DSM at 180mph

[igor@af]

I don't know man .. maybe the 400-500 lb weight did act as a major factor. I am going scientific here.

[carrrnuttt]

Originally posted by igor@af
I don't know man .. maybe the 400-500 lb weight did act as a major factor. I am going scientific here.

Nah. I doubt it.

Take the case of a GS-R motor in a 2200lb hatch.

Take that same motor in a.........GS-R.

Even though the hatch might get there a bit faster, since it's lighter(weight is a MAJOR factor in acceleration), if they have the exact same mods, tune and wheel sizes, both cars will top out at the exact same spot and stay there. This is with the Integra weighing in at about 400-500lbs more than the hatch.

This is because they have similar frontal areas, and similar power. We are talking about top-end here.

Think gravity. We were all taught in elementary school that two objects, no matter how different their masses are, will always have the EXACT same acceleration when falling, with wind resistance the ONLY variable.

Example: a marble and a bowling ball. Even though the other is much heavier, when dropped, they will accelerate similarly.

Why? Due to their shape, they have similar wind resistance, regardless of their size difference. Because the force causing their acceleration towards the ground is constant and equal(gravity).

Now, if you change the force between them...say you drop the marble from the exact same distance, at the exact same time, on the moon, and the bowling ball on earth.

This configuration is more reminiscent of the top speed factor. The marble, which is lighter, and has no air resistance to speak of, is going to drop a lot slower than the heavier, atmosphere-resisted ball, because, as we all know, the power behind(or underneath) the bowling ball is greater.






There.

[SkylineUSA]

Theory is great, you can sit there and talk about it all day long

Screw theory, Like Nike say's just do it

[davewin]

In a nutshell using those numbers I get 255 hp needed to go 180 mph. But more realistically you need about 300 hp at the wheels. The long story is below.

Vehicle drag is slightly dependent on weight (because of rolling resistance) and independent of gearing. It simply takes a certain amount of power to move a vehicle a certain speed, regardless of gearing.

Let me explain.

There are two parts to figuring out how much power it takes to move a car a certain speed. Here's how you calculate the drag force (using metric units).


========================================
1. Aerodynamic drag (call it D)
========================================

Assuming ambient air temperature (and therefore density) at about 77 F (25 degrees C) and no wind the equation is then

D = 0.5 x Air Density x Cd x Frontal Area x (Velocity^2)

Air Density at ambient is 1.184 kg/m^3

So D = 0.592 x Cd x Area x (Velocity^2)

========================================
2. Rolling resistance (call it R)
========================================

Assuming passenger car tires at 30 psi on concrete.

R = fr x Weight

fr depends on velocity and is calculated with this equation, which was determined from experiments at the Institute of Technology of Stuttgart:

fr = fzero + 3.24 x fs x (Velocity/100)^2.5

From a chart in my textbook fzero and fs are roughly 0.01 and 0.005. So all you have to do is plug in your velocity, frontal area, and drag coefficient and you'll know how much power it THEORETICALLY takes.

So fr = 0.01 + 0.0162 x (Velocity/100)^2.5

Now, different cars are limited by different factors. Some are drag limited, some are gear limited. This equation simply tells you how much power it would take a vehicle to go a certain speed. If the engine and gearing in that car don't make enough power at that speed, then the car is gearing limited.

Assuming the DSM car has a Cd of 0.29 and Frontal area of 20.4 square feet (which is equal to 1.895 m^2) and weighs 3,000 lbs (13,351 N at sea level) then total drag at 180 mph (which is 80.5 m/s) is:

F = [0.592 x 0.29 x 1.895 x 80.5^2] + [(0.01 + 0.0162 x (80.5/100)^2.5) x 13,351] = 2,108 + 259 = 2,367 N (N is Newtons)

========================================
To calculate Power
========================================

Power is Force x Velocity

W = F x v = 2,367 x 80.5 = 190,544 Watts = 190.5 kW (kiloWatts)

1 kW = 1.34 hp so at 180 mph you need 255 hp

Which is all fine and dandy but this is under ideal conditions based on assumptions of drag coefficient, frontal area, tire drag and such.

If I'm not mistaken, drag figures are calculated from wind tunnel models without side mirrors, gaps in the windows and body panels, and smooth wheel covers. These things all contribute to Cd and can increase frontal area. So I think a more realistic Cd estimate for a real world DSM would be more like 0.35 which would bump the HP needed up to around 300. That seems more reasonable.

