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.
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.
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"
Mod Note : Please do NOT cut-copy-paste text from other word software (MS Word etc.). That results in a formatting error on our Forum.
Click here to know how to remove FONT Tags from your post.
Going forward, do keep in mind that it's best to type directly into Team-BHP.
Thanks