Bonneville America Forums Forums Technical Forums Exhaust Ideal exhaust size and length

I read on the mikuni site that the ideal over all pipes should be 28"-32" of 1 3/4" header with a short (12" -19") open tip. Sound right? What is the scoop on the "new" (we did it with car headers 20 years ago) step design where they step from 1 3/4 to 1 7/8 to 2 1/2? Does this eliminate the need for a restrictor/ reducer? Any one know? I'm thinking of building a custom set of pipes.

Are you building headers too?

Thats the plan anyway...

Here ya go then:
Determining motorcycle exhaust lengths

Read all up on expansion chambers. If you get 'em just so, the pipe interior will create a low pressure area that will hasten the smoke out the pipe, so to speak. It works well on 2 strokes - maybe not so much on 4 strokes? I'm not sure.

Disco, Baby! Thanks Dinqua- you da man...

Anyone know the BBDC? I check the shop manual and no dice...

Let me know how the headers work out! Always wanted an SS set...

I agree with the stainless headers. Any blue is eternally polishable unlike chrome. Only way you wouldn't be able to polish out stainless is if you polish your way all the way through!! Thats ALOT of polishing!! LOL

From my Megacycle spec sheet: (I know they're different, but you can use this as a baseline for measurements)

Quote:

---

Timing Card: Make and Model: TRIUMPH 904cc Speedmaster
Cam# 520-x1 in/ 520x3 exh
Timing at 0.040" lift at valve
Timing checked with ZERO clearance
(see running clearance - lash below)
Intake:
open - 24º BTC
Close - 52º ABC
Valve Lift - 0.408"
Duration - 256º
Lobe Center - 104º
Lash or Running Clearance (cold) - 0.006" to 0.008"

Exhaust
Open - 54º BBC
Close - 18º ATC
Valve Lift - 0.385"
Duration - 252º
Lobe Center - 108º
Lash or Running clearance (cold) - 0.010" to 0.012"

---

And you need to figure out the difference between BBDC and BBC to get an accurate number. Aren't they the same???
Good luck, sounds like a fine project.

Okay and thanks, Pat. Yes, they are the same.

So that makes it roughly 12" of 1 3/4" into 16" of 1 7/8" into 22" of 2 1/2" for 50" total for a set of "long" pipes.

Cool.

LOL@yellowing matching. Plus Triumph chrome seems shaky at best so never have to worry about chrome problems either.

That isn't a link

Well... this is curious. I'll bet those megacycle cams made a BIG difference. I found the specs- they are in the front of the shop manual under general information. T100 and Thruxton are the same. AM abd SM are differentfrom T100 and Thrux, as well as each other. The specs for the SM are:


Intake:
open - 0º BTC
Close - 24º ABC

Duration - 204º


Exhaust
Open - 39º BBC
Close - 7º ATC

Duration - 212º

Which would make the steps 12"/15"/20". Not much change in the exhaust, but a lot of difference in the cams...

Those megacycle cams in Pat's bike must pull like hell on top in the upper rpm range with all that overlap.

Wow, this can be a very long discussion - but I'll resist.

There are 2 different ex system design theories: amplify the exhaust flow at a particular rpm point (usually peak power) through certain pipe lengths, diameters, collector design and collector dimentions. The second way (my favorite) is named 'anti-reversion' and has a design which tends to kill the exhaust pulse (which is amplified in the first design). Doing this dampening of the pulse tends to broaden the power over the rpm range. (I built anti-reversion headers for my sm blk Chevy dragster engine back in the early '80's.)

The most important dimension in header design is the diameter of the primary pipe, followed by the primary length and then the collector size and design.

Oh, the old separate pipe systems don't make the power of a collected system. (If it did, every drag car going down the track would have individual exhaust pipes - fuel cars excepted.) So, design a 2 into 1 collected system and put a Coanda cone on the end of it. Design programs should be able to be found on the net somewhere so one can set his pipe dimensions. By and large, longer pipes with not too large primary diameter are best, as they don't kill bottom end power and throttle response.

Stepped header design is an attempt to amplify the exhaust pulse over a broader rpm range, thus increasing overall power. It's tricky to design and build; they're best left to the pro header builders and, to me, are much too labor intensive to be worth it for a street engine.

By the way, putting a muffler on the end of an exhaust sytem, pretty much screws up the design, short of hanging on one of those specialty race mufflers, such as some drag cars use. (They're intended to kill noise, not ex flow.)