Ideal design for pipes?

wannabridin

XS650 Guru
XS650.com Supporter
Messages
1,319
Reaction score
849
Points
113
Location
Hurst, TX
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!
 
the step down sounds good. when I rebuilt my pipes, I was advised to keep the legth no less than 36 inches, that's with the stock 1.5 inch inner pipes This helps scavenging. I then put mikesxs small glass packs on, and at idle it is a nice sound, but will bark when prompted.
Its a 6th over engine, with john's needles in 34's, stock cam
 
As a rule, the larger the diameter that the head pipes are the higher the rpm they work. Shorter pipes are tuned for higher rpm. Stock pipes are 1 1/4" so these with very free flowing mufflers will give power everywhere with a stockish engine.

Most people think a larger diameter pipe flows better but that is not always the case with a pulsing flow in an exhaust. An interesting modern headpipe is stepped up in size down its length. You could use a stock innerpipe to after the first bend, then 1.375", then 1.5" after the bottom bend to the muffler. I'm just musing but there is a good chance this stepped pipe would work great.

Tom

Posted via Mobile
 
Re. head pipes, length impacts the power peak, diameter impacts the torque peak. Straight pipes do nothing but narrow the power band, which is why you do not find them on performance modified sport bikes. You do find them on heavy cruisers and choppers because some folks think they're going faster if they're making a lot of noise and/or because some riders think it enhances their manly image if they're putting their noise in other people's ears.

Torque peak can be corrected on fat pipes with tapered outlet inserts like Michael Morse's TPO's from 650 Central; otherwise stick to 1.5" head pipes 34" to 36" long as measured from the port outlet on the outside radius. If you want performance, mount a good pair of straight-through glass pack mufflers; Emgo 17" megaphones do a good job. Michael's exhaust port optimizers do a nice job of boosting exhaust gas velocity into the bends and controlling reversion. The combination of EPO's and TPO's in 1-3/4" headers will give good results; it's what I run in a 700 cc. mill with Jacks's port work and a Shell #1 cam, and I'm very happy with that setup.

Baffles inside straight pipes give the worst of all possible worlds in terms of performance; you get the narrowed power band of a straight pipe plus nasty restriction. They're strictly for "the look."
 
Last edited:
Baffles have a bad rep. I was running straights and baffles, they worked fine, but t the raspy noise they made was too much. I'm really happy with mikes glass packs, even if I have to repackage them at the start of every riding season.
Gris, I also have header wrap, does that indeed help increase exhaust gas velocity?
It's a great way to hide fucked up pipes and shitty welds, other than rusting out the pipes quickly, what say you?
 
What the wrap does is retain exhaust gas heat, and some claim that yields a performance benefit; but you won't find that stuff on race bikes or serious performance builds. I can't see heat retention as a good thing, especially in an air cooled motor, but under normal riding conditions I doubt that it does any harm either.
 
All great stuff guys, thanks for that.

Looks like I'll reduce the overall diameter of my pipes then. I'm likely going to run some small diameter mufflers, I'm trying to find one now. cone engineering has some good stuff that I'm considering, unfortunately it's pricey...
 
I kinda view exhaust configurations like shoe-shopping for the wife. With all the various engine setups and rider demands, there is no "one size fits all". And, there's almost no telling the result until you're committed to the installation. Then THIS happens:
BagHead.jpg

Maybe a suitable solution, for the non-dyno-equipped builder/assembler, would be to have a table of various engine configurations, each with recommended exhaust designs and their foibles, including exhaust designs that don't work well in that configuration.
 
Amen to that, 2Many! But it's easy to go AR on the exhaust, and for street use on XS650 motors in a reasonable state of tune there's a pretty wide range of things that can work well, and a few pretty simple rules of thumb to follow.

1. Get your motor tuned on the stock system before you fool with performance pipes so you won't be blaming ignition or other defects on the exhaust system.

2. Regardless of displacement, cam grind, etc., you won't go wrong with 1.5" head pipes or 1.75" pipes with outlet inserts.

3. In selecting length, a bit long (up to 40") is better than too short; high end power loss will be minimal, midrange grunt will improve.

4. Avoid 2-1 systems. With most of them you can fool with jetting until you're sick of it and still wind up with midrange flat spots due to poor collector dimensions, and cornering will be limited.

5. Pick free flowing mufflers with good volume.

6. If your goal is performance, avoid high pipes with tight radius bends that impede flow.

If you're using an exhaust system calculator program, remember that they specify length in terms of the head pipe center line from exhaust valve face to outlet. Michael Morse shared a shortcut with me--on the XS650 motor, measuring the outside line of the pipe yields a result that's so close that the difference doesn't matter.
 
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
 
Last edited:
If it were me ,I'd focus my attention at getting the head ported,then proceed to build a stepped header with a functional megaphone strapped at the end to pull at the E/P and use the stepped header to prevent any negative reversion from re entering,deluding the intake charge. Stepped headers are unique in design as they offer a low and high pressure points to combat reversion and the smaller primary pipe inserted in the larger primary act as a Anti Reversion insert .Look at the the old Super Trapps megaphones for the BMW airheads,with the disc plates removed and a custom baffle installed ,you won't find a better megaphone that'll pull as hard as that muffler does,you want it to pull not just make noise.
 
Last edited:
.Look at the the old Super Trapps megaphones for the BMW airheads,with the disc plates removed and a custom baffle installed ,you won't find a better megaphone that'll pull as hard as that muffler does,you want it to pull not just make noise.

Jack,

My experience with the Supertrapp systems is that they are not true megaphones. Look at the modern BillBuilt, Barracuda (formerly Byrd), etc. set ups. Megaphone with a reverse cone - the muffler slips over the taper down part.

No way do you want the closed end cap. Causes big reversion problems.

Sans using a megaphone, go with a real nice Burns Stainless / Coast Fab. muffler
dirt-tracker-1-625x416.jpg

note: this bike has a stepped primary, he just hit it inside a larger tube.
 
Last edited:
Back
Top