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Old 20th July 2005, 01:29   #16
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The ghad ghad is the knocking sound...it happens when the firing/combustion happens before the piston reaches the Top Dead Centre and hence the crankshaft gets a push in the opposite directions....
Basically at a high rpm, the piston takes a very little time to reach TDC...to ensure full combustion, the spark plug sparks always before the piston reaches TDC
.on the other hand, at a high gear/low speed and load on engine, there is a situation where the spark plug ignites the fuel but the piston takes Looong to reach the TDC...in the process providing the olt of this combustion to the crankshaft in the wrong direction....hence the sound
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Old 20th July 2005, 01:31   #17
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also ..the ghad ghad could be also engine stalling...starting again due to the momentum provided by the wheels...stalling...starting again...basically it depends on which ghad ghad you are referring to...
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Old 22nd July 2005, 14:37   #18
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My contribution to this discussion:

To drive correctly and using the Tachometer, you
need to also have (and memorise) the Torque-RPM
and Power-RPM curves for your car engine. Normally,
the torque falls off before the power...

You need to also know the gear drive ratios for
you paritcular gearbox.

Then, you need to decide on what you want out
of your driving, if you want to use the least fuel,
or if you want to reach your destination in the
shortest time.

Based on all of the above factors, you can
calculate at what RPM to shift up from each gear
to attain you goal...

Of-course, we don't bother about DOING all this,
so we simply TALK about it!

- mvk

Last edited by DrunkenMonk : 22nd July 2005 at 14:43.
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Old 22nd July 2005, 14:50   #19
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Quote:
By the way, this Engine knocking - the ghad-ghad which one hears if driving or picking-up in a high gear - why does it happen?
No, it is not knocking. This is car stalling. In high gear-low speed combo, your engine can not produce enough torque (which further gets reduced at wheels due to gearing) to move your car ahead. The RPM drops and car stalls.

Engine knocking is generally high rpm phenomenon. At high rpm, there is very little time available for compression/expansion stroke. The charge (fuel+air) explodes randomly before spark plug ignites it.
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Old 22nd July 2005, 15:08   #20
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Quote:
Of-course, we don't bother about DOING all this,
so we simply TALK about it!
1st 3.73/6.8 -44.8
2nd 1.96/12.9 -85.14
3rd 1.31/19.3 -127.38
4th 0.95/26.7 -176.22
5th 0.76/33.3 - 219.78

max torque - 3200rpm
max power - 6000rpm ..
gear changes are well below (2000rpm) the max torque ...
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Old 22nd July 2005, 16:28   #21
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Thumbs up Ghad-Ghad query is clear

@RX135 & @Buffetfan

Thanks for explaining the "ghad-ghad" thingy - It is crystal now.

Cheers
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Old 26th October 2006, 17:18   #22
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I do it this way. Generally I am not worried about RPM. If I feel theres loss of pickup I shift down, revv it and go up. I do it differently between highway and city. In city its more like shift up as soon as you can (based on the feel of pull & engine wroom).

On high ways its like each gear has a streched band, quick pick up: to 20 in 1st, 35-40 2nd, 55-60 3rd, 80 4th. 80+ in 5th. In city it is more like: 10-15, 25-30, 35-45, 40-50, 50+

It varies with the load too, you shift up late if the vehicle is loaded for pickup and pull.
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Old 27th October 2006, 11:51   #23
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Quote:
Originally Posted by GTO
But do make sure not to stress the engine by running in a higher gear at an extremely low rpm. GTO
Taking a cue from that - say I shift up to 5th at 50kmph and then drop the speed to around 35 and drive on without any semblance of stalling .. In such a scenario
a. Is the engine being stressed ?
b. Is it consuming less fuel than it would if I were cruising at 50kmph ?
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Old 27th October 2006, 12:35   #24
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Btw, shouldn't this thread be in the Technical section ?
It is literally in the Shifting Gears section now
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Old 9th May 2014, 19:46   #25
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Default Effectivly using the Tacho for VW Polo Petrol

Hello All,

I'm getting familiar with my parents' new VW Polo Petrol. Having only driven the Maruti 800 (and a pretty old one at that), there's a lot to learn. One of the features is the RPM dial.

I've only ever used an RPM dial in racing video games for gear shift when the dial went to the "red" zone. Is this similar to real life ?

What pointers can you give for using the RPM gauge as an aid for changing gears, specifically in the Polo?

Thank you.
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Old 9th May 2014, 21:17   #26
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Default Re: Using the RPM Gauge for VW Polo Petrol

Quote:
Originally Posted by Kumar R View Post
Hello All,

I've only ever used an RPM dial in racing video games for gear shift when the dial went to the "red" zone. Is this similar to real life ?
Have you tried doing that in real life? That will be the last time anyone will agree to sit in the car when you drive. Have an annoying lift seeker every time you take out the car, do this

Quote:
Originally Posted by Kumar R View Post
Hello All,

What pointers can you give for using the RPM gauge as an aid for changing gears, specifically in the Polo?
Try to keep the engine from revving too high or too low. Too low is somewhere less than 1200 RPM and high is like above 3k RPM. Of course you can revv it once in a while as you please. But for a good Balance between performance and economy, shift to a higher gear once the engine revvs past 2.5k RPM and once it drops below 1k RPM.

