Strange feeling when going downhill

[davesz4mc]

Manual transmission has 3 phases

1. Clutch out in nuetral, stopped coasting
2. Clutch out in gear, driving engine braking
3. clutch in, in any gear or nuetral

So clutch in clutch pressure plate and the flywheel are bolted together but spring assembly is pushed/engaged and is not making contact with the clutch disk. Transmission output shaft, drive shaft and rear differintial are always connected. transmission input shaft is in freespin can be left to free spin or engaged to a gear on the output shaft. This is where synchros speed up/slow down the input shaft to match the gear speed of the selected gear on the output shaft.

Clutch out in nuetral everything is connected except the input shaft is not slotted into a gear on the output shaft.

Clutch out in gear input shaft gear is mated to an output shaft gear (1-6,r) and everything is machinically locked together, at a specific gear ratio that does not ever change (unless you change gears).

This is why manual transmission cars engine brake really well and why it will always do so the same each time and is a reliable means of scrubbing off speed. During engine breaking with no throttle input the engine is always still running in a gas engine as fuel is a lubricant for the upper cylinder section. All flex fuel or multiple displacment vehicles will use a "soft touch" system and it will never shut off the same cylinder each time or damage could occur.

Desiel engines are diffrent in that they do not even have a throttle body or anything of the like. They are completly controlled by adding or removing fuel, when they are coasting the will truly use no fuel.



Automatic transmissions are completly diffent in almost every aspect for manual transmissions.

Torque converter - a donut shapped enclosure that has two fans in it, an input fan and an output fan. The fans effect each other with the movement of transmission fluid. the input engine side fan speeds up moving the fluid with more and more force untill it makes the transmission side output shaft move. *in overdrive these two fans touch and create a physical bond.

In automatics park and nuetral will be the only time the transmission input and output shafts are disconnected from each other. In gear the engine is connected to the rear wheels at all times, this is done with the torque converter. Now it is only a matter of selecting drive, reverse or nuetral to keep it basic. This is done with a stacked planatary gear setup and works by stopping one gear set from free spinning to mechanically step the output gear ratio. When a car shifts hard and fast they are slamming the clutch to that planatary gear set hard and fast, when it shifts smooth as butter they slowley and evenly engage the cluth that stops the next planatary gear set.

So you are in drive in your automatic, at no point is there any seperation from the engine all the way through to the rear wheels, it is always engaged/connected. Under engine braking or no throttle input while in motion the torque converter eates most of the deceleration feel you would get in a manual. It can also change if you are going down hill etc. there is no exact or constant correlation between engine rpm and vehicle speed as in a manual.

[davesz4mc]

Ok that could be better but I got tired of typing it lol.


Also in a higher performance car they need stronger gears in the transmissions and rear ends. The more helicaly cut a gear is the quieter gear to gear contact it has. the flatter the gear tooth profile the stronger but louder it gets with gear to gear contact. Now stiffin up motor and transmission mounts that pass more vibrations on into the cab and you can hear all these differances even more. They say if you put in a almost solid tranny mount it is almost unbearable to ride in the car from this gear noise...

[bhad]

Great stuff. Thanks for that info. So my car is normal

[MikeCOUPE]

Sometimes when I'm on a downhill but at a standstill in neutral, say a red light, the rpms drop really low like the car is about to shutoff and than the engine sputters a little and rpms go back to idle.

These cars do weird things lol

[psygnosis]

Originally Posted by esqu1re

I see now. I didn't know injectors shut off when coasting in gear. I always thought the engine was running, because I still heard the exhaust when coasting.

Air is still going through the engine, just not any/much fuel.