Can you replace a 66rfe with a 68rfe?

Does anyone know? Basically, if I buy a 6.4 Hemi truck instead of a cummins, can the transmission be replaced if need be with a 68?

Good question. I wouldn't see why not as long as you programmed the ECM/TCM accordingly.

Although I wouldn't see the gain in doing so since they share the same gearing. That is unless you are planning to add a lot of power and torque to the 6.4L via a supercharger.

I'm buying a new 4500 with the 6.4/ Aisin as66rc tranny. That may be the easiest swap.

I think you'd need to look aftermarket for actually mating the transmission to the block since they are totally different. As well as tc, driveshaft, etc

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6.4 dude said:

I'm buying a new 4500 with the 6.4/ Aisin as66rc tranny. That may be the easiest swap.

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That is interesting. I would love to read a comparison between the two transmissions once you get the 4500.

I'm asking only bec if I decide to get a 6.4 instead of a cummins, and I run 37s on it, and it dies or sucks or whatever... I'm curious what the options are. I don't want to be trapped with no way out. Like the GM 8spd trucks for example.... the 66rfe has been out for awhile and no one seems to do anything with it from what ive seen. I like to know what I'm getting into.... as for power adders, I'm incredibly interested but the lack of available knowledge, tuning, etc related to Rams in general, never the less a 6.4 2500, makes me think its not a great idea. Eveyone and their cousin has a power adder GM truck, tons of tuners. Mostly true of ford, but ram you basically got like a handful of places tuning them. Cummins would be WAY better for aftermarket performance, strictly from a knowledge and resource aspect. I just 1) don't really want to delete 2) don't want to deal with DEF, regens, and any other drama related to emissions

I wish they would come up with a real solution to diesel emissions vs the hack job they have now. And that's a jab at industry, not any manufacturer specifically.

I mean when you tell a normal, average person you need some Exhaust Fluid they just laugh. "Make sure you get the blinker fluid too".

Oh wait, you are serious :/

First off, I don't think you'll have an issue with a 66rfe with a 6.4 even with 37's so it's likely not an issue you'll need to worry about. Second, I don't think you'd benefit as much as you think you will swapping to a 68rfe. One of the main upgrades done to the 68rfe for it to handle the power/torque of the Cummins was the torque converter. A diesel torque converter isn't going to work behind a gas engine at all because of the difference in rpm range. So you'd be taking that out and going aftermarket anyway. Which leads to my next point... that torque converter would likely be hard to find.

I'm not sure if there are any differences between the bolt patterns on each transmission case but I would guess there's not. However like mentioned above, you'll run into differences in the transfer case and driveshafts as well.
Then there's the huge issue of tuning it. We all know that would be a huge task in itself and finding a programer and tuner that is capable of doing that could be a needle in a haystack as well.

that is true, I also read something about the torque converter being totally different on the two physically. So the answer is basically no. Even though you see a lot of people saying "they are the same". Easy enough. Thanks!

I should just plan on stock trans, LTs and tune, and gears. Less fun that a 2nd gen turbo swap cummins lol.

According to Whipple, they are a month out from releasing their Supercharger kit for the 6.4. That may be very optimistic but even so, we should start to see just how much the 66RFE will hold before too long.

My understanding is the cases are completely different as the 66RFE was designed to bolt up to the Hemi, so no easy way to retrofit. But that doesn't mean that more of the 68RFE internals won't fit, along with the typical upgrades. The 66RFE is somewhat of a hybrid, with parts from both the 65 and 68 trans. I was told by a couple of people that the TC was a weak spot, but that's easily remediated. Hemifever cautioned me about loading 4th gear on the dyno, but in real world situations you really shouldn't have that issue.

I know Edge has some mixed reviews, but honestly even with a 2600 stall the new converter is night and day over the stock unit. On mine, I replaced the pan with a Mag Hytec to get the extra fluid capacity. Changed all filters and as well but the old fluid and filters looked great, with no signs of excessive wear.

If I were going to 37's I would get a gear swap done first (4.56's?) as it will also help relieve stress on the trans. I really want to do this even with 35's since the gearing is so lazy.

You probably already saw this, but some basic info on the trans compared to other FCA units.
The 68RFE and 66RFE automatic transmissions for Ram trucks

There shouldn't be any issue loading up 4th gear on the dyno. 4th is one of the strongest gears as it's straight through and therefore is why you want to be doing your pulls in 4th. I've seen a ton of Cummins trucks do 4th gear pulls on the dyno with no issues.

