Question: what does the Rolls Royce Merlin engine (as seen in the Spitfire, Hurricane and Mustang World War II fighters) the Cosworth DFV V8 Formula 1 engine and the comparatively humble GSX-R750F have in common?
Answer: under-piston oil cooling.
It’s no big surprise that when engineers get their sums right and build amazing motors that (ahem) things get copied. Or perhaps other engineers arrive at a similar solution, sometimes decades apart?
Legendary Cosworth co-founder Keith Duckworth had a good, long look at the classic 27-litre, four-valve per cylinder, V-12 Rolls Royce Merlin when developing the DFV engine for Formula 1, which was funded by Ford and which won first time out in 1967. Like many engineers he needed to keep the engine cool to get maximum power and one way the Packard-built Merlin did this was the use of oil cooling whereby the underside of the piston crowns were sprayed with oil. Of course the Merlin was also using ‘conventional’ liquid cooling via the use of ethylene glycol and water and the DFV was also liquid cooled…but…
Suzuki had examined the design of the DFV when they began designing their Twin Swirl Combustion Chamber head in the late 1970s. Come the early 1980s and Tansunobu Fuji was the man in charge of engine development for the GSX-R750F, which was due for a 1985 launch. He had to design a relatively new engine which had to provide 100bhp.
Cooling was the issue for Fuji-san. And unlike the Merlin or DFV he couldn’t afford the luxury of liquid-cooling on his much smaller/lighter motor. Water-jackets had been tried before by Suzuki and others and as effective as they could be in the early 1980s they also added too much weight as the target weight for the whole GSX-R750F was just 179 kilos. Instead they needed some form of efficient cooling to maintain a good working temperature. Too much heat meant too much power loss!
Suzuki developed what later became known as SACS – or Suzuki Advanced Cooling System – specifically for the new machine and the basic idea was there from the Merlin and the DFV. The system used two oil pumps. The first did the traditional job of lubricating crankshaft bearings, the forked rocker arm assemblies and the cam-shafts as well as squirting the oil onto the underside of the pistons.
The heat generated at the top-end of the motor was drawn away from the hot spots and back to the 5.5 litre sump by the second of the two oil pumps, which was basically the cooling pump, which then pushed the oil to galleries in the head. Suzuki also used a large capacity oil-cooler on the GSX-R750F along with a thermostatically controlled fan – making this an oil/air-cooled motor.
Thanks to SACS, parts such as the crank-shaft, con-rods and pistons could be made lighter. In fact, some parts were based on the previous GS750/700 and destruction tested during development to see what leeway they had. Development showed what parts could be made lighter, thanks to the lower operating temperatures they had to endure. Lighter parts meant higher revs, which also gave more power. During testing, engine oil temperature always stayed around the 100 degree mark and never exceeded the 135 degree level: this was a significant 15 degrees down on what Suzuki claimed for a purely air-cooled machine. For racing where tuned engines produce more revs, power and therefore heat: things to be cooler still. This is why Heron Suzuki in the UK used oil pipes that were 20% larger than stock on the first iteration of GSX-R750F race machines.
Later versions of the GSX-R had tweaks to the SACS system. The Slingshot 750J of 1988 featured a still-larger oil cooler which was bigger than most radiators on water-cooled machines which were now in showrooms. Oil flow in the cylinder head was also improved by around half again. Even with these changes cooling could be an issue, so a number of race machines around the world would sprout cooling scoops and their associated pipes to get cold air down to the engine.
Come 1990 Suzuki had already introduced liquid-cooling on its GSX-R400RR-L, but the oil-air-cooled motor soldiered on with the 1991 GSX-R750M before the water-cooled 750W-N came out in 1992. By now the GSX-R could wear a water jacket as technology had made them leaner and more efficient than oil-air-cooling, even if the purists mourned the passing of the old, oil/air-cooled lump…
Other Suzukis would have oil/air-cooling alongside the 750, including the GSX-R1100 of 1986-1992 and numerous versions of the Bandit 600/1200 family as well as the GSX600/750F. Not a bad family tree at all, we say…