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Send me a note: darin AT metrompg D-O-T com, or here |
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Project 'nerd gear': taller tranny transplant nets +5.2% MPG
Posted Monday, December 18/06 in Mods & Tests
"Nerd gear" is the description my brother in-law and I came up with for the key feature of our ideal transmission: we decided that auto manufacturers should offer a much taller-than-usual top gear (a nerd gear) as an option in an otherwise "regular" transmission. Why? Because too many small engines spin at 3000+ RPM at highway speeds, which we declared to be unnecessary and wasteful in the hands of an attentive driver. It's true that a car with a nerd gear would see slower top gear performance (acceleration & possibly max speed), and it would require the driver to shift to & from top gear more often (depending on load/terrain). But if you're willing to tick the box labelled "nerd gear" on the option sheet, we figure you're probably willing to pay a little more attention to proper gear selection for the fuel economy payoff. So I did it - or something very close to it: I swapped transmissions in the Blackfly, and was rewarded with a healthy improvement in fuel economy. Tranny transplant overview ...
Beyond the nerd gear ... I actually went much further than installing a nerd gear. The fact is, there is actually no readily available extra-tall 5th gear that I could just drop into my stock transmission. So what I did was swap the whole transmission from a 4-cylinder Suzukiclone into my 3-cylinder car. The gear ratios are the same, but the taller final drive in the 4-cylinder transmission makes all my gears taller than stock. It's a mechanical mix 'n' match that the Suzukiclone engineers never intended to happen. Terms & definitions ... Before undertaking this project, I'll admit I wasn't totally sure what the term "final drive" even meant. So, for the benefit of anyone else who may be shaky on transmission concepts, here's a brief run-down, with links to more info, pics & fancy animations:
Swap motivation ... I'd never gotten deeply mechanically involved in a front-wheel-drive car before this project. To be honest, I had always been a bit intimidated by the magnitude of FWD drivetrain work: disconnecting ball joints, removing axles, working in confined spaces - it all seemed so much more complicated than working on a rear-wheel drive car. So I avoided it.
Another point that delayed me giving this idea the green light: I was reluctant to swap out a perfectly good transmission with only 8,000 km (5,000 miles) on it! But eventually, it grew too hard to ignore the bee in my bonnet which grew louder as it fed on tantalizing info from a number of sources:
The deal was clinched when it turned out that a 1993 Suzuki Swift 1.3 that a friend and I bought to convert to an electric car was too rusty to save. Its transmission was good, though, and it had the taller final drive that would work in the Blackfly. Doing the math: gear ratios for dummies ... FYI, here are the gear ratios for all the 1.0L Suzukiclones and the 1.3 SOCH hatchback:
1st gear: 3.416 Final drives:
4.39 - 1.0L garden variety (1995 and up), 13 inch wheels Usually, the lower the final drive number, the lower the engine RPM for a given road speed. The garden variety 1989-2000 3-cyl cars actually have similar engine RPM because the difference in their final drives accomodates different wheel/tire sizes.
The graph below shows the change in RPM I could expect from the swap. It was a deciding factor to green light the project. What you see in the graph is a comparison of engine RPM in top gear at 80.5 km/h (50 mph) that my car was turning in stock form compared to the XFi, the Swift 1.3 donor car, and then a calculation of what the Blackfly's RPM would be using the Swift's transmission (it's even lower than the Swift because the Blackfly came with taller tires, and that lowers engine RPM even further). It also shows that in 4th gear with the Swift's final drive swapped in, the Blackfly's engine would be turning at practically the same RPM that it turns in 5th with the stock transmission. So the swap would effectively gain me another gear. Running in 5th with the 3.52 would be MPG gravy. Can you overdo overdrive? I recall reading about someone who tried this (of course I can't find the reference now), who was not very happy with the results. The new gearing was so tall that the car's already slow acceleration became unbearable. He ended up using lower gears longer and more often, and with much more throttle than before, which negated the theoretical fuel economy benefit of the swap.
So is it possible to overdo things with gearing changes? Sure. Your OEM final drive was chosen by the engineers to balance fuel economy & performance. Depending on how & where you use your car, you could easily go too far:
Fortunately for me, none of these scenarios apply in my typical driving. Doing the swap: transmission plug 'n' play ... Despite having never done anything like this to a front wheel drive car, it wasn't all that difficult. I really shouldn't have been so wary of taking the FWD plunge. Suzuki's engineers made the job pretty easy, too: the Swift's 3.52 f.d. transmission is a direct plug 'n' play replacement for the Firefly/Metro's 4.39 unit.* Yes, the 4-cylinder car's transmission uses different mounts & shifter rods (because it's offset further to one side of the engine compartment to accomodate the extra length of the 4-cyl engine block), but all of that hardware comes off the transmission casing, and the 3-cyl hardware fits in its place.
Here's the checklist:
Reverse the steps to install the replacement. In the hands of an experienced mechanic, this job should only take a couple of hours. But I won't lie: it took me a whole weekend (partly because there were some family activities going on too). The most difficult step was sliding the replacement transmission back onto the engine. (Hint: be patient getting the input shaft lined up straight with the flywheel, and leave the transmission in neutral so you can turn the shaft to help it slot into position. Also, the engine mounts are pretty soft, and the engine will squirm around while you're trying to wiggle things into place.) (* One teamswift.net member in Germany reported that when he tried this swap, the transmission casings did not match and he had to also swap over the shifter stabilizing rod from the 4-cylinder car.) Blackfly mpg results: before & after the swap ... For baseline data with the original transmission, I used the readings from the Speed vs. MPG experiment with the original gearbox. After swapping transmissions, I went back to the same road on 2 different days and collected more readings from bi-directional runs.
The graph shows the uncorrected average MPG difference between the the two transmissions is 7.9%. But it doesn't take into account the effect of different ambient temperatures on the days the readings were taken (fuel consumption varies measurably with temperature). So, plugging the temperatures into a formula for rolling resistance and aero drag, I calculated that 2.5% of the 7.9% increase could be attributed to the temperature differences, which leaves 5.2% of the FE improvement owing to the transmission change alone. Post-swap observations & re-learning to drive ...
True gearheads: corroboration from other transmission tinkerers ... A number of Ecomodder.com members have done transmission swaps for better fuel economy. Some were changes from automatics to manuals, like SVOboy's 91 CRX DX (shown) and DaX's 90 Civic DX sedan (shown). Others were upgrades from "regular" manual transmissions to taller manual transmissions from the efficient VX, HF & CX cars, like Matt's '89 Civic LX sedan, and Budomove's '91 Civic hatchback (shown).
Update, Jan 23/2008: I received a message from another transmission modder who performed and documented a 5th gear swap in his '98 Ford Escort ZX2. Getting straight to the juicy bit: he saw an average mpg improvement of 6%.
I recommend a look at his thorough web site which details the modification and the ultimate results (he ended up removing the gear from the vehicle): Modifying Gears for Taller Overdrive Resources ...
darin AT metrompg D-O-T com, or here
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