For those of you with heavier cars (read: anything over 3600 lbs), you have undoubtedly been told that putting your engine combo into a lighter car will make you go faster. I am told this by people on a daily basis, and by gosh, they are right! Unfortunately though, they are missing the point. My car tips the scales at 3950 lbs on race day with me in it. Moving that much weight around takes a lot of power... I calculate 401 rwhp from my 109.21 mph trap speed using the popular formula :
hp = weight * (trap speed / 234)3
Lets side step here for a moment. I am the first to be skeptical about finding real world horsepower numbers from a formula that only takes in two variables. I trust this formula in particular as I have made dyno runs that were within 1hp of the number predicted from the formula with three different engine combos. I can't vouch for it's accuracy on anything above 500hp or below 350hp, but in my experience it is very accurate within that range.
Ok, back to the discussion at hand. Light cars are great for going fast at the track, but you and I are driving our cars on the street for the other five days per week. Driving on the street means having a spare tire, floor jack, tools, book bag/briefcase, girlfriend, and maybe even more riding around with you. In a lighter car, this has a devastating effect on performance. Here's an example:
It's Saturday night and you are cruising around with four other friends in your 4000 lb Caprice. The Caprice has a hot motor and is running 109 mph in the quarter (roughly a 12.30) with just you in it. So, 4000 lb Caprice + 4 people (750 lbs) + spare tire, jack, and misc (100lbs) = 4850 lbs total.
You pull up next to a Honda in the same prediciment. The Honda weighs 2500 lbs and also goes 109 mph in the quarter (only with Nitrous, right?). Not bad, he is putting 253 hp to the wheels. However, tonight he is driving his buddies around as well and is carrying and additional 850 lbs around, just like yourself in the Caprice. That gives us 3350 lbs total.
Goaded on by your friends, the two of you line up for a race. This should be good... both cars went the same in the quarter. Let's see who won.
Rearranging the above formula and solving for mph, we come up with 102.2 mph, or about a 13.35 for the Caprice. Not bad for carrying around all your buddies and assorted other junk... that had to have been a pretty exciting ride for them! Now the Honda. Again, solving for mph we arrive at 98.9 mph or about a 13.80. Thats 3.3 mph and almost one half second slower in the quarter. Assuming equal skill at driving and equal traction, yourself in the Caprice came out the victor by more than a few car lengths. I bet your passengers were a lot more comfortable along the ride as well!
As you can see, going into the scenario with the same track times, the loaded Honda went a lot slower than the Caprice with the same added weight. The ratio of weight added to the Caprice, 850 / 4000 = 21.25% was signifigantly less than that added to the Honda, 850 / 2500 = 34%, thus giving the heavier car a performance advantage. Some of you may cite this as an extreme case, but it proves the point well. At the same time, I've noticed that I am more likely to race someone when I have other people in the car egging me on. The peer pressure from others in the car is definately a factor in racing, and in this case that peer pressure is adding a lot of weight.
So yes, it does take more power to get a heavier car going fast, but once you are there, it is really tough to make it go slow again. I am not even getting into the added benefits of having a full frame, large engine compartment and truck, added legroom, hauling abilities, intimidation factor, enormous back seat, etc!