aDigitalPhantom

As the title says I am looking for information on the 4+3 transmission. To be more specific info on the Overdrive, and overdrive ratios. How the overdrive works, and how many gears it has.

I have an idea, but don't know if it is possible. I also don't know how to explain it.

mazdaverx7

the 4+3 unit is an automatic overdrive unit that's attached to the Doug Nash T10 4 speed manual transmission. the overdrive can be activated when the engine temperature is above 184 degrees F. the overdrive unit is activated with either a console mounted switch on early cars and on the gear shift **** on later C4's. the overdrive can be engaged in 2nd, 3rd, and 4th gear. the overdrive ratio is .067. if the overdrive is engaged in any of the listed gears and the throttle is above 50%, the overdrive will automatically disengage, giving you your direct drive gear on 2nd, 3rd or 4th. the only time this does not apply is when the vehicle speed reached between 110mph and 114mph. when that speed is reached, the computer automatically engages overdrive in 4th gear, acting as though there is a 5th gear. 1st gear is always a direct gear with no overdrive available.

you can choose to drive the car as a regular 4spd and engage overdrive when the engine rpm's in 4th gear are high enough that you believe that you should be in another gear, then you can engage the overdrive and the engine rev will lower when overdrive is engaged. i always depressed the clutch when shifting in and out of overdrive. less shock load on the overdrive unit. you also can just drive it with overdrive engaged all the time and let the ecu do the down and up shifting for you.

aDigitalPhantom

What I am thinking would work close to this with 2 changes from what I can tell. The first would be to change the the overdrive from computer controlled to manual control. The second would be to change the ratio from 0.67 to 0.5.

With the ratios I used when thinking this up it would in theory (maybe wrong) allow 2-4 to be used as overdrive gears with ratios of 0.82, 0.64, 0.50.

This would probably cost more then it is worth, but it is an idea.

Vern66

I love my '88 with the 4 plus 3. In fact, it's one of the reasons I bought the car! I found like the previous post, it's easier on the drive train to clutch when changing the OD unit in and out. I also found it better to let the ECM put the unit in OD when going to 2nd gear and then leave it there. I also changed out the OD unit fluid and screen filter as recommended. The 4 plus 3 is not as scary or as bad a system as some make it out to be.

Axial

I feel like the best alteration is to simply leave the ratios alone and modify the system so that the over-drive unit is off at start-up. Having the computer control is nice when you want it, it's only intrusive because it's on by default every time the vehicle gets fired up.

aDigitalPhantom

Well I don't have a 4+3, I have a 93 with a zf-6 speed. I also don't have access to a 4+3. I started thinking about this kind of idea not for fuel economy, but for the possibility of a higher top speed.

jimkmyc5

You won't run out of gear with the 6 speed. It has some crazy 250 mph top speed at about 6 grand. Just figure out how to get enough power.

I'm talking from 15 year old memories so don't quote me on this...

Axial

Personally, I thought the 4+3 car I drove was plenty fast; the OD behaviour was just annoying. Reducing the gears could increase top speed, but a far easier way to do that is to simply reduce the ratio on the rear differential. Since it's just one reduction gear in the OD, the net result is the same. Even disabling the computer control, if you want to increase the top speed you don't want to do it in the OD. Why? Because that unit was not meant to be used under the strains of WOT.

Of course, the one trade-off about reducing gears is that you also reduce the rate of acceleration. Personally, I'd rather have faster acceleration than overall top-speed because I will rarely, if ever, be in a situation where the car's top speed becomes obtainable and relevant.

aDigitalPhantom

I have heard it both ways. With about a 0.5:1 OD some say that using steeper gears allows 6th gear to accelerate better. I can also see that when using a transmission that has 1:1 and no over drive taller gears would allow for a higher top speed.

On a different subject, after I get a few more parts the fun will begin.

Ok I will. How much speed do you think an ls swap will add?

Axial

For the record, I also have the ZF 6-speed in my car.

Serious question: why do you want to increase the top speed using gears? Cost? If not, wouldn't it be all-around more beneficial to improve the power-plant?

aDigitalPhantom

I don't necessarily need to use gears for that. This was just me thinking too far outside the box. I did mention in my last post although not very clearly that I am planing on improving the power-plant. Here it is reworded.

I just need a wiring harness, fuel system upgrade, a computer re-tune, and whatever other small things I have forgotten.

coupeguy2001

Some overdrives had a .67 overdrive, and some had a .5 overdrive ratio.
I put a 4+3 in my 57 chevy truck, and it is a ,67, and manually controlled.
The hiccups are:
1. forgetting to take the overdrive off at a stop light and it kicks in in 2nd.
2. being in overdrive and downshifting instead of turning it off sometimes. Depends on the situation and traffic.
otherwise, it works like a dream.

QCVette

Try taking a look at
http://www.5speeds.com/dne.htm
and explore the links there. A lot of good info especially the chart that shows the operation in the following link.
http://www.5speeds.com/43table.htm

The ratios are 0.59 (MH5 code) and 0.68 (MK2 code)

I have the 0.68 4+3 in my '85 and had the 0.59 in my '87.

I find that I use it like a 5 speed. I use the 4 speed normally and then turn on the overdrive on the highway. I also turn it off when I slow down. I even use the clutch when shifting in or out of OD most of the time since I thought it might be easier on the trans.