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Old 12-22-2010, 12:49 PM   #1
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Default Trans swap, ratio change questions?

I have a feeling my 4t65 is gonna bite the dust. Maybe not right away, but certainly after I top swap the L36 in spring.
I am thinking of going the route of a swapped in used HD trans from Morad. My trans has the 2.83 (?) gearing, which has come up as being less reliable than starting with a 3.08 ratio due to internal diff issues.
But, Ive also read that swapping a trans in with a different ratio than stock is either a huge pain, or impossible, since the PCM doesn't recognize the new ratio, and it either can't be programmed, or is hard to find someone that will. Anyone have a definitive answer on the ratio change?
I'd like to have the better gears, not only for acceleration, but my general drive for work is up and down hills. I'm always in 3rd, never in 4th anyway. Even in 3rd, it still has to work to lock and unlock the converter for that extra few 100 rpm.
Basically, I'm going to buy a trans soon, even just to make sure I have one on hand. I don't want to buy one with a different ratio, only to find out that its not possible to reprogram the PCM to accept it. And yes, I know I need a different driver side axle for the HD trans.
If its up to the dealer, and a dealer only to program it, my local ones wont touch my car with a 10' pole after the FE5 swap. They don't want to have anything to do with a "modified" car, but they're happy to sell me parts.
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Old 12-22-2010, 01:20 PM   #2
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Your stock trans should be 2.93, and your new trans I assume is a 3.29 final drive ratio. Changing this ratio is not a problem, providing this is in a 97+. It'* changing one cell in HPT and clicking a button. Upload the tune and your all set. I did this in my car.
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Old 12-22-2010, 01:38 PM   #3
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Ok. Good to know. I remember someone posting about swapping ratios, then the car threw codes, and the end result was swapping the old ratio into the new trans or something like that, but it was something like the dealer couldn't adjust the parameter, or it wouldn't take, or something like that.
Do you have to have HPT to do it, or can it be changed at a dealer with a Tech2 or something like that?

I just checked the FSM, ratios for the 04 were either a 2.86 or a 3.05 according to that? I knew I was probably off a little, but I didn't remember anything like a 3.29 being available, at least as a factory option anyway. I know you can get gear ratio change kits in the aftermarket, but thought the 4T65E only had the 2 options.
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Old 12-22-2010, 01:47 PM   #4
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Not sure if a dealership can/will do it, and if they do they will charge you.

You can always send the PCM to someone like me and have it tuned. Not real sure about the gear ratios. Will have to wait for Bill to come back and see what he has to say.
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Old 12-22-2010, 02:02 PM   #5
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I think I'd have to stay away from a jump in ratio that big. While it would be nice in a weekend car. But for something that gets me to work daily, to the tune of 400 miles a week on average, with the price of gas, I don't want to sacrifice that too much. I just need a fresher HD trans that will hold top swap power, with a little more gear. I think a little more gear will actually improve mileage, with all the hills I climb to work and back. Going overboard on gearing will make it quicker, but lose the drive-ability I need it to have.
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Old 12-29-2010, 04:28 AM   #6
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Some of the PA models (and a few Bonnevilles, among others) actually came with crazier granny gears than 2.93 - they did have something like a 2.86 FDR. If you simply top swap, you shouldn't see too crazy of a change in mileage even with a jump from 2.86 to 3.29.

If you get a swap-in transmission, do no work to it, and have an HD diff, you're going to have a 2.93 FDR. Putting in the gears that will let you use stronger chains will get you to that 3.29 ratio unless you use a non-HD differential, and then it'* 3.06 or so.

Here'* a sort of table or so of what I think could happen:

Stock: 2.86
Swap in HD transmission, fully stock: 2.93
Swap in HD transmission, add stronger chains: 3.29
Swap in non-HD transmission, add stronger chains: 3.06

These numbers may be off a bit since they might be 2.83 and 3.05, but overall it'* generally right I think.
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Old 12-29-2010, 12:23 PM   #7
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So, are HD trans available in only one FD ratio then? So by swapping a stock HD, it would only marginally increase the FD? I need a bit more gear than that. But I also will need the HD diff. 329s sound pretty high, but ill have to do the math to see how it will affect cruising rpms both in drive and OD. Since I rarely use OD unless out on the interstates.
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Old 12-29-2010, 06:12 PM   #8
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I didn't actually notice much of a higher cruising RPM or a higher exhaust pitch when going to 3.29. People say they see maybe a 600 RPM shift.

The HD differential changes the gear ratio down. 2.93 is relatively low, and you get to it when you take a 3.06 non-HD trans and give it that better diff. All HD transmissions had the 2.93 gear ratio until about 2006 or so, and that'* when the Competition Group, Comp G, Grand Prix package was added. Using different gears and the same differential, it got to 3.29. Until then, 3.29 had been limited to 4-banger, 3400, 3100, and most NA 3800s.

That being said, check with any 3100 or NA 3800 GP guy to check gas mileage. They get about 30 still, I think. If not that, then high 20s like I get.
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