Print

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

[Del Boy]

BMW quote 525i 2006 as 152 MPH top end, the cars have a red line at 7000 RPM, of course on the german autobahns I just followed a paramedics car up to 145 MPH at 6000 RPM in 5th gear now still have 1000 rpm to go plus whatever 6th brings. That's more than 152 I know considering on the autobahns you have to watch for people pulling out at that speed so you can't go flat out. So how do they work out top ends at the quoted figures?

[Lizzie_Zoom]

BMW quote 525i 2006 as 152 MPH top end, the cars have a red line at 7000 RPM, of course on the german autobahns I just followed a paramedics car up to 145 MPH at 6000 RPM in 5th gear now still have 1000 rpm to go plus whatever 6th brings. That's more than 152 I know considering on the autobahns you have to watch for people pulling out at that speed so you can't go flat out. So how do they work out top ends at the quoted figures?

I believed Del it was decided between running the cars on the companies own test tracks (like Millbrook for Vx) and mathmatical engineering calculations

[Del Boy]

Then they're wrong 

[Omegatoy]

lol,
yep they are wrong, i have seen a carlton gsi slightly modded hit 173mph with about thirty people standing around who also saw it! however that was a top speed run on a rolling road not real life!! it still had a hundred or so rpms to go!!!

[Kevin Wood]

BMW quote 525i 2006 as 152 MPH top end, the cars have a red line at 7000 RPM, of course on the german autobahns I just followed a paramedics car up to 145 MPH at 6000 RPM in 5th gear now still have 1000 rpm to go plus whatever 6th brings. That's more than 152 I know considering on the autobahns you have to watch for people pulling out at that speed so you can't go flat out. So how do they work out top ends at the quoted figures?

The thing is, it's not just about how many revs you have left. Not many cars will hit the red line in top gear. In fact, many cars won't hit their top speed in top gear. If the red line is at 7K power probably peaks at 6K or slightly above so you're on the back end of the curve above there. Every little bit of extra speed, you have less power output.

Small factors can have a huge difference to top speed too. Rolling resistance of tyres, temperature, pressure and humidity, wind, whether there is a slight gradient to the road surface,etc. It's not an exact science (unless you have a 155MPH limiter ).

Kevin

[geoffr70]

Don't forget on a rolling road there' no air to push out the way as the car is stationary

[Del Boy]

BMW quote 525i 2006 as 152 MPH top end, the cars have a red line at 7000 RPM, of course on the german autobahns I just followed a paramedics car up to 145 MPH at 6000 RPM in 5th gear now still have 1000 rpm to go plus whatever 6th brings. That's more than 152 I know considering on the autobahns you have to watch for people pulling out at that speed so you can't go flat out. So how do they work out top ends at the quoted figures?

The thing is, it's not just about how many revs you have left. Not many cars will hit the red line in top gear. In fact, many cars won't hit their top speed in top gear. If the red line is at 7K power probably peaks at 6K or slightly above so you're on the back end of the curve above there. Every little bit of extra speed, you have less power output.

Small factors can have a huge difference to top speed too. Rolling resistance of tyres, temperature, pressure and humidity, wind, whether there is a slight gradient to the road surface,etc. It's not an exact science (unless you have a 155MPH limiter ).

Kevin

Still pulling mind you loads left in 5th, peak power is at 6500 rpm apparently  :-/

[pedroMV6]

What Kevin and geoffr said. ^^^

Also, the faster you travel, the 'heavier' air gets to be able to push through it.

[Mr Skrunts]

What Kevin and geoffr said. ^^^

Also, the faster you travel, the 'heavier' air gets to be able to push through it.

Explained nicely by james may regards the veyron ss on TG tonight, the extra 6mph need 200 bhp to achieve it. :-/

[Broomies Mate]

Yeah, it's quite simple really.  Air 'weighs' something.... you can put a figure on it's density, or 'Air Pressure'.

When you are at a standstill, the Air Pressure is generally around 1000mb at sea level.  As you accelerate, you are forcing your way through more and more air molecules.  The faster you go, the more dense the air becomes.

Cars are designed with aerodynamic properties to 'cut' though the air, just like an aircraft, but they have the benefit of being 30,000ft up, where the air is much thinner, giving them a clearer path.

[2woody]

this is really simple to answer.

top speed is the point at which the engine power is the same as the aerodynamic drag. provided, of course, you're on a level road.

there's an equation for it, but it still boils down to drag vs. power.

drag = aerodynamic drag + gradient drag (uphill) + rolling resistance.

in reality, rolling resistance is fairly constant across all speed ranges, whilst aerodynamic drag goes up as the square of the speed.

as an example, Omega B 3.0 gets to 150mph with 210 BHP, whilst the Lotus Carlton (basically the same car) needs another 170 BHP to go an extra 25mph or so.

[Chris_H]

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.

Then the car is measured on a straight, level piece of road often by timing between two measured points, and to (try to) rule out ground airspeed make two runs in opposite directions within a short timespan.  The assumption is that wind speed doesn't change very quickly (yeah right).

The average time between markers for the two runs is used to calculate the claimed top speed.

Rolling road speed readings are a nonsense due to the removal of wind resistance.

Oh, and it would be best if the measurements were done at sea level!

[omegadan67]

this is really simple to answer.

top speed is the point at which the engine power is the same as the aerodynamic drag. provided, of course, you're on a level road.

there's an equation for it, but it still boils down to drag vs. power.

drag = aerodynamic drag + gradient drag (uphill) + rolling resistance.

in reality, rolling resistance is fairly constant across all speed ranges, whilst aerodynamic drag goes up as the square of the speed.

as an example, Omega B 3.0 gets to 150mph with 210 BHP, whilst the Lotus Carlton (basically the same car) needs another 170 BHP to go an extra 25mph or so.

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

A better example imho would have been a 1986 sierra 2.0 gt 130mph top end and the cosworth 149mph differensc in power standard 2.0 130 bhp cossie 204 bhp and then theres the torque difference as well

[Kevin Wood]

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

[Lizzie_Zoom]

There was me thinking (obviously very naively reading the informed comment on this thread) that they just raced their new car round a racetrack, did some involved mathmatical calculations using onboard and off board computers, and arrived at 'the figures'!!!

Well I suppose I am only a women after all!!