I will experiment more. weight does really mattter on our cars I think. the logger I am using is pretty accurate on times. I was getting very consistent 5.02 sec ~ 5.15 every day after the cam installation. I prolly use my 250lb friend later

 

yes, mate. weight does matter. How much depends on the initial weight of your car.

All things being equal, your acceleration is mainly down to power/weight ratio so if your car is a 1500lbs elise, your 110lbs friend will take a dent out of the power/weight. Say if the elise has 200bhp then we're going from 0.133 down to 0.124hp/lb.

If you're driving an M5 with 500bhp and about 4200lbs in weight, before and after your friend sits in the car will be 0.119 and 0.116hp/lb which is a lot less of a change.

Now people who drag race their car already know this, but incidentally, to counter the effect of your gf in the elise, you'll need more power, in this case around 15bhp more!!

I don't know the weight of my evo off the top of my head, but I'd say that your 110lb friend will cost you some acceleration (and braking and cornering) for sure. But 1/2 a second is an awful lot. I'd put it down to other factors also....

 

yeah half a second is a lot. 1-2 tenths, maybe. the new z06 vette out-accelerates the viper coupe with almost the exact same power numbers, but the new z06 is lighter and quicker. Statis vs. rotational is a good point as well. Pocket dataloggers do have some flaws, and other factors surely contributed.

 

HAHAHA . Warrtalon you make me laugh, that was funny . I was thinking the same thing.

 

I am begining to think that may be adding or subtracting the weight doesn't correlate to the speed linearly. maybe I hit the point of weak spot of that correlation. because adding like 40 lbs(full tank of gas) didn't do anything different. may be the engine favors certain weight ratio? I don't know, it is kinda worth the experiment. who knows? may be in the future, you have tune your car's weight also.

 

this guy is really small and super skinny. Nice guy though

 

This wasnt a street race But I tried passing a m3 with 3 of my friends in my car who are 160, 180, 210 and im 205. The m3 gunned it and I still kept up with him but after my friends were like "OOOO YOU GOT BEAT YOUR CAR SUCKS" and I was like yea with your fat asses in its slow compared to a m3 convertible with 1 person in it.

 

x838nwy is exactly right. Laws of physics, and they are laws, power to weight ratio will determine acceleration. Although the rotational mass vs. static mass will throw a monkey wrench in the equation, but rule of thumb is, every # of rotational mass lost is worth 4#'s of static mass. So just convert everything to static mass and use the following for an idea that is EVO specific.....

In warrtalons case, he plus car weighs 3400#'s and he makes 294 awhp. A power to weight ratio of (actually a weight to power ratio as most manufacturers report it) of 11.56. That's whp, not crank, so again don't get too caught up in manufaturers numbers, this works for demonstration purposes.

For warrtalon to accelerate at the same rate with a 110# sissy sitting next to him, he'd have to add 110/11.56, or 9.5 whp. Not much, but significant.

And if you do the math, .5 sec at 100 mph is 146.6 ft, or about 7 car lengths.

 

Who does third gear pulls with the groceries in the trunk? For me, that's up there with saying the car feels slower when their is a baby in a car seat in the front.

 

If you care about your girlfriend you need to get that girl some eating disorder therapy. She would have to be 4'10 to barely squeek by in the normal range for her BMI. Then again maybe she is 4'10, but a lot of girls will lie about their weight so who really knows right?

Back to topic, you can feel some acceleration due to weightloss, but not much. I stripped out my old 85 mr2 and it seemed like i felt a little difference. I'm sure there was a little, but usually not much to justify ripping your interior apart. WHen i put the batery in the front trunk i felt a huge difference in handling though. 40 lbs weight transfer on a 2200 lbs car is noticable.

Taenaive: According to the rule of thumb(.1 sec for every 100lbs) your passenger was acutally 500lbs. Sorry to break it to yah..

 

The thing about the rotating mass goes a bit like this;

The energy pumped out by your engine goes mainly into energy in various forms, most importantly kinetic energy and energy dissipated through wind resistance.

The translating mass (like the body of the car) and the rotating mass kinetic energy are actually the same thing. The first one you think of the whole body as a point mass flying through space and the other you think about how the elements of that body as they move with respect to the centre of mass.

If we just think of an alloy wheel, say 22lbs in weight and let's say its about 21" diameter (includes tyre), so m = 22lbs and I ~= 0.7kgm^2

If we're going at 100mph then that's: Moving mass ke = 9680J, Rotating mass ke = 9529J. The total energy stored will be the total of the two = 19209J

This is as much energy as 43.7lbs of non-rotating mass on the same car!!! And there are 4 wheels!! This is why your car will be slightly quicker with lighter wheels. Or more correctly wheels with less inertia.

Now, if you look at a 7lbs, 10" flywheel going at 4200rpm car at 100mph, you have about 12750J of energy which is equiv to about 29lbs on the same car.

So rotating mass is important. How important depends on how fast it rotates and generally its rotational inertia which depends on where the weight is. Weight in the centre results in less rotational intertia than the same mass at the edges.

 

Incidentally, rotational inertia is of the trasnmission is where the Sti has an advantage over the Evo. Its gearbox is simpler, has fewer bits and therefore accelerates (and decelerates) quicker.

Since, as we all know, the Sti is not as quick as the Evo, we can only conclude that something on the Sti sucks. I have yet to find it.

 

I had over 300lbs in my car because it was beener packed and my friend in his modded Talon was almost keeping up with me. I was like wtf!!! (Not trying to be racist by saying beener packed.)

 

Im suprised nobody mentioned the hot air. he said his worst run at lunch was a 5.28. So actually were looking at more like a .3-.4 depending on his best run. Add the extra weight combine with the heat maybe lose .1-.2 so that really only gives you maybe .1 off. Plus maybe that 5.58 run was the worst run he had yet. With only one run with the friend it a little hard to say that he couldn't have maybe run a 5.4 considering his morning runalso fluxed by.1. Does this sound reasonable?

 

i ran 12.48 with 380+whp w/methanol injection evo....u are such a good driver