I'm not completely ready for this weekend yet (of course...that would be too easy), but thanks to my friends Brad and Eric, I'm a lot closer.

As you can see, the car is painted and mostly re-assembled, which is good, and with their help we've managed to get it put back together enough I'm sure I can make it in time.

The paint job, not that anyone really cares, is excellent as long as you stay 20 feet or so away from it--just about right for a race-car, in other words. But, our reassembly work was not accomplished without a certain amount of amicable controversy...

The reassembly debate

During our somewhat frenzied wrenching on the car, we got into a which-is-better? debate. We race in a class organized around power-to-weight ratios. Weigh the car, put it on a dyno, do some division, either you're legal or you're not. In our case, we're shooting for no less than 14.5 pounds per horsepower.

What that means is if you have a 100hp engine, the combination of car and driver can weigh as little as 1,450 lbs. If you have 400hp, your car will need to weigh at least 5,800 lbs (or you'll have to move to a different class).

So, the question is, which should be faster on the track, more power or less weight?

Given otherwise equal cars and drivers with identical power-to-weight ratios, it seems to me the cars should be pretty much dead even on acceleration in the lower gears since that, after all, is what power-to-weight ratios are all about.

In the higher gears, like at the ends of the longest straights where aerodynamics come more into play, I expect the car with more power to have the edge as that's more about overcoming air friction than moving weight.

On the other hand, the lower weight should have an edge in braking and cornering because it has less weight to stop and turn. Personally, I would really like to think lower weight will result in an overall faster lap time but it became obvious last night we don't all agree on this.

So, I'm wondering if anybody knows the real answer (if there is one). Inquiring minds want to know.

1. Stan Rogers04/12/2008 05:55:08 PM
Homepage: http://stanrogers.blogspot.com

Well, you're right, Scott.

Lower-speed acceleration, braking and cornering are all about overcoming inertia, and horsepower can only get you past one of them. Once you get past a certain speed, drag (rather than rolling resistance or inertia) starts to become the limiting factor to speed (and thus to acceleration). How big a factor? Inertial resistance and mechanical friction are linear; drag increases with the cube of the speed, so it takes roughly twice the horsepower to go from 80 to 100mph looking only at the aero (mechanical would only require an extra 25%). To take a look at some closer-set numbers, you'd need nearly 16% more horsepower to make up the aero difference between 100mph and 105mph, and that would mean adding something in the neighborhood of 12-13% more lard to the all-up weight. (Remember that there's a baseline of power that isn't considered in the aero equation.)

So -- are the straights long enough and fast enough to make the top-end speed worth the weight penalty, or are the twisty bits twisty enough to take top priority, keeping in mind that a small power gain gets you a smaller gain in top speed? My racing experience may be out of date, but I remember a lot more time spent in reducing-radius and off-camber corners than on anything resembling the Mulsanne Straight. I don't think I'd add inertia everywhere on the track in order to get a couple of mph in the last eighty feet of one good, long straightaway. But that's just me.

2. Scott Good04/13/2008 07:36:36 PM
Homepage: http://www.scottgood.com

Stan,

It turns out, you are right on the mark (at least from the Imperical testing we did this weekend. See my post on 13 April for a few more details.

Scott