OK, I know I must have too much time on my hands - actually I'm in full-blown avoidance of real work and responsibility - but my loss is your gain. Here's some fun facts about our Crossfires.

If you dropped your Crossfire from an altitude of greater than 3900 feet, and assuming the nose stayed pointed downward, it would reach a terminal velocity of 408mph at sea level. That's a fast Crossfire!

Drag - or air resistance - saps horsepower from making your car go faster, and uses it to force the air out of the way. So how fast - theoretically - could your Crossfire go? That depends on which XF you have, and if you've modded it or not.

Use this equation to figure out how much drag your car has at different speeds:

D = 0.002684 * Cf * Af * S^2

^2 indicates squared

Where D is the amount of drag in pounds of force. Cf is the Coefficient of Drag of the car. Af is the frontal area of the car in sq. ft. And S is the speed in miles per hour.

Hard-topped Crossfires have a Cf = 0.370 and Af = 20.56.
Convertible Crossfires have a Cf = 0.384 and Af = 20.62.

So when a hard-topped Crossfire cruises at 65 mph, the air around it is pulling backwards on the car with 86 pounds of force. Convertible @ 65 = 90 lbs.

That doesn't sound like much util you realize that air is in effect draining power from your ride. You can calulate power in horsepower. So how much is it loosing to the air? Hard-top @ 65: 15 whp, Convertible @ 65: 15.5 whp.

You can calculate the whp required to fight drag with this equation:

Pl = (0.05904 * Cf * Af * S^3) / 8250

^3 indicates cubed.

Where Pl is the whp required at a speed of S mph.

Here's a quick table:
_____________Hard-Top____________Convertible____
Speed_____Drag_____HP_Loss_____Drag_____HP_Loss
=========================================
10_________2.04______0.05________2.13______0.06
25________12.76______0.85_______13.28______0.89
55________61.76______9.06_______64.29______9.43

65________86.26_____14.95_______89.79_____15.56
75_______114.85_____22.97______119.54_____23.91
85_______147.52_____33.43______153.55_____34.80

100______204.18_____54.44______212.52_____56.66
125______319.03____106.33______332.06____110.67
150______459.40____183.73______478.17____191.24

160______522.69____222.99______544.05____232.10
170______590.07____267.46______614.19____278.39
180______661.53____317.49______688.57____330.47

190______737.08____373.40______767.20____388.66
200______816.71____435.52______850.09____453.32

As you can see, the horsepower requirements for keeping your car going increase dramatically as the speed goes up. Using stock whp measurements of the base model (~175) and SRT (~290), you can see that a Crossfire's top speed is physically limited by aerodynamics.

Speed limitations due to drag:
Base Hard Top: 147.6 mph
Base Convertible: 144.6 mph
SRT Hard Top: 174.7 mph
SRT Convertible: 172.3 mph

Now, if you really want to talk about fast, and join the 200+ mph club, you would need to be getting more than 435 horsepower at the wheels just to overcome drag at that speed. That's about 50% more power than a stock SRT has. I don't think anyone will be joining the 200+ mph club in a Crossfire any time soon.

Keep in mind that the above numbers do NOT account for the rolling resistance of your tires. So the theoretical top speeds of our cars would be even less than listed above.

 

Geeez ! You really do have too much time on your hands !

Just my Dos Centavos

 

It's amazing what I can do when I _really_ don't want to do something else.

 

Tom,

I may be wrong but I think you overlooked one possible variable. How much HP would the Crossfire require to achieve 200 mph if we had a draft vehicle? Now the question is where will we find a 200 mph tractor trailer. LOL

 

Let me know when you find that 200mph trailer. I definitely want to try that out!

I'm still trying to figure out tires and rolling resistance. I've found that the coefficient of rolling resistance for normal tires on concrete is about 0.010-0.015. But I can't find specific numbers on specific tires. I also don't know if this coefficient will work in the equations above or not. Probably not, but it would stand to reason that instead of frontal area of the vehicle that you would use the contact surface area of all four tires. But that's just conjecture. If anyone knows the real deal, let me know.

 

Speed limitations due to drag:
Base Hard Top: 147.6 mph
Base Convertible: 144.6 mph
SRT Hard Top: 174.7 mph
SRT Convertible: 172.3 mph

Keep in mind that the above numbers do NOT account for the rolling resistance of your tires. So the theoretical top speeds of our cars would be even less than listed above.[/quote]

Thanks a lot... now my Limited Roadster seems absolutely slow....

 

Tom here is a place to start http://www.bankspower.com/im_TDR_Feb03.cfm

 

or maybe a Ferrari.

 

Top end is higher then then 147.6 mph for the coupe, I have had my 04 to 150 mph on the spedo and 152 mph as verified by GPSr for over a distance of 25 miles on the Nevada border and I wasn't matted so I don't know where my car tops out. I don't think I am going to find out either because that little stunt left a lot of miro pits in my windshield and would have done the same paint if I didn't have the bra on the car at the time.

 

Well, if it makes you feel better the 6-speed manual transmission in the non-SRT has a redline of 6,000 rpm, and in top gear it would go 170 mph. The non-SRT 5-speed auto would go 171 mph.

Of course, an SRT has a redline of 6,400 rpm, so it would go 194 mph.

So, without re-gearing the car, nobody is gonna go 200+ mph in a Crossfire, unless they drop it off a 3900' tall cliff.

 

Well, all of these numbers change due to altitude. So you'll get some small advantage the higher you go up - less dense air = less drag = faster car. The irony of it is that the less dense air also gives you less horsepower. So I don't know where that balances out.

 

I believe for the SRT-6 you would likely want to use 348HP for calculations as that is the stated HP for the look alike MB AMG engine. I suspect it was an internal Daimler Chrysler political decision to assign a lower HP number to the "inexpensive" Chrysler SRT-6 engine. The company sure wouldn't want to lessen the preceived value of the SLK 32 AMG roadster by having a cheap Chrysler product with the same HP.

BTW, the normally aspirated SLK320 engine makes 215 HP according to MB. The same as the look alike XFire 3.2L NA engine.

 

You're right, 350 for the SRT and 215 for the base, but that is hp measured at the crank, not at the wheel. You have to factor in 17-18% drivetrain loss. I figured 18% to be conservative, which gives 287 for the SRT and 176 for the base.

 

HAHA yeah been there

 

Just curious......What do you do for work ??

 

are those #'s for the roadster with the top up or down??????????

 

The Chrysler specs don't say, but I think we can assume that it is with the top up. The coefficient of drag would be much higher with the top down.

 

Long story, but the short version is server administration and user support.

 

Ahhh, essentially a network administrator like me!
Man, you DO have waaaaaaay too much time on your hands! You gotta stop futzing around with this drag coefficient stuff & defrag something. Funny, this almost sounds like the paradox of trying to go faster than light: the closer you get to traveling at light speed the more the mass of the object grows so that you cannot theoretically attain the speed.
A question, Tom: as the car moves forward & the air is passing around & underneath it it should create a 'lift' similar to an airplane, hence the spoiler. I can only assume that the spoiler doesn't completely cancel out the 'lift factor' so how do you figure that into the equation? I would think that makes the tire resistance slightly less the faster one travels.

 

No problem, all we have to do is drop our Crossfires from 3900 feet.

Airplanes are in the same situation when it comes to flying faster. A good rule of thumb is that if you want to fly twice as fast, then you will have to quadruple the horse power. So a Cessna 150 with a 100 hp engine can maybe do 100 mph while a typical late WWII fighter needed a good 1500 - 1600 hp engine to hit 400 mph.