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6l90 behind a 6.5

I'm starting a list of needs, it sounds like this would be a upgrade to a 4L80, and the truck was a auto so most of the stock items and wiring is still their, and if I'm going to purchase a tranny I might as well go the upgrade route as this will be mostly a towing rig..
 
I should have been more clear I guess. I was comparing the Turbo 350 to the Turbo 400. I assume the Turbo 400 was a stronger tranny in its era. I know those were 3 speeds.

Could the 6L90 be considered the Turbo 400 of modern day, but much improved?
 
ak diesel driver said:
I've heard it's stronger but the biggest benefit is two extra gears
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An extra tall high gear on these 4:10 differentials would be a nice bonus, especially where 80 MPH is the speed limit on the interstate and there is very little traffic to slow anyone down. Also, an extra 5 MPH can be bumped into the equation as the HP wont even look Your direction at 85 MPH. Now, 86 they might and at 87, they`ll come after You, a whole 20 dollar fine and can be paid on the spot is nice too. LOLOLOL
 
I thought that the 4L and 6L had the same OD ratio, the difference was that the 6L split the difference with an extra gear between 1-2 and 2-3 to make it close-ratio and keep the engine in the power band without huge jumps in the upshifts. I could be wrong, and it wouldn't be the first time, but I will admit it if I am as automatic transmission internals and specs isn't my thing. To that I'll always defer to our resident auto tranny guru, @THEFERMANATOR .

As far as the TH350 and TH400 and their morphing, essentially the TH350 morphed into the 700R4/4L60(E) by adding an OD section and later electronic shifting control. The 700R4 had a very low 1st (but a huge jump to 2nd) and a very tall (.70) OD which made it have good off the line performance with an "economy" rear like a 3.23, 3.07 or 2.83 ratio and with great hwy mileage to keep the CAFE happy. They were great in lighter vehicles like the F body Camaro/Firebird, but when used in vans/pickups/burbs the TCC would often burn out and/or the OD section would just go bye-bye due to the weight/load/stress and you were back to a TH350.

The TH400 morphed into the 4L80/85E by adding an OD section to it. Many TH400 internal parts swap directly in, such as a racing-spec drum or the 32 element sprag vs. the wimpy 16 element stock one to get more HP/torque capacity. First was not a granny low like in the 700R4 and OD was a .75. This transmission was intended for heavier duty use with 3.21 to 5.38 rear ratios available in the 10.5" 14 bolt rear. As with the 700R4, the TCC and OD section were the weak links in the morph (as we all know) and would fail under heavy towing/performance, but was beefier than the 700R4/4L60 section.
 
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The 4l and 6l are completely different transmissions. The 6l trans has no direct gear, 4th is close to direct, but not 1:1, 5th & 6th are both overdrives, but there big advantage is the LOW 1st gear ratio, and the closer ratios between shifts. They have such a low 1st gear that they can still take off good loaded with 3.23 or so gears, but drop down to next to nothing for rpm's in 6th.
 
Man, I really wish somebody made a 3.21 ring and pinion for the front diff on my '98 K2500 Burb. The 3.21 for the 10.5" 14 bolt is scarce as hen's teeth, but can be found (for a price) as when I did the math, using stock LT245/75R16E tires 3.21 would drop me exactly into the rpm range I want driving Interstate at 80mph with a 4L80E in OD. A 6L90E in 6th with 3.42's would put me just about there, too, but with 3.21 it would be ideal in 5th or 6th depending on cruising speed, terrain and headwinds AND still give me the ability to pull up to a 5,000 lb trailer with no problem from a stop/in city.
 
Husker6.5 said:
Man, I really wish somebody made a 3.21 ring and pinion for the front diff on my '98 K2500 Burb. The 3.21 for the 10.5" 14 bolt is scarce as hen's teeth, but can be found (for a price) as when I did the math, using stock LT245/75R16E tires 3.21 would drop me exactly into the rpm range I want driving Interstate at 80mph with a 4L80E in OD. A 6L90E in 6th with 3.42's would put me just about there, too, but with 3.21 it would be ideal in 5th or 6th depending on cruising speed, terrain and headwinds AND still give me the ability to pull up to a 5,000 lb trailer with no problem from a stop/in city.
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Lowest they go for the 9.25" is 3.42. And until recently the 3.42 was hard to come by.
 
