[Rudra Sen]

Originally Posted by Shan2nu

in this case it's the velocity of the vehicle that's causing the overrevv and that can/will damage the engine.

Yeah Shan, talk about broken crankshafts, broken bearings, shattered flywheel...

[Shan2nu]

Yeah Shan, talk about broken crankshafts, broken bearings, shattered flywheel...

Can't leave the gearbox out. hehe

Shan2nu

[subodhchand]

Originally Posted by Rice_tuner

subodh... gettin a decent fuel economy depends more on how light/heavy ur foot is while driving...
if ur in 5th gear at 45Km/h and at bout 1200 revs...n u floor the pedal to get to 60, it'll consume more fuel than being at 2200 revs in 3rd gear n gettin to 60 with a slight depression of the throtle

cheers..

Thanks for the response. What you are saying is right but having the TAch should give us some edge over those who do not have one. It should therefore be possible to have some engine speed that is regarded as the best for the consumption and for the performance of the engine.
Subodh

[V-16]

its a debatable topic guys, if u want the optimum of everything from ur car (for car freaks like us) then the tacho is a great help. but for a novice...dont think he even sees it.and yes who redlines an engine?? (except many of us). i guess one can figure out when to change when one listens to the engine (o what a lovely sound)
especially in a turbo charged car with turbo lag, if u wanna see others lick ur dust, get a tacho to wipe em out with the proper revs...VROOOOM VROOOOMMMMM

[Modifiedsachin]

I had some problems with my zen at low speeds and was thinking that I may figure out what exactly is happening using a tacho.
But after reading the whole thread I am really confused.

My Problem: When I am driving at a speed of 10+ KMH I normally shift to second and then when there is a sudden requirement of slowing (needed in bumper to bumper traffic) my car is running on 2nd gear but the speedo has already droped below 10, so before accelerating further I change my gear to 1st and suddenly the car feels as if I am using the 1st gear with parking break on. The smooth drive gives suddeen jerk, if at the same moment (below 10) I change to 2nd gear it again becomes smooth.

I was gussing that even the speedo has dropped below 10 but the engine is still running at enough RPM to keep in 2nd and looking at speedo I shift to 1st which gives the effect of engine breaking.

[GTO]

My Problem: When I am driving at a speed of 10+ KMH I normally shift to second and then when there is a sudden requirement of slowing (needed in bumper to bumper traffic) my car is running on 2nd gear but the speedo has already droped below 10, so before accelerating further I change my gear to 1st and suddenly the car feels as if I am using the 1st gear with parking break on. The smooth drive gives suddeen jerk, if at the same moment (below 10) I change to 2nd gear it again becomes smooth.

I think you are experiencing engine breaking. Try releasing the clutch (after engaging first gear) in a more linear fashion. Also, search within our archives to know more about engine breaking.

[Modifiedsachin]

Hello GTO,

I do know a little bit about engine breaking, but my main problem is when the speedo is showing below 10 then as per gear to speed count, I should go to first gear in normal drive but that gives me a jerky feel. So is the gear to speed ration also has a RPM as an hidden factor?

i.e. even if the speedo shows below the minimum rated speed of the gear is the RPM against which it should be decided which gear one should switch to?

[manim]

In Civic-AT, during paddle-shifting, even if 'down' is clicked instead of 'up', the electronics does not shift. So it is impossible to over-revv the engine in an AT . Also the opposite, stalling the engine cannot occur in the AT, since it shifts down automatically.

My two cents.

[Shan2nu]

i.e. even if the speedo shows below the minimum rated speed of the gear is the RPM against which it should be decided which gear one should switch to?

Lets say your first gear ratio is 3.2, 2nd gear ratio is 1.9, final drive ratio is 4.5 and wheel circumference is 72.8 inches.

So at 10kmph your car will be doin 770rpm in 2nd (which is where engines usually idle and can't possibly continue) so you shift down to first gear which makes the engine spin at 1297rpm. This 527rpm diff is what causes your car to jerk.

Irrespective of how low your vehicle speed is, there is bound to be a diff between each gear. You can find out how big this diff is by caculating the overall ratio for you car.

gear ratio x final drive ratio = overall ratio

EG: In this cars's case
1st - 3.2 x 4.5 = 14.4
2nd - 1.9 x 4.5 = 8.55

Now 8.55 is around 59.375% of 14.4 so the engine rpm in 2nd will always be 59.375% of the engine rpm in 1st. So if you calculate 1297 x 0.59375 = 770rpm.

So no matter what rpm your doing in 1st, the moment you shift into 2nd the engine rpm has to drop to 59.375%.

