6L90 in my #1 M998

Mogman

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I was at a machine shop auction the other day and picked up a 6L90 with a MP1626XHD transfer case for $25.00 it is complete with T/C, the front shaft, cooler lines and shift linkage so it looks like it was pulled out of a salvage vehicle (could have been the elusive 8.1L)

So the wheels started turning and after some research it does look possible to install one in a HMMWV, there is one company selling stand alone controllers that only MUST have RPM and TPS to operate, it is actually about 1/2" shorter from the bell housing to the transfer case adapter VS a 4L80E which is amazing for a 6 speed, of course that little advantage is lost unless one adapts a smaller transfer case.

It has double overdrive with 6th being 0.677:1 VS 0.75:1 for the 4th 4L80E (the Allison is 0.614:1 in 6th)

It is much more efficient than that dinosaur 4L80 (This means more of the puny Detroit input HP will get to the output)

It has a MUCH lower 1st gear than the 4L80, 4.030:1 VS 2.480:1 but because it is a 6 speed the ratios are actually closer together than the 4L80.

Adding a ton of power to my DuraVee made a big difference but the 6 speed Allison with its double overdrive was the game changer for sure.

Phoenix Transmissions in Weatherford, TX is going to rebuild the T/C with a 1400-1600 stall and of course upgraded lockup clutches.

I will swap out the electric shift transfer case for one of the manual MP1226XHD units I have here. (the electric shift MP transfer cases have trouble in stand alone without a matching ECM)

I believe I will be the first to install a 6 speed in a HMMVW, except for the Alpha and Duramax conversions of course.
 
Last edited:

Mogman

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I have been gathering more information and have started to get the mock-up ready.
It seems I fell into a bucket of poo and came out smelling like a rose as it turns out this 6L90E came from behind a 2008 8.1L gasser, that engine was a Vortec BIG BLOCK not an LS engine, (and relatively rare as it sucked TONS of gasoline) so it is virtually a bolt up proposition rather than all the stuff it takes to bolt up a LS transmission to a Gen 1 engine.

the only coupling issues are some .4" spacers must be used between the flex plate and the converter.

Also the 6L90 converter has a 11.1" (or 11.07 depending on information source) bolt circle and there are two basic Gen 1 flex plates, a 11.5" and a 10.75"

So all that needs to be done there is to elongate the holes to match, I have both flex plates and because the 10.75" has a thicker therefore smaller counter weight welded to the flex plate it appears it will be much easier to mod the 10.75" as there will be no interference from the counter weight.

Here is the 6L90/MP1625HDX next to the TH400/NP218, despite what the camera seems to show the 6L90/MP is only 4" longer than the TH400/NP from the bell housing surface to the center of the rear U joint.
IMG_20210914_135050790.jpg

Of course the slip joint is fully engaged on the 6L90, I wish I had a 4L80/NP242 to compare it with, I may luck out and be able to use an A2 rear shaft.
That TH400 came out of the DuraVee, this is the engine which I am fortunate to have so I can mock up before trying to install.
IMG_20210913_124703778.jpg
It is a damn shame, this engine only had 6K on it when the motor pool decided to pull off the injection pump and then at some point pushed it outdoors without the air horn on the intake, on the HMMWV this is directly under the drip line on the rear of the hood.
Of course this was obvious to everyone so I got it relatively cheap and I was planning the Duramax conversion anyway and it gives me something to use for the mock up, would be a little handier if I could rotate it.

One of my concerns is the relief area for the starter nose is cast into the converter housing on the 6L90 and that may be an area that may need to be modified, but again the mock up will show this.
IMG_20210914_135107081.jpg
IMG_20210914_135702202.jpg
 

Mullaney

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I have been gathering more information and have started to get the mock-up ready.
It seems I fell into a bucket of poo and came out smelling like a rose as it turns out this 6L90E came from behind a 2008 8.1L gasser, that engine was a Vortec BIG BLOCK not an LS engine, (and relatively rare as it sucked TONS of gasoline) so it is virtually a bolt up proposition rather than all the stuff it takes to bolt up a LS transmission to a Gen 1 engine.

the only coupling issues are some .4" spacers must be used between the flex plate and the converter.

Also the 6L90 converter has a 11.1" (or 11.07 depending on information source) bolt circle and there are two basic Gen 1 flex plates, a 11.5" and a 10.75"

So all that needs to be done there is to elongate the holes to match, I have both flex plates and because the 10.75" has a thicker therefore smaller counter weight welded to the flex plate it appears it will be much easier to mod the 10.75" as there will be no interference from the counter weight.

Here is the 6L90/MP1625HDX next to the TH400/NP218, despite what the camera seems to show the 6L90/MP is only 4" longer than the TH400/NP from the bell housing surface to the center of the rear U joint.
View attachment 845162

Of course the slip joint is fully engaged on the 6L90, I wish I had a 4L80/NP242 to compare it with, I may luck out and be able to use an A2 rear shaft.
That TH400 came out of the DuraVee, this is the engine which I am fortunate to have so I can mock up before trying to install.
View attachment 845163
It is a damn shame, this engine only had 6K on it when the motor pool decided to pull off the injection pump and then at some point pushed it outdoors without the air horn on the intake, on the HMMWV this is directly under the drip line on the rear of the hood.
Of course this was obvious to everyone so I got it relatively cheap and I was planning the Duramax conversion anyway and it gives me something to use for the mock up, would be a little handier if I could rotate it.

One of my concerns is the relief area for the starter nose is cast into the converter housing on the 6L90 and that may be an area that may need to be modified, but again the mock up will show this.
View attachment 845164
View attachment 845165
.
Nice to come out smelling like roses - rather than the other option.
Good for you!
 

Mogman

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Steel Soldiers Supporter
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Location
Papalote, TX
I would be interested in the converter housing to rear u joint measurement but it is not a big deal, I would wait until I have the transmission installed before measuring for the drive shafts anyway.
The XHD transfer cases use a large 31 spline output shaft, the normal u joint is a 1480 and one of the critical areas is the u joint fuel tank clearance. on the DuraVee I found a yoke with a 1410 u joint (the smallest available), I had the drive shaft shop build a 1350X1350 shaft and they used a 1410X1350 conversion joint.

I just ordered another 1410 yoke and this time I am going to have a machine shop reduce the dia. as much as possible in the u joint area, I ended up using a grinder to reduce the dia. on the DuraVee which also got rid of the knife sharp edges.

Also you need to run the yoke as far into the transfer case as possible because it just gets tighter the further back that u joint rides so a custom shaft will be most likely needed as I am not THAT lucky, I tried to run the DuraVee with only 3/4" slack, just enough to get the shaft installed but they did not get the message and gave it another 2" for axle travel which of course the HMMWV does not have with the frame mounted diffs, ended up being an expensive mistake.
 
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