Photos of progress, the first ever Yamaha Genesis 4-cylinder (YG4) 165HP EXUP aircraft.  This is going on an Air Command gyroplane - again. 

To refresh for those who have forgotten, or perhaps are new to this stuff,  the first ever Yamaha aircraft was Todd Reick's 120 HP YG3 Air Command single in 2007.  The first ever successful YG4 (RFD's was plagued with electrical, starting and clutching issues along with power loss due to intake and exhaust design errors, which to the best of my knowledge were never sorted out when it was sold to someone out west) was my personal 140hp Air Command tandem.

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 again with EXUP, a titanium 4-into-1 header collecting at a valve actuated by a computer-controlled electric motor that is 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.  That's what I'm talking about when I say exhaust is so important to power.  

EXUP produces 160-165HP, a gain of fully 10% not just on the top end by opening up the back pressure on the pipes, but throughout the ENTIRE power band by simply closing down the flow at the collector with a butterfly valve.  This produces not just a smooth, predictable power curve but actually increases HP in the midrange as well!  Amazing. 

I swore I would never build another conversion for a customer, but Geoff Resney, PRA Chapter 18, begged me for a year to do one for him when the time came that his Subaru quit.  He said when, not if.  That wasn't me.  I told him there just wasn't any money in it for me to do installs, and that if I was going to do it again I was going to get paid $28.50/hr, machining included.  Strictly T&M.   

We agreed to a schedule, some ballpark prices, and earlier this year he showed up with his wife, dog, and three kids at my door, Air Command in tow behind his RV, sans 320# of broken Subaru.  

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 this type of aluminum round tube 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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