MetroMPG.com mailbag: VW Polo transmission swap not very successful

[MetroMPG]

Got this interesting message from a visitor commenting on the tranny swap I did for the taller final drive (MetroMPG.com post ... EcoModder discussion).

His transmission swap didn't return the expected results. He figures it dropped the engine too far out of its best BSFC range at cruising speeds (sounds like he cruises fast, @ high loads):

Just a brief set of thoughts on gearing etc.

A few years ago I had a VW Polo, a car of a reasonably similar class to your Metro. It could be astonishingly frugal given that most of the mechanicals were built for robustness rather than performance OR economy, and the initial body model had been mad e with lego bricks instead of clay. Possibly due to the fact it had even less power on tap than yours... but it was reasonably lightweight and so had good in-town and countryside performance, and could still put up a decent fight to reach 90mph (with 30 mpg at that speed, still).

However, it was obviously under-geared, with a barely-overdrive 4-speed (0.89 top, just below peak power at top speed) and adding a tacho to the instruments only highlighted that - 4000rpm at 70mph, eek. So after a lot of discussion on a Polo owner's board (with both encouragement and discouragement), I set about finding a 5-speed transmission, from the surprisingly even-more-efficient (on the open road) 1.3 litre version.

Eventually secured for a small fee, and after an epic saga of installation (they really didn't make it easy for the home mechanic on that car, it took *days*) I took it out for some test runs.

The first four gears seemed mostly the same, and it drove just as fine as ever. 5th was still surprisingly punchy, as it was only about 20% higher overall, pulled reasonably well from around 30-35mph and could still hit the mid-80s top speed.

However, it didn't deliver the expected economy boost.

Maybe it was just my driving style, or the terrain, I don't know. Personally, and with the input of some of the other guys online, it might be that the engine itself was simply tuned to have quite a high "sweet spot" when the throttle was open enough to reach highway speeds (and lower around town, etc), with the effect being so dramatic (coupled with the poor aerodynamics) that I might have seen *worse* cruising economy if I'd managed to find the +25% or higher gear that I originally wanted.

Peak torque with the original box was around 50mph, given the seemingly-crazy gearing (with which it would hit 70mpg under test at 45mph/45mph at 70), and about 60 with the new gears (turning in similar results, rather than giving better mid-50s to mid-60s mph economy).

It *was* quite a "buzzy", freely and high-revving motor after all, which was part of the fun on twisting roads - forcing it to run slow was probably making it quite unhappy!

Unfortunately I didn't have chance to give it much more than a few hundred miles of testing, but I made what I had count as much as possible, as I locally have access to quite a decent array of high-standard roads. Oh well. At least it lowered the rpms some, so the engine wasn't as likely to burn out quite as quickly (a second time; i already had to overhaul it once from the thrashing it got)... 3333rpm at 70 instead of 4000, etc.

So a bit of a wierd transatlantic tale in how best intentions in improving economy mechanically don't always work, subject to the vehicle's engineering.

Happy to see that yours turned out fine, however, even if it was only 5% or so.

Currently I'm considering doing it again, some years further on, with my current car (larger, heavier, bigger engine)... it already has a lower-revving transmission (and an engine thats much happier at low speeds than high) than the polo could ever have dreamed of, but the economy is lacking, both around town and recently on the open road. Needs some aero mods perhaps or at least if I can find a 6-speed to put on it... higher top gear and closer first 5 (requires quite a lot of throttle to pull off the line with other traffic, some days, as the first gears are tall).

Best of luck with the rest of your experiments

PS the above are imperial mpg measurements, so they make yours look even better... 125?!

[extragoode]

Sounds to me like he needed to add some aero mods to reduce the load at those crazy high speeds. I'm suprised it could even go that fast with that high of gearing.

[SVOboy]

Indeed, I think if he could reduce the load a bit he might be better off.

Sad to hear since the Polo is such a sweet ride.

[robchalmers]

There are 4 different gearboxes that will fit the 1.0/1.3/1.4s all range from 'city' boxes for the 1.0 to 'bahn' boxes with stupid tall ratios for cruising (30mph/thou)

Personally I'm looking into getting a bell housing from a 1.6 much taller cluster throughout and then puting the 1.9 diesel sedan top cogs and final in it

[JasonG]

IIRC,
Gasoline engines are at their peak efficiency at wide open throttle.
If the factory tranny kept it loaded and floored at 70mph then it was on its most efficient spot of the curve.
Changing the gearing pulled it off peak. I wonder what the MPGs at a higher speed would've been?

[robchalmers]

mine will cruise at 90+ (cough-where legal limits allow )and still return 30MPGus

[theunchosen]

Its about torque/ per rpm.

He pushed his engine very far outside of that. There is still plenty of power but not much useful force for every increased TP. For example as he reaches 50 mph in (I guess) 4rth he was at peak FE as far as the engine was concerned. Putting in the least amount of fuel to get the most torque. Any higher and its not available so you have to stack and push further.