[rajdoll]

Hi Guys,
I have a query about 4-5 gear. Assume I'm in 4/5 gear and going at 100 km/h, and then I start slowing down by applying brakes without shifting down then at what speed my engine will stall(will it stall or not), if I don't shift down. I understand that I'll not be able to accelerate but will engine start giving jerks once I cross the lower thresold for 4/5 gear or there will be no effect. Reason I'm asking this because my understanding is that the engine will always be running at 800 or so rpm even if no throttle, which is quite a high speed in 4'th gear. clarifications solicited.

[Deeps]

The engine will eventually stall as the rpm will drop less than 800. The engine will jerk 'cos it'll try to turn itself off. At this time or even better slightly before this point you should shift down. The car would still accel;erate evene if you didn;t shift down but the pick up would be very bad until the rpm reaches >1500 rpm. It'd be easier for u to notice if u had a tacho in the car. I am sure the tohers 'll be able to explain in a more comprehensive manner.

[Shan2nu]

The engine will eventually stall as the rpm will drop less than 800

I guess that would be more dependent on the engine's idling speed. Idling speed is the minimum rpm at which an engine is capable of running.

The car would still accel;erate evene if you didn;t shift down but the pick up would be very bad until the rpm reaches >1500 rpm.

Here again, it will depend on how much torque is produced and at what rpm. On a high revving engine like the Vtec where max torque comes in at 4700rpm, you will have to be beyond 2500-3000 rpm for the engine to be able to pull cleanly and quickly.

A Baleno for eg: which produces the same amount of torque as that of the Vtec but, at a lower rpm will be able to accelerate quicker than the Vtec, from 2500 rpm until the Vtec reaches a more comfortable engine speed.

Shan2nu

[speedsatya]

hey rajdoll,
first of all trying to slow down from 100kph only using ur brakes is a not such a great idea.as after sometime ,the brakes will get heated up and cud lose their efficiency.so u cud use engine braking if its not an emergency stop.
like say if u r approaching a toll booth at 100-120kph ,u can lift off the throttle and then shift to 4th from 5th.and at the same time pump ur brakes.in this way u can warn others behind u of ur intentions .and then when the rpm falls below 2500-2000 maybe u can downshift to the next lower gear.in this way u can use engine braking and not lose control also .
but if its an emergency stop then u have to brake .
now for when to change gears so that the engine does not stall....[b]when u dont want to accelerate u can shift from 5 th to maybe 3rd or 4th and wait till the engine is about to stall i.e when the rpm is abt 1000 but the car is rolling due to inertia. and then shift to the appropriate gear.
but if u want to gain speed and u have shifted from 5th to 4th ,then maybe u have to change to a lower 3rd or 2nd gear to accelerate.

[RX135]

Originally Posted by Shan2nu

I
Here again, it will depend on how much torque is produced and at what rpm. On a high revving engine like the Vtec where max torque comes in at 4700rpm, you will have to be beyond 2500-3000 rpm for the engine to be able to pull cleanly and quickly.

A Baleno for eg: which produces the same amount of torque as that of the Vtec but, at a lower rpm will be able to accelerate quicker than the Vtec, from 2500 rpm until the Vtec reaches a more comfortable engine speed.

Shan2nu

Even if you try to accelerate in lower gear, the engine RPM can not increase proportional to the throttle position. Reason being that the engine does not provide enough torque at wheels in 4/5 gear. (With lesser load, it might be able to pull through). This will result in fuel rich mixture supplied to the engine, effectively screwing up the ignition timing. One has to shift to lower gear to accelerate in this case or, do what most truck drivers do. Hit the clutch and disengage engine, let the engine gain higher RPM, engage it. It will be able to accelerate again.

[Shan2nu]

Reason being that the engine does not provide enough torque at wheels in 4/5 gear.

Equal amount of torque is produced at a certain engine rpm, irrespective of the engaged gear. Ofcourse in higher gears the acceleration is reduced due to the taller ratios used for that gear.

