THE FINAL FLING
In the third instalment of our series documenting the evolution of Porsche’s air-cooled engines, we chart development of the M64 flat-six, from discontinuation of the Carrera 3.2’s boxer, through 964 and 993 production…
Though nobody realised it at the time, Porsche’s flat-six wasn’t even halfway through its career when work began on a new engine to power the 964. All-wheel drive and other new-to-the-911 technology had been earmarked for the model at an early stage in its development programme, but with it came a seemingly unavoidable increase in weight. This presented a major challenge on two fronts. First, it was unthinkable a new generation of 911 could offer less performance than its predecessor. Secondly, extra weight worsened the car’s fuel economy and emission of pollutants, precisely when regulations governing these aspects of engine operation were getting tougher. Pressure was being driven by clean-air legislation in California, one of Porsche’s most important sales markets. A perfect storm was created for the manufacturer’s engine designers and developers, each tasked with working out how to enhance performance while also increasing efficiency.
Four-valve-per-cylinder combustion chambers were finding favour among other carmakers at this time. Porsche considered introducing the same for the 911, but concluded it impossible within the packaging constraints of the flat-six (as was). The biggest hurdle was the need to provide a sufficient surface area of cooling fins to keep the engine at the optimum operating temperature under all conditions. A twin-spark arrangement was conceived, a key aspect of the design being a patent filed in Germany by Porsche in 1982, crediting August Hofbauer and Michael Beer with the invention. The electrodes of the two spark plugs were not equidistant from the centre of the cylinder — each was optimised to suit fuel-air mixing in the cylinder at different engine speeds.
The mixture motion and position were additionally influenced by the shape of the piston crown, with a smooth oblong bowl cast into the middle. The patentable aspect of the design was the combination of the shape of the piston top and combustion chamber to cause two separate swirling motions within the cylinder, working with the two spark plugs to improve complete combustion of the air-fuel mixture.
Though this arrangement required an extra distributor (driven via a toothed belt) and six extra plug leads, it was found to be of significant benefit. Much of this was down to the extra stability of combustion at low speeds. In any petrol engine, one of the limiting factors is combustion stability at idle. Complete combustion of all the fuel in the cylinder is the end goal. If this doesn’t happen, then some of the unburned fuel passes into the exhaust, a condition bad for keeping a lid on pollution and fuel economy. This situation also manifests itself as uneven idle and can even generate pops and bangs in the exhaust.
What are the factors influencing this?
It boils down to the speed at which the flame front can propagate through the combustion chamber. The start of the flame front is when a spark plug ignites the air-fuel mixture. The flame front then moves through the mixture. If this movement is too slow, however, unburned fuel can end up escaping as soon as the exhaust valves (or valve in the case of the air-cooled flat-six) open. One major factor in the speed of the flame front is the motion of the gases in the cylinder. This is largely determined by the shapes of the intake port, the valve opening and the shape of the top of the piston. It should be noted that raising engine speed also increases this motion, but doing the same for idle speed increases fuel consumption and emissions. Not a desirable route to take, especially if avoidable.
The combination of a twin-spark head design and the shape of the piston crown resulted in a more stable idle in the 964’s engine, to the benefit of efficiency.
Leaner air-fuel ratios were also possible, reducing the need for enrichment of the air-fuel mixture on warm up, saving more fuel. That said, the twin-spark concept did much more than this. For example, it also allowed the engine’s designers more freedom with valve timing to increase valve overlap. This is the period of time where both the exhaust valve and inlet valve are open at the same time, toward the end of the exhaust stroke and the beginning of the induction stroke. During this period, the rapid flow of gases through the exhaust port, in conjunction with the design of the whole inlet and exhaust system, creates pressure waves which can cause suction through the intake valve, effectively allowing more air in for a given cycle, providing the exhaust valve is closed before the fresh intake air exits. This is known as the scavenging effect. Valve overlap is particularly useful at high engine speeds, where gas flow rates are higher and the scavenging effect is even stronger.
THE 964 CARRERA 4 WAS EQUIPPED WITH A DUAL-MASS FLYWHEEL TO DAMPEN VIBRATIONS AT LOW SPEEDS FROM THE OFF
YANK YOUR CRANK
Unfortunately, valve overlap is less useful at idle and low engine speeds. It’s not difficult to envisage some of the incoming air-fuel mixture exiting the exhaust, is it? Additionally, if overlap is too long, it can make for rough idling. This can be explained by the duration of the valve overlap: with fixed valve timing, the overlap is a set number of degrees of crankshaft rotation, but the slower the engine speed, the longer it takes for the crankshaft to turn that number of degrees, hence, at low engine speeds, valve overlap lasts longer. This can allow exhaust gases to flow back into the cylinder before the exhaust valve closes and, if the proportion of these gases is too high, it can cause incomplete combustion of the fresh air-fuel mixture, which manifests itself as rough idling. That’s just not acceptable in a road car and is the limiting factor on overlap when there is no variable valve timing trickery at work. The 964’s engine didn’t have variable valve timing, but because of the stable idle, Porsche could increase the overlap period, enhancing the scavenging effect and therefore performance.
Naturally, it was more of a challenge to package two spark plugs per cylinder