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Lets talk Chevy 3.9L & 904

RaginCaucasian

Active member
Joined
May 26, 2020
Member Number
1496
Messages
33
Loc
Arvada, Colorado
A while ago I picked up a 904 for my auto swap that I was going to put behind a 2.5L that I had. I was originally under the impression that the 904 that I bought rebuilt from a local tranny shop was from an early XJ. It has the 60* bellhousing and a 23 spline output. However when I got around to addressing the flex plate that I still needed, I found that the flex plate for a '91-'98 Jeep 2.5L had been discontinued and very difficult to find, but I did find one. When I checked to see if the torque converter would work, I found that the pilot hub on the converter that came with the tranny was slightly too big for the back of the 2.5 crank. When trying to source a new torque converter, again, it too was discontinued and I couldn't find one in a yard. After finding how many discontinued parts I had to acquire to make it work with the 2.5L,I figured I might as well go the path of a better motor. I wanted to keep the small packaging, but with a bit more power/torque, so I landed on the Chevy 3.9L out of a 2006 Chevy Monte Carlo. I found the motor quickly for a price I couldn't pass up, so I jumped on it, however, it did not come with a wiring harness or ECU.

So, here is where I now stand. I have been told that ideally a 904 transmission out of a 1986 XJ with the 2.8L V6 would be what I would want my transmission to come from since it would already have converter with the correct pilot hub to match the GM pilot hub on the back of the 3.9L. Since I already have a rebuilt 904 that I don't know much about, I have a few questions.

1.) If I am able to source a torque converter from a 1986 Jeep Cherokee with the 2.8L V6 will it be a direct swap without me knowing exactly what my 904 came out of? Any reason why the current transmission I have won't work with this converter?

2.) The flex plate from the 3.9L is a 3 bolt pattern and the converters from the 904 seem to all be 4 bolt. Would re-drilling new holes in the flex plate to match the 904 Converter work? Or does anyone have a better suggestion?

3.) Any expected issues from getting a wiring harness and ECU from a random Monte Carlo off Car-Parts.com be of concern? i.e. does it have to be the same model, will one from a 3.5 work as well since it is the same block just different model car, etc.?

4.) Any other support or suggestions especially from someone that has done this or a similar swap?
 
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Jesse Haines / mudtruck44 used this combo years ago. Might search old site fo see if any info is still there.
 
JohnnyJ said:
Jesse Haines / mudtruck44 used this combo years ago. Might search old site fo see if any info is still there.
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I tried searching there a few times in the past few weeks, but not coming up with much in answering the questions I have. However there is quite a bit on the 904 that I am still reading up on. I just tried searching by Jesse's user name, and it doesn't seem to pull up mudtruck44 as an existing name. I hate what that site turned into.
 
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I have a 2008 3.9 and 2001 904, but havent fully sorted it out. I do know there are several torque converter and pump combinations. You can't simply swap the TC as it may not engage the pump. My engine is on a stand and I havent pulled it off to check the pilot/TC, but I assume I'll have to figure something out due to the later model trans.

Search for user patooyee on the old site. He, hurleygo3, and mudtruck44 (Jesse) have a few threads

TF904 lockup vs non-lockup confusion​

TF904 rebuild​

adapter for ecotec to a tf 904/gm metric pattern​

 
j.mcconnell said:
I have a 2008 3.9 and 2001 904, but havent fully sorted it out. I do know there are several torque converter and pump combinations. You can't simply swap the TC as it may not engage the pump. My engine is on a stand and I havent pulled it off to check the pilot/TC, but I assume I'll have to figure something out due to the later model trans.

Search for user patooyee on the old site. He, hurleygo3, and mudtruck44 (Jesse) have a few threads

TF904 lockup vs non-lockup confusion​

TF904 rebuild​

adapter for ecotec to a tf 904/gm metric pattern​

Click to expand...
I'll give those a look, thanks.

I ended up talking to a few people that have done this combo and sent them some pics. At first was quite concerned as one person told me that it is a 4 cylinder 904 based off the look of the case, and confirmed that the converters between the 4 and 6 would not be compatible between the two. Then when talking to Jesse, he felt fairly confident that it was a 6 cylinder input shaft based off the pics I sent which gave me a bit of hope.

Now I had previously dropped the original torque converter back off to the tranny shop with the flex plate from the 3.9L to see if he could come up with anything while I continued my search. Then yesterday I received a call that he had a converter for me that might work. In further discussion with him, he cleared some confusion that I have a case from a 4 cylinder, but the input shaft and some other parts are for a 6 cylinder version. I was able to exchange the original converter for the one he had built. I'll have to take some pics today, but I am hopeful what I have will work. It does have the correct diameter pilot for the 3.9 and has German writing on it. I have been told that the correct converter was German made.

My first concern is that the flex plate doesn't sit flush on the cleats of the converter. It has a very small rocking motion to it. The converter has a slight dome shape to it that is contacting the FP before the cleats, so if I just drill the holes and send it, it will distort the FP a little (Maybe 1/16" total). Should this be of concern, or should I put a thin washer/spacer between the two?
 
Let us know what you figure out. I had my 3.9/904 stuffed in a corner and just recently thought about selling it to LS swap my 2.4L TJ since I wont be building a buggy anytime soon. I think the 3.9 would be great in a light'ish build. Word is that the 3.9+904+np231 would be lighter than a 4.0 alone

brandt487 and alannajamie (James Treacy) on instagram also build a lot of 3.9/904 buggies. I been meaning to ask James if he'd do a set of headers on the side
 
brandt487, sasquatch motorsports (alan) and James Treacy seem to be at the front of what all is needed for the 3.9 to 904 swap. Custom headers seem to be the norm with these. SteerAndstuff? has a JHF chassis build on here where he's the 3.9 but 700r4 instead.

I'm still 10 years back in the L67 3800 mindset for my build, otherwise I'd be scratching for as much info on these like yourself. 3.9 is definitely the way to go for the buggys I drool over.
 
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I just saw the SteerandStuff thread on here. Looks like that 84-85 2.8L 700r4 is a bolt in. I make go that route with a 231c/231j hybrid if my converter is an issue.

JHF used a 4l60e with a 96-02 Camaro bell housing and converter in his Pokey buggy on the other board.
 
Anyone here know if you can do a 30rh from a 2000 2.5 4cyl wrangler. From all I have read they are the same as a 904, just with a lockup converter.
 
Well i got a 91 YJ 4cyl 904 and dana 300 this past weekend.

I have a 3.9 lined up. Need to find someone to strip and delete the harness. I have used Jims performance in the past for v8 stuff, so Ill check with him about the 3.9
 
pennsylvaniaboy said:
Well i got a 91 YJ 4cyl 904 and dana 300 this past weekend.

I have a 3.9 lined up. Need to find someone to strip and delete the harness. I have used Jims performance in the past for v8 stuff, so Ill check with him about the 3.9
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I had Steve at Humphrey's Harnesses build one for me. Very good price and very quick. He just had me send him 1 plug that was on the original harness. He's built quite a few for a lot of the 3.9 cars out there. I'll shoot you a PM with his number.
 
RaginCaucasian said:
I had Steve at Humphrey's Harnesses build one for me. Very good price and very quick. He just had me send him 1 plug that was on the original harness. He's built quite a few for a lot of the 3.9 cars out there. I'll shoot you a PM with his number.
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How dose this set up run? Any tune issues ?
 
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