Fiat at the Auto Expo 2010

[GeekSrik]

can someone here tell me what difference a VGT or a non VGT MJD would be like for the layman?

[sandeepmdas]

Originally Posted by GeekSrik

can someone here tell me what difference a VGT or a non VGT MJD would be like for the layman?

The multijet engine that the GP uses now is FGT (fixed geometry turbo) with 75bhp output. You can find the same Fiat engine in Swift, new Indica etc etc.

The "new" engine is the same one but with a VGT (variable geometry turbo) with an output of 90bhp; you can find the same engine in Linea and Manza.

Now you can see the reason for excitement here: a 90BHP GP can eat the competition alive or dead.

[GeekSrik]

thanks for the explanation but i am still missing something here. what i want to know is other than the BHP difference what would it mean in terms of driving, say for eg at lower speeds in higher gears, and in comparison with swift's 1.7k rpm turbo, etc.

[MihirC]

Originally Posted by sandeepmdas

The multijet engine that the GP uses now is FGT (fixed geometry turbo) with 75bhp output. You can find the same Fiat engine in Swift, new Indica etc etc.

The "new" engine is the same one but with a VGT (variable geometry turbo) with an output of 90bhp; you can find the same engine in Linea and Manza.

Now you can see the reason for excitement here: a 90BHP GP can eat the competition alive or dead.

Please correct me if Im wrong herebut doesent the VGT mean that the turbo kicks in sooner , effectively lowering the turbo lag, which would itself be more of an advantage in addition to the 15 more horses.

[sandeepmdas]

Originally Posted by GeekSrik

what i want to know is other than the BHP difference what would it mean in terms of driving, say for eg at lower speeds in higher gears, and in comparison with swift's 1.7k rpm turbo, etc.

Only a test drive can answer the questions.

Theoretically, it should somewhat look like this (graph from petes tuning, which can propel the FGT output to a level that slightly exceeding the VGT; the VGT has a torque of 200Nm, petes boasts 210Nm). "Power PSI" is the tuning box-ed outputs:

My 2c.

[GeekSrik]

@mihir i dont think it means what you are saying. i believe its more about having greater torque at lower rpms or something to that effect.

[sandeepmdas]

Originally Posted by MihirC

Please correct me if Im wrong herebut doesent the VGT mean that the turbo kicks in sooner , effectively lowering the turbo lag, which would itself be more of an advantage in addition to the 15 more horses.

There's an increase of torque also in VGT. FGT = 190NM and VGT = 200NM, I think. Yup, in Indian city conditions, having more torque is better rather than having more BHP. And yes, VGT reduces the turbo lag and increases "on request" power delivery.

Read more on VGT here:
Variable geometry turbocharger - Wikipedia, the free encyclopedia

[Dr.AD]

Originally Posted by MihirC

Please correct me if Im wrong herebut doesent the VGT mean that the turbo kicks in sooner , effectively lowering the turbo lag, which would itself be more of an advantage in addition to the 15 more horses.

Originally Posted by GeekSrik

@mihir i dont think it means what you are saying. i believe its more about having greater torque at lower rpms or something to that effect.

I think Mihir is right. VGT changes the turbine characteristics with speed (because the "Turbine Aspect Ratio" is now variable and can be now optimized for different engine speeds instead of optimizing it for a a-priori fixed engine speed, which happens for FGT), and overall, the turbo-lag is lowered. Thus, theoretically, the turbo effect is spread nicely over entire engine RPM range, and the driving experience will be better. However, this is in theory and should be easy to experience when the turbocharged engines are very high powered (such as 250+ bhp). I don't know how much this effect can be perceived for smaller engines such as the Fiat 1.3.

But in theory, I agree with Mihir that lowered turbo lag and better turbo characteristics over wide engine RPM range are added advantages over the basic advantage of 15 bhp power addition.

Originally Posted by sandeepmdas

Only a test drive can answer the questions.

Theoretically, it should somewhat look like this (graph from petes tuning, which can propel the FGT output to a level that slightly exceeding the VGT; the VGT has a torque of 200Nm, petes boasts 210Nm). "Power PSI" is the tuning box-ed outputs:

My 2c.

I don't this this makes sense. As mentioned earlier, VGT changes the characteristics of the Turbo with engine RPM (in effect, the the Torque vs. RPM curve is changed in "shape"), whereas a tuning box will only increase the magnitude of this curve (e.g. linear amplification, to some extent), while retaining the "shape" of the Torque vs. RPM curve pretty much the same (as also evident from your pictures).

