headlights reduce top speed?

[stuartswan]

just wondering does anyone know if having the lights up on a mk2 turbo has an effect on top speed?

thanks

[DAN_REV1_TURBO]

Well, you would think that the car is slightly more aerodynamic with the lights down flat and flush.. But I doubt it would be a massively noticeable difference would it?

[Peter Gidden]

I remember a post on a US forum where tests were done on an airfield circuit. 3mph IIRC. And 3mph for mirrors.

[Bobby (aka Shaggar)]

I remember a post on a US forum where tests were done on an airfield circuit. 3mph IIRC. And 3mph for mirrors.

So if we turn the lights off and pull the mirrors in at night we should save a load of fuel hey?

[cantfindausername]

Does this mean during the day if we need to emergancy brake from high speed we can "deploy the lights" as airbrakes?

[JJ]

Couldnt we fit a couple of motors to the boot lid and get a Mclaren SLR effect !!??? Best setup a net too when it falls off !!

[Bobby (aka Shaggar)]

Couldnt we fit a couple of motors to the boot lid and get a Mclaren SLR effect !!??? Best setup a net too when it falls off !!

I hear ya mate but think the way forward with this is to remove the catch from the bonnet and release that from inside. very easy as the 'control' is already in place. Might try this later.......

[Si_Crewe]

While driving along at 135mph with the engine bouncing off the red-line in top gear I have often been frustrated and angered to find my speed drop to barely more than 130mph when it gets dark and I switch on the headlights.

Clearly this is totally unacceptable and we should do everything in our power to remedy this obvious design fault.

[Kongaroo]

I guess from the responses it's not seen as much of an issue, lol.

But if you want the geek answer to the original post... Putting up the headlights will increase your car's frontal area by approximately 1 square foot.

At speeds of around 150mph the amount of extra power that must be expended in order to overcome aerodynamic drag will be roughly 7 to 8 hp more when you have the headlights up as opposed to down on a mk2 MR2.

This does actually equate to a loss of about 3mph purely from aero drag at that sort of speed so the quoted airfield test result is pretty much exactly what it works out to be using the relevant equation for aero drag HP loss - ie. (cd x frontal area x velocity cubed) / 150000.

Wing mirrors will have slightly less effect as they contribute less to the frontal area than the headlights.

Hope that helps

[Mike 'MR2' McHugh]

At what sort of speeds would mirrors become air brakes i wonder ? Low speeds im guessing there wouldnt be enough resistance to make a difference but say at 90 ?

[Kongaroo]

At what sort of speeds would mirrors become air brakes i wonder ? Low speeds im guessing there wouldnt be enough resistance to make a difference but say at 90 ?

The mirrors don't contribute that much of a difference to the frontal area if you are comparing when folded over extended.

Took a few quick measurements and the difference looks to be about -0.31square feet for both mirrors when folded in.

This would mean at 90mph you would save about 0.4 BHP.
At 150mph you would save about 2 BHP. At around 150mph this might cost you about 1mph off your top speed so not a large effect.

[cantfindausername]

Do aerodynamic mirrors help?

[John145]

Frontal area doesn't change.

If you look at the front of the car with the headlights up and down, the sillouette of the car doesn't change and the sillouette is the frontal area, it'll be the coefficient of drag that is changing to create the extra resistance.

[Kongaroo]

Frontal area doesn't change.

If you look at the front of the car with the headlights up and down, the sillouette of the car doesn't change and the sillouette is the frontal area, it'll be the coefficient of drag that is changing to create the extra resistance.

Good point - I'd forgotten about the windscreen! Although if the airfield test is valid it looks as though the amount of BHP lost from the increased drag coefficient is similar to the amount you lose from increasing the frontal area by about 1sq.ft.

Shame there's no wind tunnel facility that's cheap and easy to rent out to find out the cd with the lights up.

The rough calculations for the wing mirrors still apply though.

[John145]

If I get bored enough at uni i'll see if i can run something through CFD!