OldSStroker

Win what?

Best power to weight wins an acceleration contest, but higher hp should have a higher maximum speed.

In other words, car B gets up to it's maximum speed sooner, so it's ahead of car A at something around 170 mph. If it stays there, eventually car A, with 10 more hp should catch and pass car B...if the road is long enough and all else is equal.

Here's the rub...with 300 more pounds, all else is not equal. There is more rolling friction on the tires and bearings due to the 8.5% or so more weight. This throws a curve at us. With typical rolling resistance of street tires, the 10 extra hp just about overcomes the extra rolling resistance due to increased weight so top speeds aren't more than 1 mph or so different. So the heavier car (with higher hp) might actually be marginally slower just due to rolling resistance. In the real world nothing is simple!

By going to Bonneville tires, we might gain 8-9 mph with both combinations because the % of the total drag due to the rolling resistance drops from 25-30% to about half that.

OK, so now I've muddied up the whole problem. Sorry.

Practically, both cars will have the same top speed. Remember that if a Winston Cup car at Daytona can average ONE mph faster than the second place car for 500 miles, he'll lap the second place car, but it will take about 475 miles of the race to do so.

Is it time to move on yet?

95 Silver TA

Thats exactly what I wanted to hear
I Thought the lighter car would accelerate faster winning a 70 to 135MPH run as I orignally asked.

Thanx for all the input guys...Much Appreciated!
Claude

WS6 TA

Get a 3rd gen f-body. Even the worst had Cd’s in the low .3x range. The best (assorted firebirds/TA’s) had Cd’s in the .28-.29 range, and if you care to strip/fix some of the offending bits (antennas, the way some of the trim fit…) you can get them into the mid .2x range without much work.

WRT to the rest of this, assuming the same Cd and other drag being the same, the car with the higher power to weight ratio will accelerate faster, though the heavier car will be more stable and often have an easier time maintaining top speed. Some of the Bonneville cars find that higher weights can significantly increase top speed by keeping the tires in better contact with the surface.

Dr.Mudge

Yes, but the regular FRC (99/00) is slightly more aerodynamic because it doesn't have the brake ducting in back.

Chrisbequick

That wasn't the discussion. Plus, Chevrolet has built Hardtops since '99. They didn't stop production after the '00 model year. The only 'Vettes that come from the factory with the rear brake ducts are the Z06s.

One thing I'm curious about is why the Corvette team races hardtops in endurance racing if the the coupes have a lower CD. The weight can't be much different with the Lexan rear window.

-Chris

OldSStroker

With all the aero add-ons for downforce, I expect the C5R has a much higher Cd than the production C5. My guess is that the notchback allows more air to get to the wing.

Dr.Mudge

Vette hard top was CLEARLY compared to a Z06 as above, the standard Vette hardtop is the FRC made in 99 and 00, and yes the drag numbers are different.

Z28SORR

Chrisbequick,
They don't, the C5R's are fast backs not Hardtops. ie. they do not run the Z06.

jimlab

Just curious if anyone really thinks that the minor difference in coefficient of drag accounts for anything when the frontal area of the car is relatively unchanged? That's a serious question...

In my opinion, small changes in the drag coefficient mean almost nothing without a change in frontal area, which affects CdA, or the relative size of the "wall" (or traditional "brick") that you're trying to push through the air.

The base model (no spoilers) 3rd gen. RX-7 has a Cd of 0.29. The R1 model (front and rear spoilers) has a Cd of 0.31. Both have a frontal area of about 19.1 sq. ft. The difference is basically irrelevant where their quarter mile or top speed capabilities are concerned because the frontal area remains unchanged. 1 mph either way is not something to jump up and down about, in my opinion.

It's been proven that you can push a car that has roughly the aerodynamic equivalent of a barn door down the quarter mile at extreme speeds given enough horsepower. Worrying about changing aerodynamics when you can much more easily change weight or power is sort of silly. Unless you're heading for Muroc or Bonneville, and you're looking to break 250 mph, a difference in Cd of +/-0.03 is pretty much immaterial. Both the Corvette FRC/Z06 and Coupe can be pushed to well over 200 mph with enough power. What's the point in quibbling over a couple hundredths where Cd is concerned?

