OHV exhaust system

[Ian]

I have been tossing up what style of exhaust system to put on my OC. The standard system was a two into one with a barrel muffler. Optional appears to have been a two into one with just a fishtail. I have also seen photos of a straight through two pipe system similar to some of the race bikes and the DT. I am thinking of trying the latter on mine - any pros and cons ? Do these machines need back pressure on the exhaust or is straight through OK ? If they need back pressure I was thinking of crimping the pipes at the end into a small fishtail. I would like something quite sporty sounding as the bike has been raced before so would like to keep some of the heritage !! (actually I just want it to sound and look good  )

[trevorp]

Exhaust theory is in a world of its own when they say back pressure they actually mean at a certain rev range a pressure wave goes back up the exhaust to stop the inlet mixture from running thru on valve overlap crimping the end of the pipe may be good at around 12000 rpm
Then there is tuned length which means each exhaust valve opening pushes and then pulls the next exhaust charge so each pipe length is made the same length or slightly longer or shorter depending on engine config eg v twin flat twin in line 2 etc
so my advice is ian straight pipes use a bit more fuel a muffler makes it more eco friendly some hotdog type mufflers are straight thru but take the noise out best of both worlds i think

[Daren W Australia]

Hi some of the harley bikes have a muffler inside the pipe so they look like straight pipes for looks but reduced sound Daren

[Doug]

Go for the original look. It would take some experience, and a lot of experimentation, to develop a tuned exhaust system. For all you know, you could knock one or two horsepower off and not realize it, unless you tried numerous experiments.

Someone else was just asking me about two-into-one verses dual systems, so I was looking through the published and catalog pictures. Douglas had a dual system on the 1923 RA models, as seen on Tom Sheard and Alec Bennett's senior TT mounts. The pipes were equal length, so the front pipe ended short of the rear axle and the back pipe extended all the way to the rear. Then they went with the two-into-one system as seen on the DT. The two-into-one systems you see on these now, upswept and other variations, are I think later fitments, or the owner's own idea as to performance. Douglas did not catalog another dual system till 1935 on the big ohv road machines, the OW and OW1 models. These were unequal length pipes, and both silencers terminated at the same point.

Douglas might have been very clever with the two-into-one system, it is possible that the pulse from the rear cylinder joins the pulse from the front at a critical point, timed as it were, to create an advantageous effect. Interestingly the front pipe is about half the length of the entire system. It would be hard to know for sure, and I have never heard Douglas make any claims for a scientifically designed exhaust system. So the probably just did it to conserve tubing!

-Doug

[Ian]

Thanks for the feedback folks - gives me a bit to think about. My thoughts re the standard exhaust being "tuned" is that they would have been very lucky or clever. I have an original set of headers and the read cylinder pipe just takes the shortest route to the joing possible !! As we can do the pipe bending and welding ourselves I may try a few variations before I get any nickel plating done.