Motorcycle vs car engine

[sgtlethargic]

"All else being equal", would a motorcycle or car engine of the same displacement be better for a high-MPG car? In general, don't over-analyze it, please.

[Frank Lee]

Bikes are developed more for maximum horsepower output than cars thus are tuned for high rpm operations thus "all else equal" are worse for fuel efficiency.

[drmiller100]

car engines have transmissions with reverse.

[Ryland]

Frank is right, motorcycles have cams that are ground to open the valves wider and to help the engine rev higher, higher revs tend to give poor mileage because of slow flame spread, this type of cam grind also requires a lot of motorcycle engines to idle at a higher speed.
There are a few motorcycle engines that are exceptions to this, I think Honda Goldwings are one of them as they tend to turn rather slow because a 1,800cc engine on a motorcycle isn't working very hard to move it down the road, then of course there have been a few motorcycles like a version of the Honda CB400 that was sold mostly in Japan that had a V-tec engine and was getting pretty decent mileage but still had a high power out put at high revs (of course using a lot more fuel at that time).

[Frank Lee]

My GoldWing 1100 revs too much and the fe is pretty poor.

[user removed]

Typical bike engine is about .45 pound per HP per hour in fuel.

Best giant diesel container ship engine is about 35-40% of that rate of consumption.

Best passenger car engine listed on Wicki is the 5 cylinder 1990 Audi direct injected diesel at about 45-50% of the typical bike engine. Best gas engine in a car is the Prius motor. Possibly the lean burn engine in the first gen Insight might be better than the Prius when in lean burn.

There may be others that are slightly better but I got tired of looking .

I'm sure Frank can find a few.

regards
Mech

[darcane]

Definately the car engine.

Car engines are designed for more torque at lower rpms. Often small displacement car engines will be undersquare to help optimize low end torque. Parts are often heavier for more durability. Economy of scale means more development time for EFI maps. Example: Geo Metro's undersquare 1.0L engine makes 49-55hp at about 5000rpms (depends on exact model) and 55ft-lbs at 3300rpms.

Motorcycle engines are designed for peak power, and thus higher rpms. Often these will be oversquare to reduce stress on the crankshaft at high rpms and allow for larger/more valves. Extremely lightweight parts are used to allow for high rpms and durability is not as critical. Often won't have as much refinement in EFI mapping. Example: Kawasaki ZX10R's oversquare 1.0L motor makes ~210hp at 12500 and 83ft-lbs at 8700rpms.

(Note: these aren't quite apples to apples since the Geo is a 3cyl and the ZX10R is a 4cyl. But, there aren't many car and bike engines that the displacement matches up)

[bandit86]

A good larger displacement V- twin would make lots of torque and get good fuel economy if driven properly. I had a Yamaha virago with a 1000cc engine, 50hp and 50 ft lbs torque, never drove it for fe but remember good mileage. It made so much torque it would go from 25 just off of idle to 115 mph I top gear. Apparently converted to single carb setup (dual to make it sound like a Harley) it will make almost 70 hp. I wonder how aerodynamic a bike is compared to a car

[redyaris]

A motorcycle engine & drivetrain is designed to move a 600 - 800 lb vehicle/rider and a car engine & drivetrain is designed to move a 2500 - 5000 lb vehicle/passangers. If you build a 800 to 1000 lb car, a motorcycle engine & drive train would be fine. most motorcycle engines do not like to run below 3000 rpm in top gear, in contrast my 1500cc 4 cylinder Yaris car engine is quit happy at 1500 rpm accelerating in 5th gear.
Many cars have a Cd of 0.3 to 0.4, most motorcycles have a Cd of 0.65 to 0.9, even the sport bikes are in that range aerodynamicaly. The sport bike fairings adds almost nothing to the aerodynamics of a motorcycle.

[Frank Lee]

The m/c drivetrain may function just fine but it won't be the ultimate for fuel efficiency. If a guy was in a position to do an extensive development campaign he could re-cam and re-tune the intake tracts and exhaust system for peak torque and efficiency at lower rpms.

I haven't looked into it, but perhaps there are some motorcycle-like drivetrains that are more tuned for lower rpm torque: thumpers, dirt bikes, ATVs?

V Twins aren't necessarily all that torquey and efficient... at least my 500 SilverWing isn't. 45mpg is the norm, maybe 50 on a good day and yes it's all tuned up.

That reminds me of another point: motorcycle drivetrains appear to me to be quite lossy. By that I mean, unlike in most cars where the power goes from engine to trans input shaft to output shaft to differential to wheels, bikes usually (always?) employ chains or belts between engine and trans, and after trans too. More chains and belts and shafts and **** means more drivetrain losses.