Does anyone else think the Enzo might be underrated in HP ?

If an Enzo needs 650hp to do 211, then it only needs about 560hp or so
at 201. The increase in hp required to increase the speed is roughly a cube function at these speeds, not a square function.
An increase from 201 to 211 is a an increase in velocity of about 5%. The increase in hp is therefore about 16% (1.05x1.05x1.05).
(As an aside, this shows why that kid in MN wasn't doing 205mph when he got that speeding ticket on his motorcycle. i.e. 180 mph is fast on a bike, but it would take almost 50% more hp to do 205mph than 180mph).

 

I don't think the Enzo is overrated. I think it's rated correctly (as opposed to most Ferrari engines which seem to be overrated). 650hp for a 220+mph top speed is very reasonable.

 

What kind of difference are you thinking about, 5-10hp; or something more, like the Ford GT, 50+hp?

When Auto Motor und Sport did their topspeed test, they achieved 355.1km/h (220mph), with a quoted 660hp (there seems to be some variation of quoted hp, some say its 650hp & others say 660hp, anyone know the reason for the discrepancy?), at Nardo test track in Italy. 660ish hp seems pretty accurate to me, considering the amount of downforce (and resulting drag) that car makes.

 

DIN versus SAE. 660 across the pond= 650 here.

 

The owners manual says that the NNO produces a maximum torque of 657.3 Nm at 5,500 rpms... i guess some folks didn't achieve the full torque quoted and others rounded off...

 

I wasn't specifically talking about the drag coefficient in relation to the downforce produced, you are right, they are not directly related. Drag coefficient is a constant for a certain shape, it is not affected by speed; however, that does not mean downforce does not produce its own drag. Downforce has its own penalties in drag which are separate from parasite drag (which is measured as coefficient of drag) but since this is not aircraft aerodynamics, and the Enzo has no wings, I'm not sure how induced drag (drag as a result of lift, i.e. downforce) affects the Enzo. I am almost positive that the Enzo's high downforce increases the amount of drag it produces at high speed, and lowers its topspeed significantly (it certainly increases rolling resistance, but that isn't a huge factor in comparison to aerodynamic drag). If someone who is more knowledgeable in automotive aerodynamics would like to step forward and explain, feel free.

 

100 times? Hardly.

Aerodynamic drag at 211 would only be about 10% more than the drag at 201.

 

I'm gonna talk to one of my Aero E friends and see if he can give me a straight answer (on the parasitic/induced drag issue).

 

Since aero drag is proportional to the square of the velocity (at least in this speed range),

at 211 MPH, the drag would be k(211)(211)
at 201 it would be k(201)(201)

So the ratio of the drag at the 2 speeds is k(211)(211) / k(201)(201) = 1.10
ie, the drag at 211 is 10% higher than the drag at 201

(k is just a proportionality factor which disappears in the ratio calculation)

 

I think you got the general idea but I would suggest a slightly different wording:

"square the actual numbers and take the RATIO as a percentage"

You're trying to figure out how much more the drag is at the higher speed compared to the lower speed, hence the ratio. eg, if it was twice the drag, the ratio would be 2 (ie, the higher drag is 200% of the lower drag or 100% more drag...)

 

1) Drag coefficient is not constant for a certain shape. See #2.
2) Drag coefficient is affected by speed. The parasitic drag coefficient is an inverse function of the Reynolds number.
3) Parasite and induced are both included in the drag coefficient.
4) Drag affects bodies creating downforce the same as it affects bodies creating lift. Turning the airfoil over does not magically give it "special" drag properties. The entire body of the Enzo (or any car) acts as a wing.

 

this requires way too much thinking for me....what happen to "my car can kick your cars butt"?

 

Lol, I'm sure the Enzo owner will still be able to say that to pretty much any car he sees.

 

Sorry, but couldn't this be a lateral-gee handling issue as well? The Enzo makes a ****eload of downforce, much more than those other two 'ordinary' cars.

Maybe it just stuck better and could carry more speed through the turns?

