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United States, AZ, Phoenix
Joined Jan 2016
253 Posts
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Question
zenoah g80 stock muffler mods
I recently started flying a P47 (~ 31 lbs) that came with a Zenoah GT80.
I did some testing on the ground: with a Xoar 24x10, I get 6,000 rpm. Switching to a Menz 23x8 I get 6,800. It came with the Menz, but so far I have only flown with the Xoar. I tried tuning the HSN, but these are the best numbers I can get. Doing some research, I read that it is possible to get a few 100s rmps by drilling holes in the baffle of the stock muffler, but I can't find much details. How many holes? What size? Where (sides that mates to the engine or other side)? Also, the previous owners used to pieces of high temp hosing to route the exhaust outside the cowling. Are they robbing me of RPMs? Thanks for you suggestions |
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Des Moines, Iowa USA
Joined Nov 2007
7,494 Posts
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Yes you are down on power with those mufflers. I only flew the GT-80 once with stock mufflers and they got changed to aftermarket ones. My prop of choice in that time period was the Menz 24-10 (really a 23.5-10 as Menz props are shorter than marked). I looked for at least 6800 RPM as anything less for that engine is well below its power band. Never tried a Xoar prop on the 80 because they weren't available back then. My favorite all around prop on the GT-80 for large heavy planes was the Menz Ultra 24-10. A narrow bladed prop that hasn't been available for many years now. The Ultra props usually turned around 7200 on the ground. These days, I'd probably try a Falcon wood or carbon prop as they load an engine similarly to what a Menz did. You might want to try a bit shorter diameter, increased pitch props on the P47 once you get it dialed in.
I heard about modifying the stock mufflers but never attempted it. I have seen planes that had modified mufflers on them that worked well. Whatever you do with props and mufflers, shoot for 6800+ RPM as the performance of these engines falls off quickly at lower RPMs. |
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United States, AZ, Phoenix
Joined Jan 2016
253 Posts
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Thanks. I should mention that the plane came with the 23x8. Running the numbers through a static thrust calculator, the 24x10 @ 6,000 and the 23x8 @ 6,800 achieve similar performances in terms of thrust, speed and power required (~ 4.8 hp).
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Des Moines, Iowa USA
Joined Nov 2007
7,494 Posts
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But you are still on the very low end of power for what I would expect from that engine. Heck, a good 55 CC engine will turn a 23-8 at or above 6800. Zenoah's like to rev and like to breath. I would think the performance of a 31 Lb. warbird would be a bit anemic with that amount of power. There's lots more to be had from that engine.
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Mandeville, Jamaica, West Indies
Joined Aug 2007
759 Posts
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Several years ago when I just transitioned to gas engines I asked a similar question about my Zen G23 stock muffler which was very restrictive. Though I can't remember who made the suggestion I was advised to cut open the stock metal muffler, remove the internal baffle and weld it back up. Well I did as instructed and was rewarded with a 500 rpm increase and just a tad more noise.
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Des Moines, Iowa USA
Joined Nov 2007
7,494 Posts
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Quote:
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United States, AZ, Phoenix
Joined Jan 2016
253 Posts
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Interesting approach. But it requires some welding skills. On my end, I fixed a poor connection in one of the plug caps, and that gave me 150 more rpm on the top end. I will ask if somebody around the club has welding skills
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springdale, pa usa
Joined Aug 2006
978 Posts
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I drilled mine 15 years ago or so. While don't remember the exact details I'm pretty sure I used a 1/4" drill bit through the entry port. I think if you study the muffler it will become apparent where the holes are needed to by pass the internal baffle plate.
Here a post from 2011 I found; "OK cheap trick #1 Get the biggest drill you can fit into the muffler port and drill some holes in the baffle. If you enjoy the extra power then look at aftermarket mufflers. I think that J&A Engineering makes mufflers for the GT 80. Way back when the Zenoah first came out Joe had a set of HP mufflers, a little noisy, but they actually showed a power increase over open exhaust (some tuning was happening) Yes its Terry, cant stay away" |
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Mandeville, Jamaica, West Indies
Joined Aug 2007
759 Posts
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Unfortunately that's very true and one of the main reasons why several of our knowledgeable old timers have stopped visiting here. Likewise, most times I just lurk rather than make a comment.
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Des Moines, Iowa USA
Joined Nov 2007
7,494 Posts
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Like all gassers, I just set the idle speed to where the plane doesn’t roll easily on a hard surface. A bit faster on grass. I use a flight idle up when doing hard maneuvers. If it sounds about right I’m OK with it.
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