Quote:
Originally Posted by Hawk77FT
For all of you who want the dip gone here is an explanation from someone who knows a thing or two about tuning/engines:
"As posted somewhere previously, the “torque dip” is a result of engine internal design not tuning. The only thing tuning can do is move the dip somewhere else.
There’s a long discussion here, but the best way to think of it is to look at the torque curve… if you permit me some artistic license, the torque curve looks like two hills with a valley at either end and another in the center. Each of the hills is a cam profile… low speed on the left and high speed on the right. The peak of each hill is the maximum breathing that cam can provide… either side of the peak the cam doesn’t breath as well and that will only change by re-profiling the cam's lift and/or duration - you can only change the hill shape by re-profiling the cam. Changing the VVTi timing just moves the peak left or right... the problem is if you move, for example, the high speed peak left to fill in the valley between the two peaks, the valley to the right of the high speed peak gets deeper very quickly and the engine falls on it's face."
Therefore, there is no tune out there that removes the dip..yes it might either reduce it or move it elsewhere, but you cant completely remove it without upgrading the cams. I am sure that in the following months/years we will see some modified cams ready to go for our cars, but until then we will have to deal with it.
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While i agree with that statement to some extent (tune alone will not remove the dip, but i don't think the dip is to do with the cams at all) the question was about why the aftermarket headers are able to remove the dip.
I assume the real answer is way over my head and has to do with harmonics and wave theory. The way I imagine it (note i am not saying this is the case, just the way my tiny brain tries to wrap around it) the design of the factory header creates a sort of standing wave in the exhaust at the "dip" rpm range... thus creating excessive back pressure which kills scavenging and causes the air flow to drop significantly in the dip (as i have logged over and over again).
If my magic wave unicorn theory is correct then a change in primary length, primary diameter, collector design etc COULD change or remove the restriction. As we have seen, not all of them seem to do that, but a few certainly do.