Service

SERVICE

High on compression

I have learned the hard way over the last 100,000 miles to do frequent compression checks on my motorcycle engines to look for the early signs of mechanical trouble. I take readings on all the cylinders and compare them to the figures in my shop manuals. My question is, how are my readings affected by altitude? Here in Reno I’m at 5000 feet, and I once did a compression check in Leadville, Colorado, at 10,000 feet. I assume the shop manual figures are calculated for sea level. What, if any, effect does altitude have, and how can I calculate around it in the future?

Matt McDonough Reno, Nevada

Compression readings vary in direct proportion to the relative air density (RAD). So, obviously, compression at sea level when the temperature is 60 degrees Fahrenheit and the barometric pressure is 29.92 inches of mercury.

So, regardless of altitude, if the RAD is, say, 10 percent lower than standard, any compression reading readings will be lower at higher altitudes, where the air is less dense, than they will be at sea level.

RAD numbers are a measure of air density compared with a standard number of 100, which is the air density will also be 10 percent lower. If the RAD is 5 percent higher; the compression reading will be 5 percent

higher. Any decrease in barometric pressure, or any increase in altitude and/or temperature, will cause a drop

in the relative air density, and viceversa. That's why most good tuners use an air density meter, which takes much of the guesswork out of jetting and ignition-timing adjustments.

Thus, the correction factor for your compression readings is easy to figure if you know the relative air density at the time the readings are taken. The easiest way to determine air density is with an air density meter; or it can be calculated either with a formula whose components are altitude, temperature and barometric pressure, or with the chart we have included at left. But to do either of the latter with accuracy, you frst have to know the actual barometric pressure, not the one given in weather reports (which is corrected to sea level).

For tuners of high-strung racebikes, knowing the precise RAD can be crucial. But for your compression-checking purposes, you can get close enough without knowing the actual barometric pressure simply by determining the local altitude and the ambient temperature, then cross-referencing them on this chart. For example, if you do a compression check at 5000feet when the temperature is 70 degrees, the relative air density will he 84. Thus, the compression numbers you get will be 84 percent of what they would be under the conditions used as a standard for the shop-manual figures.

Getting down

Does shifting down through the gears using engine braking cause accelerated engine wear, and should it be avoided?

Stan Hildebrandt

Wichita, Kansas

There's an old saying that states,

“Brakes are cheaper than engines. " And there's some truth therein. All engine components have a finite life, and so every time a piston goes up and down in a cylinder, every time a valve opens and closes, every time a sparkplug fires, it marks one less time that component will do so. Technically speaking, then, any unnecessary revving of the engine brings its components that much closer to their wornout point.

But realistically speaking, it makes little difference whether or not you downshift as you slow down if—and that can be a big if—you do so with discretion. If you don 7 overrev the engine in the process; if you don 7 execute full-on, right-to-the-redline, roadrace-style downshifts every single time you come to a stop; if you make your downshifts cleanly and smoothly; if you simply gear down intelligently, the amount of wear and damage to your engine should be somewhere between negligible and none at all. S