Why do cars seem to top out in speed around 200 MPH regardless of power?

Factual Questions
1

I am no motorsports aficionado but there is still the kid in me and I like looking at supercars on occasion.

One thing I have noted is that regardless of the supercar top speeds of around 200 MPH seems to be a limit. To be sure there are 200+ MPH cars out there but they rarely go way past it and most supercars hover around that speed. A 600 HP engine may get you to 195 and a 1200 HP engine may get you to 205.

I understand that the faster you go the harder it is to go faster. In other words, each mile-per-hour in speed increase takes a bit more power to achieve than the previous mile-per-hour increase. Still, it seems a bit past 200 MPH and added power barely gets you anything. I would also argue it gets you little in acceleration from a stop since there is a limit to the grip of the tires on the road.

Clearly planes go a lot faster than 200 MPH and while a commercial jet has more thrust than a car they are also pushing a lot more weight yet they seem to have little problems with it.

So what is it that keeps supercars/race cars glued to around 200 MPH?

(Someone will be sure to post a link to a rocket car/jet car…I am asking about internal combustion/electric engines.)

2

The Bugatti Veyron tops out at 400+ kph / 267 MPH. But it can only sustain that speed for 12 minutes before it’s fuel tank is empty and the special tyres for a Veyron cost almost $40,000. The Veyron is big and heavy and doesn’t corner very well, it’s not a race car.

Most people that buy supercars want something that can corner better with a lower top speed, about the only place you can actually reach the Veyron’s top speed in practise is on the VW test track in Germany. So why waste weight and money on something that can never be used? (even most race tracks don’t have a straight long enough for the Veyron to reach top speed).

3

The simple answer is inertia and friction. A car needs friction to get it going, get it round corners and to stop it. Inertia is doing it’s best to stop all those things from happening.

4

Why can jets get past this with ease?

5

the force due to drag goes up with the square of the speed of the moving object. in short, if you want to go twice as fast, you need to overcome four times the force from drag.

a ton of thrust.

6

In a word: cost. In a few words: velocity squared rule. Ballpark power required for traction vehicles like cars to double their speed is the square of the desired speed increase. If you want to go from 200 mph to 300 mph with your super-super car you’ll need around 2.25 times the power. You already needed 500 bhp to go 200. Now you need a powerplant capable of 1,125 bhp. The power required curve just doesn’t let up.

Pity propellor aircraft even more. For various reasons they’re under a cubed power required curve. That is, to double the speed of a prop plane you eight times the power.

7

[QUOTE=jz78817;19397212…]
a ton of thrust…
[/QUOTE]

Sure but they weigh a crapload more than a car.

Is there a power to weight ratio comparison for cars and planes? (I mean, I am sure someone can do the math but I am hoping someone has already done it for us.)

8

No ground friction is a big part, massive thrust is the other. Remember those $40,000 tyres I mentioned? They only last 15 minutes at top speed. Also there is not many obstacles in the sky, the Veyron at top speed takes 500 meters to stop. Apart from a salt lake or a closed test track where you can you ever drive the thing that fast? There’s just not much demand for car’s who’s only purpose is top speed, it’s purely a bragging rights thing for the companies involved, a car optimised for straight line top speed is objectively worse in almost everyway than a 200MPH car which can corner.

9

One word: tires.

As the speed increases above a certain threshold speed, the lifespan of a tire exponentially decreases. I read somewhere that, at its top speed of 254 MPH, the tires on the Bugatti Veyron will only last 15 minutes. (And the gas tank can only hold 12 minutes worth of fuel!)

On edit: looks like coremelt beat me to it.

10

Race cars have been limited to speeds around 200 mph because of safety rather than sheer engine power. The margin of error and reaction time is just too small. Driving a car in any real world environment is a moment-by-moment encountering of changes, obstacles, decisions, and maneuvers. Planes have almost none of that, except when landing and those are done at speeds well under 200 mph. The space shuttle landed like a brick at 200 mph and only the most skilled pilots in the world could handle one.

11

Well, there’s only so much power one can get out of a 2200 cc engine, that’s smaller than my lil’ Toyota pick-up. Racing circuits put these limits on for safety.

12

Ok…I see there are some practical limits but we have gotten rocket cars to far higher speeds. There is a 1000 MPH vehicle going for a land speed record.
Barring running out of gas and tires flying apart (not sure how rocket/jet cars managed) there still seems to be a serious limit at around 200 MPH for most production cars. The Veyron certainly zooms past that and there are more than a few others that will get you to 220+ but they are rare and super expensive cars and they are not exactly flying past 200.

