Print

Please login or register.
1 Hour 1 Day 1 Week 1 Month Forever
Login with username, password and session length

[omegadan67]

i dont think that is a fair comparison 2 total different cars 1 has a v6 the other a straight 6 1 is a 3 litre the other is a 3.6 litre

Why not? It doesn't matter what's producing the power, just the amount of power required. Could be a gas turbine, steam engine or an electric motor for all we care.

Torque doesn't matter either as it's not a measurement of power. Talking about the torque output of an engine is like saying I've got a 2 amp lightbulb. It gives half the story. If it's designed to run on 240 Volts it's going to be a 480 Watt bulb - pretty bright. But if it's designed for a 12 Volt car electrical system - 24 Watts. With an engine it's the product of torque and RPM that's interesting - the power output.

The point of the Omega - Carlton comparison is that the bodyshell is basically the same shape aerodynamically, so we know that it takes 210BHP to get it to 150MPH and 377 BHP to get it to 175. Knowing those datapoints we could calculate the maximum speed for any level of power output.

This does, however, assume that the vehicle has sensible gearing so that the engine's maximum power output occurs at around its' maximum speed.

The usual way to measure top speed is to first of all get some calibrated measuring device.  The dial in most cars is approximate for a number of reasons but locals law often dictate that the error is always an over-read irrespective.

A car's speedo is often out by 5% or more, and all are designed to read high. Not a great deal of difference at low speeds. 5% out at 30MPH means it reads 1.5 MPH high. At 150 MPH that's gained you another 7.5 MPH that you aren't really doing so the first thing you need to do is to get an accurate speed measurement.

Kevin

doh forget torque is the twisting force of getting the car to move the greater the torque the quicker off the line its sorry blonde moment there

[Mr Skrunts]

But how do you know the Elites 210 Bhp would not achieve 175mph, we cant find out basically because the gearing wont allow it.

In 1985 I saw an indicated 149mph at max revs on my sylvia turbo, thats a 1.8 single turbo with a mere 135 bhp, 2nd time I had a passenger to verify it

[2woody]

while we're on the subject, weight is also not an issue. am Omega B filled with concrete is goingto reach the same top speed.

I specifically included the Omega A / Omega B comparison because they're so similar - probably more so than even the Sierra example.

[TheBoy]

But how do you know the Elites 210 Bhp would not achieve 175mph, we cant find out basically because the gearing wont allow it.

In 1985 I saw an indicated 149mph at max revs on my sylvia turbo, thats a 1.8 single turbo with a mere 135 bhp, 2nd time I had a passenger to verify it

The Omega will not redline in top...


...apparently

[Kevin Wood]

while we're on the subject, weight is also not an issue. am Omega B filled with concrete is goingto reach the same top speed.

Absolutely. Try keeping up with an Omega in a similarly powered Westfield. Not that anyone would have tried such a thing. The lighter car will leave it for dead from a standing start like it's not even moving but I'd be very surprised if the Westfield got much, if any, above 130 MPH, so the Omega wins - eventually, despite over a tonne more weight (but a much better aerodynamic performance ).

Kevin

[Chris_H]

while we're on the subject, weight is also not an issue. am Omega B filled with concrete is goingto reach the same top speed.

I specifically included the Omega A / Omega B comparison because they're so similar - probably more so than even the Sierra example.

You will need a longer straight or may only be able to do it once!

[David.R]

The faster you go, the more dense the air becomes.

Hate to disagree with you, but although it may feel like that when you stick your hand out of the window, air does not change density.

The reason that so much more power is needed to go just a bit faster is because the drag experienced on a body in a fluid (like a car in air) is a square law with respect to speed.  Going from 20mph to 40mph is a doubling of speed, but a factor of 4 in the power required. Same is true at higher speeds.  Go from 50mph to 100mph and it's another factor of 4 needed.  This is borne out in power and top speed figures.  A 100hp car that makes 100mph would need 400hp to achieve 200mph.*

*ish.  Unfortunately Cd for a car is not constant over that range of speed.  Manufacturers quoted Cd is always the lowest Cd measured, not the average.  In general, higher speeds induce higher Cd as the airflow will start to detatch from some surfaces, causing eddying and more drag.

