GP Engine Muffler

Following on from the GP 38 exhaust noise issues I decided I really want to fly the Extra 300 so accepting a loss of near on 1000 rpm I have again fitted the OS 33GT muffler. With that done I now have the redundant GP exhaust on the bench. Go for broke I sawed the end cap off and lo and behold an empty can and what's more unforgiveable the outlet pipes don't reach up to near the top of the can. No wonder there is so much noise, the gases have nothing to slow down for.On a DLE 20 I had a rear mounted standard muffler fitted as supplied. These are definitely like the GP can above - hollow, minimum amount of pipe outlets! It was a little bit "barky". so I found some decent gauge ally tube that was the same size as the inside diameter of the outlets. The ally tube was cut to a length which when slid in would just fall short of the top of the can. In the area of tube that would be inside the can I drilled 4 holes in each tube, two one way and two at 90 degrees to the others. I then hacksawed a longitudional slot so that when the tube is squeezed it will be an interference fit in the outlet pipes. Worked perfectly. The gases now have to expand and will be slowed by the holes drilled in the tubes. The size of the four drill holes in each tube would equal the diameter of the outlet.So I have a plan, I intend to have go with the GP exhaust and do something similar.

Was planning on Cashmoor today but somehow it did not happen. So, with idle hands let's have a bash at the GP muffler. The GP outlets are 17mm inside diameter. Ideally some 18mm tube would be nice but did not have any. I did have however some 19mm aluminum tube at 1mm thickness. Using Pi to see the difference in diameters I reckoned on a 9.4mm reduction for the ally tube. I made a small quick test piece to see if my reckoning was about right. Near enough! So, I cut two 80mm tubes that when inserted will just clear the top of the canister. I drilled three 6mm holes at right angles to each other 10mm down from the top part. Placing the tubes in my vice marked two lines 9mm apart. I then hacksawed along these lines to create a slot. Squeezed them gently to nearly close the gap. Inserting the first tube and then gently tapping the tube all the way in till came just short of the top the can and the other end is 10mm up inside the outlet. Followed suit with the second tube. The 9.4 mm gap I think was a bit out, my maths but not an issue for a muffler. Aluminum expands faster than steel when heated so they should not slip out plus it took some good tapping to get the inserts in.Theoretically now the gases have to swirl around to find the exits and therefore losing some velocity and hopefully some noise without gagging the engine. Just need to get someone to weld the end cap back on and find out if my plan worked.

End product. Will it work. Will I get my RPM back?

Well, the can is aluminum so Kevin was unable to help welding the end cap back on. I thought, oh what the hell, should I have bought a OS33GT or DLE35RA instead? Although I have found a specialist aluminum welder online who is going to have a go at capping it.

Yes, I could get a complete canister set kit from MTW but with import duty, etc, it could well work out over £200.

Today I went over to see Steve in Poole. He cut out a new end cap in 1.2mm aircaft grade ally. It was then soldered on and excess ground off. (Kevin - TIG current was 12 amps not 20 ) Back home now and refit the muffler to the Extra and see what happens with noise and RPM this afternoon before the weather arrives for the weekend.Steve can weld most things in alloy, stainless and mild steel as required.

Difficult to identify the "steve" with his mask on

Kevin Ross wrote: Difficult to identify the "steve" with his mask on

:lol:
You two would get on well. Old school engineers. Uses British machine tools.

Resprayed the muffler with exhaust paint and refitted it after a "race" down to Halfords in holiday traffic for some high temp gasket goo. A quick run up of the engine and the awful bark at the top end of throttle appears to have gone. As I am at home I could only give it some quick bursts of throttle and so unable to verify the top rpm although the idle is nice and steady. Looking forward to getting up to Cashmoor next week to give it a flight.

On a spur of the moment I rushed up to Cashmoor on Saturday afternoon to give the Extra a flight with the modified GP38 muffler and maybe a tweak on the jets. The exhaust note now has lost it's harsh bark sound and is more akin to a DLE muffler. The transition issue, dying or cutting out, from low to high on the throttle has gone which I had when the OS muffler was fitted. I did not for the first flight adjust any jets. Prior to the first flight I checked the RPMs. 1850 idle and WOT 7550. Now it performs. Prior to second flight I leaned the low just a smidgeon but still a bit rich (engine is still fairly new) and it flies extremely well. Happy now! Now maybe I can get on with maintenance on other stuff.