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GTX_Engines

Well-Known Member
Joined
May 17, 2013
Messages
45
Location
Columbia, SC
I got the exhaust system laid out and have the primary tubes tacked together. I used the original flanges, the flexible couplers, the merge collector and tailpipe. I tried to keep the primaries equal length. A couple tubes moved when I tacked them so they are approximately equal length. I have a feeling they are close enough!

It was quite a puzzle! There were several ideas that didn't work out but I like how it ended up. I am going to hang it on the airplane before I finish weld it. I have a muffler to fit up and hang up somehow. The tailpipe may need to be adjusted for the muffler so final welding will wait.

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The length of the pipes from flange to the top crotch of the header needs to be 21-22 inches, measured through the middle of each curve. No shorter, no longer. Length is determined by the shock wave distance traveled during the cycle of the valve opening and closing. The slower the engine is turning, the longer the distance of wave travel. At WOT & 11,000 ERPM that distance is very short and you can take the pipes off and run a 3" stub if you want. But then you are either running idle or WOT, there is nothing in between.

So what you need to tune for is best cruise speed, and that which produces a smooth, crisp throttle response. A flat spot - throttle lag - indicates loss of midrange power. With a pipes that are too long you tune for low ERPM and lose everything else.

With the old 2-valve heads air flow is a mess anyway through the head ports. Hence do whatever you want to a Rotax 912 or Lycoming header and it works just fine.

When you produce so much of the power by extreme air flow tuning, packing 5 valves in the head, watch out. Even very small changes to intake and exhaust are going to slap you down.

This engine is quite sensitive to tuned length of exhaust and intake - even more so then the 4-cylinder big brother RX1 or Apex, especially since there' not a whole lot of power to begin with. You lose 5 HP across your power band with 150 HP and it's just not a deal breaker. Lose 5 HP with only 80 and you're already looking at 6% of your top end and a whopping 9% of your midrange, which means you aren't flying.

Are these pipes perhaps a bit too long? At least with too short you still have WOT, but with too long you really screw the pooch.
 

GTX_Engines

Well-Known Member
Joined
May 17, 2013
Messages
45
Location
Columbia, SC
140 HP from 160 lbs. is not bad! Time will tell if it is practical with that engine.
I agree, Marc! I've been flying a stock, 145HP Yamaha Genesis RX1 which weighs about 160-170 lbs installed since 2012, lol. And when all was said and done it cost me but $600 for the engine with 2000 miles on it, just enough to break in the rings and bearings. Most people recognize it as 140HP engine but later dyno tests after the sleds were broken in proved 145 HP time and again.
 
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Lifted1

Well-Known Member
Joined
Aug 13, 2014
Messages
96
Location
turlock, CA USA
wow, you are doing a great job! Thanks for taking the time to document your process. I love what you did with the cowl so far. the way you find things that work and the whole creative thought process.

I recently left the CX-4 group. I picked up an old Macair Merlin and plan to run the phazer also. Needs a lot of work so its going to be a while. Planning to get the Skytrax gearbox next while they are available.

Isnt there 3 to choose from? Which PRSU gearing did you get to swing the 68" prop? Im hoping to swing that or larger. 70"???

Keep the progress and pics coming!!


Here's a pic of what i hope mine to look like when done. not the paint but the mods, etc.
 

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Marc W

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Joined
Mar 31, 2017
Messages
771
Location
Colorado
Thank you. The project has been patiently waiting while I take care of other projects. I hope to get back to it by the end of the year. I did pick up a prop. It is a NR ground adjustable 3 blade and measures 67". The price was right and I have to start somewhere.

Teal is only offering one gear ratio so far. I don't remember the ratio but it gives about 2375 prop rpm at WOT. Teal did say he was going to offer more ratios but they are not available yet. As you can see, his current offering will not overspeed a 70" prop.

The Merlin is an attractive airplane. It looks like an efficient design and I looked at it seriously when I was looking for a two seat airplane. I finally decided I preferred tandem seating so I bought the Rans S7 pictured above. Good luck with the Merlin.
 
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