One of the pervasive myths surrounding sohc Honda fours is that aftermarket 4-1 exhausts can improve their power. In a word, this is untrue.

The reason is simple: Wave tuning, the actual physics that makes alternate exhausts affect engine power, can't have any effect in the vintage Honda four because these engines have so little valve overlap. There is no opportunity for pulse waves to reach the cylinder.

This is not to say that a 4-1 exhaust is worthless. There are indeed some benefits. First, since all 4-1 exhausts utilize the "extraction" principle it is possible to reshape the powerband slightly, stealing from one area in the rev range to enhance another. Thus at high speed a power advantage might be gained. Or not. And when it happens, it usually is at the cost of a glitch elsewhere in the rev range. Another benefit is the weight savings, and this is felt as an acceleration advantage, for sure. And, not the least benefit, the significant reduction in metal mass means the exhaust is much less prone to rusting internally, a problem that plagues the massively constructed stock exhaust. These are all good things.

But perhaps the greatest benefit of a 4-1 aftermarket exhaust is the reduction in engine vibration. The original 4-4 exhaust on the early Honda fours adds a resonance to the already buzzy 180 degree four engine that many find irritating. Happily, the aftermarket 4-1, carefully mounted, can often significantly reduce this.