My understanding is that with exhaust, there are 2 benefits to work for
1) lack of backpressure - a larger diameter helps with this.
2) tuning/extraction - where the exhaust pulses resonate in such a way that it helps the exhaust gasses travel out quicker...and reduces backpressure. A larger diameter *sometimes* helps with this.

Since 2) is a function of 'tuning' the resonance of something, things like manifold material, number and size of welds, tube lengths, tube curve diameters, placement of mounting bolt holes all affect how the exhaust systems resonantes.

its likely that the aforementioned items make little difference, and in this case the calculator listed above would suffice for TW racing (...but maybe not for F1 or MotoGP).

I must defer to BC here. A lot of things make a difference on paper but dont affect anything when bolted to an engine. I'm losing my touch of these things.

-John

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