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      01-04-2013, 03:39 PM   #28
bruce.augenstein@comcast.
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Quote:
Originally Posted by MKE_M3 View Post
There are engineering advantages (and some disadvantages, of course) to generating horse power at high RPMs, vs. low RPMs.

Consider two engines, both putting out 414HP, one puts it out at 8000RPM, the other at 4000RPM (think big turbo diesel). The first only makes around 250 lb-ft of torque, while the second makes around 546 lb-ft of torque. The drive train strength for the car making 250 ft-lbs is much lower than the needs of the engine producing 546 ft-lbs. This means that the high revving engine can be partnered with a lighter drive train.
There are some difficulties with this explanation. First, let me change the pairing. Let's use the current Corvette ZR1 engine paired against the four liter M3 engine.

The ZR1 makes 604(!) pound feet of torque, or a little more than double the M3 figure. That means the ZR1 clutch needs to be a bunch beefier than the M3 unit, and it also means the transmission input shaft needs to be very beefy, and/or made out of unobtanium, and/or heat treated to within an inch of its life.

However, consider that the first gear ratio of the ZR1 is at 2.29, vs the M3 ratio of 4.06. First, the torque capacity of a given transmission varies depending upon its first gear ratio - meaning that multiplication of torque is trying to tear the rest of the transmission apart. One of the ways you increase the torque capacity of a transmission is to numerically lower the gear ratios, and the Vette is a prime example of this. The Grand Sport's transmission is rated at around 480 pound feet of torque with its 2.95 first gear, but the ZR1 box (same case, gear sizes, etc.) is rated to take 600 foot pounds, in large measure because of its 2.29 first gear ratio.

Yes, there are other changes, but the ratio change is the biggest enabler. The Vette trans weighs around 220 pounds, while if memory serves, the M3 box comes in a a hair over 200, largely because it needs to be beefy to enable that aggressive first gear ratio.

Now, let's compare torque at the transmission output. In the Vette, it's a massive 1383 pound feet, but wait!, the M3 shows a not inconsequential 1198 pound feet (295 X 4.06), meaning the Vette is only showing about 15% more thrust at that junction. 15% means that the differential and half-shasts need to be beefed up, but not by a ton.

My overall point on this is that big torque requires less aggressive gearing, so driveline weights don't change very much. Consider the ZR1, with more than double the M's torque, only needs 15% beefier components, except for the transmission input.

Quote:
Originally Posted by MKE_M3 View Post
Additionally, less torque requires(generally) less displacement (or lack of hardware like super/turbo charger), which also saves weight.

Lighter is better for sports cars.
In general, I agree, but consider that the Corvette Grand Sport engine at 6.2 liters and 436 HP is slightly more compact than the M3 unit, and according to manufacturers' published weights, it's a tad lighter at 435 pounds vs 445 for the bimmer. In short, it's safer to go example by example instead of broad strokes.

Quote:
Originally Posted by MKE_M3 View Post
Additionally, there are gearing advantages to high revving engines, the car revving to 8000 RPM can get twice the mechanical advantage (half as tall of gears) as the low revving engine while covering the same speed in each gear.

Lower gearing is better for sports cars.
Mechanical advantage is essentially of no importance in this type of venue. It's power and weight that matter at any given instant. As an example, take an M3 at 8300 rpm, side by side with another identical car (save for engine and gearing) making an equal 414 HP, only at 4150 rpm. These cars will accelerate equally at that instant, with the low revving powerplant making twice the torque with half the gearing.

I do admit that the lower revving car would likely have an advantage at that point since it would likely have less rotational inertia, only needing half the rev increase for any given gain in speed, but I hope you get my point about horsepower and weight, regardless of gearing (i.e. - mechanical advantage).

Quote:
Originally Posted by MKE_M3 View Post
So if you like light cars that are aided by mechanical advantage, then high revving is for you
Most folks in this venue would tend to like light cars, but nobody directly cares about mechanical advantage. I don't think anybody objects to high revs, though, unless that characteristic robs low rpm power.

Bruce
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