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6l90 behind a 6.5

Twisted Steel Performance

Anything worth doing is worth overdoing.
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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..
 

BIGR

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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?
 

MrMarty51

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I've heard it's stronger but the biggest benefit is two extra gears
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
 

Husker6.5

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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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THEFERMANATOR

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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.
 

Husker6.5

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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.
 

THEFERMANATOR

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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.
Lowest they go for the 9.25" is 3.42. And until recently the 3.42 was hard to come by.
 

Husker6.5

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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.
 
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