These figures may vary from car to car and person to person, but I have given a rough range. You can always optimise your shift points based on the conditions.

To give you an example, when I drive alone in my SX4, I shift as early as 2k RPM and shift down only below 1k RPM. If it is loaded with passengers then I will shift up only after 2.5k rpm and shift down much earlier. In my WagonR, under load I shift up only after 3k RPM and shift down once the engine goes below 1.5k RPM.
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Old 9th May 2014, 21:40   #27
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Hello Kumar, congratulations on the Polo!

RPM basically means revolutions per minute. The meter has readings from 0 to 7 i.e, 0 rpm to 7000 rpm. The ecu doesn't let the engine rev above 5500 or 6000 rpm(this is where the redline starts). When you start the car it idles at around 1000 rpm. For normal driving it's better if you shift by 3000-4000 rpm as always taking it to the redline will return pretty low fuel efficiency(and also stress the engine). You can rev all the way to the redline occasionally when you feel the need for more power. Shift down when you feel the engine is going to start lugging.
After you get a little used to the car, you'll be able to upshift/downshift without looking at the RPM meter

Hope this helps!

Last edited by Suraj25 : 9th May 2014 at 21:42.
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Old 9th May 2014, 22:25   #28
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Default Re: Effectivly using the Tacho for VW Polo Petrol

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Originally Posted by Suraj25 View Post
Hello Kumar, congratulations on the Polo!

RPM basically means revolutions per minute. The meter has readings from 0 to 7 i.e, 0 rpm to 7000 rpm. The ecu doesn't let the engine rev above 5500 or 6000 rpm(this is where the redline starts). When you start the car it idles at around 1000 rpm. For normal driving it's better if you shift by 3000-4000 rpm as always taking it to the redline will return pretty low fuel efficiency(and also stress the engine). You can rev all the way to the redline occasionally when you feel the need for more power. Shift down when you feel the engine is going to start lugging.
After you get a little used to the car, you'll be able to upshift/downshift without looking at the RPM meter

Hope this helps!
Thanks Suraj! The Polo was a gift for my mother - she's a hardcore driver. I'm only sparingly conditioning to the Polo by going out for an hour drive at 6 am each day to get used to the brakes, clutch and dimensions (at least until I get a used car for myself) I find it a lot more challenging (obviously) as the pedals are much more sensitive and the engine is super-quiet (so pedestrians don't hear the car behind them).

I like the idea of using the tachometer for gear shift (feels very scientific) but that's one more thing to keep an eye out for at this stage. Right now I'm changing gears at 2.5k (upto 3rd, not tried 4th yet).
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Old 10th May 2014, 00:33   #29
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Default Re: Effectivly using the Tacho for VW Polo Petrol

Kumar, even though the 800 didn't have a techometer, weren't you able to drive it smoothly without revving too hard or at the other end, stalling it?

Same applies to the Polo. Agreed, new age petrols are silent and smooth but come on, they aren't electric motors. You will be able to feel the motor, you will automatically understand when to upshift or when to downshift. Just because they have provided the tachometer, that doesn't mean that for every up-shift or downshift you have to be a stickler for the perfect RPM count. One has to improvise alot.
Now, you will decide that you will upshift at 3-3.5k and downshift at 1.5k. But sometimes, you will have to upshift or go down at very different numbers.
It depends greatly on the situation; and while driving in the city, I would suggest keeping your eyes more on the road and being aware of the surroundings is more important than keeping an eye on the tacho.

Drive the car and try to understand other factors like while revving after what point does the engine feel stressed or when does it start to lug..
Finally, even if you want to use it, just a split second glance while shifting is enough to understand when to shift. And slowly a time will come when you won't even look at it and you will be driving even without looking at it.

-Bhargav
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Old 10th May 2014, 00:57   #30
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Default Re: Effectivly using the Tacho for VW Polo Petrol

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Originally Posted by Octane_Power View Post
Kumar, even though the 800 didn't have a techometer, weren't you able to drive it smoothly without revving too hard or at the other end, stalling it?

-Bhargav
I never had a problem with the 800. Definitely no stalling although sometimes I think I rev the engine too much for the 2nd to 3rd gear shift.

Quote:
Originally Posted by Octane_Power View Post

Now, you will decide that you will upshift at 3-3.5k and downshift at 1.5k. But sometimes, you will have to upshift or go down at very different numbers.

-Bhargav
Bhargav, my mother who's driving for 35+ years (and is one heck of a seasoned driver) shifts gears (in the Polo) from 1st to 2nd immediately on moving and 2nd to 3rd even while rpm is <2000, sometimes <1700. I wonder whether this early gear shift up is a good idea. That's why I started paying a lot of attention to the rpm (esp. after reading on TeamBHP that 2.5k was a good point to change)

I take your point on not getting overdependent on the rpm - but I think its a good guide to avoid stressing he engine at low gear because sometimes in peak city traffic with the windows up and AC on its difficult to hear the engine being on high rev (at least for me)
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