I have an 86 L98 with the 4+3...When I had the 3.73 gears in last, the shop who did them had a speedo shop wire in a little black box to adjust the speedo. The problem was that my ecm didn't have a clue what was going on (I assume) so my OD unit was wacked! I was getting a shift-now light in 4th OD!!

Also, she was disengaging OD in 2nd at random. More than a few coffee stains when that happened...so I finally gave up and reinstalled the 3.07s

So, for those of you who have the 4+3 and done the change from 3.07s to 3.73s...how did you make it work?

Thanks




[Modified by redvette1986, 7:12 PM 8/30/2003]

 

I have no personal experience, but *I* would get rid of the "black box" and install the proper speedo gears. Good luck, and...

RACE ON!!!

 

Yes, I think thats the right way to go. So where are these gears located in the car (tranny?) and where can I buy the ones I need?

 

Yes. On the rear output shaft. Chevrolet.

RACE ON!!!

 

If you have a stock L-98 in yor 86 putting in 3.73 gears will slow you down.
The TPI is designed for low rpm torque. With 3.73's you blow through the power band in a flash and spend the rest of the time spinning the motor going nowhere. Been there already with an 86.

 

To be honest, when I did have them in for a few months, I really didn't notice much difference. I thought maybe the ecm was to blame.

 

:iagree: I went through the 3.73 gear swap with my car also. You will need a 45 tooth driven gear and 15 tooth drive gear to calibrate your speedo. If you had 3.07 previously, you can reuse you Vehicle speed sensor, if not you will nee a new VSS to fit teh larger driven gear.

 

If you want to dump the 3:73's let me know.

 

dgoodhue...

I am curious...you have the 86 with 4+3 and saw no difference when you went from a 3.07 to the 3.73s?

 

I recently installed 3.73s in my 88 4+3, If the vss input to the instrument cluster is corrected with a ratio converter, or actual gears, the ecm will also see the corrected vehicle speed. I use a dakota digital sgi-5 interface for the ratio conversion.

Your od problems stem from the ecm not recognizing the correct gear. Within the ecm the rpm is divided by mph, to determine which gear you are in. For example at 5000 rpm in 4th gear at 100 mph, 5000rpm/100mph = 50. The ecm is expecting a value between 29 and 43 to recognize 4th for the 3.07s. This value is called mph ratio in datamaster, 1st gear and 1st od also have defined limits, that will be out of whack with the 3.73 gears.

Here are the parameters that need to be changed:

1st gear direct: FF(hex) -> FF No change required
1st gear od: 55(hex) -> 67(hex)
4th gear min: 1D(hex) -> 23(hex)
4th gear max: 2C(hex) -> 36 (hex)

Look for the string FF 55 1D 2C (should be near location 0x28C in the bin file) and make the appropriate changes.

For correct od function, You need to correct the n/v limits, For 4th, 1st and 1st od by multiplying the current n/v limits by a correction factor (3.73/3.07) and edit the ecm bin file, at the appropriate address. Otherwise the wrong tables will be used for od control, since the wrong gear is recognized. See my example above.

Unfortunately this is not covered by tunercat, which is the editor I use, but can be done manually. Don't forget to open, and save the file with tunercat, so that the checksum is updated properly.

Good luck, let me know if you need any more details and I can provide.

 

Tequilaboy, your help is much appreciated...however, my head is spinning. I don't have a clue how to access or edit my ecm data. Can the 86 be changed? Also, if I change the tranny speed gears, do I still need to edit the ecm?

 

Yes, your 86 is tuneable, but it requires erasing and reprogramming the factory chip or memcal. See the FAQ in the scan and tune section to get all the info you need for ecm tuning. It covers where to find programming tools, editing and data logging software. All of which is available at very reasonable prices.

Quick summary of what is involved: Get a programming device, read out and save the original file from your memcal. Erase the chip with a UV eraser. Edit the file, and re-program the chip. Test and repeat. I would recommend getting your hands on a spare memcal, in the event anything goes wrong with the original. Any aftermarket chip makes a good spare, and may be a good place to start with performance oriented calibration.

I have installed a socket in my memcal to permit rapid chip replacement, but this is only for convience and use with an emulator but is not really necessary.

Note: I use a pocket programmer from xtronics, tunercat editing software and an AKM cable and datamaster software for data logging and testing.

If your lucky, maybe you can find a tuner in your local area that can help.

 

Tequilaboy,

I just realized that you are not too far from me at all. I am across the river from Algonac MI. We are about 46 miles apart. Perhaps if I choose to put the 3.73s back in I could limp over your way and contract you to do the reprogramming? I have an extra chip that I had in and pulled cuz I didn't see any difference so maybe it could be the one that is re-burned.

In the meantime, I am still trying to figure out if I need to change the tranny speed gears and what effect this mod will have on my car.

Just so that I have this straight....

1. Change rear gears from 3.07s to 3.73s
2. Change tranny speed gears
3. Reprogram ECM
4. Hold-the-hell on cuz now she kicks butt bigtime and the OD and speedo both work properly...

Or am I missing something?

 

I'd be happy to help. You can contact me when you're ready and we can meet somewhere...Maybe at Milan Dragway for a test and tune session? I should be headed to to Milan in mid september with some guys from work. I'll let you know when we set a firm date. You can e-mail me at tequilaboy@earthlink.net.

Subjective impression of the 3.73s: much easier to launch. I can launch anywhere between idle (900 rpm for me) and 1600 rpm on drag radials. This used to be 3000+ rpm with the 3.07s. Easier on the car and clutch too. Acceleration flattens out at about 100 mph in 4th, so you probably want to go into od earlier. I am using my IAC as a launch control, with the a/c on (and compressor disconnected) I add 700 rpm to my idle, with the a/c adder, which brings me up to 1600 rpm for launch when I press the button. It's pretty cool.

The 3.73s only really help acceleration in 1st assuming you have the traction to take advantage of the extra torque. 0->30 mph, or so.

I have also made a few changes to my od control to work better with the 3.73s, for example I force an upshift from 4th to od at 97 mph to avoid the flat spot at 5100 rpm. I'm still dialing it in.

 

I have a 700r4 and I had 2.73 and went to 3.73. I ran the same ET at the track and with a slower mph. The Slower MPH I think was because I was out of gear at 92-93 mph in 3rd, so I had to go into OD or over rev the motor in third (tried both with the same results) I went to 3.07 but I did other mods before I wnet back to the track...

 

The speedometer can't be changed in the ECM, only your +3 OD shift points. I personally wouldn't use a speedometer box to cailbrate the speedo, I have heard it messes up the ABS.

 

..so I need to do BOTH the tranny speed gears AND reprogram the ECM?

 

The ABS is completely independent from the vss signal used by the speedo and ecm (At least in our cars)

The only concern with electronic ratio converters and ABS is if you would attempt to correct the ABS individual wheel speed sensor signals with a ratio converter, you would most likely get ABS lights, since ABS systems have extensive failsafe monitoring of the wheel speed sensor signals, including both electrical and plausibility comparisons with the other wheels, etc.

Therefore a ratio converter when applied to an ABS wheel speed sensor, would likely result in an ABS fault.

 

tequilaboy,

Your hex addresses and information above are priceless to me, thank you! I'm probably going to be going with a 3.54 or 3.73 gear to help me get around a nasty bog at launch with my own 87 4+3 soon. That information will be of great use to me. If there are any other gems reguarding the ECM you'd like to share feel free to send em' on over. :)