HP and Top Speed

[Barry123]

Ok homies... This is just from another thread where we tried to establish how much power would be needed to get a Saxo to certain speeds...

I'm kind of hoping the thread will throw up a discussion where we can make the equation a bit more accurate and include more factors...so feel free to add inputs, criticisms etc

NOTE: These are fly hp figures. The wheels used are VTS standard 195/45/R15 and assuming that the tyres are capable of doing these speeds.

The Equation
Power required = 1.0505x10**(-3) x Cd x A x V**3
Cd = drag coefficient (no units)
A = front area of vehicle in m**2 (metres squared)
V = Velocity in m/s (metres per second)
1.0502x10**(-3) is a correction factor. Thats 1.0505(to the power '-3')
NOTE: It doesnt fully take into account rolling resistance although I've tried to factor it in.

This can be reversed so you can work out the top speed of a Saxo if you know the hp... obviously it wont be totally accurate as you could get peak hp at low rpm i.e well before you get near your top speed, but its a good indication of what is possible if you had the correct gearing.


Standard VTS
Power Output: 120hp @ 6600rpm
Top Speed: 127mph @ 6485rpm

Miles Per Hour to Metres Per Second
127mph to m/s is calculated by:
127*0.4444
which equals: 56.44m/s

Saxo Coefficient of Drag
the Cd (Drag Coefficient) of a saxo is: 0.34

Saxo Frontal Area
and the frontal area is: 1.82m**2

Therefore Power is calculated by:
Power = 1.0502x10**(-3)*0.34*1.82*(56.44**3)

Power:
116.9hp - which is about right as top speed does not occur at peak power for the VTS, although the calculated value is probably a bit low as the power curve tends to plateau as peak power it met. However it'll do for the sake of this text.

so thats cool.


HP requirement to reach 164mph
Using readily available parts the VTS engine in standard tune is capable of 164mph @ 7250rpm running a VTR box

Power required = 1.0505x10**(-3) x Cd x A x V**3

We want 164mph
which is 72.88m/s
so the power equals 1.0502x10**(-3)*0.34*1.82*(72.88**3)

which gives around 252hp required.


HP requirement to reach 176mph
Using the same parts as above but with the limiter raised to 7800rpm a theoretical top speed of 176mph can be achieved.

We want 176mph
which is 78.22m/s
so the power equals 1.0502x10**(-3)*0.34*1.82*(78.22**3)

which gives around 311hp required.

HP requirement to reach 200mph
Ok, there isnt a box in the saxo range that'll provide the correct ratio's to achieve this speed but just for a laugh lets see...

Power required = 1.0505x10**(-3) x Cd x A x V**3

We want 200mph
which is 88.88m/s
so the power equals 1.0502x10**(-3)*0.34*1.82*(88.88**3)

which gives around 456hp required.


HP requirement to reach 300mph
Right, now I'm taking the biscuit here but lets say you wanted a saxo(standard shell) top fuel car for trips to McRags.

Power required = 1.0505x10**(-3) x Cd x A x V**3

We want 300mph
which is 133.332m/s
so the power equals 1.0502x10**(-3)*0.34*1.82*(133.332**3)

which gives around 1540hp required.

As you can see there is a considerable rise in the required horsepower to achieve even relatively small leaps in top speed.

Hope you find this helpful/interesting
Any questions or abuse then post away


ads

[James]

How bored are you...

Im only playing, good information sayer

[frankie]

i asked the question lol.

[frankie]

by the figures and the way my brain works to get a saxo to 400mph you will need 3200hp

so seeing a road car being produced that will hit 300mph in the next 10-20years could infact be pretty lkely?

[Barry123]

I didnt go any further dude because the cars beyond about 300mph (well before that tbh) begin to experience the compressibility effects of air and all the nasty stuff associated with that. most aerodynamic calculations go out the window after about Mach0.3

[frankie]

yea i know what your saying..

[nazz1]

Imagine a saxo with like 2000bhp. WOW

[Karl]

~ nice info Sayer.

[-Jason-]

Very interesting

[Scottp]

Nice work Ads.

I expect Ryan may have his work cut out for him here with deleting posts that go roughly like: "My friend has an xfactor air filter he got at the market and he has gone off the clocks in his vtr."

[Scattle]

Nice one good info.

[vidal]

Ads, if you get chance, check out David Lean's 1952 film "The Sound Barrier", good clips of planes hitting an almost impenetrable barrier as the aircraft got faster, pretty piss-poor 1950s stiff upper lip dialogue, but an entertaining way to spend a rainy afternoon. Follow it up with a hefty dose of "The Right Stuff (1983)"

Originally Posted by The Right Stuff

There was a demon that lived in the air. They said whoever challenged him would die. Their controls would freeze up, their planes would buffet wildly, and they would disintegrate. The demon lived at Mach 1 on the meter, seven hundred and fifty miles an hour, where the air could no longer move out of the way. He lived behind a barrier through which they said no man could ever pass. They called it the sound barrier.

V.

[Barry123]

Originally Posted by vidal

Ads, if you get chance, check out David Lean's 1952 film "The Sound Barrier", good clips of planes hitting an almost impenetrable barrier as the aircraft got faster, pretty piss-poor 1950s stiff upper lip dialogue, but an entertaining way to spend a rainy afternoon. Follow it up with a hefty dose of "The Right Stuff (1983)"



V.

Cheers man

my aerodynamics lecturer suggested watch The Right Stuff actually. Never had time although its on my list of things to do once Im finished

Originally Posted by Scott

Nice work Ads.

I expect Ryan may have his work cut out for him here with deleting posts that go roughly like: "My friend has an xfactor air filter he got at the market and he has gone off the clocks in his vtr."

I'm waiting for someone to mention their physics defying air filter scott.... only a matter of time.

[TU-Tuning]

I think you two should move in together and film it as a sitcom for BBC4


On a serious note, theyre some impressive calculations

[Colin]

Good read Ads. Spank you.

[Joesnow]

Good work! The figures seem quite realistic. I could have done with this info a while ago when someone claimed he could hit the limiter in 5th on his cammed VTS with VTR box! Are these calculations assuming the saxo is a plane/missile?? What happens when you have a boot full of sand, 5 fat people and a dog in the car? What power would you need then? Or would you just need loads of gears?

[Ryanp]

Originally Posted by Scott

Nice work Ads.

I expect Ryan may have his work cut out for him here with deleting posts that go roughly like: "My friend has an xfactor air filter he got at the market and he has gone off the clocks in his vtr."

lol!! once again scott you have managed to get me to laugh loudly in pc room at uni.

adam if you dont mind i might add this to the FAQ/sticky for a few weeks.

[Toad]

What am I doing wrong Ad, I can't get the calculations to work...

For the first example of 127 mph top speed, you have given the following calculation:

1.0502x10**(-3)*0.34*1.82*(56.44**3)

If I do the math there, I get 366.783, and not 116.9 like you. ???

1.0502x10^-3 = 0.010502
56.44^3 = 56440

So, 0.010502 * 0.34 * 1.82 * 56440 = 366.783. What am I missing ?

[Barry123]

Toadster, you're a few orders of magnitude out on the correction factor...
1.0502x10**(-3) in decimal form = 0.0010502


and the V^3 is 'Velocity cubed'.
so 56.44^3 = 179788.13

hope that helps man

ads

Originally Posted by Ryan

adam if you dont mind i might add this to the FAQ/sticky for a few weeks.

Go for it man, not much point in posting it if people dont have easy access to the info. As I said earlier though the correction factor is up for discussion as I personally think it's slightly out.

[Wayne]

some good information there