CincyScott

I was doing research last night on keeping my 993TT, buying a 965, or a 930, and I read that when the 993TT's came out that the measured braking capability was 2000 hp on these cars.

I guess I don't understand how the force to stop can be measured in the same way as the force to go, but I'm happy to know that I'm in the 2000 hp club

No HTwo O

Use that to market your car, and raise the price $10K.

ScottMellor

Yes, but nobody will believe you until you do an instumented 130-60 test!

ca993twin

I recently returned from a road trip with three other "nitwits". I noticed that my GPS "max speed" indicator was at 93 mph (I know... really paltry). The next day, after some spirited driving I checked again... it now reads 288 mph. Surely I am also a member of the 2000 HP club!

Take THAT, Mellor!

bb993tt

That's cold Scott; true but cold.

TB993tt

As usual it has been exaggerated it was 19XXhp, I have not got the original quote from Porsche but remember it well it was said that 19XXhp when braking from maximum speed (300kph)

I never quite understood how they could get to that number but presume it is estimated by measuring the average decleration....

On the 997GT2 (only because there is published data on deceleration from 300kph) it takes 6.8s to go from 300 to zero so average deceleration of about 1.25G in 6.8s

I still can't fathom how they convert this into torque (which hp is simply derived from)

Maybe they are saying it would take 19XXhp average to accelerate the car for 6.8s at 1.25G..... ?

CincyScott

I think that was where I got lost too. I get G-forces and all that, but the conversion to horsepower to me really didn't make much sense (at least in my very small mind!)

ca993twin

TB993tt,

Please. Do not use facts to dissuade me from my delusions. 288 mph. My GPS says so, so it must be true (just like everything you read on the internet). And the panties will come FLYING off when you say "oh, I dunno, about 2000 hp".

Basal Skull

I think it is a relatively straightforward calculation to get torque or hp from acceleration and mass.
F=M*A so if you know the mass and acceleration, you would know force (in Kg or lbs or Newtons) then you would need to convert that to toque using wheel diameter. To get engine numbers there will be correction factors for trans/r&p etc.

TB993tt

This is the bit which foxed me, how can one convert a total amount of retardation force which is calculated from the negative acceleration and car's mass in Newtons and is not a twisting force which can be measured in NM, will you show the calculation for us

Basal Skull

Someone who knows this stuff please help me out. I'm just a simple physician...
I would assume that it's F=m*a
and to get torque it's: torque = F*r (radius of wheel) = m*a*r
you can change acceleration (tangental acceleration) to angular acceleration but it doesn't make a difference: a=r*@(angular acceleration in radians/sec squared).
so torque = F*r=m*a*r=m*@*r*r
I don't think it matters where the mass is -ie rotating on the outside of the wheel/radius or at the centre of the axis of rotation and moving (as in a car slowing or accelerating) as in this situation.

Felix

Isn't it simply the amount of kinetic energy at speed divided by the braking time?

For example, take a 1500 kg car (approximate weight of a 997 GT2 according to http://www.einszweidrei.de/porsche/p...gt2-2008-2.htm) at 300 kph. 300 kph = 83.33 m/sec. So the KE =

0.5*1500*83.33*83.33 = 5,208,333 joules

If it takes 6.8 seconds to go from 300 kph to 0 then those 5.2M joules are being dissipated at a rate of 765,931 joules per second. Joules per second are watts. 735 watts equals 1 Metric hp (PS). So the brakes are absorbing 765,931/735 = 1042 horsepower. Hmm, where did I go wrong?

Felix

Further to the above 300 kph to zero in 6.8 seconds is an average of 1.25g.

I suppose stickier tyres and a higher friction surface would reduce the braking time...

Reverse engineering my above calcs says that 2000 bhp brakes would mean 300 kph to zero in 3.55 seconds. That's an average of 2.4g. It that realistic?

Basal Skull

I think other corrections like air drag and moments of inertia need to be calculated in too... but I'm starting to get a headache...
I've got a gtech pro somewhere - I'll see if I can find the manual for it, seem to remember some descriptions of some calculations in it. Seem to remember being able to estimate drag by coasting or something.

bb993tt

I love it when you speak engineering, Felix. Must be all that DOD/MOD experience.