straight pipe length

Does anyone have the recommended straight pipe length for best midrange and/or racing performance? No mufflers or megaphones.

Starting with 1-1/2 pipes but also looking for info for 1-3/8 & 1-5/8

Let me know if measurments are along centerline etc.

thanks

Jim

I suggest the question you really need to ask is 'what is the optimum straight pipe length for a standard commando ? ' The optimum pipe length for any bike is a function of the revs at which you want maximum power, it can be calculated by dividing the speed of sound ( about 5000 feet per second) by the frequency ( revs) to get the wave length, and then using multiples of it. However you then must think about the characteristics of the cams you are using and whether your inlet ports are large or small (top end or bottom end motor). (On 650cc Triumphs with the E3134 race kit cams and small ports, revving to about 6,500 rpm, the optimum length has been said to be 38 inches, a commando would be somewhere near that). If you have a radical short stroke motor with extreme cams and porting, and use pipes fitted with large megaphones, your bike can turn into an unridable killer on a race circuit, so the application also determines the characteristics you need from your pipes.

Paul Dunstall's Notes:

Road use
separtate headers joined by balance pipe close to head.
1 5/8" should reduce to 1 3/8" at crossover on. 30" total.
Mo Power to 5500 than race megaphone, which then make
mo power around 7000 rpm.

Race use Must match the cam profile by trail/error.
Ballpark best start points...
Street cam, 1 3/8, 32" + 21" megaphone, 3" outlet
Race cam likes 1 5/8", 28" lenght, 24" mega 3" outlet.

This last was Ms Peels 750 set up and a real dullard breathless bogger depression on first few attemps till I opened up megaphone more than heavy cluster of super trap plates allowed, then OMG > would flat run out from under so first reflex to take off was to lock on white knuclked before throttle twist G's hit. Peel was 2 into one [~9 lb lighter] and didn't have much off idle bog as that's how I found out that tail light is not a good cushion or butt retainer when sudden un expected arm-wrist stretching opens throttle even more... Point is got to diddle up and down as no one size fits all. Tight bends have no effect Dunstall found but its generally reccomended to have some straight shot right out of head before the bending begins.

The variation in pipe diameters is a vexed question. A friend of mine raced a manx years ago in A Grade which was ex-works, and always used a skinny pipe. Some people have claimed better performance from manxes using large diameter pipes. On my own bike I use a small diameter two into one pipe with a tail pipe of the same total crossectional area as the two header pipes. I advance the 850 cam to compensate for the back pressure , and the midrange is excellent. My motor is a 'bottom end' item using small inlet ports, and only revving to 7,000 rpm (- set up for torque).
I found that with the old 4 speed CR box I was using, it was impossible to gear for the end of the straights on a race circuit and still be able to get a good clutch start without cooking the clutch.

Peel will need new headers so will try to keep in mind the cross sectional effect acetrel mentions. Also each change of ID step up or down is a source of sound reflection waves that can help or hurt in some rpm zones. Harelys can rev higher than our clunkers so 7000 rpm is red zone top end power Norton, much as it may not sound that fast.

The speed of sound is 1100 fps.
Jaydee

jaydee75 said:

The speed of sound is 1100 fps.
Jaydee

Click to expand...

Glad you caught that.

ugh, sound speed varies with density and temperature. Normal hi power air cooled british iron engine can expect EGT over 1300'F so more like 2000 fs to plug into trombone tone calculator for rpm ratification wave front to arrive at exhaust valve in the nick of time. Could just start with too long a pipe and cut an inch off till it gets gooder then gets badder then remake a correct lenght.

Sound vs temp calculator
http://www.sengpielaudio.com/calculator-speedsound.htm

Whats the plan Jim?

Early on in my 500 Dominator development I tried a straight pipe on the dyno. A whole range of lenghts from about 28 inches to over 42. None of them even came close to the performance of any simple megaphone with a reverse cone. (at least 5 bhp down from memory)

I alos experimented with pipe diameters. As a simple rule a long header pipe with bigger diameter gave the same peak hp as a short pipe with smaller diameter . But the long pipe had a much broader power range. By long I mean 40 inches plus. Pipe sizes 1.125 up to 1.5 inch for the 250 cc cylinder. 51 bhp from the 500 at the end.

So unless you have a vey particular reason to go straight pipe my experience is not very promising for road racing anyway. (And looking at any starting grid almost everyone has come to the same conclusion)

When Peels long dong megaphone mount fractured off her power fell below average decent Commando, could hardly idle and barely made head way off idle w/o reving and then hardly worth giving it much gas as just sounded too loud gathering speed.

I'm building 1-1/2 pipes for my cafe Atlas. I wont' be able to get anywhere near the recommended straight pipe length of 32" because of fitting considerations - I have custom short cans with glass packs with an ID of about 1-3/4 that will add to the length. The headers will be about 39" and the 1-3/4" ID cans add about 8" for a total of 47"

Overall length is not far off from what you want with megaphones (see Axtell specs below). It just depends on what the short cans do.

Info from Ken Canaga below (C.R. Axtell specs)

Back in the day I tried a variety of exhaust
systems on my race bikes, and settled on one recommended by Axtell. I used
1 5/8" pipes 28" - 29" long, with Axtell reverse cone megas. Ax's megas
were only 17 1/4" long, with an o.d. of 3 1/2", and a reverse cone 1 3/8"
long, with a outlet diameter of anything from 2 1/4" up. I generally used
them with a 2 3/8" outlet. The outlet diameter really depended a lot on the
engine tune. Ax told me once that the best was to put the bike on the dyno,
and keep opening up the outlet until the engine didn't rev as freely anymore
(his term was a "happy" engine), and then go back to the previous size.

Jim, fwiw,

I'm running a pair of Commando Specialties 1.5" ID head pipes that are butted to the 1.5" ID of my stock peashooters. The head pipe is joined to the muffler by a 1 5/8" ID sleeve, slitted to allow clamping over the OD of both the head pipe and the muffler. Runs fine by my butt dyno.