Just seems a weird barrier. Watching this video on rare super cars prompted this question since, regardless of power, they all hovered in a 200(ish) speed range.

13

I remember Michael Schumacher in his Ferrari pomp saying that if they took all the downforce off his race car, and eliminated all the software cutouts, it would comfortably do 500 km/h (310 mph). A journalist challenged him to do it, and he said that at that speed, if he hit a pebble the size of a pea he’d lose control immediately. He added he was just making a point about the power/weight ratio.

14

I asked an almost identical question a few years ago. Some of the answers were fairly comprehensive although, I admit, it still doesn’t make a lot of intuitive sense to me.

Why is is so difficult to engineer a car that can go over 200 mph?

15

Just to clarify, I appear to have been wrong re the power required of cars: It looks like they, too, are bound by the cubed ratio of power required. Hey I learned something!

16

I would speculate that it’s not a limit, but rather a goal. A car manufacturer gets more press if the car can reach 200 vs 199. I’m sure some number crunchers factor the revenue gain from the extra press against the extra costs required to engineer the car to go 200mph.
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17

There was a thread not to long ago about tractor trailers traveling at high speed. Weight of the vehicle doesn’t matter much when it comes to top speeds. It’s all about using horse power to combat friction loss. Tractor trailer engines need higher toque to get them moving but the actual horse power is in the 300-600hp range. The sports cars do better with engines that size because the aerodynamics.

Planes are more aerodynamic, have more power and have better means to apply that power at high speeds. A car with a jet engine that isn’t limited by applying friction to the ground can go really fast too. Land speed records are set by ‘cars’ using turbo jets, setting records at 760mph.

18

Cars are pretty aerodynamic too.

Clearly rockets and jets can propel things to super high speeds.

I guess I am asking why internal combustion engines on cars seem limited regardless of the horsepower they output. Why is it going from 600HP to 1200HP on an internal combustion engine gains you only a few MPH? At that rate even jets and rockets would seem limited.

19

There have been some developments since I started my version of the question in 2013.

Chevy now makes a fiery version of the Corvette for a decent price. The Corvette Z06 can easily top 200 mph straight off the showroom floor and it only costs less than $80,000. My father recently bought the track version, the Z07, and it has true supercar performance with a 0-60 time of about 3 seconds and a top speed of 205 mph. You can get one of those for a little over $100,000 and they are both street and track ready. You can take them to the grocery store and then the track without changing anything.

It can be done for a semi-reasonable price but it has only been in the last couple of years that you could get that type of performance from an unmodified factory car and still be able to drive it around like a normal vehicle.

20

The primary physical limiting factor is not tire grip, since wheel-driven cars have achieved 470 mph: Land speed racing - Wikipedia

However air drag increases as the square of velocity. Most of the engine power at high speed is spent overcoming this, not rolling friction or other losses.

For actual racing a significant amount of aerodynamic downforce is required, which in turn increases drag and requires more power to overcome. In a pure speed record attempt, this downforce can be trimmed considerably but the vehicle still faces the “velocity squared” problem from air drag.

For example at 200 mph the drag force (at sea level) of a car with Cd of 0.39 and 22.3 square feet frontal area would be 888 pounds of force. However at 300 mph (just 50% faster), that increases to nearly 2,000 pounds of force. Drag force calculator: http://www.wolframalpha.com/widgets/view.jsp?id=bcc8ab4a61a1d1f3f102846d9617eb8

Jet planes go much faster because (a) they have much lower drag (b) they operate at high altitudes where the air is thinner hence drag is further lessened, and (c) they have much higher power.

E.g, the drag coefficient for a Lockheed Jetstar was 0.0126, which is 1/23 of a Veyron. At 30,000 ft the air density is 1/68 that of sea level: https://people.rit.edu/pnveme/MECE356/Drag%20coefficients.html

There is no generalized direct equivalency between jet engine thrust and horsepower. However we can compare two planes with roughly similar design and performance to approximate this at cruise speed and altitude. The Tu-95 has four 15,000 hp turboprop engines and the B-52 has eight 15,000-lb-thrust jet engines. Thus the very approximate equivalence at cruise speed is 2 lbs of thrust per hp. This is very inexact since it does not consider various issues like propulsive efficiency.

A 747-800 has four jet engines of 66,000 lbs thrust each, or 266,000 lbs total. If we use 2 lbs thrust per hp, that would be 133,000 horsepower.