16 replies to this topic

[nicklouse]

the same reply as last time you asked this.
 
it depends on more things than you are asking.
 
my 120hp (non KAD) will do a max of 160KMH and do Zero to 100KPH in about 6 seconds. A KAD engine using the same gear ratios would do about the same.

Don't agree with that, the characteristics of the 2 engines are completely different

Which has nothing to do with the mechanics. Ok saying that both engines would send a car to about the same 0-60 is maybe a bit fanciful.( as there are so many variables other than the engine) but top speed is a mechanical relationship to the engine revs. If the have the same max rpm then top speed is the same.
Yes how they get there can and will be different.

 
That depends on whether the car is limited purely by the maximum rpm of the engine, or whether it is limited by whether it can produce enough power to overcome the road load at a given speed.
 
If you're top speed is limited by available power (there's some rpm headroom at top speed), then top speed will vary depending on the rpm at which the engine produces peak power, rather than the max engine rpm, and gear ratios will not have such a great effect on top speed, only by shifting the road speed at which you reach peak power. If, however, the gearing is shorter, then you will be limited by the maximum engine rpm, and changing the ratio will give a proportional increase in top speed.
 
I agree though, that just having the same peak power won't give a very accurate indication of 0-60 times, as this depends on the whole of the engine's torque curve, not just the peak power value.

Which goes back to the OPs question that is 1/2 meaningless an engines that does 9k will do the same speed as any other at 9k on the same gear ratios.
Now if we look at coopermans comments. Sure match gear ratios and final drive to the engine. But a rally set up will be very poor on a hill climb which will also not be great on the circuit. And then for road usage.... For fun or cruising down the motorway?

Asking a question without qualifying it is a bit like having a piss in freezing temps into the wind. Something sticks but it might not be what you want or need.

Also not quite sure what has happened in the world of 5 speed boxes have all the over driven 5ths gone? Not that I liked them.
And then there is the question of final drive ratios. What is available for the diff you chose? There are just so many variables that should be answered first/at the same time.

[Cooperman]

Absolutely correct.

The stuff I posted was a way of gearing the car for absolute maximum speed in a straight line and how to do the calculations.

For best acceleration it is necessary to decide what absolute maximum speed is acceptable for the use to which the car is being put. For example, driving on gravel through a forest on a rally stage might require a maximum speed of, say, 110 mph in a Mini, that should be the speed at which the4 engine is delivering maximum power. If you then select a final drive ratio to give that speed in top, then the acceleration through the gears will be the best possible right up to maximum speed.

For example, my historic rally Cooper 'S' is geared to give 100 mph at 7000 rpm in top. The fastest it has been on a special stage was 6700 rpm on gravel and 7100 rpm on tarmac.

For circuit racing a slightly different way of doing things applies as each circuit will permit a different absolute maximum on the longest straight. That is why different ratio drop gears are available; so that the absolute maximum can be delivered without having to remove the engine between races to adjust the FDR for different circuits.

Hill-climbs are a bit of a 'black art' to me as it is difficult to determine what actual maximum speed will be achieved, so some experimentation will be needed for different venues.