Equal length pipes provide equal scavenging of both cylinders . In the real world, however, the difference in equal/unequal length pipes is insignificant as most street engines aren't in a high enough state of tune to benefit from scavenging. Equal length is preferred, but as you've seen on custom bikes, style usually takes priority over function.

Bob

 

Scavenging only occurs when there is a 2 into 1 system with a good collector design. A single pipe per cylinder, does not scavenge.

The battle you are trying to win with single pipe per cylinder tech, is reversion.

That said, until you have a significant difference in pipe length, you are chasing parasitic losses.

 

old pic from when i was building the exhaust... i was worried about the difference in length along with the curves. the rear has less curves and is a bit less than 2/3 the length of the front pipe.

between these pipes and equal length drag pipes that i've switched back/forth in the same day: can't tell a difference.

this is a street motor, never seen a dyno, never seen a drag strip. just seat of the pants comparison.

now... i did try a 2 into 1 similar to a thunder header for a short spell. there was a noticeable difference in part throttle and roll-on acceleration, for the positive.

 

Wondering how to get equal volume on 2 exhaust pipes? Forget trying with a tape measure. Take the shorter of the 2 and block off (duct tape) the spicket end. Fill with water. Pour into the other pipe and mark where it fills to, cut there. This tip was given to me by my 10 year old neighbor kid that was watching me struggle with a tape measure. I was amazed how well this worked. Guess there is hope for the next generation.

 

that's what we used to do when building "equal length" headers... works great (it's how i 'measured' the length of the pipes above)

 

^^^ +1

 

You are correct sir. In that the pulse wave can be timed to create a bit of a scavenge. Or, is it that due to the correct timing of the pulse, that we have reduced/eliminated the reversion we want to overcome, rather than creating scavenge?

What most folks don't understand, present company excluded, is that in the search for intake/exhaust tract efficiency, our biggest enemy for getting the bad air out, is atmospheric pressure. Atmospheric pressure also happens to be the biggest ally in getting the good air in.

Nature abhors a vacuum.......

 

Thanks guys, its been bugging me. I know about scavenging and wave pulses, though not as well as I'd like. Really my thoughts were also of one cylinder running richer then the other and thus there being a problem. I guess on a street machine its really no big deal. Thanks again.

 

two strokes rely much more on exhaust pressure to act as a "valve closure" of its exhaust port. it aids (and only aids) as a "wall" of air to cover the port at a certain instance in the stroke, to aid in compression as the piston reaches the top of its stroke. at some point, the piston will cover the hole for the exhaust port, and compression will commence. so it only aids, but its a bigger problem on a two stroke to have a proper exhaust.

on a four stroke, the valves act as the closure point, and the air is brought into the cylinder head directly, and the air is controlled mechanically (i understand i might be saying what everyone already knows) so the only effect that an unequal exhaust will have is the for the scavenging effect. with that being said, a harley davidson engine, and most of the engines we use for our builds, will not turn enough rpm to have this effect take place. especially in the case of a harley davidson engine (v twin, two rods on a common crank pin, will never turn enough rpm for this to matter)

so basically, having two unequal length exhausts is a very small problem, and can be corrected with proper fuel tuning, and will not have an effect worth worrying about on anything mild like our builds. on a high compression, high rpm drag bike? id worry a little more. cut away.......

 

I was told, years ago , you put a crayon line on the outside and cut the pipes off where it isn't burnt off? LOL

 

More info than you need...

https://www.dropbox.com/s/12ur6oquh475k7i/The Big Twin High Performance Guide-by William Denish.pdf

The Big-Twin High-Performance Guide
by William Denish

Chapter 7 - Exhaust System

 

I have that D.William Denish book,Volume 1 the Big Twin High- Performance Guide
I totally recommend it to anyone with a passing interest on hotting up engines ,his info is priceless.
Cheers R

 

Im no expert but I've been reading a lot of Vizard lately trying to fathom this 5th stroke.
From what I gather, the neg wave heads back up the pipe (as soon as it reaches the end) all the way to the carb which then creates another wave back again into the combustion chamber which draws more fuel/air mixture into it. All this happens at valve overlap.
That's what I percieve as scavenge. Reducing/eliminating reversion is achieved by a 'flare' incorporated in the header pipe directly after the port or stub, which will catch the reversion waves. Fuck, both those statements contradict, I'll go back to reading and listening for now. Please excuse, still learning. Somethin I'll never stop doing.

 

Beauty of speed had some info also. The author said it ran better with slash cut ends rather than straight cut ends?

 

hmm interesting reading thats for sure. I know from running headers on the cars i modded or turbo manifolds everythign has had equal length. UNequal has been frowned upon. interestign question!

 

If I remember my schooling correctly, reversion is caused by the piston movement downward during the valve overlap at the top of the intake stroke before the exhaust valve closes fully. The problem is that this backward flowing exhaust is met by the next wave of exhaust and the piston has to get that wave flowing back down the exhaust again which impedes the positive effects of the exhaust gasses leaving the cylinder helping to pull fresh fuel ( air and gas) into the cylinder during the overlap cycle. Yes stepped header ( some made with a shelf and lip) help stop this effect, but the best setup is a two into one header so the exhaust from the other cylinder helps stop the reversion effect. I do believe there has been some success from the use of a bolt ran through the end of drag pipes from top to bottom also but if you go to the drag strip drag pipe style exhaust ( zoomies in the car world) are only used on forced induction engines where the intake charge is being forced into the cylinder. All normally aspirated engines will run four into one headers. Motors are so sensitive to there exhaust systems that even the pitch of the pipes entering the collector will effect where a motor will make the most of its power. A motor set up to make its power in the top end should have a shallow pitch while a motor setup to make its power in the midrange should have a steeper pitch in the collector .

 

The exhaust reversion is caused by a positive pressure wave reflected from the open end of the exhaust pipe (or from some bad shapes inside the muffler).It typically happens just above 1/2 revs,just before the pipe length starts to come on song.

In round figures,a 40" pipe length (measured from the valve) will work well at about 5000 rpm.A 50" long pipe will work at about 4000 rpm,and reflect a negative pressure wave during overlap at TDC at that rpm.It's a little more complcated and involves the point of exhaust valve opening (cam timing),but the lengths work out close to that.

A staight pipe that works well at 3000 rpm will also work a little bit at 2000 rpm and 4500 rpm.
Putting a bolt across the pipe might kill the bad reversion wave,but it will also kill the good negative pressure wave too.There's no sustitute for having the right length pipe,with a diameter that's not too big.
Mufflers can kill the pressure waves in the exhaust,but not always.