I hope your mod list includes a 180 mph speedometer...

 

We are arguing from the same side, but you have to agree that formula is a far cry more accurate than guesstimations and brag analysis that exists elsewhere in this thread.

You are correct though, that a 100% accurate reading of speed can only be done externally. Sole consideration of mechical factors is inaccurate (but close enough for government work)

 

You're still only calculating gear-limited top speed, with a perfectly spherical tire, in a vacuum...

 

I have hit 170 in my 7 . I have intake, cb, dp, and PFC.

 

The drag resistance is the biggest factor. It takes alot of horsepower to over come it. I do not think it is linear or constant.

 

Drag is proportional to the square of the speed and the amount of power required to overcome that drag is proportional to the cube of the speed.

Fd = Cd * 0.5 * rho * A * V^2

P = Fd * V

Where:
P = Power in Watts (1 Watt = 0.00134 horsepower)
Fd = Force (drag) in Newtons (1 Newton = 0.2248 lbs.)
Cd = Coefficient of drag (0.29 for base/touring, 0.31 for R1)
rho = Density of the air ("Standard day" = 60°F, 29.92" barometric = 1.229 kg/cu.m)
A = Frontal area in square meters (FD = 19.2 sq. ft. = 1.78 sq.m )
V = Speed in m/sec (1 mph = 1.609 km/hr = 0.447 m/sec)

So, the horsepower required for a base FD to overcome air resistance at 60 mph (26.82 m/sec) is...

Fd = 0.29 * 0.5 * 1.229 * 1.78 * (26.82 * 26.82) = 228.17 Newtons = 51.29 lbs.
P = 228.17 * 26.82 = 6,119.5 Watts = 8.2 horsepower

At 120 mph (53.64 m/sec), the horsepower required is...

Fd = 0.29 * 0.5 * 1.229 * 1.78 * (53.64 * 53.64) = 912.68 Newtons = 205.17 lbs.
P = 912.68 * 53.64 = 48,956.2 Watts = 65.6 horsepower

But at 180 mph (80.46 m/sec)...

Fd = 0.29 * 0.5 * 1.229 * 1.78 * (80.46 * 80.46) = 2,053.52 Newtons = 461.63 lbs.
P = 2,053.52 * 80.46 = 165,226.2 Watts or 221.4 horsepower

In other words, at 180 mph, you're "wasting" 220+ horsepower just to overcome the resistance of the air.

 

BTW, the formulas above are for air resistance only. They do not account for rolling resistance or variation from a flat plane, and they assume a steady state (traveling at a fixed speed, no acceleration).

 

Hmm, I didn't realize that the R1 package increased the coefficient of drag, thanks Jim that's nice to know. BTW, what is your occupation (I'm guessing engineering of some sort)?

-s-

 

I hit 170 on my 7 too. with ecu, stage1 bnr, intake, with boost of 0.9kg/cm^2 and it was still going.....

 

Come to Detroit and I'll show you...I hit 155 all the time. That's the fastest I've gone in my car, so that's why it sticks out even more. According to the recording your speed change through 1000rpm's...I came up with this:
5th gear-
2000rpm=80km/h
3000rpm=120km/h
8500rpm=340km/h
340km/h=212.5mph

Of course, this is considering the drag is the same on my car from 3000rpm to 8500rpm...Which in this world isn't necessarily true. I think my car will top out just under 200mph. Although, I would love to hit 200mph!!!

Craig

 

page 114 of the MAzda RX7 book by John Matras :
..."Motor Trend tested top speed of the fivesome-yadda yada yadda...the rx7 came in at 163mph in 4th place"
this was a stock fd so the 150mph with enough road argument is a mute point..the fd 7 with enough HP will easily top that figure since the speed was HP and not red line limited

 

BTW the other cars were the NSX, vette, 3000gt vr4 and 300zx..top was the vette at 172.2mph..however at the race track(WIllow Springs) the 7 came in 1st around the track and the 300zx in last place..2nd place was the vette with more than a second behind...

 

where that video of that guy going 180 though the tunnels?

 

An indicated 165mph on my stock R1 FD, stock tires as well. This probably translated to an actual speed of somewhere around 161-163mph as the tire expands slightly due to heat from friction.

