Replacing the 2.93 with a 3.23 in my '96 Roadmaster Wagon

Hi all! Got a deal on a '91 OCC posi 3.23 rear for my '96 Roady wagon (120K miles, bearings quiet, will replace pinion seal and fluid). I understand it's an easy swap and should wake the car up a bit.

My questions are, will I have to have the computer re-programmed for the car to shift correctly, or is this such a small change that it won't matter, and for those that have done it, how far off will my speedo read?

I already committed to buy the rear...again it was a deal. I just want to do this right.

Thank in advance!

-Mike

you do need the computer reprogramed the speedo will be off and the shift points

mtrhead79 wrote:

you do  need  the computer reprogramed the speedo will be off and the shift points

Dangit. I've still not found anyone local to me here in Austin that does this.

81X11 wrote:

My questions are, will I have to have the computer re-programmed for the car to shift correctly, or is this such a small change that it won't matter, and for those that have done it, how far off will my speedo read?

yes, you absolutely need a retune. This is a good time to send get a tune anyway. It's the best 200$ you'll spend. If you're even considering doing other mods in the foreseable future (like headers, rockers or valvesprings [absolutely do valvesprings...]) just get ONE tune. Disable VATS, the rear O2s (on spec), AIR, etc.

3.23/2.93 = 1.1024, or 110% high. Your shift tables are speed based. Your shifts will be all over the place.

jasonlachapelle wrote:

81X11 wrote:

My questions are, will I have to have the computer re-programmed for the car to shift correctly, or is this such a small change that it won't matter, and for those that have done it, how far off will my speedo read?

yes, you absolutely need a retune.  This is a good time to send get a tune anyway.  It's the best 200$ you'll spend.  If you're even considering doing other mods in the foreseable future (like headers, rockers or valvesprings [absolutely do valvesprings...]) just get ONE tune.   Disable VATS, the rear O2s (on spec), AIR, etc.

3.23/2.93 = 1.1024, or 110% high.  Your shift tables are speed based.  Your shifts will be all over the place.

I need to find a place to do this then. I also wanted to program the electric fans to come on earlier, so I could have that done at the same time. If anyone knows someone good in Texas, please let me know!

While you are playing around with the pinion seal, check the axle seals, axles, and bearings. They can be worn, and it is easier to do while you have it out of the car.

I know a guy who does have the gear and software to re-program PCMs, can ask him if he's able to program a 96 PCM when I see him tomorrow.

don't program the fans cooler unless you add the cooler stat,..or the fans are running all the time,.

your shifts will be a little early ,. but certainly driveable till you reprogram it,.

your speedo will say 60 and your doing 55 basically,..

it gets really weird when you go from 2:56's to 4:88;s,...

me personally i wouldn't put all that effort into such a small difference,. 3:42's yes or 3:73's

but do what you want,. yer gonna anyway right?

Now you have a spare rear end sitting that you can build.

Buy some 3.73s and do the job once.

And then powder coat it, because Texas Mike.

The car will drive just fine until you can get it reprogrammed. I'm too tired to type out why, but trust me on this. Yes the speedo will be off but that's about it.

If it was me...  I would save up a few more bucks and then look at doing a Gen3 PCM conversion.  The '96 cars have very limited tuning support for the stock computer.  A guy JUST came out with a real slick and cheaper way to do it that you may be very interested in.

Thanks for all the replies. I found a place down in New Braunfels, about 40-miles to the south, that will do the mods for me. Said just bring him the car and it'll take about 30-mins. Not bad.

Speedo being a little off does not bother me, but I do want the car to shift right.

As for the fans, before I swapped from engine-driven to electric, the temp gauge NEVER went over halfway. Car was always a cool runner. Now it still runs cool as long as I have the a/c on, but with the a/c off, the gauge goes up to 3/4 or more before the fans kick on. As soon as they do kick on, the temps come right back down...until they cycle off again. I HATE that, even if it is "by design". About 9-months out of the year you can drive with the a/c off and windows open....my favorite way to drive.

If the car has always run cool, until this change, please educate me on the need to change the T-stat. I will if needed.

Thanks again all!

-Mike

81X11 wrote:

If the car has always run cool, until this change, please educate me on the need to change the T-stat.   I will if needed.  

Thanks again all!

-Mike

Sorry i misunderstood what you meant,.

