Metro taller transmission swap thread: +5% efficiency gain (taller final drive)

[MetroMPG]

Theoretically, the lower RPM should help with the moderate load/low RPM accel technique during P&G.

I definitely have to shift later now. Up by approximately 7-10 km/h per gear. (e.g. 1-2 @ 25k, 2-3 @ 35-40 k, 3-4 @ 45-50 k, etc.)

[MetroMPG]

I effectively gained another gear. I did this mainly as a way to boost highway FE. I don't think it's going to have a big impact on city mpg. In fact, a case could be made that this could hurt city FE (at least that's what someone at teamswift said who did it. He said he stood on the gas much more to get the car moving, which negated the gains from different gearing... then again, I'm not likely to do adopt that driving style...)

Haven't decided what I'm going to do with the short legs. I could easily sell it - some of the teamswift speed enthusiasts like do do the exact opposite of what I just did: put the 1.0 trannys in the 4-cyl GT cars.

I'm tempted to just hang on to it because: I'll have a back-up in case I have trouble with the taller one (it's got about 180,000 km on it); or if I ever want to revert the car back to "stock" for some reason.

[MetroMPG]

July 12/2006...

Update: I've noticed one mechanical issue with the 3.52.

Sometimes when engine-off coasting, I can't easily get back into gear to do my next clutch start. Meaning: I'm coasting with the engine off, depress the clutch and try to select a gear. And... no go - the shifter resists slotting into gear.

If I force the shifter under these circumstances, I crunch into the gear I'm trying to get. I've tried double clutching, and tried to select other gears than the one I want, but it doesn't seem to work.

The transmission itself seems to shift nicely when the engine's running though. I don't think it's synchro related.

One other thing I noticed that may be related to this: yesterday I popped the car into neutral to roll out of a parking spot (hadn't yet started the engine), and it wouldn't really roll easily, like something was hanging up a little bit. I pressed and released the clutch once, and that fixed it - something seemed to unhitch and the car rolled normally.

Mechanical gurus: all this seems to suggest what? That the clutch is not fully releasing sometimes? Is that the clutch release bearing (which I know nothing about... I'm just throwing that term out there)?

It's not a massive problem, as it only happens once in a while.

Edit: the "new" tranny also has a very quiet mechanical "whine" when the car is stopped (engine running), the shifter in neutral, and the clutch pedal released. Don't remember the original tranny making that sound. Adding this in case it's related.

[MetroMPG]

PROBLEM SOLVED!

Adjusted my clutch cable this afternoon - I believe it was engaging too close to the floor (in terms of pedal travel). That would have caused problems re-engaging any gear coasting with the engine stopped if there was even the smallest amount of clutch drag.

I ran errands this evening, and the transmission didn't balk / hang up once. In fact, it also seems to shift smoother (though that could just be placebo effect).

[MetroMPG]

July 26/06 ...

Update: since the clutch cable adjustment, I've had absolutely NO problems with the 'new' long-legged transmission in any way.

I've almost unlearned my old shifting-based-on-engine-rpm habit developed with the original gearbox (I have to stretch the RPM just a little more when accelerating since the gaps between gears are larger).

Hill starts require noticeably more clutch slippage, as my "new" 1st gear is equivalent to somewhere between gears 1 & 2 on the original gearbox. But it's not unmanageable. If I were driving around with 2 or 3 people in the car on a regular basis and lived in a hilly place, it would perhaps be an issue.

I also do believe it shifts nicer than the original unit, but that's pretty darned subjective, isn't it. My satisfaction, however isn't. I'm still very glad I did the swap.

[MetroMPG]

Originally Posted by SVOboy

Told you so, told you so!

Where's the 110 segment!?

Hell, I'll settle for 105.

Originally Posted by krousdb

Ye have little faith......

Can we split the difference?



edit: Round trip - 7.9 mi. My normal sub/ex/urban "loop". Very good, but not optimal traffic/light conditions.

[MetroMPG]

Originally Posted by krousdb

With or without the alternator belt?

Without. Best I've been able to manage with the belt is around 100.

Edit: which, now that I think of it, is actually an improvement since my previous best of 101 was without the belt.

[MetroMPG]

August 10/06:

The only other hot-weather bi-directional runs I did in warm weather before changing transmisisons was the alternator/no alternator test.

Unfortunately, it's a little complicated by the fact that I tested at 70 km/h, which was the one speed on the speed vs. mpg graph where the data seemed to get a little out of line (literally):


(should be an image above this line)

I'm inclined to think that 71.0 jump at 70 km/h was testing error, since in the alternator vs. no alternator test, the control runs (5 bi-directional) at 70 km/h in 71F temps (vs 58F for the speed runs) averaged 70.8 (i.e. same results, at higher temps, which you wouldn't expect to see).


(should be an image above this line -)

All of this is a long-winded way of saying that the FE improvement from the transmisison is likely closer to 5% (5.6%, based on the single 70 km/h point comparison) than 8.8% I claimed before.

[MetroMPG]

August 21/06:

Collected a bit more info:



I find that "bump" at 70 curious, as well as the narrowing of the difference between the 2 as speed increases.

[MetroMPG]

The taller transmission lowered RPM by 19.8%. It was lowered a further 5.8% because the OEM tires on the Blackfly are taller than the tires originally spec'd on the taller transmission car. For a total effecive gearing change of 25.6%.

With the extra information posted in the above chart, the average difference between the measured speeds vs mpg's for the 2 transmissions is 7.9% from original. That's the most reliable baseline difference yet, since there's more speeds to compare.

But that figure also includes the effect of ambient temperature difference.