Andy Stephenson Posted February 13, 2021 Share Posted February 13, 2021 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. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 13, 2021 Share Posted February 13, 2021 Some good stuff here Andy...... **LINK** for boats but might get you going.... I do remember Andy Ellison publishing an article in RCM&E many years ago about tuned pipes......I'll have to have a look through some back issues... Quote Link to comment Share on other sites More sharing options...
Andy Stephenson Posted February 14, 2021 Author Share Posted February 14, 2021 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. Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 14, 2021 Share Posted February 14, 2021 I do remember something about it Andy....it mentioned port timing in degrees & rpm & various other parameters....as I say I know Andy Ellison did an article on it many years ago. I wonder if Just Engines might be able to help.....I know they offer a range of Purple Pipes... Quote Link to comment Share on other sites More sharing options...
Steve Hargreaves - Moderator Posted February 14, 2021 Share Posted February 14, 2021 Found it.....September & October 2007....October 2007 is in the Digital Archive if you're a subscriber but not September 2007 for some reason.... Is this the bit you're after Andy? Quote Link to comment Share on other sites More sharing options...
Andy Stephenson Posted February 14, 2021 Author Share Posted February 14, 2021 Steve, So many different ways of calculating the length of a pipe, it looks like there's no one definitive method. Cheers, Andy. Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 15, 2021 Share Posted February 15, 2021 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. Quote Link to comment Share on other sites More sharing options...
Tim Flyer Posted February 15, 2021 Share Posted February 15, 2021 (edited) 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 February 15, 2021 by Tim Flyer Quote Link to comment Share on other sites More sharing options...
Tim Flyer Posted February 15, 2021 Share Posted February 15, 2021 (edited) 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 February 15, 2021 by Tim Flyer Quote Link to comment Share on other sites More sharing options...
Tim Flyer Posted February 15, 2021 Share Posted February 15, 2021 I think my next project might also be a classic aerobatic plane like a bullet with a piped 2s engine .. I have been thinking about a nice Webra or HP to power it Quote Link to comment Share on other sites More sharing options...
Nigel R Posted February 15, 2021 Share Posted February 15, 2021 49 minutes ago, Tim Flyer said: Most sport engine timing (like OS) is around 150 degrees OS61SF/RF had just 142, unless you had the bling Hanno version, which had 150. Quote Link to comment Share on other sites More sharing options...
Bob Cotsford Posted February 15, 2021 Share Posted February 15, 2021 I used to run the JE/WestonUK Genesis pipes I had a bit long with large high pitch props and got useful increases in torque on OS 61SF, ST61 and MVVS90s along with sub 80dBa noise tests, nice for aerobatics ☺️ 1 Quote Link to comment Share on other sites More sharing options...
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