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DSM Tuned Pipe Setup Instructions.


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I have been looking for the rather comprehensive instructions that came with my DSM Joker back in 1989 but lost in the mists of time. As well as the building of the airframe there was comprehensive advice on how to set up a carburettor and how to tune a pipe. There was a simple formula for doing this based on exhaust port timing, much more straightforward than other offerings on the web. Does anyone still have this formula and could you publish it here please,

Thanks,

Andy.

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Steve,

Many thanks for this one which I have actually come across it before. Although it talks about the size of the stinger, it doesn't take it into account in the formula which is what the DSM instructions did. It's not always possible to know the size of the stinger if it is internal to the muffler section but I would still be interested if someone has a record of DSM formula all the same.

Below is a sketch of a pipe I made by probing it, it came with my OS RF61. You can see instead of a reverse cone there is a baffle leading into two muffling chambers with 2 connecting pipes and a tail pipe. This is all very confusing because which one would counted as the stinger and where is the reflection point. It seemed to work though and was remarkably uncritical.

I have other pipes which are more conventional so working through some of the formulae would be good practice whilst engine tuning isn't an option to see which ones agree before I start cutting down headers.

OSRFPipe.thumb.JPG.3576edd90e3875c41ca106ca963ff55a.JPG

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Always easiest to bench run I think, start with a long header, and cut it down bit by bit, takes some time but simple method and works.

 

19 hours ago, Steve Hargreaves - Moderator said:

I wonder if Just Engines might be able to help.....I know they offer a range of Purple Pipes...

Sadly not been offered for a few years now.

Weston can supply pipes.

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Looks like you have a flat disk pipe I.e a “throttle pipe “ as in figure 2 above. The flat bit of the disk is the reflector for the back pressure wave, which you should measure from. Throttle pipes are designed to be less peaky than dual cone and have better idle etc. 
 

The Weston pipe I bought for my OS 55 Ax is also a throttle pipe.  This engine has 150deg timing as I measured  . Max rpm in the air I thought achievable might be close to 14,500/15,000 if unloaded.


My calc was (150 x1200)/13,846.
I cut my manifold to make the LI number 13inches to start with and was very impressed so left it there . Remember rpm are higher when moving in the air due to lighter load. 
 

it’s best to choose RPM close to manufacturers max power figure (when unloaded in the air). Obviously you need to know prop you will be using and do trial and error tests. I got quite used to it with model boats and dual cone pipes . The throttle pipes are much tamer and user friendly without such marked on or off peak. 

Edited by Tim Flyer
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By the way to measure timing a simple way is putting the engine by on a test stand with a propeller on . You then put a protractor in front of it . You measure how many degrees moved from when the piston passes the top of the exhaust port as it moves down after firing to when it is closed on its way up again. That is the area which the pipe has to work in to boost the engine by drawing out and reflecting back each pulse. Racing engines have higher exhaust ports to enable extra boost and rpm but that can come at the expense at idling stability and mid range power . Most sport engine timing (like OS) is around 150 degrees. Racing engines like OPS/Rossi have far more aggressive timing often more than 170. As a result their idle is far higher. ... my Rossi 28 idle is around 5000rpm 

Edited by Tim Flyer
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