BrazilianZ28Camaro wrote:You're concerned with wave tunning on forced induction engine?
I am concerned about wave tuning on all kinds of engines, boosted or not. Why wouldn't the waves matter in turbo engine? Now, with turbos there's one more degree of freedom (manifold pressure can be chosen) so it's more complicated.
BrazilianZ28Camaro wrote:The boost pressure offset an incorrect runner lenght by far.
I agree that one can boost over many flaws. But it would be nice to maximize the potential.
My impression is that a lot of N/A race motors are tuned for the 3rd harmonic whereas at least here in the US high powered turbo cars seem to have short runners that look like N/A runners tuned for the 4th harmonic. Or maybe the 3rd harmonic but at a high rpm. If I am correct, I don't know why that is. Let me speculate, however:
The resonance frequencies change with density, pressure, and temperature. When air is compressed, the density and pressure effects cancel, and only temperature needs to be considered. There's some effect on the optimal runner length from the higher temperature of boosted charge, but I believe that's minor. Let's ignore this.
Friction and losses from small radius turns appear more significant with denser charge in the sense that it's relatively more important to make the runner paths as straight as possible with a boosted engine than with a N/A engine, even if that means compromising a bit on the runner length. Things seem to fall apart real fast in boosted applications if the air speed gets too fast for what the intake port and runner can handle. That's pure speculation, can anyone confirm?
For a N/A engine, runners are tuned for the mid point of the used rpm range. For a turbo engine, one might do better maximizing the dual torque peaks right before the boost threshold rpm and at the very high end when the engine is running out of turbos. Basically find out the point where the selected turbo work the most effectively and then tune the intake to work poorly at that point. Conversely, design the intake to work well at the points where turbo doesn't work that well. The turbochargers can fill in any torque hole in the mid range. That's one strategy. I can't think of an intake design without moving parts, however, that works well under 4000 rpm and then great again above 6000 rpm while not doing much in the 4000-6000 rpm range. Any ideas?
Finally, if the turbo engine is intercooled and intercoolers have excess capacity, it sometimes makes sense to "detune" the intake. That is, intentionally design the manifold such that the charge density drops in the runner. Why? Just turn up the boost to compensate. What's the benefit? One can get hotter air to the intercooler, which means more heat is removed. Then the temperature drops further in the runner as the pressure drops. It's like the air-cycle air conditioning in air planes. For knock-limited engines, this can result in same mass in cylinder with lower temperature, which shifts the knock constraint. This is what the new 911 Turbo successors do, I believe.
And all these things have to be taken into account while dealing with space constraints. So far, the best idea I've come up with is to tune for the third harmonic at high rpm where I am other wise running out of turbos. This means shorter than typical runners.
Still, I am very, very intrigued by those second order harmonic Honda four-banger manifolds that Joe McCarthy has posted about. If I could get straight enough runners, wouldn't say 7k rpm 2nd order manifold work better than anything else?
BrazilianZ28Camaro wrote:You shoul'd be concerned with fuel distribution in first place.
The injectors are in the intake port. With any sort of non-zero runner length, fuel will not be blown out of the runner. So fuel distribution will be automatically correct as long as air distribution is equal between cylinders.
BrazilianZ28Camaro wrote:You could build a intake with short runners and a big plenum feed by the front/center ,just like the Alien intake NRE build.
The space constraints under the hood are such that I can't do a front feeder. I need to use a rear feeder or dual side feeders.
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