does your rs4 feel slow in 1 st and 2nd gear??

[r4fdud]

i do use the the good fuel 99 ron from bp..tyres are great aswell......

i dont get you when u say they shave teh rolling road?

[derdle]

Sorry mate...dyslexic fingers on the keyboard. What I meant to say was that many tuners have rolling roads, including QST who did your remap.

Right, off to "Site news and suggestions" now to suggest a spellchecker.
Doh!

[nol]

Let me try and shed some light over this with torque and power. To simplify things I am disregarding from air resistance and any losses in the gearbox and drive train over the revs.

Acceleration is about getting maximum torque to the driving wheels. The gearbox is here acting as a torque converter between the driving wheels and the engine, and the conversion ratio will change with the gear ratio of the gearbox (and of cause the final drive).

Looking at one single gear, you will get the most torque to the driving wheels when the engine has its torque peek. In other words, you will have the highest acceleration force when you are at the torque peek of the engine (for this gear).

However when comparing different gears it gets a little bit more complicated.

For the RS4, the gear ratio for 1st gear is 3.5 and for 2nd gear 1.889. This means that if you change up to 2nd gear at 7000 rpm, you will end up having 7000 * 1.889 / 3.5 = 3778 rpm in 2nd gear. Even though you are gaining torque by doing so (because the engine is delivering more torque at 3778 rpm than 7000 rpm), you are also going to loose torque to the driving wheels because of the gear change. In this case the ratio is 1.889 / 3.5 = 0.54 which basically means that 2nd gear is only 54% as strong as the first gear. In other word, you are going to loose 46% of the torque (on the driving wheels, not in the engine) over the whole rev span compared to the 1st gear. So to get the best possible acceleration we are looking at keeping the best trade off between high torque from the engine and low gear. So how do we do that?

This is where Power becomes interesting. The formula for Power is

Power (W) = Torque (Nm) * Revs (radians/s)

and where the revs in radians/s are

Revs (radians/s) = Revs (rpm) * 2 * Pi / 60

Pi is about 3.1415.

Without getting into too much mathematical details, you will find that the best trade off between high torque and low gear is when you have the highest Power from the engine. In basic terms comparing different gear ratios is the same as comparing different revs (as we just did above) so the Power curve is in some sense telling us in what rev span we should use the engine to get the best acceleration through the gears.

Looking at the standard Power curve for the RS4, you will get the best acceleration through the gears by reving it to 7000 rpm before changing gear, under the condition that you do not hit the rev limiter and do not have any wheel spin.

So to summarize

For one single gear you will have the best acceleration when you have the highest torque from the engine

When comparing two gears with each other, you will have the best acceleration in the gear that give the highest Power.

Different type of cars usually do not have the same gearing, which means that comparing performance of different cars in principle comes down to comparing the Power curves with each other. Maybe know you understand why you in a petrol car have to gear down and "use the Power" to be able to keep up with modern diesel cars.

/Niclas

[r4fdud]

my head hurts....but i do get what u mean