View Single Post
      02-11-2012, 11:34 AM   #31
bruce.augenstein@comcast.
Colonel
99
Rep
2,000
Posts

Drives: 2017 C63
Join Date: Jul 2007
Location: Manheim, PA

iTrader: (0)

Quote:
Originally Posted by swartzentruber View Post
Mind giving more detail about what you mean by this? I assume you mean something about how the engine spins up quicker perhaps? Just curious about the explanation, and what has been engineered into the car for this.
There are at least two reasons why the four-liter V8 spins up so quickly. One is the multiple throttle bodies (often given all the credit for "throttle response"), and the other is low rotational inertia. In my opinion, part of the excellent "throttle response" is really a low overall rotational inertia issue, assisted by lightweight engine components, lightweight (or disconnected) accessory drives, reduced (by German standards) engine flywheel weight, and, since the transmission also has a bunch of rotational inertia in the lower gears, reduced weight of internal gear clusters, etc.

One reason the transmission can be engineered for lower rotational inertia is that the engine makes relatively low torque per cylinder (much lower than the 335is, for instance), and torque per cylinder is all the transmission needs to be designed to withstand. Overall torque is not an issue, since only one cylinder is firing at a time.

All this doesn't matter a bunch when you're running for red line in, say, fifth gear (because you're not picking up a lot of rpm per unit of time), but it matters a bunch in first and second - hence a positive effect on 0-60 times.

If you'd like to do a basic rotational inertia test your self, get an automotive G-meter (available from several vendors, sold to measure acceleration/deceleration figures such as 0-60, quarter-mile, etc.). Then on a nice warm day on a sun-warmed, high-traction surface, measure max acceleration (expressed as max G) in both first and second gear. You'll notice that the peak G number falls well short of what you'd expect in first gear, based on observed second-gear peak G numbers.

The best numbers I've ever gotten were on a stock (light flywheel) '85 Vette. That car had a first and second gear relationship of three to two, numerically speaking, so with a baseline max G of .5 (point five) in second gear, one would have an expectation of .75 (point seventy-five) in first, given good traction. Instead, what I got was .66 G max, or 88% of what I had expected.

That 12% shortfall was due almost entirely to rotational inertia. The engine, flywheel, clutch plate and disc, engine accessories and transmission input shaft, etc. all had to accelerate 50% more for each mph gained in first gear compared to second gear, and per Mr. Newton, they just don't like to do that.

I've tested at least a couple of dozen cars since then, and 12% was the smallest shortfall I've gotten. Most were in the lower 80s, percent/wise. My '95 M3 had a shortfall of 17.5%, as an example.

The big guns from Ford and Chevy will have a lot more rotational inertia than the M3, including a big chunk from the transmission, which has to be designed to absorb a bunch more torque per cylinder. Of course, first and second gears in those cars are a lot lower (numerically speaking) than the M3, so they will be able to absorb more torque per cylinder, everything else equal. Still, they have to be more massive than the bimmer box, which costs in terms of rotational inertia, along with massive pistons and rods, a more massive crankshaft, etc.

If you're interested in more detail on the topic, get a copy of Gillespie's book, "Fundamentals of Vehicle Dynamics". It's a worthwhile read, authored by an internationally famous engineering teacher, covering virtually all aspects of automotive performance dynamics.

Bruce

PS - If you do an M3 test, you're still likely to get a significant first-gear shortfall, because there is a large gap (around 40%) in gearing between first and second gears. Quite a bit bigger than the gaps in the Ford and Chevy. However, the test is only measuring the difference in rotational inertia between gears. Not the inertia values themselves.

PPS - Understand that I have no specific knowledge of the weight of various parts of the M3 engine and drive train. I just know that the M Group has worked very hard on reducing rotational inertia in this car, since even with its overall very aggressive gearing (thereby increasing rotational inertia) it still spins up like a maniac freezing ice cream.

Last edited by bruce.augenstein@comcast.; 02-11-2012 at 11:51 AM..
Appreciate 0