Exhaust Guidance

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Exhaust Guidance

Post by Fiddler » Wed Jan 17, 2007 12:41 pm


I'd be grateful for any tips regarding exhaust design.
I need to have an exhaust made to replace the standard one on the Punsun as it would pose a serious burn risk to my inside leg if left standard.
The standard unit exits both cylinders of the v and meets a common silencer which exits at one end.
I would like to have a 2 into 2 system made using a couple of screaming eagle silencers. I havnt become a harley poser but inherited an old pair with the box of bits so it makes sense to use them if possible. I hope to mount these somewhere near the drive chain run as brackets exist for this purpose.
Im concerned about ruining the engine performance if I get this wrong.
Will two different lenghts of down pipe cause problems? Will the difference in diameter between downpipe and silencer cause problems? Will using silencers designed for petrol engines cause problems?
Are these types of engine in such a low state of tune that I'm worrying about nothing?
Thoughts and alternative suggestions gratefully recieved.

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Re: Exhaust Guidance

Post by Byrdman » Wed Jan 17, 2007 1:33 pm

Fiddler wrote:Will two different lenghts of down pipe cause problems?
It will hurt performance slightly and will have an effect on sound quality as well.
Fiddler wrote:Will the difference in diameter between downpipe and silencer cause problems?
Going from a small headpipe to a larger muffler will slow the exhaust velocity and will hurt performance.
Fiddler wrote:Will using silencers designed for petrol engines cause problems?
Only if they are more restrictive.
Fiddler wrote:Are these types of engine in such a low state of tune that I'm worrying about nothing?
More than likely. :wink:

Performance wise: If this were a two-stroke gasoline engine or a high performance four-stroke engine, I would be concerned. For an industrial engine like this, the worst you have to worry about is creating exhaust restriction.

For sound quality: This is where exhaust "tuning" plays an important role. Equal length head pipes are important so the exhaust "pulsing" isn't overwhelming.

The reason many motorcycle exhausts have a 4 into 1 design is for exhaust velocity and sound qality. The joining of all the exhaust headers into one common pipe is to reduce the audible exhaust pulses and to maintain exhaust velocity. Velocity is important because the exhaust pulse from one cylinder will pass through and then will create a vacuum effect on the next exhaust pulse thus increasing exhaust gas velocity.

If I were you, I would go with a 2 into 1 system. One muffler will sound better than two separate ones, unless you create a joining crossover pipe. That will smooth out the pulses. Harley's have their distinct sound because of the crankshaft layout which I'm sure your Punson doesn't share. Two separate headers will be very punchy sounding and won't swound as good as you might expect.

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Post by andrewaust » Wed Jan 17, 2007 1:50 pm

Hi Fiddler

For my bike I thought Stainless Steel was the go, but will cost a little more. I purchased a length of 40mm straight tube and a heap of bends which came in 90 & 45 angles, having it tig welded, after spot welding it up beforehand to see if it fitted neatly.

My system is basically a straight through, using the sports Enfield muffler, which does a good job at limiting noise.

In regard to bends! You could get the bends (complete pipe) done professionally to give you a nice looking job. You might be lucky enough to get hold of someone willing to do it for you if that's the way you would like to go.

Them Harley S.E mufflers would do the trick if they are of the less restrictive type :wink: Diesels like to be able to breath due to the fact they always take in a good volume of air - with even more volume coming out the exhaust.

Two different lengths won't cause a problem, as tuned pipes "lengths" are only used to help scavenge the fuel air charge in a high performance petrol engine.


Andrew :D


Post by Fiddler » Thu Jan 18, 2007 1:53 pm


Thanks for the advice. I thought in theory that all those things were problematic but suspected that perhaps I was worrying about nothing for this type of application.
As suggested I think I'll go 2 into 1 using one screaming eagle end can cos I have one. I suspect that they are a lot less restrictive than a standard can which is why they appear a popular mod. Probably breath freely and enchance that sound Harley owners cherish so much but I'm not at all bothered about replicating.
One last question. The simplest way to achieve this is to run one downpipe around the front frame downtube and the other behind the engine to meet somewhere around the drive sprocket.
Would it be acceptably to join the narrow downpipes in a near 90 degree angle before running back to the silencer in narrow gauge?
Or would it be better to attempt to join the pipes immediatly before the end can? If I can make this fit?
Thanks again

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Post by Diesel Dave » Thu Jan 18, 2007 4:04 pm

90 degree joins should be avoded where possible but there have been enough bikes and cars with similar setups in the early days.

Essentially you have an 800cc motor producing 22 hp - not at all stressed.

Diesels do flow full charge all through the rev range but to be honest I doubt you will notice and difference one way or the other.

If you want to get into tuned lengths it becomes very complicated but this would only be apparent when you have large valve overlap times when you are asking the exhaust to act as an extractor for the cylinder by sucking fresh air straight fron the inlet during valve overlap - forget it.

Make sure that the pipes are adequately sized and the baffle is not too restrictive in the silencer - as if you would.


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Post by Anorak_ian » Fri Jan 19, 2007 12:31 am

As your engine has a lower V than a Harley have you thought of using a 350 Motor Morini exhaust system? The V is wider apart on your engine but about the same height as a wee V Morini. :)


Post by Fiddler » Fri Jan 19, 2007 1:21 pm


Thanks mate. I'll dig out those bike magazines and see if I can find a picture.

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