Most of the drag on a car is the aero, almost 10 times as much as rolling resistance. Hope this clears things up a bit. By the way, these figures are from a textbook called "Fundamentals of Vehicle Dynamics" by Thomas D. Gillespie.

[carrrnuttt]

Nice write-up.


There is rolling resistance, where a heavier body will always exert more stress on it's moving parts, therefore increasing the friction resisting against it's movement (lube up!).

But, just like you said...and especially with modern automotive designs...the impact to ultimate top-speed is minimal, around ten-percent.

The biggest enemy at those speeds is atmosphere. If you have the power to push it despite your frontal area, and have the gearing to take you there, then you're set.

Also, the 300whp claim you made is quite valid. The Civic in my example had over 270 to the wheels when it made it's 186mph run. That's with some weight reduction, and major aerodynamic enhancements.

The heavier, less aerodynamic Eclipse would need at least that.

[davewin]

Thanks for the compliments.....

You went 186 in a Civic?! Wow. Balls out!

I think a Civic has less frontal area than a DSM... do you know what your Cd is?

Hey, here's an idea. If there's enough interest I'll try to put together a script to calculate this so that we can run "what if" scenarios quickly.

What do you all think?

[2of9]

the vid that i put up, im not even sure if that was a DSM. Like wat Flywsi said too. if u want weight reduction for sure if u wanna go 180+, u gotta have down force....then the math part comes i think. Even with 250 some hp, u wont hit 180. im GUESSING around the 350 to 450 hp range, and DSM that can hit 11s could do 180....I THINK

[SkylineUSA]

davewin,

Very good post.

I like your idea, as well

[carrrnuttt]

Originally posted by davewin
Thanks for the compliments.....

You went 186 in a Civic?! Wow. Balls out!

I think a Civic has less frontal area than a DSM... do you know what your Cd is?

Hey, here's an idea. If there's enough interest I'll try to put together a script to calculate this so that we can run "what if" scenarios quickly.

What do you all think?

LOL. I wish.

Nah...in a previous post, I used the Vortech Civic that went 186mph on its first motor as an example, then 204mph after they rebuilt it.

Fastest I've ever gone is 159-160 bouncing off the rev-limiter in a Chevy Beretta of all things. The speed was confirmed by my ex-roommate pacing me with his GSX-R 750 on the highway to California just right before Blythe.

I did go close to that in a 1990 Integra with a modified JDM-spec motor, keeping up with an old co-workers YZF. He claimed that he only pulled-away after he hit 160mph (he didn't downshift). I can't confirm that though, so I can't really claim it.

[tino]

a stock awd dsm can go 160 with 210 hp

[carrrnuttt]

Originally posted by tino
a stock awd dsm can go 160 with 210 hp

And a lighter, more powerful SRT-4 can only go to 153MPH radar-verified?

AWD DSM:
3200lbs/210HP to the crank

SRT-4:
2800lbs/220 to the wheels

[davewin]

Weight effects on drag are negligible.

What's important is to know the actual frontal area and drag coefficient of each car, as well as power to the wheels.

And remember that every manufacturer tends to be optimistic on their numbers.

[carrrnuttt]

Originally posted by davewin
Weight effects on drag are negligible.

What's important is to know the actual frontal area and drag coefficient of each car, as well as power to the wheels.

And remember that every manufacturer tends to be optimistic on their numbers.

True. I even said so myself above.

But, it doesn't deny the fact that the SRT-4 has power over-all, stock-for-stock, and has less rolling resistance, since it only has to power two wheels.

His claims are for a stock car, plus I doubt if the SRT's CD is that much worse than the Eclipse's, if at all.

[davewin]

Yeah, I would guess an SRT and the DSM are not all too different in Cd and frontal area. DSM might have slightly less frontal because it's lower (I think) though it seems a bit wider.

Here's another curiosity. What would the true power difference between the 2 cars be if they were dynoed on the same machine under the same conditions?

Does anyone know of a list of Cd and area figures for street cars? I know there's one that covers some sports racing cars and a few sports cars (you'd be surprised how many generate lift instead of downforce, ahem, 911), but I haven't seen one for most regular cars.

http://www.mulsannescorner.com/data.htm

[SkylineUSA]

I can get my GTR's speedo to read 200mph. Therefore I must be going 200mph. Speedos are very inaccurate. I have seen Ferrari's read as much a 30mph off at top speed. Speedo 199mph, actually 169mph.