This article was originally printed in the Moto Guzzi National Owners Club monthly newsletter. http://www.mgnoc.com
Several years ago while teaching engine classes, I would throw out controversial topics that were likely to spark lively, thought-provoking conversation.
These sessions would always start out innocently enough. "How many of you guys ride motorcycles?" "Okay, good." Then with eyebrows slightly raised (the tip-off) I would ask, "How many of you downshift your bikes when coming to a stop?" Now understand that the class already knew that the instructor could occasionally display a deviant mind. Many in the class already suspected that the rider of the hopped-up bike, who had raised his hand, was about to become a victim. The motorcyclist had swallowed the hook.
Of course, the students all knew that I rode Guzzi and that I liked discussing the general subject at hand. However, the rider was ready to defend what he had been taught. He knew downshifting was note a bad habit, but one that he somehow picked up on his own. Besides, all of his riding buddies downshifted.
The attention of the class was now peaked. At this point I would always get laughter upon repeating the statement, "I, the spider, have noticed that a fly has landed in my web." We did have a lot of fun.
So anyway, downshifting is always a bad thing, right? No, not at all. Trucks downshift to safely get their loads down hills. Truck brakes are secondary to engine braking that often uses exhaust restriction to further increase the engine's braking effect. The truck's brakes are thereby saved from probable overheating.
How much braking can an engine provide? Only for the sake of this explanation (read as: Don't try this at home!), you downshift on a downhill, close the throttle, hit the kill button, and then roll open the twist grip. The suddenness and amount of throttle opening determines when the back tire breaks loose and how quickly you go down. Remember, a diesel engine has no air-controlling throttle. (Its air inlet is wide open all the time.) However, a motorcyclist would wisely downshift to help prevent overheated brakes on a steep grade.
A few years back, we discussed downshifting to increase horsepower and, more pertinent to that discussion, to decrease effective compression ratio and reduce burn time BTDC. Well, downshifting is also done to save time. Some motorcyclists do indeed downshift just to save time. All things being equal, the cyclist using the least amount of time is the one who wins the road race. There remains, however, little in common between road and race shifting.
So, that leaves us with habitual downshifting and the reasons why it is bad. The rider in the spider' s web admitted to using much more than a quart of oil every thousand miles. This was due to the incredible horsepower of his breathed-on American pony, or at least so he thought. Let us assume the work on his lower mileage engine was competently performed and that the engine was correctly broken in. Even so, each downshift allows much more oil into the combustion chamber than would be the case with any other situation for an engine.
I suggested to the rider that he not downshift at all for a riding season to drastically reduce oil consumption and prevent fouled plugs. My correct assumption concerning his serious plug fouling problems is what it took to convince him to think about eliminating downshifts.
The habit of downshifting is likely the number one reason today that hotter-than-specified plugs are installed in motorcycles. That which may seemingly be knowledge (i.e. that hotter plugs have proven to last longer in a specific case), may in reality be a treatment of the symptom rather than of the cause (the cause being downshifting). Hotter plugs can be detrimental or even damaging when operating at higher and/or prolonged engine speeds.
However there is yet another reason not to downshift. As you downshift you also close the throttle. (Downshifting is typically closing the throttle on a now over-revved engine.) An over-revved, carbureted engine with a closed throttle draws excessive fuel through its idle circuit. A benefit of fuel injection is the nearly complete elimination of richness in an engine that is on the over run. In a carbureted engine, a downshift condition will result in lower fuel economy.
We all know that hot rodding uses more fuel than normal driving. However, downshifting not only uses more fuel, but you don't even get to go faster in the process. The sooner-to-be-fouled plugs and carboned-up valves and combustion chambers go hand-in-hand with oil that is more quickly deteriorated and with any possible detrimental effect of oil contaminates on engine internals.
Every time our rider downshifts four times through his five-speed gearbox (and this guy did), he more than doubles the wear on his bike's entire drive train. Additionally, have you ever noticed the louder the mufflers are, the more a bike gets downshifted?
I do occasionally downshift and roll off the throttle to aid the breaking in of a new engine. That same increase of oil in the direction of the combustion chambers flushes and cools the cylinders, rings and pistons of a tight, new engine. Conversely, slightly faster than normal acceleration (not drag racing) together with short shifting helps to seat rings in a new engine. Come to think of it, I sure wouldn't mind rolling off the throttle of a new black Jackal! Like the bug-eyed Chihuahua says in a near whisper, "Hurt me."
Think about your Guzzi's clutch parts, splines, U-joints and driveshaft. Use your gearbox to get up to speed. Use your brakes to reduce speed. The old saying that brakes are cheaper than clutches was never truer, as today's shop rates can quickly prove to you.