[cem_devecioglu]

The faster you go, the more dense the air becomes.

Hate to disagree with you, but although it may feel like that when you stick your hand out of the window, air does not change density.

The reason that so much more power is needed to go just a bit faster is because the drag experienced on a body in a fluid (like a car in air) is a square law with respect to speed.  Going from 20mph to 40mph is a doubling of speed, but a factor of 4 in the power required. Same is true at higher speeds.  Go from 50mph to 100mph and it's another factor of 4 needed.  This is borne out in power and top speed figures.  A 100hp car that makes 100mph would need 400hp to achieve 200mph.*

*ish.  Unfortunately Cd for a car is not constant over that range of speed.  Manufacturers quoted Cd is always the lowest Cd measured, not the average.  In general, higher speeds induce higher Cd as the airflow will start to detatch from some surfaces, causing eddying and more drag.

yes..

air is a fluid.. and a car accelerating inside air is subject to fluid dynamics laws..

http://en.wikipedia.org/wiki/Drag_(physics)

[johny5]

What about calibrating from a GPS device such as a tomtom that provides the speed ur travelling?

Ive seen 140mph come from a Renault laguna before! And whats that around 140bhp for a 2.0? 

[Del Boy]

What about calibrating from a GPS device such as a tomtom that provides the speed you are travelling?

Ive seen 140mph come from a Renault laguna before! And whats that around 140bhp for a 2.0? 

Was also verified by GPS

[ChevetteNick]

What about calibrating from a GPS device such as a tomtom that provides the speed you are travelling?

Mr tomtom cannot take into account gradients

[Broomies Mate]

The faster you go, the more dense the air becomes.

Hate to disagree with you, but although it may feel like that when you stick your hand out of the window, air does not change density.

The reason that so much more power is needed to go just a bit faster is because the drag experienced on a body in a fluid (like a car in air) is a square law with respect to speed.  Going from 20mph to 40mph is a doubling of speed, but a factor of 4 in the power required. Same is true at higher speeds.  Go from 50mph to 100mph and it's another factor of 4 needed.  This is borne out in power and top speed figures.  A 100hp car that makes 100mph would need 400hp to achieve 200mph.*

*ish.  Unfortunately Cd for a car is not constant over that range of speed.  Manufacturers quoted Cd is always the lowest Cd measured, not the average.  In general, higher speeds induce higher Cd as the airflow will start to detatch from some surfaces, causing eddying and more drag.

Sorry, my brief explanation was erroneous... what I was implying was;

All figures for explanation reasons only;

If you are stood still, and there is 1000mb Air Pressure, you then accelerate to 100MPH. This will result in the air in front of the you becoming compressed, therefore increasing its density (not on a molecular level) you are simply hitting more molecules at any given time-frame (pre second, per minute etc).  Essentially, as you compress it (make it smaller) it will take up less space (volume) allowing more molecules into a given area (1 cm3 / 1m3 etc)  Boyles Law states this.

Charles' law would also play a trivial part, in that the action of compressing 'air' would cause heat.... I dont think enough of a rise in temperature could be calculated in MPH as it would have almost no 'practical' influence in this type of instance.

Then take into consideration Newtons law of motion, we can assume the reduced air pressure at the rear of the car is equal to the increased pressure at the font of the car.  Of course, to put all of this theory into actual workable figures, we'd have to place a car (probably an Omega) into a Wind Tunnel and get some boffin with a Casio Calculator to do the math.

I am no expert of Physics, just remember the basics from my education..... so, if I'm talking complete balls, just tell me to go away! 

[albitz]

Its all a bit academic really. Pretty sure Mercs are limited to 155mph, as are most German cars.

[tidla]

didnt james may explain all this when recapturing the speed record in the veyrony thing.

the links on the tinternet probably there somewhere...