1st harmonic pulse???

Why do engineers always seem to tune for the second harmonic pulse and not the first when they create their intake runners. From what I understand the 1st pulse is capable of creating 125-130% VE while the 2nd pulse is only capable of making 110% VE. I know that this would require extra long runers but that is just more space to mix the air and gas, better atomazation is a very good thing. it would be worth it to me to have a long akward set of runners to get 20% more power and better fuel economy with less emissions. Is there somthing that I am not considering here? Why does no body even make mention of the first pulse? Is the thought of building an extra long set of runners that daunting a task that noone would even consider it? Or is there somthing automaticly wrong in trying to catch that first pulse? (AKA extreamly narrow powerband)

Most are accually third, four, even more pulses. If you tune for the first pulse the intake runners could easily be 5-6 feet long, even longer depending on what rpm you tune them to. Its true that it weekens the more times it bounces back and forth, but there is simply not enough room to cheeply and effiently make a manifold that would take advantage of this. Also unless your running carb or tbi, there is no fuel mixed with the air as it is going though the runners.

^^^ Ditto. Catching the first pulse on anything but a 15,000 rpm motor requires very long intake runners. Also, they can produce higher PEAK VEs, but often at the expense of average VE across the board.

I remember reading an article years ago and they said that they were going to limit the power of F1 cars not by limiting their rpm but by limiting the length of their intake runners, reducing their fuel atomazation. They were talking about it reducing their power about 100hp If that has that much of an effect maby they maby we need to reserch doing that on passenger cars. Lets face it there is no downside to good fuel atomazation, the more the better. Does that make sence to you too?

Definitely the more the better, but today's injector designs combined with high swirl chambers and higher compressions (and higher temps associated with it) have atomization handled pretty completely. Volvo is coming in with numbers like 4% of the gasoline that isn't completely evaporated prior to combustion. During combustion I'm sure almost all of that evaporates and gets combusted given the incredibly low HC emission numbers from many ULEV engines today. I'm pretty sure that my 454 with its functioning exhaust crossover, 195-degree stat, under-hood air ingestion, and water passages right in the intake does an even better job than that when its really hot :) It cooks high-temp paint off the intake so my guess is that droplets of gasoline have no chance :)

How mutch would forced induction change the optimum intake runner length. Would the extra density make the pulse have to travel faster??

as pressure ratio increases, the need for tuning in general decreases. Tuning for an intake pulse won't have much effect because the air is always under pressure anyway. So, if you're tuning for an intake pulse its wise to tune it as if the engine were not boosted. That way you would have the benefits of the pulse while not under boost, and while boosted it would have little effect.

But, in direct answer to your question; a pressure ratio of 2:1 would theoretically double the density of the air which would double its mass in the runner which would cut its harmonic frequency in half. The other huge problem for tuning a pulse under boost is that along with pressure comes heat. In practice, a boost ratio of 2:1 only increases density and mass by a fraction of that pressure because of compressor efficiency, intercooler efficiency, and the fact that boost changes widely over the operating range means tuning for an intake pulse on a forced induction engine would only be beneficial at a very narrow, magical confluence of RPM, load, and pressure. Not worth the effort.

The intake pulse tuning is very effective on race engines for two reasons; they typically operate in a narrow range near the hp or tq peak where the pulse helps, and they can spend the extra tuning time developing it because every last hp helps. On street cars that operate on a broad range of RPM, intake runner length and size is more a function of the inertia created by the column of moving air while the valve is open, not the secondary pulse available for the next intake stroke. Small long runners peak their velocity and inertia early helping low end torque, but they choke off at higher RPMs. Wide, short runners peak velocity high supporting high-rpm hp, but there is little inertia to pack in extra air at low RPMs. The intake harmonic pulse that happens when the valve re-opens is but a fleeting boost to VE on a pretty narrow range so it isn't of great consequence on a street engine.