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Engine basics => Engine set up tips => Topic started by: Richard Koehler on September 14, 2013, 08:25:17 PM
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I have a friend that wants to run his Brodak 40 on muffler pressure. I can't talk him out of this. Does anyone have any experience doing this?
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Yes, it should work fine. 8)
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Thanks for the response, I'll pass the information along.
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The Brodak muffler has no tapped holes foe a pressure nipple, what size thread do you tap the muffler? Also where do you find the pressure nipple and does it matter where you drill and tap the muffler?
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Dubro makes a pressure fitting, drill and tap for 4/40. Fox makes one, smaller diameter hole, drill and tap for 2/56. I usually put it on the flat muffler extension but doubt that it makes much difference. OS tube mufflers have them in the muffler body itself. I think the Brodak tube itself is too thin to safely accept a pressure fitting. I would try it w/out muffler pressure first. If it runs fine and does not RPM upwind vs downwind you don't need one. Many engines are like that. If you do get significant run variation in windy conditions muffler pressure may help. 8)
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I used to run a B40 on a Chipmunk. I started with no muffler pressure. I found that in windy conditions engine rpm varied considerably as you came into and then put the wind behind you. But in winds much over 10mph, the rpm didn't fall off enough and just kept building. Going to pressure solved that issue. I didn't tap the stock muffler, I just put on a tongue muffler that was already tapped.
Brian
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I have a friend that wants to run his Brodak 40 on muffler pressure. I can't talk him out of this. Does anyone have any experience doing this?
Can't talk him out of it? It's not a big life decision. Switching to or from muffler pressure is easy and worth the experiment. When I used to run Brodak 40s unpressured I would get a distinct richening when headed upwind (losing power into the wind) and leaning when headed downwind (gaining power with the wind) even in moderate wind, similar to Brian's experience with his Chipmunk. This is just the opposite of what we would want to happen even if we wanted the engine to change speed twice a lap, which we don't, at least I don't.
I agree with Pete that the wall of the stock Brodak muffler tube is too thin to drill/tap. There is a double wall thickness where the front and back halves of the muffler tube join, and I've had good luck locating the pressure fitting there. The flat "header" part of the muffler is probably a better and easier place to drill and tap. I don't think the location makes any performance difference, as the pressure is positive throughout the exhaust system. I have even seen engines without mufflers pressurized successfully by locating the pressure fitting in the exhaust stack of the engine.
I've had really good results flying several Brodak 40s, both the early and late versions. I like the fact that with its light muffler it weighs almost the same as the OS20FP with its heavy muffler, while providing twice the displacement.
Tell your friend good luck with it. ;D
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An easy experiment to test muffler pressure would be to simply push a length of silicon tubing iin the stinger. I once did that using wire to hold the tubing in place. A bit restrictive maybe, but it sure showed what the pressure would do.
Brian
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If you get too much rpm variation, try lower pitch props. I use 11x4 thunder tiger prop and the motor never cares if it's windy or not
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My favorite B-40 is the APC 10.5 x 4.5. Solid 4 stroke around 10,600 rpm.
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While I like muffler pressure, there is an alternative fix for the rich/lean crap in the wind. Just hook a hose from the uniflow inlet to a non-pressurized area inside the cowl, such as in the backplate cavity. Brett says that's his sekrit trick for windy conditions, but there's no reason I can see not to run it this way all the time. ~> Steve