Custom Exhaust

In an ideal world, the exhausts are technically the same length. That is making the assumption that both exit the same and have no bends or routing constraints. Unfortunately that isn't the case. I use old calculations from Colin Chapman of Lotus fame for doing my exhaust systems. Given the diameter and the radius of the bend, one can determine the amount of restriction created -vs- a straight flow. Trick is to get even exhaust flow (CFM) from both exhaust systems.

Crossovers are great -- provided the exhaust flow (CFM) going into the crossover itself is about the same and out of phase. Problem one runs into is that with differing flow rates and the cylinders being out of phase, you can get some really strange things to happen ala one cylinder scavenging incredibly well and the other really poorly in a poorly designed system. Thus a crossover itself isn't always to ones advantage.

Search the threads -- there are numerous solutions tried by many people to build a better or unique exhaust system.

Desmo Elmer said:

Seems like one could get very near to equal length headers that way, but my concern was whether or not a crossover would be necessary. I've seen crossovers on inline twins with identical dual exhausts so I presume there is more to it than just "equalizing" disparate header lengths. Are there any good theoretical sources out there you'd recommend reading?

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Per crossovers? I don't use them at all on my DesmoDevil BoomTube exhaust systems. There is nothing to gain with a crossover just inches from the outlets themselves -- although people always ask why I don't do one as if there was something wrong with them otherwise.

The resources I use are from Jo Marquart from my old IMSA GTP racing days. He was a designer/fabricator within Lotus racecars during their F1 / Indycar heydays years -- and later when he did his ARGO racecars for IMSA and other classes. His cars out of the gate typically won their first race and were competitive immediately.

I've talked with enough engine builders that I have a good understanding myself of what needs to be done. I'm sure the information is out there somewhere via the Internet also or via books -- I just haven't looked. If I need help, I usually pickup the phone and call people within the racecar industry that I have known for decades.

One thing I recommend is some type of baffle or restrictor ring within the exhaust itself if you go short -- ie: anything under 26" overall length. The Ducati v-twin likes a small amount of back pressure to run properly.

The only problem is this guy isn't making pipes for ducati.