So what are the ideal engine specs for those who plan togo the forced induction route? Low CR in the 8.0 to 9.0 range? Stock or mild increased cam? Over square, square or under square bore vs stroke? Air to air or water cooled intercooler? Other considerations?
If this is considered hijacking a thread my apologies to RJRuiz but hopefully it's relevant to your opening question. Plus the Rotrex is not yet available in electric drive but is in crank driven.
Doug
Like many other things when it comes to complex systems, most of those factors depend on what you want.
CR, in a FI application, could be considered a limiting factor when you are designing the system. For example, if you only want low boost, you can get away with 9:1+ on a M30 on pump gas using port injection no problem, especially with an intercooler. (at or under 7-10 psi) Much above that, and you start having problems due to the M30's head design and the limits of available gasoline. What is really going on is you are trying to maintain a maximum pre-ignition temperature and pressure so you don't get detonation. The pressure is increased by the CR, the boost, and the intake temperature. That's why you see high boost builds with a low CR. Alternatively, using higher-grade gasoline raises the allowable pressure, giving you more power. This is why you see really high power builds state two power ratings: pump and 100 octane.
I have seen a few M30s pushing 500 on 20+ psi using CRs in the low-to-mid 7:1 range, running on pump gas. 300-350 is pretty mild and can run the stock CR with intercooling up to ~14 psi. TCD and Miller have done some excellent work along these lines.
Intercooling is another system with pros and cons. On the one hand, air-water intercoolers are usually smaller for a given temperature reduction, which results in less of a pressure drop. On the downside, they are much more complicated, since they require another radiator, tank, pump, and lines. Also, the coolant eventually becomes hot under extended use, resulting in heat soak and reduced effectiveness of the intercooler. Air-air coolers are usually larger for a given temperature drop than air-water coolers, but they have no secondary systems and will never have heat soak. Larger systems also reduce the transient response somewhat, since you are increasing the volume of air that must be moved when you open up the throttle.
Oversquare and undersquare function exactly the same as a NA engine, because all you are doing is increasing the air and fuel present for the combustion event. Oversquare gives you high-RPM torque and power at the expense of low-end, undersquare gives you more low-end torque at the expense of high-end. Undersquare won't really help with a turbo though, since the turbo will only be active from ~3500 RPM and up.
Cams do get a bit tricky, and I frankly haven't looked into it enough to give a decent opinion.
RJRuiz- when you say "driver-adjustable boost" are you talking about a knob in the cabin? There are good ways to do that which are a bit tricky, and there are poor ways to do it that mostly work.
