Larger exhaust system

Schwany said:

Considering it is only a whopping 1/16" increase in the ID radius, and a 1/8" increase in ID diameter,

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Looks like about 10% more internal cross sectional area.

worntorn said:

When I tested the large bore pipes I spent quite awhile on carburetor tuning but they just don't produce as much hill pulling power as the stock skinny pipes do.
Dunstall discusses this in his tuning notes.
He was also had input to Commando engine design as a consultant. The separate skinny pipes were his recommendation. Seems to be correct, for the stock motor at least.
The reduction in power isn't huge, 4 kmh on Dyno hill vs skinny pipes. That translates to about 3 bhp loss @ 4500+- rpm.

Glen

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I put your comments in the pipe testing category where you wanted to see how a variety of pipes worked on a stock 850. I was not aware that you were making carburetor changes. Oops

The OP's motor is not stock so no telling how it would respond, but skinny pipes more than likely work to produce more torque at lower RPM. Commandos were sold with skinny pipes if I'm reading all this thread right, so I would imagine they are better on a stock motor.

I would install the larger diameter pipes if somebody gave them to me and my old pipes were falling apart. Worse yet, I wouldn't ask if they would work or not. I am funny that way.

Thanks for the info on how the testing was done, and the results.

Suck it and see . It might run differant , somewhere , somehow , so what . One Way to FIND OUT . Try It .

Matt Spencer said:

Suck it and see . It might run differant , somewhere , somehow , so what . One Way to FIND OUT . Try It .

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I'll look forward to your "which carb jets/slides?" thread.

Schwany said:

I put your comments in the pipe testing category where you wanted to see how a variety of pipes worked on a stock 850. I was not aware that you were making carburetor changes. Oops

The OP's motor is not stock so no telling how it would respond, but skinny pipes more than likely work to produce more torque at lower RPM. Commandos were sold with skinny pipes if I'm reading all this thread right, so I would imagine they are better on a stock motor.

I would install the larger diameter pipes if somebody gave them to me and my old pipes were falling apart. Worse yet, I wouldn't ask if they would work or not. I am funny that way.

Thanks for the info on how the testing was done, and the results.

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Up until about 1960, Triumph 650s had fat pipes, then they changed to skinny with a balance pipe. The 1953 Tiger 100 race kit included skinny pipes with megaphones. The 2 into 1 exhaust is a much later development. If you get it wrong, you immediately lose heaps off the top end. But for road racing, a 2 into1 exhaust can be much better than separate pipes. Separate pipes with megaphones often create an unwelcome surge of power in the otherwise usable rev range. Being smooth when you race is extremely important
If you don't have a bump in the power curve, you do not have to maintain verry high speed in corners. Without the bump, when you get balked, you do not need to slip the clutch. If you have got the bump and slip the clutch, you can lose traction at the rear tyre's contact patch
With many bikes, as you accelerate from low revs in any gear and your motor's revs rise, there is a surge of power as the cam and exhaust start to work. It is good for an adrenalin rush, but not much else.
My Seeley 850 with the 2 into 1 exhaust, spins up quickly from zero to 7000 RPM with no sudden surge anywhere in the rev range. It pulls like a train. The critical point in the exhaust is at the outlet of the collector. There must be no restriction whatsoever. The cam timing is adjusted to suit the exhaust system. My EO is 92 degrees BBDC

acotrel said:

A mate of mine fitted a megaphone to his 500cc Velocette in place of the muffler , without changing the jetting, and almost immediately burned the piston.

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Can anyone explain how/why that happened. I have heard this folklore for half a century, buy cannot resolve it with logic.
Thanks in advance.

concours said:

Can anyone explain how/why that happened. I have heard this folklore for half a century, buy cannot resolve it with logic.
Thanks in advance.

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Weakened the mixture.

Triton Thrasher said:

Weakened the mixture.

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I've heard that. But HOW?
A simple, stupid carburettor mixes fuel on ratio, by volume.

Changing the exhaust is the one reason you need to adjust your carburation. Most other changes have a minimal effect

concours said:

I've heard that. But HOW?

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Here’s a possibility:

The more open exhaust, with pressure waves enhanced by the megaphone at some rpm ranges, allowed air to enter the cylinder through the exhaust valve.

o.k. stop whingeing .

Right , Er ,

The Induction Phase would be enhanced - As in Velocity - But more - ACCELERATION - would be earlier & stronger . And leave the fuel behind , initially , as its Heavier ! ( Noble Prize ? Send to %#&=@ )

Zorsts 'n ' Cams , TWO STAGE Cams - they used to call em . Could describe the response as Pallid ( off cam ) and Psycho ' On the Boil ' If it boils up properly .

NOW , with a Big Bore system - if it FUNCTIONS while its ' on the Cam ' and does not in the least impeed breathing at Max RPM ( + ) its gunna get up & go .
It might sound like cannonballs falling down a drain pipe , down low . But ( if ) it all gets in phase ( we wont mention a certain T 120 ) The POWERBAND

( Ive tried to find a antique graph ) rather'n the useual ' Peak ' at say 6-5 unred , continues the uphill inclination to redline . Or Past . Due to Non Impediment !

