What bike engine should be used in a Mini-sized ecomodded car?
 

Hi!
I'm asking a very open question here, obviously - but...
I have in mind a conversion for a 60s car (the Hillman Imp).
The kerb weight for the car is quoted at around 750kg (100 more than an Austin Mini, but a prettier car IMO) - plus is also rear engined/rwd which suits my driving style better :D
Several Imp owners have converted their cars to bike engined high-revved speed-machines (keeping the Imp's transaxle) - but I would like to get a vehicle that is designed primarily for efficiency whilst keeping the old-school looks intact (though this may hurt the aerodynamics).
Imp engine - 37.0 bhp @4800 rpm. Torque. 67.0Nm(49 lbft) @2600 rpm

Thinking:
BMW R850 Boxer engine - produces 57 lbft , 70 bhp
Oilhead/air cooling could cause problems, but Imp has rad in back, air drawn from under car (messy). Would keep Imp's diff/ section of transaxle casting (or equivalent).
Interested in how difficult it would be to 'de-tune' for lower BHP, remapped for good MPG.
I have a gut feeling that a low-slung torquey boxer engine has plenty of potential for good thermal efficiency, plus breathing is not hurt so bad.

Or for really ambitious build
R1200GS - with 85 lbft 100bhp
- converted cam lobes for Atkinson cycle with Mild hybrid e-motor/batteries combo.

Has anyone any experience using these motors for economy? Is it a good starting unit?
Note - being all Ally the Imp's engine/tranaxle comes in at around 70Kg as standard. I can's see the point in replacing it with anything heavier without a very good reason.

Most bike engines are biased towards ultimate output (higher rpm hp rather than lower rpm torque) than economy and it seems they return econo not as good as we'd hope for when repurposed. Still, they can be made to work and I'm sure some are better than others. So after all my blather I guess I don't have direct experience with the options you listed... whichever one is more of a "torque" engine will probably give better fe.

Not sure why you would not just use a Geo Metro/Suzuki Swift 1.0L and its transaxle. The 3-cyl G10 motor is all-aluminum and very light, and the transaxle is also very light, I believe its only 45lbs. Motor is designed for MPG.

There is another bike engine I ran across that looked promising, 650cc V2-cyl water cooled, shaft drive, here it is,
Honda CX series - Wikipedia, the free encyclopedia
saw one for sale for $100 a while back, but was sold before I can get there with my trailer.

If money is no object. I had a friend who rode a 2000 Triump 955 ST. Claimed it got 65 MPG before the performance mods.

PERFORMANCE

Maximum Power
110PS (108bhp) at 9,200 rpm

Maximum Torque
97Nm (72ft-lb) at 6,200 rpm



2000 Triumph Sprint ST

I have seen a used 955 sell for under $3K.

I can vouch for this engine. I had a Honda CX650C, and it's a great motor (674cc). Built-in 5-speed/clutch, with a splined shaft output. Lots of power from a reasonable rpm, water cooling for flexibility, good power (65hp IIRC), good economy, at least in the bike (50-60 mpg basically rodding the piss out of it with 200+ pounds of knucklehead on the seat, and no fairings).

For a small front-engine/rear drive, I'd think it'd be perfect. Get a driveshaft from a motorcycle, cut off the u-joint, and have a shop weld it onto your existing driveshaft, and you're in business! Hook up a set of paddle shifters (1/2 travel to lighten the clutch, 1/2 travel to switch gears) for sequential shifting and you're off to the races! Rear engine/rear drive, I'm not sure how the drivetrain would work, but still an excellent, tough, tractable motor. Probably small enough to set up as a mid-engine/rear drive motor.

Charging system works (a little bit of a weak spot, but fixable), and has thermoswitch-controlled radiator fan, and water temp sender and oil pressure switch for instrumentation.

If you don't need quite that much power, the same basic engine was built in huge numbers as a ~500cc, and they're much easier to find.

I could see an engine like the Honda Goldwing's working ok, 1,100cc to 1,800cc depending on the year, it's a flat 4 boxer set up for shaft drive and water cooling and is from a motorcycle that is designed to be able to be loaded down and towing a trailer where alot of motorcycle engines are designed for low load high revs.

I was going to mention the Gold Wing.. but my 1100 and I guess most all of them don't return very good fe, usually in the 30's and I don't think I've seen anyone claim any better than 40. In fact I'm deciding if my bike would run better and more economically if I ditch the 4-carb setup and put a single automotive carb on there...

I also have a GL500, which has the same drivetrain as the CX500. It got 45-55 mpg, driven gently and slowly. I'm not real impressed with that and always felt it should be better.

The Suzuki 650 twin (SV650 , DL650) is also rather efficient - at least in a bike.

Why the attachment to the motorcycle engine? find an engine out of a smart car that's been crashed or out of a Geo Metro.

I'd vote for the smart fortwo's 799cc diesel. I think its around 50 hp but would have a lot of torque a gas engine wouldn't. I don't think we can get them in the US, lucky europeans... Nothings going to beat a diesels efficiency. I've been dreaming of a micro ecomodder car with a ~20 hp v twin diesel.