8L90 to NP205 info

[brandonleg]

For the 4x4 guys swapping a 8l90 to an old style passenger drop transfer case.

Had a few pm's asking about this, so I'm just putting this out here. Sorry for the jumbled post, it's just a coy and paste from a couple conversations.


I modeled my t-case adapter off something advance adapters already makes, their part number is 50-6910. Theirs is too thin to work for what I was doing, but the bolt circle mates to the 8l90 and 205, and it has the notch already for the 205's shifter rod. I just had the local machine shop make me one about 1" thicker (roughly 2 5/8" thickness). There are other t-case options in the 6 hole round pattern with a passenger side drop (241 and 208, etc) if you go this route, but the VSS makes it a little tougher.


http://www.advanceadapters.com/image...ge/SY_0050.JPG



Use a round pattern, 32 spline input, long input shaft 205. You have to find something to do for a shifter as you lose the pivot point when you ditch the factory adapter. I twin sticked mine and run a cable shifter. And I had the speedo housing modified for a VSS.

This setup also leaves you with a rather short front shaft, which could be an issue if you're lifted. I run a 4" lift and with the engine as close to the firewall as I felt comfortable running, my front shaft is 26 1/2" long. U-joint angles become pretty severe at that point.

My 205 was fixed yoke output for the rear. I have a long bed, so my rear shaft was 65" long which can become an issue for drive shaft critical speed if you run any sort of gears (I run 4.56 and 35" tires). I had a shaft made from 4"od .083" wall steel tubing which gave the highest critical speed on spicer's calculator (short of exotic materials). Even then, I'm limited to about 95 mph.


If I were to do it over, I wouldn't use a 205, or I'd run a doubler on the 205 transfer case - anything to help with the driveshaft lengths.

[brandonleg]

One more thing to be aware of... In mid model year 2015, chevy changed the VSS to a digital sensor. There is a bulletin about it floating around the internet. Mine was made in May of 15, so I didn't have to deal with this issue but I assume the VSS mod that is done to the 205's using the np241 reluctor wheel won't work with digital variants.

I had some goofy driveability issues until I got the sensor adjusted in close enough to the wheel. Trying to run without it was completely dead pedal.

[jasonblair]

Your post doesn't mention this, so I'd better ask... Did you modify the output shaft on the 8L90 at all? Or just machined the adapter plate and bolted it right up?

Also... You mentioned the loss of a pivot point for the transfer case shifter. No way the machine shop could have put one on the adapter plate they made?

Thanks so much! This is GREAT information, and you are really pioneering on this!

[brandonleg]

No, I didn't modify the output shaft. GM doesn't seem to over-engineer anything anymore, so I wanted to keep all the splines and get as much engagement to the transfer case as possible. Output of the 8l90 is 32 splines (4wd version at least), so once your adapter is the right thickness they go right together.

They might have been able to incorporate a shifter mount into the adapter, it would have taken alot more time, setup, and measuring. the original adapter was roughly 4" and some change thick, (I think, it's been a year and half so...) so the pivot would have came out right at the trans side edge. Mine was originally a NP208 truck, so when all was said and done the 205 shifter would have been clear back into the seat. Cable shifter seemed the easiest way to go, didn't have to hack up the floor pan as bad.

Had to notch the front crossmember ( I used clamshell mounts and adapter plates).

Gauges were from speedhut. The fuel and oil psi are mechanical. At the time I did it, the speedhut oil psi gauge wouldn't work over GMLAN, they might have resolved this issue by now.

the valve cover to brake booster clearance was too tight for my liking, so I converted to hydroboost brakes. Different mounts or engine placement might help with that.

Dirty Dingo was the only one offering a solution to the front accessory drive, now Holley has finally got their's on the market. I'm gonna try Holley's version out someday.

I ran a fuel pump control module and modified the 2015 fuel pump assy to fit in the stock tank. Many people aren't going that route, just using the ls1/ ls2 swap fuel system stuff.

I run a Dana 60 front axle and 4" lift. If I had less left I'd start to worry about oil pan to differential clearance at full compression. I think with the 10 bolt or 44 you'd probably be OK.

I used a BCM in my swap, a lot more stuff running through it than used to be. Don't know what the mail order guys are doing with that now.

The beauty is, there are a million ways to skin the cat. There is more and more stuff available all the time, too. I don't think we'll ever see alot of 8 speed support, though, since they are already being replaced by the 10 speed...

[urmaster85]

do you think this might work http://www.advanceadapters.com/produ...-bolt-pattern/ its 2 1/4" thick and has the vss in front of np205

[brandonleg]

I don't see why that wouldn't work, I wish I'd seen it before. You might have to shave a little off the 205's input for that, but I think it'd be worth it. I wonder how the reluctor wheel attaches to the input shaft? looks to be a press fit.

Your speedo will be off when in 4lo, but that's probably not much of an issue.

[brandonleg]

Sorry for the poor quality


IMG_20170527_210313999.jpgIMG_20170527_210253938.jpgIMG_20170527_210309135.jpgIMG_20170527_210324012.jpgIMG_20170527_210353876.jpgIMG_20170527_210415459.jpgIMG_20170527_210245574.jpgIMG_20170527_210226809.jpgIMG_20170527_210230943.jpg

[brandonleg]

Got a chance to look at the Advance adapters piece a little more. It looks like from their installation instructions that it is a slip fit, so not sure how you'd keep it in place. Wouldn't be hard I guess. Also, the output shaft on the 8L90 has a grooved shoulder with an o-ring forward of the splines. The input shaft of the magna transfer case has a machined space before the splines that rides on this o-ring. I guess the "right" thing to do would be to have your np205 input shaft machined so that it will ride on the o-ring and not mess with cutting the shaft shorter to make up for the thinner adapter.

[urmaster85]

i guess ill just have to try it out. will update with pics.

[jasonblair]

i guess ill just have to try it out. will update with pics.

Great! I can't wait to see how it turns out! (My Blazer just got back from sandblasting, so I've got a little time to figure this out, but the time to get the plan together is fast approaching!

[LSxPoweredSorento]

I'll just toss this out since I seen a few Advance Adapters comments. I can tell you from personal experience that the adapter I bought (6L90 to NP231) is a massive pile of cattle dung!! No, actually it's worse, at least cow crap can be used as fertilizer!
The size of the necked down area of the casting combined with the depth they machined the bore relief for the slip fit over the trans boss leaves practically ZERO material left. This is exactly where mine ripped around the adapter at. It's also the nastiest white fuzzy aluminum I've ever seen where it tore. I contacted AA about the adapter and after sending them pictures they said it's MY fault and the only way it could have happened is if my trans and t-case shafts were hitting each other and I torqued the adapter to them that way. Absolute horse dooky!! I measured them, they don't touch. They also made no comment about how THIN the material was either.
I myself, like Brandon ended up having my own made out of aluminum round stock.... Really, really big round stock haha.
Not trying to jack your thread man, just warning everyone about their quality and poor ethics.
I know our parts are rather different but if your interested I will share pictures of mine.

[jasonblair]

i guess ill just have to try it out. will update with pics.

urmaster85 - Did you ever try this out?

[urmaster85]

haven't yet. but i did manage to score a 1991 np205 with factory vss for $250. ill post pics when done