Unrelated, but still on the subject of test driving, BMW invited me out to their "Ultimate Drive Event", where they have a number of their cars and some of their competitors' cars-- in this case, four different Mercedes-Benz models-- available to test drive.
It seems I treated this completely incorrectly. Most folks were interested in driving the big V-8 powered $100,000 cars. I was interested in the other end of the spectrum. BMW's recently changed their low-end models from a naturally aspirated inline-six engine to a turbocharged four cylinder, and I was interested in driving that. I drove three different varieties, plus the lowest-end Mercedes they had, which had a 3.5L V6 under the hood.
I'll preface this by noting that I've never been a big fan of four-cylinder engines, and I really don't like "big" four-cylinders, up above two liters displacement or so. One might think this is because a four-cylinder is a small engine and doesn't make much power, but that's got very little to do with it. My problem with the four-cylinder engine is all in what automotive engineers call "NVH", Noise, Vibration, and Harshness.
An inline four cylinder engine (as opposed to a "boxer" or H-4 engine, like you might find in a Volkswagen or a Subaru) isn't mechanically balanced. This means they need a balancing shaft to cancel out vibrations that are caused by the motion of the pistons. They (along with their boxer cousins) also have the problem that the power pulses don't overlap-- four cylinders and four strokes means that each cylinder is doing one of the four phases of engine operation at any given time, unlike a 6 or 8 cylinder where there's some overlap. Both of these contribute to annoying vibrations.
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| Why I-6 engines (red) are rare: when you put the cylinders next to each other (yellow) , you can fit the engine in more places. |
This is the big reason why I wanted to try BMW's new four-cylinder. BMW's one of the few companies left building inline-six motors instead of V-6s, because inline 6s are in both primary and secondary mechanical balance inherently, where V-6s require balancing shafts. If they'd managed to get a four-cylinder to behave politely enough to replace their I-6, that's saying something, right?
Turns out the answer is, "Well sort of." Don't get me wrong-- it's not bad. Honestly, most modern I-4s and V-6s aren't. It wasn't the nice smooth experience you get out of an I-6 or a crossplane V-8 though. The turbocharger does mean they get adequate power out of a lower-displacement engine, which means it's not as harsh as a bigger engine, which is good.
Overall, I like the new 2L turbo-- even in the de-tuned version they have in the 320i. But it is still a 4-cylinder. I was a little disappointed that this engine isn't a case of, "Look what we can do!". It's more, "This is what we're stuck with so we can meet the EPA's goals by 2015."
Which is where the real NVH problem was with three of these (and more and more modern) cars. When a car's engine isn't spinning quite fast enough to make enough power to do what you're asking it to in a given gear, it "lugs". Unfortunately, right on the edge of this is where you'll get the best mileage out of a car; the slower the engine is turning for a given speed, the less gas it's using (generally).
The sound a car makes when it's doing this is like nails on a chalkboard to me. We grew up being taught not to lug the engine. We know it can lead to bearing failures, piston slap, all manner of nasty engine trouble. Sure, engines are better now... but it sounds horrendous nonetheless.
This is the biggest problem with the tendency toward more and more forward gears on automatic transmissions-- The E350 I drove had a six-speed transmission, the BMWs I drove yesterday had 8 speeds. Chrysler and GM have teamed up to build 9 and 10 speed gearboxes. Eventually the sum of human endeavor will be put into building gearboxes with more and more gearsets in them-- or at least everyone who isn't putting more and more razor blades on a razor will be building these transmissions,
This just means that for any chunk of road in any given conditions, there are now 6 or 8 (or soon, 9 or 10) different velocities you can travel that will have you on that fuel-efficient but nerve-grating edge of lugging the engine. And continuously-variable transmissions can always be there. Ugh.
The three cars that did this got much nicer to drive in "Sport" mode-- or, as Amy puts it, "When you push the JD button." (She's incorrect. The JD button is the one between the windshield and the sunroof-- the one that opens that sunroof.) The engine spun about 500 RPM faster and never felt like it wanted to lug. But when I put each transmission in manual mode, it was even better-- I didn't need 500 RPM to keep the engine happy, I only needed around half that. As long as I could get past the eons it takes an automatic transmission to shift (and they're SO much better than they used to be!), they all drove quite nicely.
This seems to be a developing trend as we try to add more and more fuel efficiency to cars. It seems like just about every thing I've rented in the past few years also wants to ride on that edge of lugging the engine.
Two exceptions were the BMW 328i I drove yesterday and Amy's RAV4. Neither lugs significantly unless you push the button that puts the car in "Eco" mode. I consider this a reasonable middle ground: If you don't mind that noise, you can have the extra half a mile per gallon (or however much it is.) If you do, leave it in standard mode and it'll sound fine. Hopefully more cars will begin to do this.
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