LUBA

I am planning on doing a frame off resto of my 72 Blazer. I would like to run a mast motorsports LSX 454 fuel injected small block (700 hp) and a newer 6 speed tranny and put it into my k5. I already have a dana 60 front, 14 bolt rear 4.56 gears detroit locker in the rear, maybe an ARB in the front. I know the mast ecm will control the 6l90. I plan on running around 37" tires and want to be able to drive this thing anywhere. I plan on fully boxing the frame and building a cage tied to the frame. Just wondering what kind of power levels the 6 speed auto can handle. Also wondering if I can bolt my np205 t-case to it, or if I should get a different one. Any advice?

Irish350

iTrader: (14)

would be ****** killer. And pretty different from whats out here that ive seen.

If i remember right, the newer 6 speeds can handle upwards of 600hp, but i wouldnt quote me. Hopefully some of the syty guys can say whats up with the t case.

LUBA

Well that's what I was thinking too. Just wondering on the 6l90 and np205.

Black FormulaLs1

iTrader: (10)

pics pics pics and more fkn killer pics..thats going to be beastly!!!

silverz28camaro

iTrader: (20)

Yeah what he said ^^^^^^^^

LUBA

Just in the planning stage right now. Just looking for info from the late model guys.

1972k5ls

I am thinking of the same swap for my 72 blazer and was curious about the 205 and 6l90 mating as well.... Any info?

billyjo

The biggest problem I had when I did my swap with the NP205 was getting a VSS signal, I had to pull of the tail housing and do some special machining to get a VSS that would work but it can be done.

Bill

blazerboy72

iTrader: (9)

I don't know how much lift you are planning on running, but you could possibly use a 2wd 6l90 with a divorced np205 to save a few headaches. If the lift is too high you will run into driveline angle issues though.

billyjo

blazerboy72 brings up a very good point. I have 4" springs in the back with a 1" leveling block. I had the block machined so that it moved the rear axle back 1" to center the tires in the rear wheel well. My springs are shimmed to roll the pinion slightly to allow for less angle. All this and I still ended up running a CV joint in the front to get rid of the vibration from having to short of a driveline.

1972k5ls

I was wanting to do a 6 inch lift and use pacific fabs crossmember for the swap. Does the 6l90 mount up fairly easy?

tsnow678

iTrader: (3)

I can't wait to see this project become a reality. I bought my son a 72 Chevy stepside 4x4 and have located a 5.3 I am really considering buying for a swap into this truck. I thought I would keep the TH350 and transfer case as is and just add the engine. You may have added a new idea for me. I just completed a 6.0/T56 swap in my 69 Camaro and all I can say is what a pain in the *** but how rewarding the outcome is! Make sure you post a ton of pics, please.

poorhousenext

I think you should take a little trip over to the Automatic Trans Forum and do a little research before putting 700 HP and XXX.X Ft Lbs o Tq in front of a 6L90E.

Second thing you need to consider is what rear end ratio you are going to use. The 6L's gear ratios are designed more for gas mileage performance using lower 1st -4th gear ratios to get weight moving, as in hauling load than GM's other automatics.

Trans has no 1:1 gear ratio but does have 2 OD gears so it can maintain speed and some gas mileage benefit when it has to shift out of 6th on a not so high of a grade, depending on if your rear end gear ratio choice and TQ band of engine.

These are the most quoted 1st - 6th gear ratios for the 6L80E/90E. Depending on year model of trans they have changed. If I remembered correctly, 6th gear has been changed to 0.70 instead of 0.67/0.68, and I believe 4th is now a 1.18 ratio. You can check this out by going to GM powertrain engine and transmission website.

You do have your real, real, real old style Granny 1st gear to get weight moving are crawling for a very short distance and speed.

1st: 4.03
2nd: 2.36
3rd: 1.53
4th: 1.15
5th: 0.85
6th: 0.67(0.68)

6L's are not GM's best close ratio gear transmission, thats older 4L80 trans. The 6L80E does allow you to run a tighter stall converter.

6L80/90
4.03
2.36 -- 58.56% (high) RPM Retention 41.44% RPM DROP
1.53 -- 64.83% (high) RPM Retention 35.17% RPM DROP
1.15 -- 75.16% (high) RPM Retention 24.84% RPM DROP
0.852 -- 74.09% (high) RPM Retention 25.91% RPM DROP

Now if you just have to have a 6sp auto, then you might want to look at TCI's 6X six speed that's can hold up to 850 HP and can be purchased with upgraded components to hold 1000+ HP.

It's 1st to 5th gear ratios are more like a close ratio 4 or 5 sp close ratio manual transmission, only the TCI 6X has deeper 1st gear to get weight moving.

TCI 6X (4L80E based)
2.93
2.23 -- 76.10% (high) RPM Retention -- 23.90% RPM DROP
1.57 -- 70.40% (high) RPM Retention -- 29.60% RPM DROP
1.18 -- 75.16% (high) RPM Retention -- 24.84% RPM DROP
1.00 -- 84.745% (high) RPM Retention -- 15.255% RPM DROP
0.75 -- 75.00% (high) RPM Retention -- 25.00% RPM DROP

M21 close ratio 4 sp manual
1st gear 2.20
2nd gear 1.64
3rd gear 1.28
4th gear 1.00

If you like what you see with the TCI 6X but think MSRP price is to high, they can be bought several hundred dollars less on web, and if you factor in one rebuild of a 6L90 by the only shop that I know of that has been able to get a 6L80e to hold for more than a few runs at strip, it's cheaper, and 6X has a limited lifetime warranty when used behind engine with 850 HP or less...

By the way, I'm not pushing either trans, but I can say I have one of each in a car. First one has a 6L80E an I'm building the second one with a 6X so I can beat on it some...

Do your home work if 6sp is more for go than show.

PS: Jake, Jake's Transmission is working on a 4L80E based 6 speed too.