That's why I wished somebody would make a 3.21 front to match the 3.21 available for the rear, because I had done extensive research only to find that the front is available with 3.42 as it's tallest ratio only. Which leaves me looking at swapping my 3.73's for 3.42's and then going with a 6L90E conversion to get the desired speed vs. RPM for cruising.
 
Increased tire diameter with 3.42 gear-sets will get you close to what you desire, there is also an aftermarket lower 1st gear option for the 4L80/85 e/t
 
FellowTraveler said:
Increased tire diameter with 3.42 gear-sets will get you close to what you desire, there is also an aftermarket lower 1st gear option for the 4L80/85 e/t
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No to increased tire diameter for very good reasons, including increased height of tires is also increased height of vehicle which with the logarithmic increase of wind resistance pretty much negates and increase in fuel mileage and the additional cost of new rims and tires.

I have already ran the math more than once and with a 4L80E O.D. ratio and stock LT245/75-16 tires, a 3.21:1 gear ratio puts me right where I want to be RPM-wise at 80mph.

Since my Burb is a K2500, and a 3.21 set is not now and most likely not in the future available for the front differential, my options are currently limited to really tall tires and a 3-4" body lift (NOT!), tall not wider than a 265 profile Load Range E tires, a 2" body lift and a 3.42 rear (NOT!), a stock transmission, the 3.73's, tires and suspension and a GearVendor's OD unit that would put me about 200+ RPM too low at 80 mph from where I want to be at (1,950) at that speed (HELL NO!!$$$$ to make work/driveline fabrication/vibration issues). 6L90E with 3.42's front and rear and LT265/75R-16E tires, BINGO! Pretty much a bolt in job to engine/frame/x-fer case (lengthen/shorten driveshafts or move x-members?) and a standalone controller/ECM reflash and I'm cruising 1950rpm at 80mph and getting low-mid 20's mpg!

Comments, please.
 
FellowTraveler said:
I had ran a 400 turbo behind my cheater 427 L-88 back in the day........
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I had a TH400 behind the BBC 402 in my '72 Monte Carlo with a 2.73 Posi 12 Bolt rear. Tough transmission, after putting an almost 180° twist in the shaft at the rear yoke before the rear U joint broke of one driveshaft, and then later snapping the front U joint and tweaking the yoke ears slightly (welded on a new, heavier duty yoke and rebalanced at local shop) of the second driveshaft to be behind it, the transmission finally failed on an overnight 8 hr run from El Paso to San Antonio that used 6qts of Dexron and necessitated a total rebuild in San Antonio at 130K+ miles.
 
I would NOT even think about a gear vendors in a diesel suburban. I tried that back in 04 when I bought mine because of the 4.10 gears and what a cluster frick that turned into. The harmonics in the driveline created a horrendous vibration.

As to your 3.21's, run 3.42 gears with 235/85/16 tires. That would get you pretty close.

As to the 4l80e different gear ratios, you could just about do the 6l90e swap for what the 2 planetary sets will set you back.
 
Well, @THEFERMANATOR, as you read in my options comments, I was not ever considering a Gear Vendors for the reasons I outlined, $$$$$ and your experiences with driveline vibration and gave that a big HELL NO.

Running 235/85-16 with a 4L80E and 3.42 gears leaves me running about 220rpm higher than my target rpm.

I won't bother with trying to change the internal gear ratios of a 4L80 to get a lower 1st, and a taller OD of say .65 is useless when the transmission OD section can't handle the stress of its .75 OD.

So, ideally, a 3.42 ring and pinion set front and rear, with 265/75-16 tires and a 6L90E would give me a low enough 1st and 2nd to get moving a 5,000 load like my pickup bed trailer with a full cord of firewood in it or a lighter load like a camper and a .67 6th gear ratio for high speed cruising and a .85 5th gear for rural highway/wind/hills/load but still cruising economically, with a great ratio split for city driving.
 
My '94 C2500 HD C&C had a 3.42 rear and I had no problem towing a concrete cart with a yard and a half of Redi-Mix in it around town, a dovetail with a 731 Bobcat on it or a dovetail with a 94 Dodge Caravan on it. The 'empty' weight with the utility box was right at 8K and then those loads I pulled.
 
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