So after quite a bit of research, it seems that the ideal exhaust dimensions are 1 5/8" tube that's between 38-40" long for each pipe. I've also read that a 1 3/4" pipe off the head followed by a step down to 1.5" after the bend is a great way to get good mid-range power.
I plan on running a 2-2 exhaust for my bobber, but I don't want to sacrifice drive-ability/power for looks/sound. I was going to try to keep to the exhaust length for each pipe and likely go with 1 5/8" all the way. My motor is slightly warmed up (2nd over, XS1 cam, bumped up compression, VM34's), so i think I'll be fine with the diameter.
Does anyone have any thoughts to this setup? Should I try to incorporate some baffles, or will the use of a good turn and the proper length help with back pressure and scavenging?
Thanks!
Never step down the exhaust!
The trick to building an exhaust is 1) velocity preservation 2) tune length
You start at the head: The primary coming off the exhaust port only has to be as large (area wise) as the exhaust port itself. Remember, the exhaust primary is an extension of the port. You don't put a step at the exhaust port / exhaust primary interface for the same reason you don't do it in the intake port. Remember, most drag occurs from behind.
You then add a step up in diameter at 8-12" (this is the blowdown length - so it's rpm dependent). For an engine running over 7000rpm, I would put it at 8".
Then you step up in diameter every 10-12" from there on out.
Of course, remember, a 4-6" bend radius is going to flow more than the same size diameter on a smaller bend radius. You want the pipes to be as swoopy as possible. A tighter bend radius will require a larger diameter to flow the same amount.
Length is simply a function of rpm. There are different harmonics (2nd, 3rd, etc.) each with varying amounts of 'strength'. The 2nd is stronger than the 3rd, but is typically impractical to build to. Length is dictated where your operating rpm is.
Going back to the first part....
You're treating the XS650 like to single cylinders. Your exhaust pulses are 720* apart and this is about as bad as it gets in exhaust tuning. In all exhaust tuning, you want to preserve velocity - you can't gain back what you lose. This helps in a couple areas 1) improve scavenge 2) the harmonic pules will be stronger 3) it gets the exhaust mass further away from the cylinder so when the wave comes back up the exhaust and enters into the cylinder, it doesn't 'pull' as much exhaust gas back with it 4) the increase scavenge will make for a stronger signal at the carburetor - allowing a smaller jet for a given fuel demand, making for a more responsive motor, and ultimately, allowing for a larger carburetor (read: flow) for a given power band.
The idea is to keep the 'weight of that atmosphere' at bay. A larger diameter pipe slows exhaust gas speed way down and gives the atmosphere a much bigger door to get back into the engine.
To put into perspective:
A 1.625" pipe off the head will support 120hp/L ... or a 650cc Yamaha making 78bhp or a bored out 750cc engine making 90bhp. A 1 3/4" pipe, especially on a stock or even most modified engines, isn't doing much but just making for less pumping loss and isn't providing really any scavenge benefit.
As for a muffler, you want a straight through design that doesn't reduce the effective ID (ie: no baffles sticking into the exhaust pipe).
As for exhaust port optimizers - they do not gain back velocity. The exhaust gases are leaving the cylinder at close to the speed of sound (which is higher mph wise due to the temperature of the exhaust). Velocity can only be lost, not gained back. There are other tricks for controlling reversion outside of the powerband (Fueling A/R valves, a true reverse cone on a true megaphone), but they should be nowhere near the exhaust port. The Fueling ones are typically located at the first step location, but others experimenting in the high end automotive world, are placing them further back now.
Cheers,
Bob
