Transmission/gearing question

[racprops]

I am planing on adding a old Borg Warner over drive taken off and converted from an old 3 speed with OD behind the stock transmissions.

It also can be used as a gear spliter giving me up to 8 gears.

But the main one is a large drop in forth to fifth(both ODs)

Rich

[Ecky]

Just a guess, but I'm thinking the benefits of increased load and lower friction / parasitic drag from accessories would outweigh the increased parasitic losses from the OD, but only on the highway. More gears to go through to the wheels = more losses.

[racprops]

Originally Posted by Ecky

Just a guess, but I'm thinking the benefits of increased load and lower friction / parasitic drag from accessories would outweigh the increased parasitic losses from the OD, but only on the highway. More gears to go through to the wheels = more losses.

That has been a concern as well and the added weight of the old cast iron over drive its self.

But there is Gear Venders selling add on ODs all over the place.

Rich

[EcoCivic]

Originally Posted by racprops

I am thinking that a second overdrive added to a stock 4 speed auto with a OD gear can work and in fact greatly increase a stock car/van’s MPG, just by lowing the cruse RPMS from around 2400 RPMs to 1700 RPMs.

This seems to be the best of all worlds, stock gearing until the added Over Drive is engaged then the advantage of super highway gearing.

Your thoughts and knowledge is invited.

Rich

It would basically be impossible to add a 5th gear to a 4 speed auto. However, it would theoretically be possible to swap the 4th gear to a taller one, although I don't think that would work well and you may not gain anything for several reasons:

1. The PCM would have no way of knowing that the gear ratio has been changed, so it would likely shift to overdrive early and then lug the engine or shift back and forth between 3rd and 4th, so you would probably need to lock out overdrive by putting the shifter in 3, pressing "OD OFF", or similar (if your vehicle even has this option) until you are at a high enough speed to shift to 4th gear. The PCM will likely also set a code, likely for an incorrect gear ratio.

2. The torque converter clutch will probably not lock very much or for very long since lockup is controlled in large part by engine load, and the load would probably be too high for the TCC to lock, even cruising down the highway at a steady speed on flat ground. The converter is also likely to lock and unlock quite frequently, which is inefficient and will quickly destroy the clutch. You could get around this with a manual lockup switch, but again, this will probably set a code, and may even cause the PCM to enter limp mode.

3. The trans will also frequently need to downshift to 3rd gear to accelerate a tiny bit or climb a tiny hill, which will eat into your savings, especially if you manually lockup the converter. Unlocking the TCC may provide enough power to climb a hill or accelerate without downshifting, but again will eat into your savings.

4. When driving in the city, your speed will probably be too low to cruise in 4th gear, especially with the converter locked. I have this problem with my car when I manually lock the converter in 4th gear below 35-40 MPH, which is a speed that I drive at a lot around town. A taller 4th gear would make this much worse. So you will need to cruise in 3rd gear a lot at lower speeds, which will also eat into your savings.

So for those reasons, I would advise against installing a significantly taller 4th gear on an automatic transmission vehicle unless the vehicle is driven on flat highway almost exclusively, you have workarounds for those other issues, you are okay with the vehicle being miserable to drive around town, and you don't care if it sets a code.

[racprops]

As for the CPU throwing codes I will be running a combo of a 93 Plan Jane TBI PCM (runs ONE O2 Sensor..._)and a 86 Camaro TPI PCM OBDI Running the fueling system, I don't think they are smart enough to throw such complex codes.

As for using the second OD I agree only on the highway at a easy cruse speed around 75/85MPH. SO in the city it will drive normally, one of the reasons I like the add on OD.

The nice thing is I can easily lock the second over drive out.

I plan on adding a VSS sensor/sender to the rear of the OD so the truck/car PCM will be able to see a normal speed reading.

My thinking is the convertor clutch and PCM will see 5th gear like a very tall highway gear and act as it is stock.

Good points makes me think.

Thanks.

Rich

[EcoCivic]

Originally Posted by racprops

As for the CPU throwing codes I will be running a combo of a 93 Plan Jane TBI PCM (runs ONE O2 Sensor..._)and a 86 Camaro TPI PCM OBDI Running the fueling system, I don't think they are smart enough to throw such complex codes.

As for using the second OD I agree only on the highway at a easy cruse speed around 75/85MPH. SO in the city it will drive normally, one of the reasons I like the add on OD.

The nice thing is I can easily lock the second over drive out.

I plan on adding a VSS sensor/sender to the rear of the OD so the truck/car PCM will be able to see a normal speed reading.

My thinking is the convertor clutch and PCM will see 5th gear like a very tall highway gear and act as it is stock.

Good points makes me think.

Thanks.

Rich

Oh I see. So you replaced the trans with a 5 speed auto rather than changing out 4th gear? That would be a good idea IMO

[racprops]

Well a 4 speed with a second transmission that is a in/out overdrive.

So it is a 5 speed or even a 8 speed as the over drive could work as a gear splitter, giving a First and first OD, or second then 2nd. w/OD and so on.

I used to play 6 speed in the 60s with my 56 Studebaker with a 3 Speed w/OD, I would drop OD in each gear, 3 + OD = 6 gears.

Rich

[EcoCivic]

Originally Posted by racprops

Well a 4 speed with a second transmission that is a in/out overdrive.

So it is a 5 speed or even a 8 speed as the over drive could work as a gear splitter, giving a First and first OD, or second then 2nd. w/OD and so on.

I used to play 6 speed in the 60s with my 56 Studebaker with a 3 Speed w/OD, I would drop OD in each gear, 3 + OD = 6 gears.

Rich

Oh that sounds like fun to have more gears and something else to play with LOL. But as someone else said, that makes me wonder if having a second transmission would actually be of any benefit. That would depend on the engine's efficiency at different RPMs and how much frictional loss the overdrive unit adds. It is quite possible that such a setup would be of no benefit to MPG.

But like I said, cruising RPM isn't all about gas mileage. Some people (like myself) don't like the noise of cruising at a high RPM, especially since I made this car so loud as it is. Also, even though I know that it's illogical, I don't like the feeling of cruising at a high RPM for extended periods of time because I feel like the engine is working harder than it has to. To me, driving down the highway is more relaxing at a lower RPM

[Frank Lee]

Pretty simple to slow down.

[EcoCivic]

Originally Posted by Frank Lee

Pretty simple to slow down.

Not really LOL. The people here in Memphis drive like jerks and if you go less than 60-65 you would get rear ended. I often have to exceed the speed limit just to not hold traffic back or get rear ended.