's rotary valve engine, produced with Southcott firm in Adelaide in 1950s, takes on almost-myth status
Adelaide engineer Dave Dunstan's rotary valve engine was first used in the South Maid racing boat, followed by speedcars such as Dud Dansie's. He later started, with the Southcott firm, converting Holden grey six-cylinder engines to his system.
The Dunstan rotary valve engine image courtesy Cyril Robinson
Dave (David George) Dunstan’s 1950s rotary valve (head) design for the internal combustion engine, created at Southcott Ltd (precision engineers) in Adelaide, became a much-discussed South Australian venture, taking on almost mythical status.
Dunstan, from Adelaide’s Clarence Gardens, worked for General Motors-Holden (GM-H) during World War II when H.S. McLaren submitted a rotary-valve engine concept to GM-H. The inventive Dunstan became friends with McLaren and devised his own rotary-valve variation. Southcott, where Dunstan later worked, allowed him to produce his first prototype in 1954, followed by 30 production heads. Dunstan also designed and produced a fuel injection system and dry-sump oiling system for the prototype and production heads.
Dunstan's rotary valve was first used in racing boats. It powered a Lewis skiff, the Southern Maid, that sunk, after setting a new water speed record, in 1954. Dunstan's engine was in 50 feet of salt water for a week.
Dunstan rebuilt the engine and came up with a new two-piece head that made 200hp (horsepower) on the dyno running nitromethane and 150hp on the pump fuel while revving to 7500rpm. The engine ran stock crank, pistons and rods, with a 12:1 compression. Induction was by mechanical fuel injection. Its downside was a lack of low-revolution power.
One of Dunstan’s first production engines was used in Bill Rowe's home-built Minx hydroplanes. A Minx, renamed Charmerie, was sold to Clive Fromm, with great success using one of Dunstan's rotary valve/Holden engines.
The first car powered by a Dunstan rotary valve was believed to be a speedcar owned and driven by airways pilot Roy Sands. A Southcott 1958 bulletin said it “easily won two speedcar races ... the acceleration and speed was greatly in excess of all other cars and engines”. Next to experience the Dunstan rotary valve thrust was Murray Trenberth’s thoroughbred Alta but it hit a lack of low-end torque.
In 1959, Bakewell Bridge Motors built a new special for its owner and TR2 driver Dud Dansie, first appearing at the October Port Wakefield race meeting, with a supercharged version of the Dunstan rotary-valve engine. Dansie raced at Port Wakefield meetings through 1960, blowing up the engine in an Easter race but finishing place in the June feature event. By December, he’d replaced the engine with a supercharged non-rotary Holden.
Dunstan hoped his rotary valve would become a standard for the regular family car. The nearest he came was when Southcott took on converting the post-World War II Holden Grey six-cylinder engine, party or fully, to his systems, for those seeking better performance.
Dunstan designed and built a rotary valve fuel-injected head for the 138 Grey Holden engine. His design varied from the conventional where the valve turned at half engine speed: it rotated at one-quarter engine speed, with two intake passages, 180º apart on opposite sides of the valve, for each individual cylinder. Intake and exhaust gases used the same valve port in alternation, reducing cooling problems and friction losses. The valve was hollow, with cooling water passing through, and the cylinder head is split horizontally to hold the valve, with sealing rings set into the head to seal the unit.
The prototype Dunstan Holden conversion first appeared in 1954., Engines fitted with Dunstan’s unusual head were developing about 140bhp in 1958. In 1957, the Dunstan rotary valve could be bought as stage 1 and stage 2 kits or the complete stage 3 Holden Grey converted engine for about £675.
Dunstan engines were said to be so smooth in operation they could maintain 8000rpm (revolutions per minute) indefinitely and 10,000rpm for short periods. But they had problems with the heads cracking and overheating with the bottom ends not taking the extra revs. Dunstan’s death in 1958 ended his efforts to get them developed properly.