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Old 1st October 2015, 08:06   #1
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Default VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels

Polo TSI & TDI: Torque-at-the-wheel in different gears.
I Recently put down money for a GT TSI as:
it’s the only hot hatch on the market
it comes with a dual clutch gearbox (this will be my only car, and will spend most of its life bogged in Bangalore mire, so the self shifter made a lot of sense; that it’s not a slush box sealed the decision.)

Nevertheless I test drove both GTs, the TDI and TSI. The TDI sounded like a truck, and by 4000 rpm, like a bus on the Gangtok road. Disclosure: My last car was a silky V6 that begged to be taken to the 7,000 rpm redline (cut off was at 7,200).

Read reviews on this forum and elsewhere about the superior low end torque of the TDI. Didn’t have an opportunity to drive either car again, so I had a go at analytically simulating the performance of the two.

I trawled VAG sites for the VW Polo, Seat Ibiza, Skoda Leon and Audi A1 and obtained torque curves and gear ratios for both cars. Sat down at my computer and made a worksheet of
  • Torque at the wheel vs. speed in different gears
  • Power vs. speed in different gears
(Note: torque-at-the-wheel doesn’t adjust for drive train efficiency, tire resistance etc, but I’d expect both cars to be within 2% of each other, so using 100% efficiency as a proxy won’t affect the comparison).

Here are the plots.
VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels-tsi-tdi-torque-wheels.jpg
VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels-tsi-tdi-speed-vs-power-gears.jpg

The torque vs rpm plots tell me
Up
  • to 3,200 rpm in 1st gear, the TDI puts more torque on road than the TSI. After that theTSI the puts more torque on the road in all gears, even at 1500 rpm. At 4000 rpm, the TSI delivers 47% more torque in 2nd gear and 60% more in 3rd gear.
  • From 2nd gear on, the 7 speed gearbox, which allows smaller drops in wheel torque between shifts, more than compensates for the TSI’s lower engine
torque.

The power vs speed plots tell me (the TDI’s stops at 5k rpm, the TSI’s go to 6k
  • The TSI puts more power on the ground at all speeds above 28 kmph
  • Once you shift into second, you (or the sport mode) can keep the TSI
within 96% of max power all the way to maximum speed. On the TDI, there’s a big drop in power after upshifts, particularly if you upshift before it sounds like a bus.

Here's a second plot in which the TDI shifts gears at 4,500 rpm fir those who have a low NVH threshold. In this, the power drop after upshifts is greater.
VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels-tdi-power-4500-rpm-gear-shift.jpg

Conclusion.
Off the line at a red light, the TDI will get ahead, but only for a moment. If you’re crawling in traffic and an opportunity appears to dart into the adjacent lane, the TSI may miss a few the TDI wouldn’t (though if you’re riding the clutch, the DSG would compensate for some of the split second advantage).
But above 28 kmph the TSI is in a different league, presuming you put the DSG in Sport mode and stomp on the pedal, or shift yourself. That’s what I conclude from the simulations. Anyone with real world experience care to comment?

On methodology and data
All my data, calculations and graphs are on the attached Excel worksheet. Excuse the sloppy organization and the inept graphs, but I use Excel only as a calculator. When charts and glossy presentations are required, someone else does it.

The source of my data are brochures and other material put out
  • by VAG for the Polo and equivalents (Leon, Fabia, A1) in various countries.
  • by aftermarket chip tuners in Europe and USA
.

Caveats are
  • The torque curves don’t extend till the red line, so I’ve had to interpolate and extrapolate.
  • The torque curves available are not precise (presentation pretty charts that forsake precision for appearance). For example, a document put out by Volkswagen called The TSI Engine: Environmental Considerations shows a torque curve for the 77kw TSI, with torque at 138 NM at 5500 rpm. That calculates to 79.5 kw, So I reduced torque to 133 to keep power below 77 kw.
  • The cars seem to come with minor variations of tuning (engine maps) so my estimate for the Indian cars is a guess, especially for the TDI since India is the only country it comes in a 1.5L (elsewhere it’s either 1.6 or 1.4) and I couldn’t find a torque curve for it.
I think the gear ratios are spot on since they are consistent across countries and brands for the 77kw 5-speed TDIs as well as the 77 kw 7-speed TSIs.

If anybody finds the plots useful, I’d much appreciate you checking them for errors in
  • methodology and calculation
  • assumptions about torque curves
.
The Excel Worksheet is named "POLO TORQUE AT WHEEL - TSI vs TDI.xlsx"

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Attached Files
File Type: xlsx POLO TORQUE AT WHEEL - TSI vs TDI.xlsx (117.6 KB, 419 views)

Last edited by Aditya : 1st October 2015 at 14:38. Reason: Formatting
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Old 1st October 2015, 09:14   #2
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Default re: VW Polo TSI & TDI - Torque & Power at the wheels in different gears

As you've mentioned, the precise mapping & configuration of the diesel is not available as other markets have the 1.6 diesel which on paper would be throwing out much higher torque due to the added power.

Also if I understand correctly, there are 2 versions of this particular 1.5 diesel engine, the 88 bhp version and a 103 bhp GT version, which engine have you used for this comparison? Since you mentioned TDi I'd assume the 90 ps one?

Now for ground practicalities, I'd pick the TSi. As you've mentioned its far more smoother than the diesels or its 3 cylinder petrol manual engines. It is about 100 kgs lighter than the diesel and is better balanced. Also since this is a dual clutch transmission it would always beat the reflexes of a drivers hand while the driver can keep his/her arms on the wheel focussing on the road ahead. Manual is always more fun than an automatic though. If winning a hypothetical quarter mile drag race is the concern then the TDi might have the edge due to the initial rush of torque, but that's only hypothetical. If cruising on a road without strain is the concern then TSi would fit the bill much better.

