Motorcycle Exhaust Length Theory

[Buzz_Kanter]

Over the years I have seen several different length exhaust pipes on older Harleys - especially the JDs up to 1929.

Can anyone explain this to me. Was it all over the place because they didn't know how to best tune the engine, or some other reason?

[Dave Morrill]

The pipes on this 1921 J Model racer amaze me. They are about as far as you can get from the stubby aircraft style exhausts used on the earlier boardtrack bikes. Maybe they didn't work very well, because I haven't seen them on any later racers.

[rob]

first bike is hillclimber 2nd bike is mile flat track looks like short pipes for quick spurts and long pipes flat out top end


3rd bike half mile with medium length pipes

[D.Mac]

Buzz,
I remember in the late 60's 42 inches was supposed to be the "tuned length" for panhead pipes, this was supposed to scavenge the exaust gases. We all went to the local muffler shop and had 'em bend us up drag tubes 42" long. With extra compression and a Sifton cam, an old pan would really sing, you could hear it for a half a mile. Unfortunatly the CHP didn't like that music, and the ticket you got for this was "modified exaust" which was a moving violation and went against your driving record. After numerous times in court trying to keep my licence, I decided to go with something a bit less obvious. Still ran straight pipes, but with baloney mufflers. I just knocked out the baffles and put a bolt in the end of the muffler so a nightstick could not be stuck in to check for a baffle Still got tickets sometimes, but they were just "fix-it" tickets- stick some quiet mufflers on and go down to the cop shop and get signed off. Ah, carefree youth.
Anyway, sorry for wandering off topic, the "tuned length" pipes did seem to work well and I believe it was the guys drag racing panheads that came up with this particular length.
Doug.

[Bigincher]

You must live in a rough area, DMac. I've ran straight drag pipes on my old pan/knuck since 1974, and have never ever had a cop say anything about it.

[MattWalksler]

To tune your exhaust (straight pipe setup), paint the pipes, put them on, run them for a few minutes, then cut where the paint stops burning. Old trick taught to my dad by Jim Davis!

[Buzz_Kanter]

Originally Posted by MattWalksler

To tune your exhaust (straight pipe setup), paint the pipes, put them on, run them for a few minutes, then cut where the paint stops burning. Old trick taught to my dad by Jim Davis!

What an amazing concept. Wonder how well this theory works vs. test & tune on a dyno?

[28JDCA]

The problem with tuning is that you are 'tuning' for an RPM range, and anything you gain at high RPM's you usually loose at the low (coming off idle.) I was told that the really short pipes allow cold air in there too quickly at shut-off; it'll warp the exhaust valves. Most of us equate noise with horsepower, so the more raucous the better? I like the paint idea though. ...bill in oregon

[D.Mac]

Originally Posted by Bigincher

You must live in a rough area, DMac. I've ran straight drag pipes on my old pan/knuck since 1974, and have never ever had a cop say anything about it.

This was in the San Francisco bay area, been gone many years, don't know what they do now. Back then the law was doing all they could to harrass anyone they considered outlaw. Seems like a lot of guys run open pipes here in Oregon nowadays, and I guess they get away with it.
Doug.

[haugster1340]

Originally Posted by MattWalksler

To tune your exhaust (straight pipe setup), paint the pipes, put them on, run them for a few minutes, then cut where the paint stops burning. Old trick taught to my dad by Jim Davis!

This does work but, the point where the paint stops burning depends on the length of the pipe you start with. When helping a friend with his street/strip car we discovered that the longer the pipe you start with the further away fom the motor the paint burn will end and the shorter the pipe you start with the closer to the motor the paint burn will end. I have experienced this on garden tractor engines as well. Ad a short straight pipe, then ad a much longer one and the place where the burn marks end are as much as two feet from each other. Is there an equation to figure out the length of pipe one is to start with to achieve the performance desired?