Originally posted by PhamKL@Dec 28 2005, 05:49 PM
For 4-2-1 vs 4-1: having a 4-2-1 design creates two points in which flow is maximized. In an exagerrated dyno, it should be possible to observe two small torque peaks. This results in a low end peak and a upper mid range peak which results in better power and sooner. A 4-1 only has one peak.
This is highly simplified but the longer the runners, the higher and later the peak occurs.
I don't know where you got this info, but it's a little bit inaccurate. Longer primary tubes tend to produce more low end torque. Shorter primary tubes tend to produce more high end torque. It's pretty much the same with primary tube diameter. Larger diameter makes more high end torque (higher maximum flow rate) and smaller tubes make more low end torque (better energy retention for a variety of reasons, but not capable of higher flows). We won't even start to discuss reversion, anti-reversion, and auxiliary chambers. There are a number of software programs available on the web that address the design issues and do all the calculations for you.
4 - 2 - 1 works better for street and roadrace applications except on very high speed tracks like Willow Springs. It does this by tying the two cylinders that are 360 degrees out of phase together before merging the flows into the second collector. The reason for this is to improve scavenging in the cylinder pairs that are working like parallel twins in the middle rpm range. 4 - 2 - 1 pipes excel in midrange performance and help raise the middle part of the torque curve.
4 -1 works best for drag racing because it produces the best peak numbers. Since drag racing is what it is, the best peak number with the best gearing will win almost without fail (until you get to the truly exotic classes where the clutch has more to do with winning than anything). There are also huge arguments about which way to pair the cylinders in the collector since some guys want to put the pairs side by side, and other guys think they should be across from each other. Regardless of the details, a 4 - 1 exhaust header will make the best maximum torque at high rpm at the expense of low rpm torque. This is why I would choose a 4 - 2 - 1 for a street engine.