You can see the WASSELL ' Mega's on BOTH inna da picture . These came in 1 1/2 , 1 5/8 , 1 3/4 header sizes . ( look at the CHROME ones at right for a clear view of the radius rolled ENDS . )
o.k.
Now a ' std. ' mega aint got nuffin in it ! cept maybe a ' revrse cone ' on the aft to ' further manipulate the pulse waves ' so as to get a bit of Mid - Range & sucklike . ( )

THESE - have a 2 in (odd ) O. D. fully perforated tube - to the exit ( a 1/2 in flange for the baffle flange - the pulses wreck the alloy rivets - P. K. screws , tight . are a Necessity )
they ave a little fully perforated washer alf way down the toob .



( Afterthought . nobody ever mentioned ' glass pack ' with these ' tubes ' Do Able as the baffles are removeable . BUT the ' enhancement '
from the tube being Red Hot to ignite the Unburnt Fuel - is what ' enhances ' the operation . These are 16 G ? STEEL about twice as thick
as old original Pea Shooters . At least as by the time the F'Glass has GONE - Leaving the Same Perforated Tube - But ' other ' cone configuration .
The soulfull bellow from the resonace of the material , is absent in the pea shooters - a crackling rattle ? issueing resonance from the thinner material .
We have to assume the pea shooter ' cones ' are a scientific calculation , and the Glass Pack a secondary neccesity . For a Road Bike . as were the mutes .
REGULATIONS as to how far you were allowed to disturb the neigbours - and rattle their windows . This is less of a consideration for rural types ,
as they generally slow down in suberbia , whereas town types tended to get it on ' out of town ' as they wernt used to The Wide open Spaces .



Therfore , at the CORRECT HEADER LENGTH ( to catch the p-pulses ) the heavens open . Rarther'n going flat at 90 in 3rd , under uphill ,
She runs out to 105 . PEAK shifted from 7 to 8-1 and Un Impeded past to 8-5 .

onna da Flat , just offa da full fropple , She Runs Out to 105 inna da 3rd , onna da stop she pulla da 110 , but ya gotta Be right there -
And catch that TOP Real accurate An da Sharp Like , orra she blow . but she neva did . she just sounda like she go ' Ka - BOOM . Real Big .

she go ' Ka - Boomb onna the cam , whenna the throttles on - Away she Goe - Hard Like , An dinna stop revving till like past 8000 .
way Past .
aska da Boys . any one done know . Only Hart him a De other rider - Him take em to bits too , once . ' Donna taker past 90 ' - In third '
( She comes on the pipes at 90 in third on 3/4 throttle )

270 main jets , 1 5/32 monoblocks . Top Line ' flowed ' 62 head . Dosnt impeed the ports like the nine stud .

" I Dinna go Ova 90 in third - True . " ( Nods ) " I hadem way Before that - i didnt need to " Dumb shir , Yer Change up at 90 on My Bike . Laddie !

O.K. For those that dont speak ENGLISH . A unadulterated Mega enhances Top end Breathing .

A ' enhanced ' one can go ballistic . A Revrse Cone as a Cap - as per the Dyno Man - flat track ( on the 1 7/8 inch O.D. Headers ) is used to
DEFINE THE POWERBAND . ( in this case upper midrange ) Fireing the cannon balls ( Gas Charge Pulses ) Up & Down the PIPE -
At the correct phasing to get it all synchronous - when he wants it / needs it . for Flat Track . AXTELL .

Note : at no time were ' muffler ' or ' silencer ' considered . But Cut C'do headers at outta the bend up front down under - throw a secondary
twice the X-section aft under - And Put Axtels ' cone ' on the End , and shes gunna GO , all R P M , and not a noise till WFO or past 3/4 open.
Nearly , where its pretty much that rolling rattling sort of high frequency low applitude variation pulse ( its in doubled frequencey OBVIOUSLY ),
With a mild bark & cackle on throttle shifts .

( Boyer length for the T 120 Primaries - first cut at top to pull the pipes in close - THEN to prescibed dimension . 27 1/2 If i recall correctly .)
( These'll add 1500 to a sohc 2 litre ford , easy . in you Loctite the Big ends . Cam Peak 5500 , rpm 1st & 2nd 7500 , 7000 in third . DVA carb only other mod .)
theyll get a V 4 2 litre hauling ass too , with the Od - fire pulse L & R the torque band is phenomonal , from 3200 . Dual freeflow .

Pity they put a tractor manifold on the new R - III 's !

Therefore , if its built with No Compromises , youd tend to trickle, idle , and shift up through settlements . ' Off the Idle Jets 'at 60 or 80 - with a free flow set up.
but the uninterupted pulse is uninterupted , The RESONANCE from the pulse in the headers and cans is part of the equation .

Red Hot after pulling down after a run at night , visable ( dull red - Hence BLUEING of Headers ) On hills cruising in cars with Tuned Headers - you get a light crackling
sounds like piston slap or small ends . this is the Metal Flex at the pulse as it travels in each header pipe . The load also enhances Vacum. ! As Would a Mega . In Parts .

Thus theres about six effects cross referancing - varying speeds pulse speeds pressures openings & closings . More like an Orchestra than a Solo musician ! If its right
Dischordant or sub optimum if ' somethings out ' .