Again I don't know if the 1.5 engine spoken about here is the 1.5 GT or not. Obviously the 1.5 TDI with 88 bhp will fall short of the TSi in higher gears.
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Old 1st October 2015, 10:29   #3
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Default re: VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels

Quote:
Originally Posted by Muskrat View Post
Conclusion.
Off the line at a red light, the TDI will get ahead, but only for a moment. If you’re crawling in traffic and an opportunity appears to dart into the adjacent lane, the TSI may miss a few the TDI wouldn’t (though if you’re riding the clutch, the DSG would compensate for some of the split second advantage).
But above 28 kmph the TSI is in a different league, presuming you put the DSG in Sport mode and stomp on the pedal, or shift yourself. That’s what I conclude from the simulations. Anyone with real world experience care to comment?
You have put in a lot of effort in compiling this, but the actual on road performance and an actual dyno run would be different.

Couple of points from my side.

1st Gear and 2nd Gear on a stock TDI (GT or regular) is good just to get the car moving. The toque produced by the engine is limited in 1st and 2nd gears to protect the transmission from abuse. TDI gets interesting only from 3rd gear and on. So from a standstill the TSI would be leading.

Having driven both, from standstill the TSI gets off the line quicker. Anyone who has driven both would say the same thing.

To the best of my knowledge the gear ratio is not same across the world for the 1.6 77KW TDI engine (India 1.5 TDI and India 1.6 TDI Speed vs RPM in Gear was same when I compared GT 1.5 TDI against Vento 1.6 TDI). Here in India our 5th gear roughly translates to their 4th gear based on the information gleaned from the web - Speed+RPM+Gear. Maybe its only the final drive or maybe the ratios are different.

TSI DSG will be quicker off the line. But both will go on till 190KPH in stock form where its electronically limited.

1.2 TSI 77KW
Name:  77KW1.2TSI.jpg
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1.6 TDI 55KW
Name:  55KWTDI.jpg
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1.6TDI 66KW
Name:  66KWTDI.jpg
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1.6TDI 77KW
Name:  77KWTDI.jpg
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These screen shots are from VAG SSP, and its of 1.6TDI engines. From my experience the 1.5 TDI is not very far behind the 1.6 TDI comparing stock to stock save for the linear power delivery of the former compared to the rush of the latter.


PS: The sheets mentions DPF for exhaust gas treatment. In our market we don't have it.

Last edited by GTO : 2nd October 2015 at 11:46. Reason: Trimming quote :). Please quote only the relevant stuff. Thanks
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Old 2nd October 2015, 17:46   #4
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Default re: VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels

Regarding the Diesel engine. It's the 77 kw (105 hp) one.Sorry for not making that clear.

My source/proxy for the torque curve is the curve for 77 kw 1.4L on the Skoda Fabia - which I 'massaged' to extend the peak power to 4400 rpm (the number given in the brochure for the India 1.5 GT)
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Old 3rd October 2015, 12:25   #5
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Real world experience only. Caveat: remapped 1.6 TDI and remapped TSI. The TDI shoots off the block. The TSI struggles - rev limiter in N. Through the entire range, the TDI is faster. Same stretch, TDI touches 205, TSI touches 185/193 (depending on tuner). (Please note: safe stretches, no public racing) No contest in a straight line really. The TDI kills the TSI. At best, a TSI can keep it within sight. On curves, tighter ones, the TSI takes it all back and delivers a knockout punch. No chance for a TDI to even keep the TSI within sight.

Yes, the TDI sounds like a tractor while at it. But, undeniably faster and more responsive. And yes, appreciably better mileage even when being driven this way. I barely manage 6.8/7 kmpl driving like this in my TSI.

But the convenience, refinement, handling and responsiveness of the DSG in Mumbai is unbeatable to me even though the dirty diesel is faster.

Last edited by d_himan : 3rd October 2015 at 12:33.
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Old 4th October 2015, 08:40   #6
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Default re: VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels

Would you have any details on the remaps? The torque curves they deliver and changes made to the engine maps -- which I suspect would be mostly, if not exclusively, to the ‘charge pressure control function’ (which is Audispeak for boost pressures and waste gate actuation on the petrol engine/variable vane setting on the diesel).
Got the jargon from the VAG Self Study Programs. There’s one for an almost identical 1.2 TSI engine on the A1 & A3 (only difference seems to be valves per cylinder). Google ‘audi self study 485 Tfsi’ and you should find a link to it. Decent level of detail and informative diagrams.

If I can get my hands on the torque curves, I’ll stick them into the Excel worksheet.
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Old 4th October 2015, 09:58   #7
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Default re: VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels

Quote:
Originally Posted by Muskrat View Post
There’s one for an almost identical 1.2 TSI engine on the A1 & A3 (only difference seems to be valves per cylinder). Google ‘audi self study 485 Tfsi’ and you should find a link to it. Decent level of detail and informative diagrams.
Couple of points from my side.

1.2 TSI and 1.2 TFSI both have same number of valves i.e., 2 per cylinder.
Both are same engines.
77KW variant of both have same engine code, CBZB, according to SSP.
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Old 4th October 2015, 20:01   #8
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Default re: VW Polo TSI & TDI - Simulated Comparison of Torque & Power at the Wheels

Torque at wheel (rather than power) for each gear vs road speed would be more illuminating.

Regards
Sutripta
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