I'm sorry if it is a very basic question , but its one thats nagging me for a while
Why does the engine RPM fall while you shift up , say from 1st to 2nd . Also tell me why there a rise in engine RPM when you're performing an overtaking maneuver i.e shift down then go up .
Try old mate Bill. Seems to have a simple no nonsense layman's approach.
Your car has to operate over a wide range of speeds, and the engine can not alter it's revs by enough to cater for that range of speeds, so the engine's torque must be passed through two stages to get it to the point where we can use it practically.
When you shift up or down, your wheels are still at the same speed... So engine torque vs tyre grip.
Generally grip wins so engine revs slower (faster) when upshift (downshift). When grip loses, you burnout (upshift) or lock your tyres (downshift). Another possibility is transmission breaks.
Engines make peak power, generally, at high RPM, and little power at low RPM. in order to accelerate fast you want to keep your engine at high RPM at all times (I.e near where it makes peak power).
If you only had one gear you would only be able to use peak power at one road speed... lets say at 120mph.
by adding additional gears we can use Peak power (and the engine RPM's near peak power) at more road speeds... I.e. in a 5 speed box at 5 different road speeds. and that means higher acceleration over a bigger range of road speeds.
To drive fast you want to keep your engine at the high rpm at all times, avoiding low rpm by using the gears.
lets say you are moving at 40mph in 3rd gear, and the engine is at 4000rpm. you open the throttle and accelerate. your engine speed increases and your road speed increases until you hit the red line, at, lets say, 60mph and 6000rpm. Your engine can't turn faster, so to go faster you need to select 4th gear.... the new gear at 60mph would allow the engine to rev at just 4000rpm, so the revs drop allowing you to then accelerate it again up to the red line which would now be equivalent to 90mph, where again you would need to select another gear to go faster.
if you had started in 4th gear to begin with the 40mph would've been equivalent to 2600rpm... which is generally too low for good power, so acceleration would've been slow.
When you are driving normally you don't need peak power, so you generally drive around in a high gear at low revs. if you need to accelerate quickly (e.g. to overtake) you need to get the engine into an area of its rev range where it makes high power (generally this is at high revs). The way to do this is to select a low gear... doing so forces the engine into the high area of the rev band where it makes peak power, and you can accelerate quickly. if you need sustained acceleration you may hit the redline, in which case you'll need to change up a gear as per the paragraph above.
