Siamese Exhaust

I happened to have come across a set of siamese pipes for a Commando (originally from Armour Motor Products) and tried them out last year. I say siamese because they're not an equal length 2-1 system. As you can see from the pics below, the right pipe follows the standard roadster pattern, and the left pipe crosses over in front of the frame to joins the right. Unfortunately I don't have the measurements on hand.

I was running a single mikuni and 1-1/2" headers with short universal megaphones (Map Cycle system), so I have a feeling bike never really breathed properly before I put these pipes on for a test. I ran them last season, and was pleased with the result - from the seat-of-the-pants dyno anyway. They seemed to be a bit anemic below 3000rpm, but definitely picked up after that. Hard to tell with the single mikuni, because I've always found that it stops really pulling after about 4500rpm.

Anyone else have experience with a setup like the Armour one here? I know that the common wisdom is that primary pipes in an exhaust system should be of equal length (and all other systems I've seen for Commandos are designed this way), but I'm wondering what the effects of unequal lengths on a Commando engine might be. There must be a formula out there somewhere....

Unequal lengths will cause unequal extraction of exhaust gasses from the cylinders, which will introduce a source of vibration between the cylinders. If you feel an improvement, don't worry about the induced vibration.

If you obsess over any induced vibration, then go to 2 into 1 into 2 symmetrically down the centerline.

Slick

hehe Trevor if the nay sayers were significantly right about this much mis match un-equal lengths to function well then I guess ya can't believe your butt sense. Before ya put down the Miki-roonnie-mouse single carb might fit a UFO kit then re jet for the illogical better flowing siamese pipes. Personally my sense of it is both side see it as same one long hi torque pipe

Btw this is almost exactly the design that litterally kept me from selling off Peel and quiting the clunker hobby - it takes whiplash hooked up power to impress my old bones but I swear to hi heavens I went straight from deep depression of wasted Cdo money when I half ass snapped throttle and put me on tail lens hurting my ass, so instantly angry at myself for being so depressed I'd not even tried to hold on any, so jerked neck up on that thought planted butt and squeeze locked arm and snapped again going 20ish mph to end up again shocked- lifted up seat vertical stretched arm on decortive hump of seat, then evil grin set in and I really hunked down like my P!! and smother throttle to WOT to hit 50-60 before I could let off - then a Shit Eating Grin Hit as realized easy potent enough to go change smug fatso tire attitudes. So quess what next money Pit Peel is getting. Keep farting with the mega end hole and maybe remove a gasket till you too might agree with George Kelly gp fast preference. I suspect a 34 Miki can flow close to 100 hp before choking off but sure worth finding out. There are versions that have equal lengths that meet under TS cover area but are not known for helping much so not very popular for decades now.

If ya want to be happy for the rest of your life take and ugly woman to be your wife...




Ever see clean shiny cars after days of multi leg baja rally style joys the canyon road getting there is only half the fun

Nortons offered a siamesed exhaust system as a no cost extra back in the early 1960s, for dommies..
And for performance work claimed it was significantly better than the stock twin separate pipes/mufflers.
(above 4500 rpm, when you observed the numbers they quoted).

Not seen any lengths/measurements quoted for the pipes, but the 2 pipes didn't look same length. ?
They only breathed out through one muffler too, so it cost less to make/replace....

Hi Trevor.
Re: Header Formula.
I recall reading Phil Irving's writings about induction length, porting, head and header design in the eighties (perhaps late seventies) but don't recall the publication. I believe he was working with Yella Terra / Repco in Australia around that time (45° angled induction ports and extolled D-exhaust ports on 6-cylinder Red motor heads) and I recall formulas being mentioned. Scribbled notes (veracity not established) I made in my 1973 Chiltons Norton manual says:

Induction length (inches) to rear of valve head = 228 x duration / (max. power revs - 500)
Primary pipe length (inches) from rear of valve head = (5100 x exhaust valve duration (opening to TDC in degrees)) / ((max. power revs. - 500) x 6)
Primary pipe diameter (inches) = ((cc's x 2) / 16.4) / Primary pipe length (inches)

I found these links on the internet - if they help:
Irving - https://www.facebook.com/UniqueCars/pos ... 6380559236
Formula - http://www.wallaceracing.com/header_length.php
BlahBlah - http://motodna.net/exhausted-motogp-exhausts/
Ta.

PS: ask hobot about his "crayon mark along the header to determine length".