May 26, 2006

Lessons from GM

This is a concoction from having too much spare time, but hopefully i'll make some sense at least. Just finished my second week at General Motors, and after driving a few of the vehicles, I wanted to bring to light something often overlooked in cars: torque. I think most people try to guage a car's performance based on horsepower alone, but horsepower isn't an accurate indication of a vehicle's performance by itself. Also, from reading reviews in magazines or whatever, the "experts" generally say stuff like "Acura's engine produces 200 HP from only 2.4 liters, compared to GM's engine that produces 200 HP from 3.5 liters. Acura's is a technological marvel [and therefore its better]." Fine, lets compare those two engines in a little bit more detail. GM's engine, the "3500 V6", produces 201 HP but 220 lb-ft of torque. Acura's engine produces 200 HP but only 166 lb-ft of torque. Take a look: This graph shows how much power the engine is really making at any given engine speed. Although the two engines ultimately have the same peak power rating, GM's 3500 reaches this peak much faster, and because it has more torque, it has more power at any given engine speed. (Note: The power curve for the Acura is approximated.) It's also easy to get lost because of things like i-VTEC, DOHC, and 4 valves per cylinder. The Acura has all three, the GM engine has none. Each adds complexity to an engine and generally strives to increase specific output (make small engines produce lots of power). It also, however, increases the cost of an engine. You might think that GM's 3500 engine would get significantly worse fuel economy, but this isn't the case either: In the Chevy Malibu, a car similar in size to Acura's TSX, it gets 22/32 mpg. This is actually slightly better then Acura's mileage estimates, which gets 22/31. Also note that Acura's estimates are taken with a 5-speed transmission, while Chevy's is using a 4-speed. The Acura demands premium fuel to achieve these numbers as well. Here's another comparison, GM's 3900 V6 compared to Honda's 3.0L V6: Similar story: both motors have about the same peak power, but the extra torque from GM's motor makes it more accessible and has more power throughout the entire powerband. Honda's engine is often praised for it's power, because of it's smaller displacement, while GM's is often dogged - but you can see how it really works here. Ask yourself how often you really have your car revving at 6500 RPM to get every last pony out from under the hood. Probably not often, and if it is, the car probably isn't going to last too long. I think a more effective way to measure the actual potential of an engine would be to take the area under the power curve (yes...an integral) because it would take into account the extra torque in larger engines. This could be represented by something like this: The larger torquier engines generally have the higher power integrals because they have more available torque throughout their powerband. This makes them easier to drive, since an engine usually isn't operating at its peak power. Anyway, i'm done. Hope somebody learned something from it. EDIT: A few things I wanted to make a point about - One, that yes, the 3500/3900 engines from GM find their roots back to the 1980's. Yes, they use pushrods, 2 valves per cylinder, and iron blocks. But who cares? I think every review I read concerning a vehicle with one of these engines calls them "antiqued" or "outdated" or whatever (www.nctd.com and www.consumerreports.com comes to mind). But they perform well, get good fuel economy, and for all I'm concerned, I would rather use proven technology than something loaded with new technology, given that they perform equally. Consumer Reports, by the way, is pretty strongly anti-American as far as cars go. Evidence of this can be seen in their reviews: 20/30 mpg on regular fuel is "poor fuel economy" for the Pontiac Grand Prix, but 19/29 mpg on premium fuel is just fine for the Acura TL. Also keep in mind that OHV (pushrod) engines like the 3500/3900 are much smaller than their SOHC/DOHC counterparts. And there's only one camshaft, not 2 or 4. And there's not routine timing belt/chain replacements like in some SOHC/DOHC engines. They are also cheaper to manufacture by a considerable margin - $800 comes to mind, but I can't verify that as of this moment, so take that with a grain of salt. But I am sure they are cheaper. I also want to make it clear that I think Honda makes some fantastic engines. But I think GM does as well, and they don't get the credit they deserve for it. thus ranteth Gheed at 6:44 PM | Permalink |