Exhaust Systems

4 into 1 vs 4 into 2 into 1

Perhaps you're talking about something else, but 4/2/1 headers give a different kind of torque curve (broader).

No, you're on the right lines- but I wasn't aware that the curve was different.

Yeah, any pipe join causes a reflection in the sound pulses and these can be timed to arrive back at the exhaust valve just before they open again. Since the pulse hits a solid valve, it bounces back again which leaves a little low pressure behind it, just in time for the valve to open and so help the new exhaust gasses to exit. Since the exhaust length is fixed, the RPM at which this works best is also fixed.

So, a 4-into-1 exhaust can be tuned to help all four cylinders breath really well at a very specific RPM.

In 4/2/1 exhaust at the 4/2 join, there is a reflection back up the source pipe, but there is also a pulse going up the other pipe. This can be tuned so that this "extra" pulse can help the other cylinder at a different time. I don't know the analysis well enough, but the idea is that it helps the breathing over a wider range. Note that this is in exchange for a lower peak, as compared to the 4/1. I've always had 4/2/1 headers in my cars and they've seemed to be very effective. Of course, I've never done the comparison side by side.

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