Twins vs Genesis Coupe from the perspective of GC owners

[nalc]

Originally Posted by HunterGreene

Correct me if I am wrong, but these are the primary factors affecting a top speed measurement for any particular car (in no particular order):

1) Altitude (thinner air = less resistance but possibly less power)
2) Engine power
3) Humidity
4) Road surface

Take, for example, the same car's top speed tested on a runway (smooth tarmac) vs. a dry lake bed or salt flats. The speed on the runway will be higher simply because there is less rolling resistance on the car. Throw in any of the other above factors, and you will start to see more differences.

I don't disagree, but the point I'm making is that if the car top speed is limited by the aerodynamic drag on the body, reducing the relative airspeed of the body will increase top speed correspondingly, which is the claim for the twins. If rolling resistance or gearing is the limiting factor, then the relative airspeed won't really matter. I'm not really sure how much rolling resistance varies as a function of speed.

Essentially, if you plot a graph that shows power as a function of ground speed, you could plot six humps on it for available engine power in each gear, each with a starting point (idle) and an ending point (redline). Then you could create a curve of the maximum in any gear, going from idle in 1st to redline in 6th, and make an power available curve, which would be a flat squiggly line.

If you then draw a line for rolling resistance, I am not sure what it would look like. I assume it would be relatively linear.

If you draw another line for drag, it will look like an exponential growth.

If you add the rolling resistance and drag values together, you can plot a curve of power required, and wherever that crosses your power available curve, that's your top speed.

If you're limited by gearing, these lines will never cross, and your top speed will be limited by the endpoint of the power available curve (redline in 6th gear).

If you're limited by rolling resistance and drag, they'll cross at some point before the end point. If you decrease the airspeed of the car but retain the same ground speed, your drag curve moves to the right, and unless there's a significant increase in rolling resistance at high speeds, your top speed will increase correspondingly.

[Fett4Real]

a 4mph tail wind while going 55 mph in the same direction would no longer be a tail wind. Airspeed vs ground speed for example in a aircraft, if your flying into the wind your airspeed will increase by the airspeed vs the ground speed because of the pressure difference the pitot tubes see...when you have a tail wind effectively you are going slower giving you less drag but also less lift, with a ground vehicle, you will have less drag with a tail wind but once you overcome the windspeed then you will have drag again.

[autobrz]

On my way to work today, there was a blue GC3.8 right beside me. The middle-aged guy must have got excited in the presence of a brz, he went pretty fast through an s-curve, I followed his speed but it was faster than I expected from him and the car. What's up with old guys and going fast, lol... So yeah, it appeared to be pretty good around corners, too. They have my respect.

[HunterGreene]

Originally Posted by nalc

I don't disagree, but the point I'm making is that if the car top speed is limited by the aerodynamic drag on the body, reducing the relative airspeed of the body will increase top speed correspondingly, which is the claim for the twins. If rolling resistance or gearing is the limiting factor, then the relative airspeed won't really matter. I'm not really sure how much rolling resistance varies as a function of speed.

Essentially, if you plot a graph that shows power as a function of ground speed, you could plot six humps on it for available engine power in each gear, each with a starting point (idle) and an ending point (redline). Then you could create a curve of the maximum in any gear, going from idle in 1st to redline in 6th, and make an power available curve, which would be a flat squiggly line.

If you then draw a line for rolling resistance, I am not sure what it would look like. I assume it would be relatively linear.

If you draw another line for drag, it will look like an exponential growth.

If you add the rolling resistance and drag values together, you can plot a curve of power required, and wherever that crosses your power available curve, that's your top speed.

If you're limited by gearing, these lines will never cross, and your top speed will be limited by the endpoint of the power available curve (redline in 6th gear).

If you're limited by rolling resistance and drag, they'll cross at some point before the end point. If you decrease the airspeed of the car but retain the same ground speed, your drag curve moves to the right, and unless there's a significant increase in rolling resistance at high speeds, your top speed will increase correspondingly.

Sounds reasonable to me

Originally Posted by Fett4Real

a 4mph tail wind while going 55 mph in the same direction would no longer be a tail wind. Airspeed vs ground speed for example in a aircraft, if your flying into the wind your airspeed will increase by the airspeed vs the ground speed because of the pressure difference the pitot tubes see...when you have a tail wind effectively you are going slower giving you less drag but also less lift, with a ground vehicle, you will have less drag with a tail wind but once you overcome the windspeed then you will have drag again.

But instead of a 55 mph speed relative to the air, you are going 51 mph. Aerodynamically, the car will act as if it is moving through 51 mph of air speed vs. 55.

Best example I can think of is if you have ever flown west to east, specifically coast-to-coast (or close to it) The flight west to east is usually quicker than the trip east to west, because the predominant winds in this area of the hemisphere are usually west -> east. Therefore the plane can travel more efficiently and make better time. The same types of rules apply to our cars as well, but are obviously much more variable.

[CSG Mike]

Originally Posted by Celica00

I agree. Just didn't think a 4 mph wind would yield the equal amount of speed towards the car especially considering the odds that the wind was going the exact direction of the car and etc. but what do I know, I haven't actually engaged in an physical test regarding the subject.

