First 165HP Yamaha EXUP Aircraft Build


Unobtainium Member
May 21, 2006
Columbia, SC
Air Command Tandem w/ Arrow 100hp; GyroBee w/ Hirth 65hp; Air Command Tandem w/ Yamaha 150hp
Total Flight Time
Photos of progress on Geoff Resney's Air Command tendem, formerly a Subaru powered gyro. This is the first ever Yamaha Genesis 4-cylinder (YG4) 165HP EXUP aircraft. Yet another AC Yamaha pioneering project.

Chris Toevs, et. al., was lamenting that missing from RF these days are the detailed build threads so popular on this forum here, 6-12 years ago such as Ron Awad's YG3 Dominator build using Todd Reick's popular conversion kit, Stan Foster's Helicycle build, and many others. I thought I'd take another stab at posting something new and interesting for the forum readers. I will follow this thread with a Yamaha YG2 80 HP build thread, using a Honey Bee (Gyro Bee derivative) when Geoff's project is completed.

The 2003-2005 RX1 snowmobile engines were carbbed and put out 140-145HP, depending on which dyno sheets you're looking at from the independent shop of Dynamic Research, Inc. the third-party that does all the AMSNOW qualifying dnyo work each year.

In 2006 the Apex sled was introduced with EFI and accompanying cam grinds that suit the stingier fuel flow of EFI. Those engines produce 145-150HP depending on year and which dyno sheets you pull.

2011-2018 models were upgraded with EXUP, a titanium 4-into-1 header collecting at a valve actuated by a computer-controlled servo motor not unlike your car's power window mechanism. This system was pioneered by Yamaha in 1983 on their bikes. Honda copied it the following season.

Exhaust is so important to power. Here we see a manufacturer's proof positive: EXUP produces a dyno-proven (independant manufacturer's race-sanctioned third party, Dynamic Research, Inc.) 160-165HP, a gain of fully 10% not just on the top end by opening up the the pipes, but throughout the entire power band by closing down the flow and creating proper back pressure at the collector with a butterfly valve. This produces not just a smooth, predictable power curve but actually increases HP in the mid-range as well, over and above conventional headers of any maker's design, be it stock or speed shop! Amazing, but true.

The Air Command tandem lends itself well to this EXUP build, since you can't just cut and weld titanium like steel. Leave the pipes as-is, and build the rest of the exhaust around it. But Air Command's type of aluminum round tube airframe construction presents its own set of obstacles and build peculiarities.

(I expect you could cut the EXUP valve pipes about 6" upstream, and clamp headers to them. In that way you could wrap the headers around and tuck the valve up underneath the engine.)

One thing I've read about these EXUP titanium exhausts is that they can crack. So extreme care has been taken to properly secure the heavy stock SS muffler (18 #) while isolating engine shake & vibration. Note the yoke bracket milled from aluminum billet holding the header to the right, angular, keel support. The header grommets are secured to it and the headers are free to slide up and down +/- 3/8", but the lateral movement fore/aft is +/- 1/16" and side-to-side is next to nothing.

There are easier, simpler ways to attach this muffler to the keel, but I wanted it upside down like this since the other way the muffler seam is on top, so it actually looks nicer this way. And I wanted to drill as few holes as possible into the air frame sections.

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I was planning to attend Mentone 2019, but was not 100% and decided not to make the trip at the last minute.
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