Someone please explain downshifting for acceleration to me!!

[tokes99]

I understand that in the lower gears you generally have more power, but I just not visualizing why you would downshift to get acceleration. Can anyone explain with couple of examples.

[2strokebloke]

It's like when you downshift on a hill, you don't downshift to slowdown, you downshift to maintain speed. You select a lower gear to let the engine rev higher, closer to it's power peak than it was in the higher gear. Of course this is only workable when the car is revving pretty low in a higher gear; like 40mph in 4th, come to a hill, shift to 3rd and continue going 40mph. Or in the case of accelerating, shift to 3rd and accelerate to 50mph before shifting back into 4th - of course where you shift is going to change from car to car.

[454Casull]

Because you get more torque. F = ma.

[tokes99]

Thanks for the replies, but if you rev the engine over the given rpm doesn't that "burn the clutch up"

[tonnerboy]

It will only burn the clutch up if the clutch is slipping. A properly setup clutch shouldn't slip if you don't have your foot on the clutch pedal.

You have to remember that gear ratios multiply torque. So a lower gear will give you more torque at the wheels, but at the expense of wheel speed. This is why lower gears pull harder than higher gears.

[JohnnyWash1]

The more times an engine can rotate, the more horsepower it can make, generally speaking. Therefore, you want to select a lower gear to make the engine rotate more, creating more revolutions a minute accomplishing more work in the given time. This is only a general rule; the important thing to follow in any car is the powerband--the rev range that produces the most power. But, generally speaking, you will make more power at 4000rpm than 2500. As for your clutch, unless it is extremely worn out or improperly installed, it won't burn up, spin or otherwise self destruct unless you ride it (keep your foot halfway depressed on the pedal, allowing the clutch to spin to control accleration).

Jon

[krebs128]

but what about in automatics?-is this any different? b/c i've been messing around w/ the shifting N-D-3-2-1....another recent post in here spurred my curiosity. anyway, if i'm in drive and i shift to 3rd, my cars slows down a little, then if i shift down to 2nd slows down even more, and 1st it actually jolts to about 10-15 mph. so apparently this is not supposed to happen, or could it be different in auto's than man? when i downshift like that, i can hear the engine 'de'-reving; idk if that's what it's actually doing, and idk if it's good for the tranny.

[JohnnyWash1]

krebs, are you named after the ATP producing cycle of the same name?

Jon

[krebs128]

haha, wow. just a little off topic but no, i'm not named after the Krebs Cycle-though many retarded ppl at my school acredit me for the cycle....

[JohnnyWash1]

Just checking

[krebs128]

well, anyone have an answer?

[cvcc_wagon]

well if you did that in a standard it would do the same thing if you didnt give it gas. it jolts to 10-15 mph because first gear is a lower for starting off

[JohnnyWash1]

Originally Posted by krebs128

but what about in automatics?-is this any different? b/c i've been messing around w/ the shifting N-D-3-2-1....another recent post in here spurred my curiosity. anyway, if i'm in drive and i shift to 3rd, my cars slows down a little, then if i shift down to 2nd slows down even more, and 1st it actually jolts to about 10-15 mph. so apparently this is not supposed to happen, or could it be different in auto's than man? when i downshift like that, i can hear the engine 'de'-reving; idk if that's what it's actually doing, and idk if it's good for the tranny.

Your car does that because it is changing gears, much like on a bicycle. The lower gears have shorter ratios, meaning the engine spins more times per tire revolution. This multiplies torque, but ultimately limits top speed in that gear, due to maximum engine speed. When you are at 50mph in overdrive, let's assume you are at 2000rpm. When you shift to 3rd (a shorter gear ratio) your engine has to speed up a couple hundred rpms (or whatever the ratio difference is) to keep that speed, and in the exchange, the car slows down to accomodate the engine and the gears. Doing this from a high speed into 1st gear (your shortest gear) is not recommended, due to the extreme load placed on your engine--not to mention your drivetrain (gears, transmission, driveshaft, etc.).


Jon

[krebs128]

well hey, i figured it wasn't good, so i don't do it...but thanks for the reply. krebs-cycle, haha

[tokes99]

the only time I really use the to control/slow down the acceleration is when I am in reverse, because it is easier that way? Could this cause problems?