Torque and power produced at an rpm don't change with change in gears. If a car produces 100bhp at 6000 rpm, it will produce that much power in any gear that enables the engine to revv to 6000 rpm, be it 1st or 4th.

Shan2nu

[RX135]

Originally Posted by Shan2nu

Equal amount of torque is produced at a certain engine rpm, irrespective of the engaged gear. Ofcourse in higher gears the acceleration is reduced due to the taller ratios used for that gear.

Torque and power produced at an rpm don't change with change in gears. If a car produces 100bhp at 6000 rpm, it will produce that much power in any gear that enables the engine to revv to 6000 rpm, be it 1st or 4th.

Shan2nu

Yeap! That is the point. Can you rev engine till 6000 from 1000 rpm in engine engaged conditions? In 4/5 gear, the torque available on the wheels will be much lesser. In engaged conditions (at 4/5 gear), at lesser RPM, engine can not produce enough torque. It will start misfiring if one tries to accelerate by pressing the throttle. The load on wheels (weight, slope etc.) will decide the torque required to accelerate and the engine has to match that if it has to accelerate.

[fasbond1412]

well if u are in the wrong gear the car will n the rpm is goin low the engine will start knockin.. indicatin to shift down..

[rajdoll]

So does this all mean that technically I don't need to shift down from 4-5 gear to lower gear(assuming I'm not going to use engine braking) till the engine has almost come to standstill or 1000 RPM. But as I understand if engine is engaged in 4 gear(1:1 ratio) and running at 1000 RPM, then speed at wheels has to be more 20km/h. So my question is again, whther a car can still be rolling at 5km/h in 4 gear, where I have slowed down from 100 to 5km/h(which means engine RPM has to be much lesser than the idling RPM?

[Shan2nu]

5kmph in 4th in a car?? No way. The Vtec would be doin around 170-175 rpm if i were to do 5kmph in 4th gear. The engine will definately stall.

I'll have to do atleast 28-30kmph (1000 rpm ) in 4th gear, for the engine to keep running. On a Vtec, anything below 30 kmph in 4th, would be dangerously close to stalling the car.

I personally, never let the revvs drop below 1500 rpm, unless the clutch is pressed.

Shan2nu

[RX135]

5kmph in 4th geat? That would be rather cute engine with aweful torque or absolutely pathetic gearing for a car. Suits well with a tractor.

If you really want to know what is lowest speed you can drive in gear X with your car, glance the manual or look carefuly at the speedo. It will have markings indicating speed range for each gear. You drop below that speed in that gear, engine is likely to stall.

[speedsatya]

Originally Posted by rajdoll

then speed at wheels has to be more 20km/h. So my question is again, whther a car can still be rolling at 5km/h in 4 gear, where I have slowed down from 100 to 5km/h(which means engine RPM has to be much lesser than the idling RPM?

hey dude ,
if u doing 5kph in 4th gear .then first of all u need to go to a driving school.
again i repeat slowing down from 100-5kph only using brakes is not a very good idea.
try to use engine braking.

[RX135]

Originally Posted by speedsatya

hey dude ,
if u doing 5kph in 4th gear .then first of all u need to go to a driving school.
again i repeat slowing down from 100-5kph only using brakes is not a very good idea.
try to use engine braking.

Be easy on him I guess, his intention is to know only technical details. Not doing it in real life At least I hope so!

[rajdoll]

Hey SpeedSatya, I'm the newbie, trying to learn few things, from u experts. Can u explain me again, what will happen if I slow down from 100km/h to 5km/h without changing the gear and why?

[RX135]

Originally Posted by rajdoll

Hey SpeedSatya, I'm the newbie, trying to learn few things, from u experts. Can u explain me again, what will happen if I slow down from 100km/h to 5km/h without changing the gear and why?

In one line, you CAN NOT do it.

Engine will stall much earlier as speed drops below approx. 20 kmph. Depends on which car you have. Check your car's user manual. You should start get knocking sounds and then a good bang and halt! (Unless you are on a 45 degree slope where car may roll down, still trying to run engine. Quite hypothetical)