Simanaitis Says

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THE R&T WEAR INDEX

MAYBE A YEAR ago, auto journalist pal John Dinkel asked if I knew how to calculate the old R&T Wear Index. Much to my chagrin (for it was John who hired me at the magazine back in 1979), I had no idea.

I recall seeing Wear Index values in old R&T test data with other arcane things such as Tapley Meter readings. The Tapley Meter (“Still Off-Scale After All These Years”), I understood.

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Illustration by Leo Bestgen, R&T, April 1988.

Indeed, I inherited a pair of these vintage accelerometers. But the Wear Index calculation was lost to the ages.

That is, it was lost until recently when I was looking up something or other in R&T, January 1961. I found an article there titled Road Test Terminology containing a description of the Wear Index.

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There’s good history here as well. Cars today are amazingly reliable in everyday use and durable in the long term. Back in the old days of R&T, this wasn’t the case. English cars of the 1950s, for instance, had instructions in their owner manuals for valve “decoking” every few thousand miles. Back when leaded fuel was the norm, spark plugs lasted only a couple thou before requiring cleaning and resetting the gaps.

The R&T Wear Index was John R. Bond’s assessment of how one car might be more durable than another. Essentially, it describes wear of a car’s piston; the lower the number, obviously the better. The magazine defined the Wear Index as the product of a car’s top-gear engine revolutions per mile times its piston travel in feet per mile. The resulting value, a really large number, is divided by 100,000 for convenience.

wearindexformula

The article notes, “The Wear Index is actually only an estimation of the true rate of wear, as it doesn’t take into consideration such relatively unpredictable factors as quality of materials and driver habits.”

Nevertheless, it makes for interesting comparisons among cars. Conveniently, another R&T issue, June 1961, had three Road Tests of highly contrasting automobiles.

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volvo

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1949 MG TC, R&T Wear Index: 88.2. 1961 Volvo 122 S: 68.0. 1961 Mercedes-Benz 300SL: 48.4. Images and data from R&T, June 1961.

The 1949 MG TC, tested retrospectively, defined English sports car to many Americans; its Wear Index of 88.2 was typical of the breed. The 1961 Volvo 122 S was a stolid Swedish sedan with a reputation for durability; its Wear Index was 68.0. The 1961 Mercedes-Benz 300SL Coupe, one of the best and most desirable sports cars of the era, had a Wear Index of 48.4.

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Calculated Data of the trio, from R&T, June 1961.

A little analysis suggests that the Wear Index rewarded short-stroked engines that didn’t rev much. An original Volkswagen Beetle exemplified this, as shown in R&T’s October 1956 test. Its 1192-cc flat-four’s stroke was 2.52 in. At 60 mph, it revved a mere 2910 rpm.

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1956 Volkswagen Sedan. Image from R&T, October 1956.

Though R&T Data Panels had yet to include this factor, the Wear Index of this 1956 Beetle would have been 35.6. And, sure enough, Beetles were legendary for being flogged foot-to-the-floor at their 70-mph top speed all day long on the Autobahnen.

To calculate the R&T Wear Index of your own car, start with the automaker’s specifications listing its engine’s stroke. The other essential bit of information is engine revolutions/mile in top gear, equivalently, the car’s tachometer reading at 60 mph. (This can also be calculated from its top-gear ratio, final-drive ratio and tire rolling circumference.)

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My Honda Crosstour, like other modern cars, scores well in the R&T Wear Index.

For the Crosstour, Honda specifications list its i-VTEC four-cylinder engine having a stroke of 99 mm, 3.90 in. The car’s cruise control made engine revs/mile an easy-peasy exercise: Set cruise to 60 mph on the freeway (here in California, in the slow lane), let the car stabilize, and read the tach: about 1900 rpm.

By the way, with a little arithmetic this yields another bit of R&T Calculated Data: mph/1000 rpm. The Crosstour’s speed at a not-much-above-idle 1000 rpm works out to 31.6 mph; like most modern cars, it’s long-legged indeed. Even the high-performance 300SL achieved only 20.8 mph at 1000 rpm in its top, 4th, gear. The Volvo sedan, 16.7; the MG TC, 15.5 mph.

Back to the Crosstour. To figure out its piston travel, note that a Crosstour piston travels 3.9 in. up and another 3.9 in. down for each revolution. Thus, 7.8 in. per engine revolution. And, hence, at 1900 engine revs per mile, it travels 1900 x 7.8 = 14,820 in. per mile, or 1235 ft./mile.

Therefore, using John R. Bond’s formula, the Wear Index for the Crosstour is (1900 x 1235) / 100,000 = 23.5. I suspect its valves will never require decoking. ds

© Dennis Simanaitis, SimanaitisSays.com, 2016

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