Shan2nu

[Modifiedsachin]

Originally Posted by Shan2nu

Lets say your first gear ratio is 3.2, 2nd gear ratio is 1.9, final drive ratio is 4.5 and wheel circumference is 72.8 inches.

So at 10kmph your car will be doin 770rpm in 2nd (which is where engines usually idle and can't possibly continue) so you shift down to first gear which makes the engine spin at 1297rpm. This 527rpm diff is what causes your car to jerk.

Irrespective of how low your vehicle speed is, there is bound to be a diff between each gear. You can find out how big this diff is by caculating the overall ratio for you car.

gear ratio x final drive ratio = overall ratio

EG: In this cars's case
1st - 3.2 x 4.5 = 14.4
2nd - 1.9 x 4.5 = 8.55

Now 8.55 is around 59.375% of 14.4 so the engine rpm in 2nd will always be 59.375% of the engine rpm in 1st. So if you calculate 1297 x 0.59375 = 770rpm.

So no matter what rpm your doing in 1st, the moment you shift into 2nd the engine rpm has to drop to 59.375%.

Shan2nu

ok, i am getting only little bit because I can understand the calculation but not that aware of exact working of gear ration w.r.t. speed (I hope its related to speed).

So what I understood is when ever I am shifting from 2nd to 1st gear the engine RPM suddenly shuts by 527 and hence I observe jerk at below 10 speed.

But what can be done to avoid this as my 2000 Zen do not have a tacho & every time I face this situation i know the jerk is coming my way but do not know how to avoid it without making any adverse effect to car?

[Shan2nu]

ok, i am getting only little bit because I can understand the calculation but not that aware of exact working of gear ration w.r.t. speed (I hope its related to speed).

It's simple. Rem the overall ratio we calculated for our test car. When i say the 1st gear has an OR of 14.4 it means that for every revolution of the wheel, the engine has to make 14.4 revolutions but when you shift into 2nd this fig drops to 8.55 revolutions. Which is why, as you switch to higher gears you can maintain a lower engine rpm at the same vehicle speed.

So what I understood is when ever I am shifting from 2nd to 1st gear the engine RPM suddenly shuts by 527 and hence I observe jerk at below 10 speed.

527rpm was just an example i gave. Each car has it's own drop % depending on how the gear ratios are spaced out.

But what can be done to avoid this as my 2000 Zen do not have a tacho & every time I face this situation i know the jerk is coming my way but do not know how to avoid it without making any adverse effect to car?

U can try n release the clutch slowly when you downshift into 1st. This will smoothen out the transition. But at high rpms, you need to match the revvs (using throttle) while you downshift.

EG :
Lets say you're doing 2500rpm in 2nd and feel the need to shift into first, you need to revv the engine while you downshift such that the engine is revolving at an rpm that matches 1st gear. In this case it would be 4210rpm. There is a much higher diff in rpm bcoz the drop is percentage based. As the shifting rpm gets higher the drop/rise diff increases.

This is where the driver starts feeling the need for a tacho.

Shan2nu

[Sudipto-S-Team]

Most drivers change gears intuitively. At least that's what I do.
I use the tacho in the morning while starting the car and leaving it in idle for about a minute, just to see the idle rpm. Rest of the day I have no time to see what the tacho is doing.
I think for average you and me kind of drivers it's just a frill, "nice to have, no problem if not there" kind of gadget.

[Kappa]

Hey guys! Just git a question while I was wondering about the tachometer. When we revv, and the tacho needle goes to the max(6-7k rpm), ans its time to shift up, why does the rpm suddenly go down to zero as soon as we disengage the clutch?
Thanks!

[vanam]

Reviving a very old thread. What is the purpose of a tachometer on an automatic transmission car (without a manual override/ paddle shifters etc?). is it just ease of using same instrument console across all variants, looks, or does it have a technical reason?

Thank you,
Vanam

[Kosfactor]

Originally Posted by vanam

Reviving a very old thread. What is the purpose of a tachometer on an automatic transmission car (without a manual override/ paddle shifters etc?). is it just ease of using same instrument console across all variants, looks, or does it have a technical reason?

Thank you,
Vanam

You could put a large logo of the car maker in the empty space and call it a day or fit a Tachometer instead - yep its there for appearance for the most part.

In a manual transmission vehicle it may let you rev to 5000 RPM but the owner`s manual will tell you not to run the vehicle beyond ~4000 RPM for extended period of time, you cannot always tell this by ear so a Tacho is necessary.

I have learned shifting by ear, while this works fine for older engines, the Turbos do not like to be lugged, so when the vehicle is new its good to have a Tacho to calibrate your ears a bit and it keeps me from driving at engine speeds above the recommended RPM on highways for extended period of time.