In simpler words, VGT will try to make the Torque vs. RPM curve somewhat flatter (which is very desirable), whereas a tuning box will just "pull it vertically up" without changing the "flatness" (well, it may even worsen from flatness point of view).

So the effects of VGT and that of a tuning box are fundamentally different and can't be compared directly. By the way, I am not an expert on tuning boxes, so experts on this, please correct me if I am wrong

Thanks,
-AD

[deetjohn]

We all know how turbos work. The exhaust gases from the engine pushes the vanes of the turbo to rotate it and which inturn gives additional torque (rotational force) to the flywheel of the engine.

The problem is that at lower engine speeds there is no effective volume/ force of exhaust gas that can spool (rotate) the turbo. So a threshold rpm exists for the turbo to give boost. So a 'turbo lag' exist till that rpm.

You can get around this problem by using a smaller turbo but then this will be useless at higer rpms. It will restrict the high flow of exhaust gases thereby choking the exhaust and mouting the exhaust manifold pressures.

So then we should use a bigger turbo. But this will have a higher threshold rpm and higer turbo lag. See, a classic conundrum.

So we need the best of both worlds. Enter the VGT

The Variable Geometry Turbocharger has little movable vanes which can direct exhaust flow onto the turbine blades at the desirable force.

Lets make it simpler. We all have used a hose to wash our cars sometime. When the flow of water through a hose is low, we squeeze the tip of the hose to increase the pressure and the water pours out faster and farther. But if the flow gets higher there is no need to squeeze the tip of the hose for more pressure. but conversely we need a larger tip to get all the water out at a faster rate.

So the ideal condition is a smaller hose tip when the volume of water is low and a relatively bigger tip when the volume is higer.

VGT uses the same logic.

When the engine is at a lower rpm, the variable vanes are in an almost closed angle. This narrow passage makes the exhaust gas flow to accelerate towards the turbine blades, making them spin faster; like our hose with a smaller squeezed tip.

But when the engine is at a higer rpm, the flow of exhaust gas increase and the vanes open so that they are fully directed onto the turbine blades and the black smoke is not restricted from escaping the exhaust. These vane angles are adjusted via an actuator.

So a VGT equpped Punto will have a lesser turbo lag or more response at lower rpm compared to the Punto that we have now. You can notice this if you drive the Punto and Linea/ Manza back to back.

PS: The explanation may have some mechanical discrepencies but you get the logic.

Read more: Variable geometry turbocharger - Wikipedia, the free encyclopedia

[Dr.AD]

deetjohn, thanks for the nice explanation.

Originally Posted by deetjohn

See, a classic conundrum.

So we need the best of both worlds. Enter the VGT

There is another solution to this conundrum: Twin-turbo! This can (to some extent) give us the best of both words because we can use two different turbos kicking in at two different RPMs. But this is expensive. VGT achieves somewhat similar effect at much lower costs.

[sandeepmdas]

Originally Posted by Dr.AD

So the effects of VGT and that of a tuning box are fundamentally different and can't be compared directly.
-AD

Dr.AD, I am not comparing VGT with Pete's; A TB is not a substitute for a VGT. A VGT is not a substitute for actual cubic capacity either. There's no data available for VGT Punto as of today and GeekSrik was querying how it would fare in road, which is an interesting question IMO. That's why I put the pete's data here, to explain my point theoretically. It was just my 2c as mentioned in the post

[deetjohn]

Originally Posted by Dr.AD

deetjohn, thanks for the nice explanation.

Thanks Dr.AD I too like your posts but sometimes, it just flies over my head.

Also, when I read the post again, I see that I have missed a line at the start and its now misleading.

It should read:
We all know how turbos work. The exhaust gases from the engine pushes the vanes of the turbo to rotate it and which inturn drives a compressor to feed more air into the engine. A turbine converts the heat and pressure of the exhaust gas into additional torque (rotational force) of the engine.

So the engine can now burn more air-fuel than its physical capacity producing higher power and torque.

[p'arth]

Will the TJet have FGT or VGT?

[GeekSrik]

deetjohn, thanks you finally answered it the layman way with the hose example. it has also aroused the physics interest in me

[pnredkar]

Originally Posted by Dr.AD

deetjohn, thanks for the nice explanation.

Originally Posted by deetjohn

Thanks Dr.AD I too like your posts but sometimes, it just flies over my head.

Mutual admiration society, eh!

Both your nice explanations have helped me understand VGT vs. FGT.

- Prasad.