When designing an engine upgrade it’s important to ensure that any improvement will not compromise the engine’s integrity or longevity. Often it’s felt that the two cannot go safely hand in hand, as the one is usually deemed detrimental to the other. Of course it depends entirely upon how the improvement is achieved. It can be done in three very different ways; increasing the RPM, increasing bore/stroke or via forced induction. In-depth analysis and research revealed fundamental impracticalities in increasing RPM and bore/stroke, both in reliability and cost vs performance (you can learn more about this by checking out our FAQ). Therefore, our focus went on innovative engineering in forced induction.
Compressive load is created on the power stroke when the burning gas applies pressure on the piston, down through the con-rod and into the crank. Compressive load means torque, and from torque, power. The designer needs to know the projected “peak” compressive load in order to design a big end strong enough to take the pressure. But the important fact to know about compressive load for our purposes is that it does not induce fatigue stress. In a good, supercharged engine, tensile load is effectively reduced too. As the engine produces a lower/mid-range torque, the driver needn’t rev the engine as hard, thus reducing unnecessary stress. Additionally, to mitigate the unopposed stretch of the con-rod on the exhaust stroke, there is also a mild cushioning effect at the top of the stroke created by the compressed air in the supercharged car’s manifold.
Engineers use an indicator called Brake Mean Effective Pressure (BMEP) to calculate accurately the average cylinder pressures generated over the four engine strokes. This produces a figure (in PSI), which is the single most definitive indicator of the engine’s effectiveness and power delivery. A forced induction engine is far more effective at generating power into the crankshaft than a naturally aspirated one and can effectively double the power. But, because BMEP is an average taken over the full 4 strokes and not just the single power stroke, it does not double the total load on the engine as you might imagine. The GMR Geyser conversion meanwhile, increases the BMEP of the 4.3 Vantage motor from 157 to 241 PSI! That explains the 53% increase in power measured through the wheels on the rolling road.
But here’s the point: the GMR system only uses half the typical boost, just 9.5 psi. That means that despite our 53% power increase, the peak cylinder pressure is never more than 10% higher than that of the standard car. This is well within its design parameters, and the water injection also keeps cylinder temps at standard levels, so that coolant and exhaust gas/catalytic converter temperatures remain normal.
Just because you are halving the revs does not mean that you are using half the fuel. A forced-induction engine still obeys the same fuelling rules. We are blowing more air into the cylinder and must fuel it proportionately. We maintain an Air Fuel Ratio (AFR) of 13.2:1 for maximum torque/transitional fuelling and 12.5:1 for maximum power. Gains come from the fact that lower RPM levels means lower internal drag, throttling and frictional losses to overcome. Also, the use of the Geyser System permits us not having to waste fuel (going above the 12.5:1 AFR), by using it as an anti-detonant.
The net result is that our forced-induction engine produces the same torque at 2,500 RPM as a normally aspirated engine would at 5,000 RPM, but does so using comparatively less fuel and much less tensile load than the standard car, and is quieter in operation too.