I'm considering changing the internal chain gears in my 4T65e from 37/33 to 33/37, giving me a final drive ratio of 3.69 ( vs the 2.93 I have stock). I've done a lot of reading and searching over the net, and while it looks doable, there are some folks that are negative about such a change.

The reason I am considering the change is I would like more performance at lower speeds- around town driving mostly. I do take an occasional road trip, but the gas mileage lost is not a big concern. I have 3.73 now in my El Camino, and I know that going from 3.08 to 3.73 was like installing a new engine!

Anyone done this? Any constructive ideas? I've located a set of gears for a good price, and the gaskets are readily available. I've found this thread on changing out the 4th clutches, which is pretty much the same procedure needed for the gear swap.

Transmission side cover removal 4T65e

I assume that I will have to drop the cradle for access, but wondered if anyone has done this in the vehicle. While tight, it looks doable??

I know that the PCM will need re-programming. I have access to a Power Tuner, and there are a couple of variables that are readily shown: Calculated Pulses per Rev, Pulses per mile, Drive Sprocket Ratio. I've gotten some info on these, but it is sort of confusing. There are also a ton of tables with shift points, and I assume that all of them need to be looked at and modified. I doubt that the power tuner is sophisticated enough to automatically change all of the tables based on a gear change ( maybe someone knows??). This is probably a task for Ryan, and one he may have done before. Maybe he will chime in.

Lastly, since I will have it open, should I add a Transgo Shift Kit while it is out? Vehicle has the Trans Performance Shift button enabled already.

Be glad to hear your thoughts... ( be kind!)

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'87 GT , '00 3800 Series II SC, 4t65e.

ZZP, Grand Prix Forum, or HP Tuners forum will get you the necessary inputs for 3.69 gears. I have a gear change spreadsheet. PM or Email for details.

3.69 gears will really wake up the car in the 1/4 mile sprint. Talk to some grand prix guys and I don't think you will be disappointed.

I plan on getting some when I go to a built trans as I don't care about MPG I only care about acceleration..

As you alluded to; if you change the FDR , the PCM program shift table will need to be modified relating to the speed and RPM's that it references. Tire diam will also factor in. Without reprogramming the transmission will short shift and can result in lugging and possible transmission failure. Unfortunately its not as simple as changing the drive gears and chain. Its a tricky process.

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" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Northstar TB, LS1 MAF, 3" Spintech/Hedman Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
"THE COLUSSUS"
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "

[This message has been edited by Dennis LaGrua (edited 01-28-2017).]

the 97 Z34 monte came stock with 369 gears in the 4T65E tranny only with the 3.4 DOHC engine making it another tranny with the gears and programming that match. mind you it is shift points for the 3.4 but still a start

I found this information (below) on programming while searching the net. I checked my DHP editor and it seems I have access to most of these items. A rough estimate shows about 69 tables, with about 1200 individual parameters. Most of the parameters are in MPH.

I'm thinking that a change from 2.93 to 3.69 would result in MPH at a given rev dropping by 26%, so if I reduce all of the MPH's in these tables by 26%, I should be in the ballpark. I'm also thinking I could program the "Normal " mode with the same parameters as the "performance" mode, eliminating the need for the switch.

Anybody done this with the DHP tuner? Or does this need HP tuners or Tunercat? As always, look forward to your thoughts.

CALIBRATIONS PARAMETER DIFFERENCE SUMMARY REPORT

Source Calibration File: D:\Profiles\wltm07\My Documents\BINs\97_Grand_Prix_3100_16243385_speedo.cal
Compare Calibration File: D:\Profiles\wltm07\My Documents\BINs\97_Grand_Prix_3100_16243385.cal

Total Parameters Compared: 522
Total Differences Found: 26


Constants
Transmission Parameters
2->3 Upshift Vehicle Speed In Drive 2
3->2 Downshift Vehicle Speed In Drive 2
1->2 Upshift Vehicle Speed In Drive-Low
2->3 Upshift Vehicle Speed In Drive-Low
2->1 Downshift Speed In Drive-Low
3->2 Downshift Speed In Drive-Low
System Parameters
Speedometer Input Pulses Per Revolution
Speed Pulses Per Mile
Trans Output Shaft Revs Per Mile

Tables
Transmission Tables
Normal Mode Shift Speed (MPH) Vs. %TPS Vs. Shift
Performance Mode Shift Speed (MPH) Vs. %TPS Vs. Shift
Cruise Mode Shift Speed (MPH) Vs. %TPS Vs. Shift
Hot Mode Shift Speed (MPH) Vs. %TPS Vs. Shift
Normal Mode WOT Shift Speed Vs Shift
Performance Mode WOT Shift Speed Vs. Shift
Hot Mode WOT Shift Speed Vs Shift
1->2 Upshift Speed Vs. TPS - In Drvie 2
2->1 Downshift Speed Vs. TPS - In Drvie 2
Normal Mode TCC Apply Speed (MPH) Vs. %TPS Vs. Gear
Normal Mode TCC Release Speed (MPH) Vs. %TPS Vs. Gear
Performance Mode TCC Apply Speed (MPH) Vs. %TPS Vs. Gear
Performance Mode TCC Release Speed (MPH) Vs. %TPS Vs. Gear
Cruise Mode TCC Apply Speed (MPH) Vs. %TPS Vs. Gear
Cruise Mode TCC Release Speed (MPH) Vs. %TPS Vs. Gear
Hot Mode TCC Apply Speed (MPH) Vs. %TPS Vs. Gear
Hot Mode TCC Release Speed (MPH) Vs. %TPS Vs. Gear

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'87 GT , '00 3800 Series II SC, 4t65e.

A friends 97 Monte still has the 369 geared tranny and a 3800SC that we installed and it is running fine, all programming was done with my power tuner.

quote Originally posted by FIEROFLYER: A friends 97 Monte still has the 369 geared tranny and a 3800SC that we installed and it is running fine, all programming was done with my power tuner.

Thanks for the input. Does my approach of changing the shift point files mentioned above by 26% seem like a logical starting point, or am I just re-inventing the wheel? Would I be better served buying a tune that has already been accomplished? ( I have plenty of time- retired engineer. So spending hours adjusting tables is not an issue).

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'87 GT , '00 3800 Series II SC, 4t65e.

anyone Know if the Grand Prix "Daytona" had more desirable gearing (as far as 1/4 Mile is concerned?? Thanks