4-2-1 or 4-1

[Doug Carter]

Can someone explain the needs or differences in both exhaust collectors? Where would one work better than the other, and why?

[CenDiv20]

Talk with Mike Beaumia about it since he makes most of the exhaust systems up here in the CenDiv. Varacins can also offer some good insight.


JD

[hardingfv32-1]

Generally speaking.... 4-2-1 lowers the location on the RPM scale of peak Torque and HP for a given primary length. This could also be accomplished using longer primaries, although some thing the area under the curves is greater with 4-2-1. All of this DOES NOT jump out at you on the dyno, so it is not obvious which is better. It could be somewhat situation oriented. You see them in Nascar but not F1. Closer to FV, FF has been a little indifferent about the acceptance of 4-2-1.

Framing this as a Darwinian type of activity: After 45 years, why have we not settled on the correct solution? Could it be that it does not matter? This is an easy area to do development in. I would say that the primary exhaust lengths are the only thing etched in stone.

I speak from a background of many weeks of exhaust development work on the dyno and track.

Brian

[SR Racing]

Since the 2 merge points are a few inches apart, there is actually 2 tuned peak RPM points. The logic being that the peak HP range will be longer. For example if the first merge point is tuned for peak HP at 5200 and the second at 5400 you will flatten the curve from 5200-5400. Whereas with a single merge point your peak HP would be at one point. You can see differences on the dyno but it's not always significant. For whatever reason it also seems we always see a slight stretch of the HP curve at high RPM.

Scavaging your cylinders is critical in any engine, but probably not as critical with 1200 cc and the VW design.