Some folks want a cooler running engine,. aperformance sort of thing,. so they program the fans for the 180 deg range, and then add the 160 stat,.

i wasn't quite understanding that you want the fans on just a little lower temp,.sorry.

fyi the heat gauges in these cars are like gas gauges,. all good to a certain spot then they gain rapidly,.

or put another way a lot of needle movement after a certain temp , for not much temp gain,.

phantom 309 wrote:

81X11 wrote:

If the car has always run cool, until this change, please educate me on the need to change the T-stat.   I will if needed.  

Thanks again all!

-Mike

Sorry i misunderstood what you meant,.

Some folks want a cooler running engine,. aperformance sort of thing,. so they program the fans for the 180 deg range, and then add the 160 stat,.

i wasn't quite understanding that you want the fans on just a little lower temp,.sorry.

fyi the heat gauges in these cars are like gas gauges,. all good to a certain spot then they gain rapidly,.

or put another way a lot of needle movement after a certain temp,.

Good deal. I just don't want to see it going above halfway....unless there is a real problem. I NEVER in all the years I've known this car went over halfway on the gauge until I did the electric fan swap. Now it goes up to 3/4 or more in traffic before the fans kick on. While moving it's fine regardless, but if I want to drive the car to work sometimes and get in traffic, I either have to kick in the a/c or watch the gauge climb toward red. I never gets to red, thankfully, but I still hate sitting at a light and watching it headed that way....

Have you ever pulled the temp on the climate control to see what it was really running when it was getting "hot?"

Stingroo wrote:

Have you ever pulled the temp on the climate control to see what it was really running when it was getting "hot?"

That's a good idea!

As Sherlock said.. there's absolutely no reason the rear end swap would affect shifting; it will shift exactly at the same rpm's as it used to. Think about it: nothing in the transmission, sensors, ECM etc sees that the rear wheels are now turning faster.

The speedo owill of course read wrong and that's why it needs the reflash.

I think 3.23 would be a very good ratio if you don't tow a lot, and regularily drive 60 or over.

Stingroo wrote:

Now you have a spare rear end sitting that you can build.

Buy some 3.73s and do the job once.

And then powder coat it, because Texas Mike.

Yes,yes,and yes.

81X11 wrote:

Stingroo wrote:

Have you ever pulled the temp on the climate control to see what it was really running when it was getting "hot?"

That's a good idea!

I only ask because my wagon would tell me it was running stupid hot on the gauge, but using an infrared heat gun didn't give me any sort of indicator, and coolant never bubbled.

So I basically learned to ignore the gauge. At least in your case you've got a real digital readout you can reference.

andy caprice wrote:

As Sherlock said..  there's absolutely no reason the rear end swap would affect shifting; it will shift exactly at the same rpm's as it used to. Think about it: nothing in the transmission, sensors, ECM etc sees that the rear wheels are now turning faster.

The speedo owill of course read wrong and that's why it needs the reflash.

I think 3.23 would be a very good ratio if you don't tow a lot, and regularily drive 60 or over.

I don't think anyone is implying his transmission will get damaged by not getting the PCM tuned. The TPS, line pressure, and shift speeds won't be affected (unlike if you changed the TB to a larger one without reprogramming).

Shifts aren't a function of engine RPM. Maybe I'm out to lunch (and please let me know if I am), but I'm guessing the PCM measures vehicle speed by measuring transmission output rpm. Just guessing that because the VSS is in the transmission. If that's what you meant then I would agree. The PCM then commands shifts based on TPS% and mph. If at 25% Throttle, it commands the 1-2 shift at 18 mph, the 2-3 at 32 mph and the 3-4 at 38 mph, you are shifting when trans output RPM reaches a certain threshold, since the number of tire rotations and tire diameter are constants in the PCM. Now say that instead of making one full revolution per 2.93 rotations of the driveshaft, the tire turns once per 3.23 driveshaft rotations. It's still going to shift at the same trans output rpm because the pcm still thinks you are going 18, 32 or 38 mph. In reality you are going 10% slower at that rpm. Does it really matter that you're now shifting 16, 29 or 34 mph ? not really if it's just 10% off, but it's not optimal.

Given how inexpensive it is to get the car tune, and address a slew of other issues at the same time, why not just spend the money to do it right ? The tuner will probably tweak the speeds that trans shifts as well, not just correct the gear ratio/tire constants to correct he PCM.

Maybe I'm completely misunderstanding what you and Joel meant.

I agree with Nick though, 3.23 is a very small jump from 2.93. If you feel like tackling the project go for it.