Good advice on the gear swap too. I would definitely recommend going that route before I'd be touching the transmission.

4th gear is 1:1. WOT gear drops from 5th and 6th will destroy either the 66 or 68. The 68 has a different input shaft than the 66. I believe the bell housings are interchangeable but the 68 still won't bolt up to a hemi.

With enough money, anything is possible.

spoon059 said:

With enough money, anything is possible.

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lol right you are

I see this idea being a bigger pita than it's worth. The 66 is much stronger than the 545RFE that's in my '11..........

SouthTexan said:

That is interesting. I would love to read a comparison between the two transmissions once you get the 4500.

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I'll let you know something next month. It has to be delivered to a shop in NY to be upfitted then I'll drive it home.

Doing some reading on the 68 vs 66 from another Allpar article.

All three automatics have a one-piece case, electronic control, two stage hydraulic pump (which shuts the secondary side at higher speeds, for efficiency), and similar valve bodies. They all use three planetary gearsets, three driving clutches, three holding clutches, and an overrunning clutch. The 66 uses some parts derived or taken from the 545 (mainly in the front: torque converter and pump; overdrive, underdrive, and reverse clutches; and valve body), and some derived or taken from the 68 (low/reverse clutches, planetary gearsets, and 2C clutch).

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The 68RFE and 66RFE automatic transmissions for Ram trucks

So, in comparing to the 68RFE it appears that 3rd and 5th would be the weakest link since the underdrive/overdrive and 4c clutches are not upgraded in the 66RFE. The low range and 2c clutches (1st and 2nd) are upgraded per the article. It's unclear (to me) if the 4c and UD/OD clutch assemblies from the 68 could be retrofitted into the 66RFE, but would seem like a logical place to start during a rebuild. Per Brandon, 4th would seem to be the best gear on the dyno since no clutches are engaged. I will have to follow up with HF to see what his actual concern is.

Thanks for the all the info guys! Viking... ive been watching whipple and procharger as well, I was disappointed with the Arrington truck, but we will see what happens. You should add a whipple, and let us know what happens

As for regearing, I'm actually thinking of going 5.13s lol. 4.88s would be the least aggressive I would go. Ive playing with the RPM calculator, 5.13s seem pretty good. I don't like to drive over ~65, 75 MAX in a truck that size just from personal experience.

I just need to decide if I want a 6.4 or a 6.7, I cant make up my mind. I was also considering a 6.0HD with a s300 on it, but I don't know if I can go back to GM lol. The ecoboost is kind of the ultimate platform for me, as I don't really need a 2500, but I don't really care for the f150 and they are having their own issues at the moment. Too many decisions!

I also thought about maybe trying to put a s300 on a 5.7 2500, since there are basically no emissions related items on that truck, which I what I like about the 6L HD. But I feel like building and tuning that whole thing would be a disaster, at least with the GM, its tired and true.

5.13's have durability issues in the 9.25 pinion teeth. Any pulling in reverse will likely destroy your front differential. I wouldn't go any lower than 4.88. If you need lower ratios you should probably consider building a transmission with different gears...to which you will be looking at significant cost.

By far my #1 complaint with my 6.4 is the gear ratio spacing with the transmission. I couldn't imagine spending any money to upgrade and still have the same ratios. I like idea of the Aisin though if that's possible.

Also, fwiw, my 66rfe tranny completely blew and 38K miles under totally normal driving conditions and no real previous abuse. I tow an 11K lb 5th wheel but only about 5% of the time. Even after the rebuild it's not perfect and I'm somewhat at odds with my dealer about some things it's doing.

The 6.4 vs. 6.7 debate is a tough one. I came from a Duramax and as a daily driver I'm happy with the Hemi. Towing is definitely a step down but not as drastic as I would have thought. 410HP/430 torque is nothing to sneeze at.

I had a smaller RV when I got the Hemi. If I had the 5th wheel I'm in now I would have of gone diesel. That being said, I'm in a great position with my truck and financially could easily get a new diesel but am happy enough with the Hemi. I'm just old enough to remember towing with 250HP trucks and thinking that was amazing

Did you ever get a detailed analysis on what went wrong? Would be good to understand the common failure points. Compared to the 545 and 645 the 66 does seem to be a lot more robust. The Aisin doesn't really have much more aggressive gearing other than 1st gear, so not sure it's solving the big problem. I'm still in favor of 4.56's since they seem to have proven reliability and very positive feedback from those who upgraded.

IMO, the only real solution is for FCA to put an 8 speed (or better) behind it. But then you're buying a new truck...