 

1) Drag coefficient is constant for a certain shape.
2) Drag coefficient does not increase with speed. Total drag increases with speed, but drag coefficient does not.

Drag coefficient (Cd) is the actual drag (D) divided by dynamic pressure (q) and representative area (A).

Cd= D/(q x A)

Thus as the speed increases, so does the dynamic pressure, which is wind speed (ft/sec) converted into the pressure (pounds per square inch) with respect to fluid density. Dynamic pressure and drag increase at the same rate, with the square of speed. Here is an example. Let's say a certain shape produces 5lbs of drag at 50mph, the dynamic pressure is 6.09lbs per square inch at 50mph, and the frontal area is 2 square feet.

Cd= 5lbs of drag/(6.09 x 2)=.41 so at 50mph the Cd=.41

Now, let's change the speed to 75mph, that will make the dynamic pressure 13.7lbs per square inch, and the drag will increase to 11.25lb, while the frontal area stays the same.

Cd=11.25/(13.7 x 2)=.41 so at 75mph the Cd remains .41

So no matter what the drag (D) value is, the Cd will still be the same.
If you want the formula for dynamic pressure (q), I can provide that.

If Cd varied with speed, manufactures would quote it for a certain speed, otherwise it would be meaningless to know the Cd.

The Reyonlds number measures the viscous qualities of a fluid (and relates to the air flow separation and skin friction), however it is not the only component of parasite drag.

That's enough for tonight, induced drag gets pretty technical.

 

Cd can and does change with velocity.

The skin friction component of Cd is 1.328/(Re)^.5 for a laminar boundary layer and .074/(Re)^.2 for a turbulent boundary layer. Re, Reynolds number, is a function of length, velocity, and kinematic viscosity. I'll leave it as an exercise to the reader to demonstrate that Cd also varies with velocity.
The other component of parasitic drag, form or pressure drag, also varies with velocity, but is far more complex.

A dynamic pressure of 13.7 psi interesting choice, since it is virtually unheard of in normal atmospheric conditions. It would correspond to a velocity of approximately 35000 mph at sea level.

Manufacturers should quote the Reynolds number when they give the drag coefficient for a vehicle, but unfortunately they dont. I've heard, but cannot confirm, that they're usually given for standard day conditions at sea level at freeway speeds.

Induced drag is relatively simple compared to parasitic drag. Just Cl^2/(e*pi*AR) where e is Oswald's efficiency factor and AR is the aspect ratio.

Information in this post was pulled from Introduction to Flight, Fifth Edition by John D. Anderson, Jr.




I want to make it known that this is not me writing, but a friend of mine.

 

13.7psi, that was my mistake, it should be 13.7 pounds per square foot, not inch.

Ok, a coefficient is this: A numerical measure of a physical or chemical property that is constant for a specified system.

Cd is not some gimmicky thing, it is a constant and it does not vary with speed, and there is a reason auto-makers and aircraft manufactures quote it.

 

You guys are making me remember why I hated fluid dynamics.

 

I am 90% positive they are quoting it for a specific speed. We don't know what that speed is though.

 

Some definitions for "coefficient":

This is a number that is multiplied times a variable.
library.thinkquest.org/C004647/util/glossary.html

A number being used to multiply a variable or power of a variable in an algebraic expression.
www.isbe.state.il.us/ils/glossary.html

A numerical or constant multiplier of the variable in an algebraic term. For example: 4x : coefficient is 4
www.bced.gov.bc.ca/irp/mathk7/appfa_e.htm

a number multiplying a variable in a mathematical expression or equation
dev1.epsb.edmonton.ab.ca/math14_Jim/math9/glossary/default.htm

A factor of the term. x is the coefficient in the term x(a + b) or 3 is the coefficient in the term 3y.
math.about.com/library/blc.htm

Constant? Sure, it could be, but it doesn't have to be.
In the case of drag, the coefficient is not constant during changes in velocity. You can demonstrate that to yourself using the skin friction drag coefficient equation.

I can scan and post some pages out of an Engineering Fluid Mechanics text showing plots of Cd vs. Re for a variety of shapes if you would like to see them.