Jim that is a good formula to use. I think the single biggest variable in that formula is temp/pressure. Nearly everyone gets a higher top speed in "hotter" air. My run was done at an undisclosed location in Upstate NY 3 years ago, mid August with temps that i believe to be somewhere around low 80's/high 70's with very little humidity.

 

Is this the article you're referring to?
http://www.scuderiaciriani.com/rx7/m...article01.html

Regardless, the "with enough runway" comment was made in reference to the earlier claim of an FD exceeding 150 on the back straight of VIR only running 10 psi. With a blown engine, no less.

I'm well aware that a stock FD can exceed 150 given enough highway, but it won't happen in only half a mile or so.

 

Yep, unfortunately air density is a fairly complex calculation by itself, so to avoid confusion I chose to use "standard day" for the illustration.

 

Nothing is free. More downforce = more drag.

 

Why would downforce affect coefficient of friction??

Example: Wind Tunnel Testing:

Would COF (drag) numbers change (increase) IF downforce was applied to a stationary "tested vehicle"?? I don't think so. It might even lower the COF as the vehicle is lowered (via downforce) onto the pavement??

Kinda what you get with Indy/Cart type vehicles.

Take a look at this link on Downforce vs. COF. (drag) Pretty interesting findings.
http://www.racingarticles.com/article_racing-3.html

 

Don't confuse aero forces with tire forces. More downforce will certainly bring increased cF to the tire contact patches but at the same time it increases aero drag on the car and thus slows it top speed and its ability to accelerate. The reason high downforce vehicles work in racing is because they can use the extra cF to corner at much higher speeds, even though their increased aero drag leaves them slower on the straights. There is always a balance to be struck between getting increased grip at the expense of drag and acceleration.

 

Jimlab very good formla also damonb good point.
fastcat17 you said you hit 170 did the car start to lift?Or swerve?Did you have any wings of diffuser's

 

i hit 165mph

 

The variances introduced based on ride height, tire size/offset, and various other irregularities in the laminar flow over the vehicle are far more complicated than can be described with a linear coefficient of drag.

We have computers that churn on this kind of crap for hours only to approximate our vehicle as a lump in a fluid rather than a car on a highway.

Mirrors especially create vacuum bubbles behind their flat trailing surfaces - cavitation creates turbulence and the turbulence causes a LOT of drag.

The power required to maintain a given velocity considering these factors is probably a lot higher than the 200 or so horsepower quoted, not even considering rolling resistance.

However, the variances introduced from air density due to temperature and humidity changes are very small. You'd be better off using the lower temperature air to assist your power delivery with your intercooler than worrying about air density.

Before any of you go out tonight with a GPS and try to set top speed records, keep in mind that

a) Tires and wheel bearings, among other things, fail VERY quickly at high speeds, and when they do, the results are usually fatal

b) The FD is a very nicely-shaped lifting body. Any bit of high-velocity air under the car and you're now driving a wing.

 

What will we have to do to keep wheel bearings intact?What is the best tire for a high rate for speed?
Did anyone watch that vid of the Fd at the salts?That guy turned a little bit and the car got off the ground really quick.I think he hit 210mph.

 

No the car handled very good i was very surprised. No wing or diffuser.

 

u will have to do nothing to keep the wheel bearing intact providing they are up to spec(packed right with good grease)..they will be just fine....I am currently in Germany and they drive very fast her..Italy too...there would be a speed limit set at XXX Km/h if at 250km/h(bout 155mph) wheel bearings would look up like a motor with no oil....that was a bunch of BS...Porsche would have to limit all their cars to 100mph if that were true...I have driven 150+ and was still passed by Porsches liek I was standing still and I know that he was not driving at that speed for just a few minutes.more like hours..as far as tires go you will need at LEAST Z rated tires....I would go with W rated myself...and I do...have W on my car right now...as far as taking off like in that movie-dont go 200mph and you will be fine...i can drive as fast as I want to and don't care to drive much faster than 150...100mph seems to be cruising speed on the bahn with a few people driving faster and some slower...after 155 or so all BWM's are falling behind because they are limited to about that speed(250km/h)..anyway..good luck and don't drive too fast