wkevinm

New engine, after 5 tanks on the bench, 15% Synthetic, 1,200 feet ASL, approximate 60 Deg F, low humidity; TT 40 Pro with GMS muffler - 9X7 - 16,000; 9X8 - 15,000; 9X6 - 16,500. Would like to know if anyone has experience shimming to increase RPM for 9X7 prop.

djlyon

Just wandering when I found this. This engine is what we use for 424 pylon racing. Generally we wind up with the engines shimmed to .005 or .008 but all will vary from one engine to another. Shimmed you should reach 17000+ after the engine has warmed up for about 30 sec at close to peak rpm. The head on these engines needs to get up to top operating temp to see the last 4 or 5 hundred rpm. All above is with a stock TT40 muffler. Why are you using a GMS muffler?

Denis

wkevinm

I have used a GMS muffler since I got the engine, only because the local group all use it for increased RPM (said to improve by 1,000 RPM). I have not even removed the orginal muffler from the box, engine has had about 6 flights plus time on bench. It apears I maybe should shim then test with both mufflers, by your results.

Thanks, Kevin

djlyon

Yes, your numbers indicate to me the GMS is giving you nothing over the stock muffler. I like to make about 15 to 20 flights before I start shim changes to make sure the peak rpm is stabilized. I have one engine that gets best rpm from the stock shim (.013) 17000+. You never know. So much depends on hummidity and density altitude of the day. Our field is 1500 ft but on a hot summer day the density altitude can be well over 5000ft.

Denis