Transmission issues after gear ratio swap

[U&A]

Well I just broke 52,000 i think or im about to.

So soon my transmission will fail and my cam will be destroyed[emoji23][emoji23][emoji23][emoji23]


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[SouthTexan]

I think the issue is not enough horsepower at the rpm you are at. While gears multiply torque, they do not multiply horsepower.

According to the math, it takes roughly 200 hp to tow a 10k trailer with a 7k truck up a slight 2% grade at 70 mph. It takes 230 hp up a 3% grade and 270 hp up a 4% grade. From the dynos I have seen, I don't think the 6.4L makes that much horsepower at that low rpm. In fact, it drops pretty considerably from the 410 peak hp at 5,600 rpm to 327 hp at 1,600 rpm less where it is making peak torque at 4,000 rpm (429 lb-ft X 4,000 rpm / 5252 = 327 hp). Dropping down another 1,500 rpm to 2,500 rpm, and the truck might not be making enough horsepower for even a slight 2% grade at 70 mph.

There is also the factor that these horsepower numbers are at sea level with near-perfect humidity and temp. Most will be making 10-20 hp less in the real world. Even less if you live at high elevations or in extreme weather.

In contrast, the diesel you bought is making about 310 hp at just 1,800 rpm due to the massive amount of torque the engine is making at that rpm. More than enough horsepower for even 4% grades at that speed.

 

[U&A]

I think the issue is not enough horsepower at the rpm you are at. While gears multiply torque, they do not multiply horsepower.

According to the math, it takes roughly 200 hp to tow a 10k trailer with a 7k truck up a slight 2% grade at 70 mph. It takes 230 hp up a 3% grade and 270 hp up a 4% grade. From the dynos I have seen, I don't think the 6.4L makes that much horsepower at that low rpm. In fact, it drops pretty considerably from the 410 peak hp at 5,600 rpm to 327 hp at 1,600 rpm less where it is making peak torque at 4,000 rpm (429 lb-ft X 4,000 rpm / 5252 = 327 hp). Dropping down another 1,500 rpm to 2,500 rpm, and the truck might not be making enough horsepower for even a slight 2% grade at 70 mph.

There is also the factor that these horsepower numbers are at sea level with near-perfect humidity and temp. Most will be making 10-20 hp less in the real world. Even less if you live at high elevations or in extreme weather.

In contrast, the diesel you bought is making about 310 hp at just 1,800 rpm due to the massive amount of torque the engine is making at that rpm. More than enough horsepower for even 4% grades at that speed.

THIS IS THE EXACT REASON i preach that these Hemi’s LOVE higher RPM.

I cruse in 5th at 60 at almost 2,500 rpm on the highway. She loves running at 2,400-2,500 all day long.

I wish i went 5.13’s instead of 4.88’s. Id love even higher RPM.


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[mtofell]

Turns out the overdrive clutch was on its way out the door. So I now own a 2020 3500 with a cummins and most importantly not the 68rfe. Pretty sad that I only got 75k out of the transmission in my 2500.

"Not the 68rfe" ??? Does this mean your new diesel has the Aisin or is this a typo? Your Hemi would have had the 66rfe which I'm certainly no defender of (I'm on my 3rd at 106K miles). Most diesels have the 68rfe... except for the ones that have the Aisin.

As for the original problem, I guess I'm not surprised that the problem turned out not to be due to the gear swap (right?). I've looked into gear swaps a lot for the Hemi/66rfe and was under the impression that nothing was needed for programming since it's the final drive ratio and is all past any computer "stuff".

 

[loveracing1988]

"Not the 68rfe" ??? Does this mean your new diesel has the Aisin or is this a typo? Your Hemi would have had the 66rfe which I'm certainly no defender of (I'm on my 3rd at 106K miles). Most diesels have the 68rfe... except for the ones that have the Aisin.

As for the original problem, I guess I'm not surprised that the problem turned out not to be due to the gear swap (right?). I've looked into gear swaps a lot for the Hemi/66rfe and was under the impression that nothing was needed for programming since it's the final drive ratio and is all past any computer "stuff".

I was hoping it was due to the gear swap because I had never had the problem before the gear swap. I didn't want it to be a physical problem like a trans or torque converter going bad. Yes the new truck has the Aisin, I was pointing out that chrysler had no part in making my transmission (hopefully).

 

[mtofell]

I was pointing out that chrysler had no part in making my transmission (hopefully).