This is precisely why Bonneville salt flat records require cars to do two runs in opposing directions, back to back.

[CSG Mike]

Originally Posted by HunterGreene

Correct me if I am wrong, but these are the primary factors affecting a top speed measurement for any particular car (in no particular order):

1) Altitude (thinner air = less resistance but possibly less power)
2) Engine power
3) Humidity
4) Road surface

Take, for example, the same car's top speed tested on a runway (smooth tarmac) vs. a dry lake bed or salt flats. The speed on the runway will be higher simply because there is less rolling resistance on the car. Throw in any of the other above factors, and you will start to see more differences.

From a post I made years ago on a different forum... the number are obviously different for our car, but the math is the same.

http://www.myg37.com/forums/2986820-post119.html

Drag: (FD)= CdqA = (Cd(1/2) r V2)A

Where:

Cd = Coefficient of Drag is a reflection of the aerodynamic efficiency of the shape
q = Dynamic Pressure in the Test Section
r = Mass Density of Air
A = Frontal Area of Vehicle
V = Velocity


Aero Horse Power: Horsepower required to move the shape through the air at a specified velocity.

1HP = 745.7w

P = F · V
P = (1/2)(rho)(V³)(A)(Cd)


Where,
Cd = Coefficient of Drag is a reflection of the aerodynamic efficiency of the shape = 0.29 (published figure, includes rolling resistance)
V = Velocity in m/s
A = Frontal Area of Vehicle = (roughly) Length x width * .85 = 2.53847234 m^2 * .85 = 2.15770149 m^2
rho = Mass Density of Air = 1.2kg/m3

Width (in.) 71.8; Height (in.) 54.8

Using the formula above:

1HP = 745.7w

P = (1/2)(rho)(V³)(A)(Cd)
(330hp)(745.7w/hp) = (0.5)(1.2)(V³)(2.15770149m²)(0.29)

[(330)(745.7)]/[(0.5)(1.2)(2.15770149)(0.29)] = V³
[246081]/[0.37544] = V³
655446.942 = V³
V = (655446.942)^1/3
V = 86.8652047 m/s = 194.311929 mph
V = 190 mph (using significant figures)

[CSG Mike]

Lets do the math for our cars.

Obviously, the math doesn't factor in EVERYTHING, like rolling resistance, tire deflection, etc.

This also assumes peak output at ANY rpm, which obviously isn't the case.

P = (1/2)(rho)(V³)(A)(Cd)
(200hp)(745.7w/hp) = (0.5)(1.2)(V³)(2.15770149m²)(0.27)

[(200)(745.7)]/[(0.5)(1.2)(2.15770149)(0.27)] = V³
[149140]/[0.3495476] = V³
426665.78= V³
V = (426665.78)^1/3
V = 75.2828301614 m/s = 168.403 mph
V = 200 mph (using significant figures, but you get the idea...)

[ZDan]

Regarding how much a tailwind helps, it's actually a *little* different for a wheel driven car on the ground vs. a plane. While the tailwind does cut down on aero drag at a given road speed, you won't *quite* get a 1:1 increase in speed, because you're having to spin the tires faster, which increases power required at the increased speed, even if the total drag load is the same. Power required will vary with apparent wind speed squared multiplied by actual road speed. If max road speed is 145mph with no wind, then with a 4mph tailwind it will end up being a bit under 148mph, not 149, at the same power level.

A minor point...

[ZDan]

Originally Posted by CSG Mike

[(200)(745.7)]/[(0.5)(1.2)(2.15770149)(0.27)] = V³
[149140]/[0.3495476] = V³
426665.78= V³
V = (426665.78)^1/3
V = 75.2828301614 m/s = 168.403 mph

The power figure you want to use is the rwhp, ~173. Using this and accounting for rolling resistance, I get 146mph as the theoretical maximum for the FR-S/BRZ.

[CSG Mike]

Originally Posted by ZDan

The power figure you want to use is the rwhp, ~173. Using this and accounting for rolling resistance, I get 146mph as the theoretical maximum for the FR-S/BRZ.

Correct! I was just posting for the sake of argument...

[FRSBRZGT86FAN]

Originally Posted by CSG Mike

This is precisely why Bonneville salt flat records require cars to do two runs in opposing directions, back to back.

Not only bonneville, even the Guinness book of world records for top speed runs with the Veyron, Ultimate Aero, Venom etc. I think its standard procedure

[Celica00]

Originally Posted by FRSBRZGT86FAN

Not only bonneville, even the Guinness book of world records for top speed runs with the Veyron, Ultimate Aero, Venom etc. I think its standard procedure

He might have (shot in the dark) mentioned te bonneville because I'm in utah. But yeah standard practice for accurate recordings

[Rayme]

This thread has now too much science and not enough emotions.

[Brzetto]

Originally Posted by fatoni

well 230 kph is about 143 mph so a four mph breeze explains that perfectly

Did you watch the whole video? It went to 235 km/h which is 146 mph. Yeah right a breeze when the trees aren't even moving at the beginning of the video. Hasn't your bullshit shovel rusted by now? The car does 146 mph in stock form, deal with it.