Swapping Trans in the Motor-Rolla

[jeff88]

I see all of the Civic threads about swapping parts, including transmissions and I want to look into something similar for my Corolla.

I *think* I have the A245E transmission and I have found the A246E, which I believe is swappable (need to confirm that too). According to Wikipedia, these are the gear ratios for each:


I've been using this Gear Ratio Calculator, but I'm not really good with gearing and what it all means, so can someone help me decipher the data? What I'm looking for is a peppy car in the first couple of gears and the best possible cruising gear (in terms of lowest RPM).

Can someone tell me what I need for what I want? Shorter 1st, 2nd and 3rd and taller 4th with a tall final drive? Or the exact opposite? If each gear in each trans was easily swappable, which gears would you choose?

Ahhh... I'm so confused!

[Daox]

If you're going to go through the hassle and cost of swapping a transmission, I would seriously look into a manual trans swap instead of keeping the auto.

[jeff88]

That's what I always hear. If I could, I would, but in this situation it's a no-can-do.

Any idea on what set of gear ratios is best?

[some_other_dave]

For fuel economy, you want the tallest gears that you can run at your target speed with the torque converter locked up. If you're going to be accelerating and decelerating all the time (e.g., city driving), the gearing probably doesn't make a whole lot of difference--your FE isn't going to be very good anyway.

If you've got lots of highway miles to travel, you want the tallest gearing you can find, subject to where the TC locks up. "Gearing" takes into account the individual gear (4th gear, the tallest one you have), the final drive ratio, and the tires. Larger tires effectively make the final drive even taller, reducing the RPM to run a given speed.

Speeds in 4th gear at 6000 RPM for your first transmission with 25" diameter tires are: 46/83/129/188 MPH
Speeds in 4th gear at 6000 RPM for your second transmission with the same tires are: 37/68/105/153 MPH

Since the speed at 6k RPM in 4th is almost 20% higher for the first trans, you can figure that the RPM at any given speed in 4th is about 20% lower with the first trans than the second.

So given the two of these, you want the first trans--the 245 one.

-soD

[jeff88]

Thanks for your insight!

I didn't think about the 20% faster means 20% lower RPM part. That makes sense, but I'm mostly only in 4th gear at any given time, since I'm usually on the highway. So would it make sense to have shorter gears in 1st and 2nd to accelerate faster/sooner/easier and then have 'cruising' gears in 3rd and 4th?

I re-did the calculator, so that it was at 2000 RPM at 60MPH with 22.5" wheels/tires. It still shows that the 245 is a better transmission for lower RPMs, but is more accurate to what I would see. (my tires are currently only ~22.5", taller tires will definitely help)

A245E Transmission Gear Ratios
A246E Transmission Gear Ratios
Hybrid Transmission Gear Ratios

I guess at this point, my question is would it make sense to have shorter gears for acceleration and then taller gears for cruising?

[D.O.G.]

Originally Posted by some_other_dave

For fuel economy, you want the tallest gears that you can run at your target speed with the torque converter locked up.

If you've got lots of highway miles to travel, you want the tallest gearing you can find, subject to where the TC locks up.
-soD

Very true soD.

When I did my trans swap in the Datsun, I was advised by those that had done this swap (on another forum) to keep the original diff ratio (3.54).
Top gear went from 3400rpm at 110kph to 2300rpm, about 32% taller

The L28E engine handles that ratio OK and it's great on the motorway, but the (non adjustable) TC lock-up is around 10kph above highway PSL (80kph, a third of my commute).

I have a 3.9 diff that will still give me a 25% taller top gear than standard, but will drop TC lock-up down to a usable margin below highway PSL.

I still need to find time, opportunity and assistance to swap it over though.

[some_other_dave]

Originally Posted by jeff88

I guess at this point, my question is would it make sense to have shorter gears for acceleration and then taller gears for cruising?

Not really for fuel efficiency. Might be worthwhile from other perspectives, but pretty much the most efficient RPM you can run at is the lowest RPM you can stand.

I don't know how the torque converters in either of those lock. It might be worth checking into, because if one will be locked during cruise and the other won't be (for whatever reason) that would be an extra factor. An unlocked TC wastes energy...

-soD

[Occasionally6]

Calculating gearing is quite straightforward, if easier with an online app. (or a spread sheet).

Divide the engine rpm by the particular ratio (the taller gears will be multiplying engine rpm as you are dividing by a number <1) and then divide by the final drive (always a reduction).

That will give the wheel rpm - which is nominal with an auto. transmission. Multiply by the tire circumference to give a distance (in inches) covered per minute. By 60 gives distance per hour and then convert inches to miles for MPH.

It's nominal with an auto. trans. because the torque converter slips, varying with engine torque/load, when it is not locked up. That effectively lowers the gearing. That is why auto. trans. are geared "taller" - numerically lower - than manual trans. and can use fewer gear ratios. Swapping in the different ratio trans. may not be the only option if different TC are available.

I don't know how you would find out (easily) what the exact characteristics of the TC are, particularly because they also depend on the torque and load it is subject to. Size is one indication; physically smaller are usually "looser" i.e. slip more at a given load. The stall speed - measured with the driven wheels held stationary - of a TC might be found in shop manuals though.

TC clutch lock up is usually determined based on road speed and engine load, either directly from a MAP sensor or derived within the ECU from engine air flow and rpm. If the road speed sensor is "downstream" of the final drive, that will remain the same with altered gearing. With taller gearing the engine load will be higher - good, in terms of getting the TCC to lock - and rpm lower - may not be so good, depending - for a given road speed. Really high engine load will also cause the TCC to unlock.

To calculate "ideal" gearing with some precision you really want a full BSFC map. For some more insight, find any BSFC map that has the lines of constant power on it (like the VW one). Pick an arbitrary - but typical - power output (from measured engine MAP or TPS %). Then see what effect your new rpm will have by running along the constant power line to the new rpm and reading off the lower(?) BSFC.

A couple of other things to consider. The trans. shift points will be determined from engine load, engine rpm and road speed. The different installation (and wheel circumference?) may affect those. If the trans. is shifted electronically you may have issues with compatibility with the PCM or need an associated stand alone trans. control module.

[jeff88]

O6- I still cannot find a full bsfc map for the 1zz. It's certainly holding me back, so I can't fully understand my car. Any ideas on where I can find it or at least start my search, other than Google?

Availability aside, you can swap TCs?

I'm wondering if this is a(nother) dead-end project, only coming to fruition in the corral...

[user removed]

.892X2.655 (245)
.982X2.962 (246)
Looks like the final drive and diff ratios are going in the wrong direction for lower revs per mile. You would really be ticked off if you did that swap.

regards
Mech