Yeah, it's funny... Dodge/Ram/Chrysler has been churning out terrible transmissions since I was a kid. I love my truck and feel my experience is a bit out of the norm having 2 failures but still... it's not like the old "Maytag Man" (sorry youngsters - Google it if you don't know what I'm talking about )

My first job ever was at a carwash when I was 14 and I always remember this old guy bragging that his LeBaron tranny made it to 75K. Granted this was back in the 80s when vehicles were far less reliable but even then, 75K was nothing to be proud of!

 

[6.4LMegaMan]

I'm coming to the conclusion that RFE transmissions are wear items. Like brake pads. I'm fully expecting to have to do a replacement in the next 70k miles. It's such a shame, the rest of the truck is awesome.

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[yoda]

I have built allot of Chrysler Transmissions over the years, the main weakness in what I've seen in the RFE, is it doesn't like wide open throttle downshifts, especially 5-4 or worse yet 6-4 downshifts.
When I tow I always use tow haul and if towing in hills, lock it in 4th or downshift to 4th if your approaching a hill that will force a 5-4 downshift under load.

 

[loveracing1988]

I have built allot of Chrysler Transmissions over the years, the main weakness in what I've seen in the RFE, is it doesn't like wide open throttle downshifts, especially 5-4 or worse yet 6-4 downshifts.
When I tow I always use tow haul and if towing in hills, lock it in 4th or downshift to 4th if your approaching a hill that will force a 5-4 downshift under load.

This might have saved my transmission but I'll never know. I know it wasn't heat related, towing my fiver it never would get above 172 or so but generally would stay right at 170 no matter what gear I was in.

 

[U&A]

I have built allot of Chrysler Transmissions over the years, the main weakness in what I've seen in the RFE, is it doesn't like wide open throttle downshifts, especially 5-4 or worse yet 6-4 downshifts.
When I tow I always use tow haul and if towing in hills, lock it in 4th or downshift to 4th if your approaching a hill that will force a 5-4 downshift under load.

Good info. I never force a downshift with the gas pedal with my Rfe for that very reason. If i need to down shift I eas off the gas a bit, manually down shift and once the torque converter grabs THEN i us the extra torque i was looking for.


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[SouthTexan]

I have built allot of Chrysler Transmissions over the years, the main weakness in what I've seen in the RFE, is it doesn't like wide open throttle downshifts, especially 5-4 or worse yet 6-4 downshifts.
When I tow I always use tow haul and if towing in hills, lock it in 4th or downshift to 4th if your approaching a hill that will force a 5-4 downshift under load.

Yep. This the reason why I try to anticipate when I need to go to wide-open throttle and downshift into 4th via the select shift feature before I hammer down. I didn't do this with my original 68RFE and it lasted 80k miles. Although that is not bad considering I tuned and deleted the truck at 30k and always ran the 450 rwhp tunes. With my current custom-built 68RFE with the Sonnax overdrive hub with performance clutches, I keep it on the 500+ rwhp tune unless I am towing. When I do tow, I lock out 6th to keep it in 5th which puts me around 1,800 rpm at 65 mph with my 3.42 gears.

 

[mtofell]

My first 66rfe failure occurred while sitting in traffic with basically zero warning. The second was slower and most definitely first noticeable at heavy throttle downshifts - usually 4/5-2. Whenever I wonder how it's doing I'll do a heavy throttle downshift to see if I sense any problems.

When it was failing the second time the best way I can describe it is it felt like engine was over-running the transmission. Basically, the tranny would shift back up to a higher (numerical) gear after accelerating but the engine would keep revving and then I'd get a big tranny "clunk". That was usually associated with going into limp mode. Post-second rebuild it failed again within 50 miles (so, I basically just group this with the 2nd failure) and it was due to the valve body not having been replaced. Not sure why they didn't do that as part of the rebuild but they replaced it and all is well. But, during that 2nd failure of the 2nd rebuild it was the same thing with the engine over-running the tranny.

Needless to say, with two failure before 100K miles I'm paranoid it's going to go out again so I do check on it by forcing some WOT shifts from time to time. I have a MaxCare Lifetime Warranty so it's really no monetary problem if it fails. I guess it's been 15K since any problems so I suppose it's "fixed" for now. After two rebuilds I always figure the inside of my tranny looks like a tangled web of duct tape and baling wire.

 

[loveracing1988]

Good info. I never force a downshift with the gas pedal with my Rfe for that very reason. If i need to down shift I eas off the gas a bit, manually down shift and once the torque converter grabs THEN i us the extra torque i was looking for.


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Which is complete BS that you have to do that to get these things to last.