How much horse power is needed for Xmph?

[XitUp]

If you know the CdA of a car, how do you work out how many horses it would need to hit 100/150/200/etc mph?

Sorry if this has been covered on the forum before, but the search function is crap.

Ta.

[Marf]

Depends on W too.

[Heretic]

you are being too vague. There are so many variables you have left up in the air, that the question is unanswerable

[Noger]

It doesn't really. W just tells you how long it takes to get there

CdA = bhp (at the wheels) * 8400 / mph ^3

So, bhp = CdA x mph ^3 / 8400

Edited by Noger on Sunday 23 October 20:00

[XitUp]

Marf said:

Depends on W too.

Not really.

Heretic said:

you are being too vague. There are so many variables you have left up in the air, that the question is unanswerable

What like? I know rolling resistance of the tyres will have a bit of an impact.

Noger said:

It doesn't really. W just tells you how long it takes to get there

CdA = bhp (at the wheels) * 8400 / mph ^3

So, bhp = CdA x mph ^3 / 8400

Thanks.

[davepoth]

Also depends on the rolling resistance of the tyres.

[XitUp]

Thanks.

So if a car had a CdA of 5.10, roughly how much would it need?

[Noger]

That is quite slippery

So 5.1 sq ft is 0.474 sq m.

So...

100mph needs 56 bhp at the wheels
150mph needs 190 bhp at the wheels
200mph needs 451 bhp at the wheels

[kambites]

Planning on getting a mk1 Insight to 200mph?

[rsv gone!]

The only statistic I have to hand is that a faired motorbike only needs 30bhp to do 100mph.

[XitUp]

kambites said:

Planning on getting a mk1 Insight to 200mph?

Good guess!

I was bored driving home from Leicester yesterday and my mind was wandering thinking about it. All I need to do is find one for sale, get some forged internals and a turbo off a diesel train.

[Noger]

rsv gone! said:

The only statistic I have to hand is that a faired motorbike only needs 30bhp to do 100mph.

Yes, CdA (guess) of .19 gives 23 bhp needed for 100mph.

Bikes are not very aerodynamically efficient, but there ain't much of em

[Heretic]

after power, gearing is the next thing to look at. Wind drag also changes with speed. As you go faster the wind will actually try to "flatten" out the front of a vehicle. Wind drag increases with speed, and can change with atmospheric conditions. Tire size will increase with centrifugal force, and that will change the drag under a vehicle. Proper alignment and tire inflation can be contributing factors to top speed. Even the medium on which you are driving can make a difference in tire resistance.
never assume that speed is a matter of a figure on paper. It will bite you in the posterior every time. The numbers on paper are just a starting point. It is something that should be researched, but that number will always be low

[XitUp]

I know.

I was just looking for a ballpark figure.

I'm not actually going to try building a 200mph Insight. My girlfriend wouldn't let me.

[kambites]

It probably wouldn't be that hard actually - take out the existing drive-train and stick in a transverse turbocharged V6, old Noble style. There's a decent amount of room back there.

[The Nur]

kambites said:

It probably wouldn't be that hard actually - take out the existing drive-train and stick in a transverse turbocharged V6, old Noble style. There's a decent amount of room back there.

That sounds like a challenge if ever there was one. Now who on here has a disposable insight?

[CraigyMc]

XitUp said:

I know.

I was just looking for a ballpark figure.

I'm not actually going to try building a 200mph Insight. My girlfriend wouldn't let me.

It'd be interesting trying to keep that on the ground.

If you chuck a car out the back of a plane, it will fall at approx. 110mph (at least, according to some skydiving types who have tried it - albeit a car with no engine).

Think about a car shoving itself along the ground at 200mph and that's how unstable it is, unless it's got some sort of downforce.

C

[kambites]

I suspect that all you'd need is an air-dam. It will presumably already be designed to sheer the airflow at the back quite effectively to reduce drag·

[XitUp]

Noooo. I was pondering what the best way to beat the land speed record for a <1000cc car was.

[kambites]

XitUp said:

Noooo. I was pondering what the best way to beat the land speed record for a <1000cc car was.

A formula Ford with a turbocharged motorbike engine?