Subaru

Just looking at some of the engine options listed in the forum, the one that makes the most sense, at least on paper, is the Subaru Robin 28 HP fuel injected EH72 low profile UTV engine.
Reasons:
1. 28 HP
2. Low profile for more room under the cowling
3. Fuel injected with altitude compensating computer for easier starting and fuel efficiency
4. Drop forged crankshaft with large ball bearing mains
5. Electric starter
6. Alternator
7. Price, about $1,800 new
8. Available in splined or tapered shaft

Downside: about 100 lbs.

I'm a newbie at these planes, could someone tell me why this wouldn't work?

The additional weight might be an issue with the weight and balance. It seems like I recall something about them moving the wings forward about 3" on the Q2 in order to accomodate the 0-200 engine for the Q200, not sure where I read that, but it might be worth checking some of the online articles. The extra hp would easily carry the weight, but the weight and balance would be greatly affected, and without suspending one at the CG and actually adding 25 lbs. to the engine area, and then seeing how it is affected, I don't know if anyone out there has the math skills to recreate the needed engineering data to evaluate it.
Like Dan said, if you try it, we'd all be interested in hearing the results.

Robert

Sorry, on that last post it should say "wing" and not "wings" forward, I think they just moved the canard forward about 3".

Robert

This is a very interesting thread for me. I have been debating on rebuilding or replacing with like the 20HP Onan - maybe even going with the 24HP version.

But at the end of the day, the Onan is very old technology - not very fuel efficient and will be very difficult to maintain (they don't make this engine any more).

Something to remember on the weight is that you will eventually strip off much of the factory engine as you install / mount it. You will make your own baffling - you can remove flywheels since the prop is enough. You can even take off the starter, if you like. I therefore think that the new engines will not weigh much more than the original Onan. Please note that I have not verified this yet - if anyone has any actual weight data, please post it.

I have been considering the Kohler CH745. Let's compare it with the EH72 Low Profile UTV:


Max Power - 28 at 3600 vs 28.9 at 4000 RPM (26 at 3600)
Displacement - 725 cc vs 720 cc
Peak Torque - 42.7 vs 38.2 ft-lb
Bore - 83 vs 84 mm
Stroke - 67 vs 65 mm
Length - 14 vs 12.5 in
Width - 17.7 vs 18.8 in
Height - 26.5 vs 16.1 (note the Kohler includes the air filter)
Weight - 94 vs 101 lbs (note the Kohler includes the air filter)

The Kohler has a thrust bearing option - It looks relatively easy to install.

In summary, it looks like the Kohler has more power and weighs less.

My current concerns are that both of these engines are taller from the crank to the top than the original Onan. This means the engine cowling may be higher than the original, possibly blocking the view. I may want to lower the engine mount to compensate, but need to keep the prop out of the ground. I have resigned myself to re-fabricating te cowling, but I am not worried about that.

Another concern noted previously is weight. Again, I think that I can take enough weight off to make it work. The extra HP will make up for the extra ~10# weight, but the balance is a concern. I would want to move any possible weight to the rear of the plane such as battery, transponder, etc... It would be critical to keep the engine center of mass as far rearward as possible. On the original Q1 plans, the rear flywheel is actually inside of the cockpit (in the pie pan) to keep the weight of the Onan as far rea as possible. IF the balance is maintained, it should be fine - some wing loading calculations would be in order, though.

I agree that the electronic ignition and electronic fuel injection are real nice. I expect the fuel efficiency would increase dramatically as compared to the Onan, especially at the higher altitudes. The original design did not have any mixture control, so as you flew higher, you were wasting gas.

Remember more power means more heat to get rid of - baffeling needs to be well thought out and constructed. Also, both the Kohler and Subaru have O2 sensors. It doesn't take much leaded fuel to ruin them. So, you need to only use the (cheaper) autogas. Also, I would want to know what would happen if the O2 sensor fails - I believe the Kohler defaults to a standard mixture ratio.