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best 100 HP auto/motorcycle engine conversion?

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13brv3

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Greetings,

I've recently been looking at the BMW R1200 conversion. Air Trikes sells a nice gearbox for it, and the engine has a great reputation for reliability. I love the thought of fuel injection rather than carbs, and it would be a fun project to create the engine management system. The engine is really compact, which would make it easier to fit in the cowl as well. The downside I fear would be smoothness, since two big cylinders likely wouldn't be as smooth as 4 smaller ones. As nice as they seem on paper, they sure don't seem very popular here in the US. Is anyone actually running one?

In looking at the Air Trikes site, I see that they sell gearboxes for lots of different engines, but I haven't had a chance to sort through all the various models to figure out what is a good 100 HP option. If you wanted to make a 100 HP alternative to the 912S, using a modern engine. What's the best engine to start with? I don't have an immediate need for this, but I thought it might make an interesting discussion.

Rusty
 

rv7charlie

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Have you gotten an honest 'fighting weight' for the R1200? Would the Yamaha RX1/Apex/etc be in the same ballpark? There are already at least two vendors offering drives for the Yamaha 4cyl engines, and Air Trikes says that it will be the next engine they build for. There seem to be a *lot* of them flying here in the USA on STOL a/c and trikes, and I haven't read anything negative from anyone who's installed one.

The Yamahas make more HP than you're looking for, but if they're light enough (and they're really light), you might be able to just let the engine 'loaf'. Another option would be the 3 cyl version; only a little lighter than the 4cyl but might be enough to matter in your case.

Charlie
 

13brv3

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The only numbers I've seen on the complete R1200 were in the upper 170s. I happened to think that the power pulses per prop revolution are actually higher with the 2 cyl geared engine than with the traditional 4 cyl aircraft engine, so smoothness may not be a real negative.

While VW engines are the most common alternative engine, they have enough limitations that it would seem the market should be ripe for VW weight conversions made from modern engines.

I've seen some comments about the Yamaha engines, but haven't looked into it very much. There are a handful of engine conversions that are available new, but if the price reaches what a good used 912 sells for, then it becomes harder to justify. Just a mental exercise at this point.

Rusty
 

rv7charlie

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IIRC, the Yamaha 4cyl core engine is around 125 lbs. Supposedly, it can fly at around 150 lbs. The 2cyl 4 stroke is bound to have pretty brutal pulses, with 1 per rev instead of 2 per rev from the 4cyl (assuming same HP). While the prop will see more pulses per rev, the gearbox has to handle those massive pulses, and the inertial 'damping' (resonance frequency) will have to be much lower to avoid issues. Gearboxes have HP ratings that go down with cyl count.

As long as you can swing the large dia prop, almost any geared engine should radically outperform a direct drive VW on a slow airframe.
 

TFF

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Yamaha seems to have a head of steam, no pun intended. Corvair will be the old work horse. This is at 100 hp. The criteria of being cheaper than a used 912 is going to the the buster. How much cheaper? Once all the nuts and bolts are payed for, it will be a wash. If spending the money, the Yamaha might be the one with less woo behind it.
 

13brv3

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Have you looked into the Aeromomentum AM 13? As far as auto conversions go, it looks like the "easy button" for 100 hp.
Yes, but it's in the same price you could probably get a decent 912S, which has to be considered a better engine until proven otherwise. It would be nice to have EFI and cheap parts availability from a mass produced core engine though.
 

billyvray

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There's a good effort by a guy on Facebook putting a 2cyl Yamaha Phazer engine on a Kolb. It will be a good lightweight alternative, but not on par for a 912 replacement - maybe 503,582,612 Rotax or HKS range.
For 100 hp it seems like the Apex Yamaha is the best deal going right now.

The snowmobile engines are hot right now. I've been eyeing the latest crop of watercraft engines. There are some really good etec 2 strokes and 4 strokes.
 

Pilot-34

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Has anybody ever put any effort into the six cylinder motorcycle engine light Honda built?
 

Vigilant1

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Has anybody ever put any effort into the six cylinder motorcycle engine light Honda built?
The Goldwing/Silverwing, etc water cooled engines? I remember a Contact article by someone converting one to acft use ( was planning to use a VW Rabbit wheel bearing as a prop bearing, or something' like that). The engines are very smooth and well made, but his conversion ended up quite heavy for the HP it made.
 
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rv7charlie

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There's a good effort by a guy on Facebook putting a 2cyl Yamaha Phazer engine on a Kolb. It will be a good lightweight alternative, but not on par for a 912 replacement - maybe 503,582,612 Rotax or HKS range.
Any chance I could get a link to his info? I've been toying with that idea to get rid of the 503 on my Twinstar. As much as I despise FB, I'll at least look at it if there's some useful info.

Thanks,

Charlie
 

stanislavz

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If you wanted to make a 100 HP alternative to the 912S, using a modern engine.
Just buy used rotax and add efi to it if you want / need. It is still cheaper if your consider all special parts to be made and tested for one off
 
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TFF

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Different engineering points. Rotax was designed to be exactly what it is. No matter how nice any conversion is, it’s cobbled together adaptation. If Rotax wanted ultimate put their mind into a fictional 947UL and out do the Yamaha they could. It’s not apples to apples either way.
 

wsimpso1

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Yes, but it's in the same price you could probably get a decent 912S, which has to be considered a better engine until proven otherwise. It would be nice to have EFI and cheap parts availability from a mass produced core engine though.
Only if the Rotax is quite used. New, the Rotax is about $7k more than the 100 hp AM. If you are going to compare used, then maybe you go used to used and look at rebuild costs.

Billski
 
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