Quote:
Originally Posted by rstringer
Hello,
My technical knowledge about cars is rather limited therefore I don't quite understand the advantage of a high reving engine vs a torqy one.
In my mind if one can produce a lot of torque at lower rpms that seems to be more useful than reving the engine really high to achieve the same output.
Now It seems that BMW traditionaly always goes with the high reving concept (except the E39 M5) and I like to think that there is a very good technical reason behind this which I unfortunately do not understand.
I would appreciate it if someone could shed some light on the subject.
Cheers,
Rob
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If you can stand yet another interminable post...
First of all, there are many factors that go into rating a really good engine, but a key one (in my opinion) is that it should offer "balanced" power. That is to say, the peak horsepower and peak torque numbers should not be too far apart, and ideally (again according to me), the horsepower number should be a bit higher than the torque number.
Engines that make a lot of power compared to their torque numbers are often considered "peaky", meaning you have to wind them up in order to get any real performance out of them. (As an aside, these engines are rarely "peaky", in that they will typically have very broad torque curves. It's just that in order to extract a lot of performance out of them you have to use the gears to their utmost.)
An example of a power-laden, relatively torque-deficient engine is the one in the Honda Civic Si. It makes 197 horsepower at 7800 rpm, and only 139 pound feet of torque at a very high 6200 rpm. When driving behind such an engine, you'll tend to feel that the last half of accelerator travel is the good half, while the first half won't do a bunch for you. In addition, from rest, you can't get anywhere close to that torque peak unless you are completely out of your mind, so the car will be a bit lethargic at low speeds, and especially off the line.
Engines that offer a large surplus of torque over horsepower will offer a good deal of lunge off the line, but then die when really pressed. When driving behind such an engine, you'll tend to feel that the first half of accelerator travel is the good half, and the second half won't do much more for you, since you'll need to shift before anything really cool happens in terms of "winding it out" acceleration.
Think diesels, including the upcoming spate of turbo diesels that we'll be getting to drive soon in the U.S. (now available elsewhere).
"Balanced" engines tend to have a really good first half, followed by an even better second half. In my opinion, these are the most rewarding to drive.
There are exceptions to this "balanced power rule", but only if the engine in question makes a good deal of whatever it's comparatively short of. As a for instance, the latest Ferrari (the 599) makes a bunch more power than torque, but it makes a *lot* of torque (448 pound feet, I think), combined with really excessive horsepower.
Another example is any one of the current "65" Mercs. They make 738 pound feet of torque, which is pretty much a river of torque in any chassis they make - but they're way down on power - at only a little over 600.
How much torque is enough? Good question. My personal opinion is that, for a good and lively drive (not the same as flat out racing), you'll want no more than about 15 pounds of vehicle weight for each foot pound of torque, or something in that vicinity. Somewhere in that neighborhood is where cars can be "lazy fast". That is to say, such cars can be herded down the road with alacrity without having to wind them up using a bunch of throttle along with the resultant noise and unwanted attention. The driver will get a sense of effortless speed, which is never a bad thing.
Consider that Civic Si: Since it has to haul over 20 pounds around for every foot pound of torque, it'll never be lazy fast. Not to say that it isn't a quick car when you really want it to be, but you'll need to wind it way the hell up in order to get it to accelerate really well.
How does the new M3 stack up? Well, it's pretty horsepower centric, but on the other hand, it makes just enough torque to make it pretty fast in everyday driving. It only suffers a bit when you compare it to the 335 (or RS4) in that arena. Otherwise, good to go.
A note about torque rich, lower powered cars: Unless I'm overlooking something (quite possible),we only have one in the U.S. right now, which is the Mercedes Bluetec diesel. It's a fun drive with its seven-speed automatic, but I believe such cars will work a lot better with an auto than with a stick. With a stick, you'll get a series of truncated lunges at full power, and the driver will feel as if he's frantically batting the tachometer down with the shift lever. Just driving around, however, all that torque again makes these cars effortlessly fast. We're about to get a bunch of such cars starting next year, and it will be interesting to see how it works out.
Bruce