Is There a Limit?

Autos race for ever-more transmission speeds

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Technical Editor Kevin Cameron shares his wealth of motorcycle knowledge, experiences, insights, history, and much more.Cycle World

During the all-out 1960s years of classic GP road racing, narrowly-tuned racing engines were given more and more transmission speeds to keep them pulling hard in their ultra-narrow 500-rpm powerbands. Then in 1968 common sense prevailed as the FIM just said no to 18-speed Kreidler 50s and to 12- and 14-speed Suzuki 125s and 50s. Henceforth, they decreed, six speeds will be the maximum number. That's where motorcycles are today – racing or production. It was a good decision because it forced manufacturers to seek – and to find – ways to make much broader engine power.

Now we hear that at least two automakers have developed ten-speed automatics. Why are they doing this, and will the trend affect motorcycles?

To understand the why we must look at the specific fuel consumption maps that are prepared from engine test data. Specific fuel consumption is the amount of fuel (in pounds or grams) required per horsepower (or kW) produced, per hour. A typical ballpark figure for four-stroke engines is 0.5 pound/hp-hr. This tells us that if our engine is giving 30-hp as we cruise up the Interstate at steady speed, it will use 0.5 x 30 = 15 pounds of fuel per hour as it does so.

But specific fuel consumption, or SFC, is not constant over an engine’s operating range of rpm and load. At lower throttle, pumping loss increases, causing SFC to rise. At very high rpm, friction rises steeply, pushing SFC higher. Somewhere in the lower middle there is an optimum balance between pumping loss and engine friction.

If we plot this as curves of constant SFC on a graph whose axes are rpm and load, we get a nested set of roughly elliptical closed curves of SFC. The smallest ellipse is that engine’s island of optimum fuel efficiency.

It is quite common for the closed curve of maximum SFC (worst fuel economy) to have a number that is 2 to 2.5 times greater than the small central island of minimum SFC. This is why my little econobox gets 43-47-mpg in steady Interstate cruising, but only 25-28-mpg around town.

This also explains why "She's Real Fine My Four-Oh-Nine" can't get very far on a gallon of gas; closing the throttle enough to stay at legal speed on the highway pulls the engine down from the island of best SFC into the stormy seas of higher SFC regions at lower load. So, to get better mileage, we build car engines dinky enough to maintain highway speed on the more open throttle of the high SFC island. And it also explains why, when Japanese WW II pilot Saburo Sakai was severely wounded in combat, 600 miles from base across empty ocean, he immediately reduced engine rpm so that he could open the throttle against the load produced by pitching up his propeller. This combination of lower rpm and higher throttle gave him the fuel economy he needed – drifting in and out of consciousness - to reach his base and land. Charles Lindbergh taught American pilots this very same technique.

All automakers are holding their collective breath these days, wondering how in the world they can meet the coming Corporate Average Fuel Economy (CAFÉ) standard of 54.4-mpg. One obvious technique is to keep their cars’ engines operating as close as possible to that innermost ellipse of best SFC. Yet looking at the graph you can see that it’s not very wide in terms of rpm, and not very high in terms of engine load. The more speeds there are in the transmission, and the closer they are together, the more likely it is that the engine ECU can find a transmission speed that keeps the engine humming on its island of best SFC. With four speeds you spend a lot of time pretty far from best SFC, but as you add more speeds you can stay closer.

Lots of tranny speeds are also handy when you’ve chosen a really small engine, for the smaller the engine, the farther open its throttle has to be to keep it “on the island.” And they help when pulling away from stoplights; a little 2-liter ‘banger’ needs a lot of help to heave a 4000-lb. luxo-liner into motion.

What’s this got to do with motorcycles? Up to now, regulators have pretty much let motorcycles off easy, but I don’t expect that to be permanent. So if motorcycle fuel consumption does come under official pressure to improve, all the techniques that have worked for cars are available, with the exception of aerodynamic refinement. So, ten-speed transmissions? Yes, if one day we need them.