Service

SERVICE

The numbers game

Could you explain how bore and stroke dimensions affect engine performance? How about compression ratios?

J. Harris Vancouver, B.C., Canada

Bore and stroke variations have no direct affect on engine torque. Four-stroke engines with large bores generally have matching large valve areas and ports, however; and that can reduce power at low speeds while increasing it at the top end. But it's the combination of valve area, port dimensions and cam timing that controls the powerband, not the bore-to-stroke ratio.

In general, higher compression ratios give more power and better fuel efficiency, while requiring higheroctane gas to prevent detonation and pinging. Generally, compression ratios are set as high as the readily available fuel will tolerate.

Custom chambers

I have a 1972 Suzuki T500 twostroke Twin. Eve looked all over, and no one sells any expansion chambers for it. Do you know of anyplace that would have chambers for this soon-to-be bottle rocket?

Mark Wisniewski Plainfield, Illinois

Near us, a company called Power Pros (817 N. Lakeview, Unit J, Anaheim, CA 92670; [714] 777-3278) does very good custom pipe work.

There are surely many other shops that do custom expansion chambers; if readers would send us the names of any they're familiar with, we'll compile a list for a later Service column.

To lug or to rev?

A recent Kawasaki advertising brochure stated that engine lugging is more damaging than redline use.

A recent issue of CAR AND DRIVER stated that lugging is not dangerous to car engines (except a few old British cars). Who’s right? And what constitutes lugging on your typical 10,000-rpm engine? Is running my CSR 305 at 25 mph and 2700 rpm on level ground bad practice? Or my 550 Seca at 25 mph and 2000 rpm? Is it better to downshift and go up a hill at 3000 rpm in second or speed up to increase the revs in in a higher gear? Finally, should you keep the revs high or low while warming a bike up, and how long should the warmup be?

S. DeGalan San Francisco, California

It depends on how you define “lugging., "Simply using a tall gear to run an engine slowly shouldn 7 cause damage if: 1) there's no audible pinging from detonation: and 2) the engine isn 7 making objectionable noises, often caused by the bottoming of the shock-absorbing springs or rubbers in the driveline. As automotive companies have rediscovered in the last few years, the best fuel economy results from running an engine as slowly as possible, with the throttle opening just short of full open. Running under those conditions, however, increases loads on engine bearings and transmission gears compared to running in a lower gear and spinning the engine faster.

Neither example you cite sounds as if you 're running your engine too low on level ground, and 3000 rpm in second up a hill should cause no problems. Finally, we'd recommend warming an engine briefly at medium rpm (above 1500 rpm, below 3000), and riding off as soon as it will respond to throttle. Prolonged warmups at low rpm can be particularly damaging, as they prolong the period in which engine oil is cold, thick and poorly flowing. But neither should an engine be operated in the top half of its speed range until it's thoroughly warm.