Well we see a lot shorter but if you don't want an airbox a VM type (non CV) carb is probably more tunable. Grzld refresh me but the longer the header, the better the low end?
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Troubles tuning BS38's for open intake/exhaust and 750 kit
- Thread starter Rexxis
- Start date
Two things are going on with head pipe dimensions. Shorter length tunes for a power peak at higher rpm. Optimal for the old twin is 32" to 36" valve face to outlet measured on the center line. MMM shared a neat trick that makes life easier--on our machines you'll get close to that measurement by measuring the head pipe on the outside radius. Diameter is what determines the torque peak for which the pipe is tuned: larger diameter tunes for peak torque at higher rpm. Optimal for our bikes is 1.5" OD.
Availability of pipes is limited. It seems that the aftermarket suppliers all want to sell 1.75" OD pipes. MMM at 650 Central has the cure for that. My own pipes are 1.75" OD and longer than optimal at 40" because it's hard to find stainless, and an upkick prevents use of torque peak correcting inserts if the pipes are shortened. But the motor delivers good power. Bottom line: there are some things that would make a fraction of a second's difference at the drag strip that you'll never notice on the street. There are other things that will turn a motorcycle into a slow, noisy [expletive deleted].
Availability of pipes is limited. It seems that the aftermarket suppliers all want to sell 1.75" OD pipes. MMM at 650 Central has the cure for that. My own pipes are 1.75" OD and longer than optimal at 40" because it's hard to find stainless, and an upkick prevents use of torque peak correcting inserts if the pipes are shortened. But the motor delivers good power. Bottom line: there are some things that would make a fraction of a second's difference at the drag strip that you'll never notice on the street. There are other things that will turn a motorcycle into a slow, noisy [expletive deleted].
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