Top Speed & Power - More flippin sums

[S2central.net]

Been looking at more of the wonderful maths at Puma Racing with regards estimating top speed from BHP. I now have a spreadsheet which captures the sums for rolling and aerodynamic drag and uses the following assumptions to get the official maximum speed of 154mph from the Coupe at 230bhp with the following.

Weight = 3355lbs (Jees thats heavy)
fA = 20.6sqft (Frontal Area)
Cd = 0.3
25% transmission loss

Anybody ever measured the fA on the S2 or seen reference to a quoted Cd to see how close reality is to theory here ?

The numbers appear to work so far - certainly good enough as rules of thumb.

Using the same constants I get 200mph with 482bhp at the flywheel !
A 280bhp S2 Coupe should be good for 165mph by the same maths...
A 360bhp RS2 conversion gets you to a frightening 180mph on paper !

I'll eventually create a web-based calculator that we can play with on S2 central - or maybe just produce a lookup table as weight, fA and Cd are pretty static on our cars. Its amazing how little effect that weight has on maximum speed - 100mph is achievable with 73bhp - which is why heavy cars with small engines can get up to high speeds - EVENTUALLY. It is acceleration, governed by the god of torque, which determines how quick you get there.


Paul

[Doug_RS2]

I doubt we can get to 200mph on standard ratios and wheels. I think I worked out that if I had aenough power I could get to 199mph (IIRC) with the rpm limit increased to 7600 and my slightly larger wheels - I'm off out to test it.

[SteveH]

You can see from the maths that it's a good idea to buy an RS2 speedo when you do the RS2 upgrade

[APNess]

The width of your tyres will slow you down in a straight line. This weeks' Autocar proves that with near identical RX8s except one has wide 19s on and loses something like 6mph over one with standard wheels.

I would have thought you'd have more chance of getting to 200mph with a longer 6th and standard wheels than your 19s Doug.

Oh, and a snow plough on the front might help too, instead of the square 'scythes through the air like a brick' front we have.

If you fancy testing it out we can time your car over a set distance with your 19s on then my ickle bitty 18s. Bet ur faster on mine

Oh and according to certain rolling roads, transmission losses are nearer 30% for the S2... Didn't we have a thread once with rolling roads reports on it? November should be interesting to see how much everyone loses between f/wheel n floor...

[Doug_RS2]

I'll just swap my longer 6th gear cog in the box now I'll pump up my tyres - it is the resistance not width that slows you down

I'll need to bolt on my bumper properly - or it will end up under the car - come to think of it, I need a turbo on the car as well

[4WheelDrifter]

Does anyone know how accurate the standard speedo is in the S2?

[S2central.net]

I doubt we can get to 200mph on standard ratios and wheels. I think I worked out that if I had aenough power I could get to 199mph (IIRC) with the rpm limit increased to 7600 and my slightly larger wheels - I'm off out to test it.

You are right of course the standard box and wheels max out at 7000rpm at 172mph by my calcs.

A 'longer' final drive ratio (F/R) would help of course... Like a 3.59 from some versions of the All-Road or 2.5TDi 01E box. Not worth thinking about unless you plan on lots of trips to the Autobahn or the MoT testing tracks !

This is pure theory here !!!!


Paul

[CHEN]

We have a 3B S2 with a 270bhp chip and no cats, yet it goes of the clock which is 170mph + which it shouldnt do on paper?

[4WheelDrifter]

That's why i asked about accuracy. It's not just me then. I figured that can’t be right, then wondered how fast it’s really going.

[S2 Ant]

i know my clock is about 5mph out at 100 and this increased to about 7mph at 150... Assuming accuracy of the GPS system in my car at the time of course, was a constant speed tho not a accelerate there and slow down again so it had time to keep up...... was going like that for about 20 miles LOL!!!

[4WheelDrifter]

You quickly eat up the miles at that speed. I find it difficult to find a private road long enough.

[APNess]

Don't know about anyone else but I find the car a bit floaty over 165. Below that it seems pretty rock solid - enough even to take your hands off the wheel - that rear spoiler DOES have a use - other than to pour rainwater into your boot of course.

Not sure how the avants get on at that sort of speed though the Labrador might complain it's face is getting warm when stuck to the back windscreen...

Has anyone actually fitted another box (e.g. from diesel) or is that all theory about whether they fit straight on?

Also would disconnecting the front drive shafts a) work b) make the car faster (less transmission losses)?

Oh, and Thank da lawd for private test tracks

[4WheelDrifter]

Yeah. Maybe something feels different at 165+. I thought it might be me beginning to poo my pants.

[Fat Audi 80]

I try and work out my top speed runs on my private test track by going off the speedo and working out the gearing. Its a slightly better route tnan using the speedo, although I am still using the speedo to calculate the "inital" rev counter multiplying value for the upper speed calculation....

[4WheelDrifter]

It'd be nice if my speedo worked again. I wouldn't need to think about revs and gearing calculations then. I've got a sticky one! It sticks at 20mph and won’t budge ‘til you go over about 40. Then it refuses to go over about 80.
The first time it happened at last winter, I wondered why everyone else on the road was going so slow!