How about forced induction?
We can safely double the engine’s power, retaining the original RPM limits without doubling the structural loads placed upon the components. It’s not necessarily all about getting a bigger bang, it’s far more important to make the bang last longer, thus increasing torque. It works like this:
There are two types of load that an engine is subject to.
One is particularly destructive, as anyone who has seen a con-rod “let go” will attest. That type of load is known as “tensile“. It occurs when a pulling or stretching force is applied to engine components. The highest tensile load within the engine is applied to the con-rod at top dead centre (“TDC”) on the exhaust stroke. At that point, the piston is changing direction from its upward to its downward stroke, when both inlet and exhaust valves are open, so there is no air pressure within the cylinder to oppose the piston’s travel skywards. This stretching force creates a tensile load within the con-rod which, if rpm’s exceed design limits, can create metal fatigue.
(The destructiveness of tensile load is easily demonstrated: it’s much easier to strain your shoulder punching the air than punching a punch bag.)
Obviously the higher the RPM the greater the load. In fact the loads are huge and directly proportional to the RPM difference squared! So at 8,000 RPM the loads are 16 times greater than at 4,000 RPM. 8,000 RPM is fine when the loads have been factored into the design, but problems can occur when loads exceed the original engine designer’s parameters. So where does that take us? We need to be very careful indeed when considering raising the RPM limit. It can only take a few hundred extra revs to halve the fatigue life of your engine. This is why engine builders use lighter rods and pistons – i.e. to offset the inertial forces caused by increasing the RPMs.
This brings us on to the other kind of load: “compressive“. This load is at the other end of the scale but no less important to the basic design of the engine. 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.
So there are two kinds of load: one good, the other bad, but both necessary and both fine when calculated and managed.
But what do you get with a supercharged engine?
Firstly, greater compressive load. This is why we supercharge – for greater torque and power.
What about tensile load? In a good supercharged engine, that is effectively reduced too. The engine produces much more low and midrange torque, so the driver need not rev the engine as hard – remember, less rpm means less tensile load. Furthermore, to mitigate the unopposed stretch of the con-rod on the exhaust stroke, there is a mild cushioning effect at the top of the stroke from the compressed air in the supercharged car’s manifold.
Sounds like something for nothing! – Surely not?
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. As shown in the graph on the above, a forced induction engine is far more effective at generating power into the crankshaft than a normally-aspirated one – it 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 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 boost illustrated in the graph, just 6.5 psi. That means that notwithstanding our 53% power increase, peak cylinder pressure is never more than 10% higher than that of the standard car – well within its design parameters, and the water injection keeps cylinder temps at standard levels, the consequences being that coolant and Exhaust Gas/catalytic converter temperatures are also no higher than standard.
What about fuel economy?
Just because you are halving the revs does not mean that you are using half the fuel. A Forced Induction engine still has to obey the same fuelling rules – although we are halving the revs we are blowing more air into the cylinder and must fuel it in the same proportion (we maintain an Air Fuel Ratio (AFR) of 13.2:1 for maximum torque/transitional fuelling and 12.5:1 for maximum power). The gains come from the fact that at lower RPM levels there are lower internal drag, throttling and frictional losses to overcome. Secondly, that 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 the Normally Aspirated engine would at 5,000 RPM, but it does so using less fuel with much less tensile load than the standard car, and is quieter in operation.
We are always happy to discuss the science behind our engine enhancements – please contact us. We want you to be as confident as we are in the quality of our work (and you can impress those people who point at your engine and ask questions)!