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. We’ve actually been doing it in the print magazine for years. So, feel free to 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.
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WHAT TO WEAR
Do engines that feature cylinder deactivation always shut down the same cylinders? If so, I’m wondering about the wear and tear on the cylinders that are always activated versus those that shut down.
—John Bolduc, Hubbardston, MA
The term “cylinder deactivation” is a bit misleading because none of the pistons stop moving. What actually happens is that the valves of specific cylinders are deactivated. Some engines, such as the Volkswagen Group’s 4.0-liter V-8, run the same cylinders when the engine operates as a V-4. The most sophisticated cylinder-deactivation system on the market, General Motors’ Dynamic Fuel Management that’s fitted to some of its V-8s, has 17 possible firing orders, allowing the engine to vary which cylinders fire and which don’t. GM also employs a simpler system that’s similar to the VW Group’s setup, and Joe Folk, GM’s assistant chief engineer for small-blocks, tells us: “Comparing always active cylinders with cylinders with deactivation capability on high-mileage test engines has not shown appreciably different levels of wear in the cylinders or within the valvetrain components.” —K.C. Colwell
A HARD CELL
In your March 2019 issue, you compared the Hyundai Kona Electric with the Chevy Bolt EV [“Always Be Chargin'”]. In the specs, both cars have prismatic cells from the same supplier, LG Chem. The Kona’s battery pack boasts 4.0 kilowatt-hours more than the Chevy’s yet has only six more battery cells to do so. Could you explain how a seemingly small difference in battery size yields such a difference in capacity? How large are these individual cells anyway?
—Bo L. Movement, Pataskala, OH
The energy stored in a battery, measured in kilowatt-hours, is calculated by multiplying a pack’s voltage by its amp-hour rating and dividing by 1000. Both the Bolt’s and the Kona’s individual cells are rated at 3.7 volts, but the Chevy’s hold 57 amp-hours while the Hyundai’s store 60 amp-hours. The packs are wired similarly—battery cells wired in parallel increase the amp-hour rating of the pack while cells connected in series increase the voltage—with three cells in parallel and 96 (Chevy) or 98 (Hyundai) triplets connected in series. The difference in total energy capacity, then, comes from both the Kona’s six additional cells and the fact that each one has a higher amp-hour rating than the Bolt’s cells. Neither the manufacturers nor battery-maker LG Chem were willing to explain the difference in amp-hour ratings, but it likely comes down to the chemistry or construction of the cells themselves. They’re nothing like your typical household battery. Each of the Bolt’s 288 cells measures 13.1 inches long and 3.9 inches tall and is roughly as thick as a cellphone. —Eric Tingwall
The “Crank Yankers” sidebar in the Ford Mustang Shelby GT500 piece is misleading [“GT500,” January 2020]. I understand the point that a twin-turbo cross-plane-crank V-8 with crossover exhaust tubing can provide 180-degree exhaust pulses to each turbocharger. How does the proposed alternative (hot V) provide 180-degree spacing without crossover tubing?
—Dean Lekos, Litchfield, CT
Turbos should be positioned as close to the exhaust ports as possible in order to maximize their efficiency and response. Situating the turbochargers next to each other in the valley of the engine’s V simplifies the plumbing problem when, as in the case of BMW’s S63 V-8 in the M5, it does employ crossover tubes. Remember, unequal-length crossover headers aren’t ideal for a road car, as energy is lost in those long runners. —K.C. Colwell
ROCKY MOUNTAIN LOW
The Car and Driver article “Should You Be Buying Premium?” by Eric Tingwall was well done and offered great information. At an elevation of 6000 feet in Colorado Springs, gasoline comes in three flavors: 85 octane (regular), 87 octane (intermediate), and 91 octane (premium). Is there any harm in using 85-octane gasoline at this elevation when the manufacturer recommends 87?
—Al Ziegler, Colorado Springs, CO
A modern gas-powered vehicle should always run the fuel that meets the manufacturer’s minimum required octane, regardless of altitude. We haven’t found a single manufacturer that condones the use of any gasoline below 87 octane at higher altitudes in its current products. Many explicitly warn against the practice. Regular 85-octane gas is a relic best suited to vehicles built between 1971 (when automakers started engineering for unleaded gas) and 1984 (when fuel injection became common). Lower atmospheric pressure at elevation reduces the density of the air-fuel mixture and makes it less likely to knock. Today’s engines—computer controlled, fuel injected, and often turbocharged—compensate for ambient conditions by adjusting fuel delivery, ignition timing, and boost pressure, when applicable. Using 85-octane gas risks a situation where the engine cannot retard timing enough to eliminate sustained knocking, which can cause serious damage to the engine. —Eric Tingwall
Now that I’m old enough to buy a Porsche 911, I’d like to know the difference in acceleration times that translates to a seat-of-the-pants difference. One second? A half-second? Damn, Porsche. Every new, more powerful variant of the 911 seems to be at most a half-second quicker than the last.
—Robert, Spokane, WA
On one end of the acceleration spectrum, a half-second improvement from 3.0 seconds to 2.5 seconds is the very obvious difference between average acceleration of 0.91 g and 1.09 g’s. In slower cars, a half-second is less noticeable. Going from 7.0 seconds to 6.5, for instance, results in a subtler difference of 0.39 g and 0.42 g. It’s perhaps better to think of it in percentage terms. A 10 to 15 percent improvement in zero-to-60-mph times is about where you can start feeling it in the seat of your pleated khaki Dockers. —Ed.
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