TTJunkie

I just read the post about 190 MPH runs, and it brings to mind a question I've had for some time now. This question is aimed at others who bring their 993TT's to maximum speed regularly. I have found that if I shift to 6th before 6000 RPM, the car settles in at an indicated 178 MPH, but if I delay the final shift to 6700 RPM the car continues up to an indicated 190 MPH. Can anyone explain this? Do the aerodynamics overtake the horsepower if you shift too early?
Oh, and please do not use this thread to preach about speeding, I am only interested in physics.

ca993twin

It seems that if you engage 6th gear with an rpm that's out of the max power band, then it will struggle against the huge aero forces (and loses the battle). If you delay the shift and engage 6th in the peak HP band, you have enough power to overcome the aero forces.

Not preaching... but where the heck can you go that fast?

ebaker

The ecu could be pulling the timing more when you shift into 6th sooner.

Stummel

Is this consistent? At 178mph you are more in the maximum horsepower range than at 190mph, so it should have nothing to do with the shifting before.

If you shift before 6000rpm to 6th it will take way longer to get to 178mph.
If you have stock intercooler maybe the engine gets to hot as you need more torque in 6th with low rpm than you would need in 5th with higher rpm.
Than the ECU would pull back some of the ponies you need to get to 190?

Perhaps someone could shed some light on torque vs. horse power when it comes to hi-speed?
Do you need torque to accelerate and horses to stay at a certain speed?

VerySideways

Torque is the rate at which power is achieved.
There is a formula for this, i believe it is:

HP = (torque X rpm) / 5252

So yes, you need torque to accelerate and horsepower to overcome drag.
Therefore to increase your top speed you need to increase power or decrease drag.

Remember also that drag is exponentially increased as you go faster (twice the speed = 4 times the drag, 4 times the speed = 16 times the drag, etc) so you need an awful lot of horsepower (and in our case gearing too) to get from 180mph to 200mph.

I hope i have explained that clearly!

VS

Carl Young

Interesting circumstance, especially since in both cases of shift RPM you are still in 6th before about 145 MPH. I wonder what the difference in boost pressure is in each case? Carl