You have a lot of questions and occasionally we find ourselves with enough time and in a good enough mood to answer them. Joking aside, we strive to answer any and all of your question about our procedures, tests, or the mysteries of the automobile. Use this chance to broaden your knowledge or even settle a bar bet. And remember, there are no stupid questions—well, if they’re really stupid we won’t answer them. Email us at: firstname.lastname@example.org
In the September issue, you had an enlightening article about new Porsche brakes titled “Dust Busters.” In it, you stated that the brake dust that accumulates on wheels comes from the rotors, not the pads. If that were true, then that dust would be iron particles and thus would stick to a magnet. Well, it doesn’t, which means it’s pad dust. Care to explain?
—George Knab, Buffalo, NY
We should have said the brake dust that accumulates on the wheels comes primarily from the rotors. Dust from the pads is certainly a part of the mix, but only a small portion. With impressive specificity, EBC Brakes says that 92 percent of the brake dust on a wheel is ferrous metal material. That comes mostly from iron rotors, with a smaller portion originating from the pads, which are made up of as much as 30 percent steel. The metallic dust particles are often hot enough to melt into a painted wheel’s surface. Your magnet is too weak to pull them free. Finally, consider that vehicles with carbon-ceramic rotors leave virtually no brake dust despite having brake pads. —Ed.
What is the difference between engine horsepower and horsepower measured at the wheels?
—Bao Duong, Encinitas, CA
All the horsepower figures you see in this magazine are supplied to us by the manufacturer. Automakers measure engine output at the crankshaft. These performance statistics don’t factor in the power that’s lost to friction in the transmission, differential, and other driveline components. A chassis dynamometer is spun by the drive wheels and, consequently, its measurements include these parasitic losses. Conventional wisdom says that output at the wheels will be roughly 15 percent lower than at the crankshaft, but that’s an outdated notion. Improved bearings, lubricants, manufacturing tolerances, and computer-aided designs have reduced losses to single-digit percentages in new cars.
We recently put a Toyota Supra on a chassis dyno. At the wheels, the Supra put down 339 horsepower—higher even than its advertised 335-hp crankshaft output. Factor in a 5 percent loss and the engine appears to be making nearly 357 horsepower, which helps explain the Supra’s bullet-like 12.3-second, 113-mph quarter-mile run. —Ed.
Why aren’t turbos electric? Why have all the hardware of an exhaust-driven turbo when an electric motor would be so much simpler? Plus, it would speed up quicker and be more controllable. You can borrow my hair dryer to test this theory.
—George Hovany, Gig Harbor, WA
Your description of a turbo spun by an electric motor and without the exhaust-driven hardware is actually an electric supercharger. Mercedes-AMG and Land Rover both employ one in their inline-six engines to provide boost before the turbocharger wakes up. However, electrified turbos, where an electric motor assists the exhaust-driven turbine, are the next frontier. Combining them, as Formula 1 powertrains have since 2014, is all but inevitable. With the turbine, compressor, and electric motor all spinning on a common axle, the motor can assist with response and harvest energy when off throttle without completely abandoning the efficiencies of a purely exhaust-driven turbocharger. Garrett, a leader in this field, says electric turbos can provide boost so quickly that passing maneuvers often don’t require a downshift. After the $2.7 million AMG One debuts this tech next year, we expect to see these e-turbos trickle down to more accessible production cars by mid-decade. —K.C. Colwell
COOL FLEX, BRO
In your comparison-test scoring, there is an objective category called “flexibility” [“Deep Roots, Long Shadows,” October 2019]. What is flexibility in terms of vehicles and how is it measured? I can touch my toes. What score would I receive?
—Rob Gardiner, Madoc, ON
In your case, you get a four because you’re bending your knees. We define flexibility as the difference between the time it takes to hit 60 mph—achieved by any means of aggressive launch necessary—and the 5-to-60-mph test. A small gap between those two times indicates that a vehicle’s acceleration performance is more accessible and attainable, which is what we reward in our scoring. Two things that sometimes lower flexibility scores: turbocharged engines that require extended brake torquing for a strong launch and low-torque engines where a redline clutch drop is the quickest way off the line. These types of engines, which can otherwise be described as laggy or peaky, tend to produce a wider gap between the two metrics. In comparison tests, we dock a point for every 0.4-second difference. —Dave VanderWerp
YOUR MILEAGE MAY VARY
I read in another magazine that the odometer in a new German car can be off by 25 miles for every 1000 miles driven. So it may show 1000 miles on the odometer after having traveled only 975 miles. In many new cars, the warranty runs out when the odometer shows 36,000 miles, which would be 900 miles short of the true distance. On a per-car basis, that’s a bit less coverage. However, if that company sells 100,000 of that model, it really adds up. If this holds true for all cars made by that company, well, you get the idea. This also makes the fuel mileage appear better than it is.
—Paul Messier, Portsmouth, NH
It’s not just German cars, Messier. To track this issue, we perform an odometer calibration on every vehicle we test and very few are perfectly accurate. The amount of error varies not just from brand to brand and model to model, but even among multiple examples of the same vehicle. Most odometers underreport miles traveled; it’s rare for one to count more miles than were actually driven. Some error is inevitable, as odometer calculations rely on tire revolutions per mile, which change with tire model, size, pressure, and wear, along with manufacturing variance. There are no regulations governing odometer accuracy in the U.S., although the SAE industry standard recommends a tolerance of plus or minus 2.5 percent. On the extreme end, we’ve seen more than 5 percent deviation, with vehicles covering fewer than 5000 feet or more than 5600 feet while their odometers indicate they’ve driven one mile (5280 feet). C/D uses odometer calibrations to correct its observed fuel-economy figures accordingly. If you’re truly worried about maximizing your warranty, you can always mount larger tires, but don’t expect the manufacturer to pay for your speeding tickets. —Ed.