Yes, every little bit helps, but since most people don't have the ability to safely test the effects of changing the aerodynamics of a car, or the consistency to know if their changes are having the desired effect, it's sort of pointless to dwell on the subject. If you're not going fast enough, just make more power.

GordSipko

Finding more power isn't always an option. Sometimes better results are found by reducing the load that engine power must overcome.

A 10% reduction in frontal area results in a 10% reduction in drag. But it's difficult to change frontal area without massive body work, ie, top chops, sectioning etc.

A 10% reduction in Cd results in a 10% reduction in drag as well and might be easier to achieve. GM was able to reduce the Cd on their electric S-10 project from 0.44 to 0.34 ( a 22% reduction) by adding two belly pans, a cab to bed seal and a smaller front air inlet. These are very simple changes some of which can be applied to cars as well as trucks...

Every change counts!

Z28SORR

Also, not every one drag races. There are several races a year. where you can run unlimited speeds for 90m or more. The current record holder averaged 207.7801mph over the 90m course. See:"www.silverstateclassic.com".
That's what the ORR(Open Road Racing) in my sig. stands for.

arnie

"These are very simple changes some of which can be applied to cars as well as trucks..."

They (adding the belly pan) can be, but you wouldn't want to. I once thot like that. After a lengthy (and informative) thread a few months back, I learned the belly pan is great for a lower speed, high mpg/efficiency vehicle, but not for a vehicle attempting high speed use. Rather than smoothing the flow of air under the vehicle, you are better off (stability wise) to reduce lift by preventing, as much as possible, the (any) flow of air under the vehicle. How was the high speed handling characteristics described, when allowing smooth air flow under the vehicle, without the necessary downforce? Like pushing a bar of soap up the side of the bathtub with your foot?

NH-Z-28

One way to think about which Z28 is faster (A or B) is to use the old equation a lot of us learned in our physics class, F=MA. Assuming that the Cd is the same for both cars and that horsepower is directly proportional to force (which it isn't but if we assume it is it will make the following equations work a lot better) all other variables that make one car faster than another should cancel each other out and leave us with A=M/F. Where A is acceleration, M is mass, and F is force.

So, if car A is 3600 lbs and 410 horsepower, and car B is 3300 lbs and 400 horsepower, we will call 400 horsepower = 1 and 3300 lbs = 1. This gives:

Car B, Acceleration = 1/1, so for Car B it has a relative value for acceleration of 1

Car A has 9.1% more mass and 2.5% more force than Car B, so it's accelertaion equation looks like this:
Car B acceleration = 1.091/1.025, so car A has a relative value for acceleration of 1.064.

This means that Car B will accelerate faster than Car A, and will win the race.

What do you think?

GordSipko

My post at the start of this thread mentioned the VW Beetle, a classic upside-down wing design with a very full belly pan. Drag racers that were getting into the 130's with this design were becoming airborne all too frequently and with disasterous results.

Eliminating the flow under the car would be ideal, but it's very difficult to do. The current generation of F1 cars with high noses have profiles that are very flat on top and provide a longer flow path under the car exploiting downforce wing technology to it's fullest. In fact all high speed vehicles whether F1, Indy, Bonneville have a very smooth and streamlined bottom. So it must be worth the effort in spite of the side effects.

Minimizing drag and parasitic losses is still a very desireable objective whether they occur above or below the vehicle.

Z28SORR

GordSipko,

You are not wrong in what you have said about belly pans, just some what out of date.
Most high speed modern race cars do all they can to keep as much air as possible out from under the car. Take the corvette C-5R, it has a large front air dam and side extention that end in an air splitter. The front dam forces the air out to the sides of the car and the splitter keeps the high pressure air from curling under the car. Unfortunately some air still gets under the car and would indeed cause lift if the pan were flat from front to rear. But it isn't. It's flat under most of the midbody but opens up at the rear diffuser. Creating and inverted wing.
When Ferrari built the 360 they achieved something like 250 to 300 pounds of down force, at speed, eliminating the need for a rear wing. There is as much detail put into the bottom of an F-1 car as there is in the top.