Mad_Scientist_565

I have a problem and a plan. I'd appreciate your comments on both.

The plan...i have a K1500 with a pretty stout engine and a 1 piece 60. I found a 2 peice from a 99 Tahoe. Pretty sure it'll just bolt in. So what i would like to do is build it for 550hp plus towing duties. The current engines gonna be due for a refresh and when it gets pulled will be going into an older malubi or olds. The truck will be ls swapped. At that point, id change the 298 shaft and bell housing over too the 300mm ls style along with the front pump. In other words, I'll only need to build one trans. Sound feasible? Am i leaving out any parts for the change over (other than the pump and TC)


the problem...because of the power band of the current engine i find myself needing to be in 3rd alot. On many many occasions i manually pulled the trans into 3rd. Sometimes with throttle sometimes not and with intention to be used as decel or downhill control. Is it likely that the 4 to 3 forced broke the input sprag? (I dont have the old trans apart yet) ... if this is the likely problem ...or if it is a problem regardless... how do i fix this in the new build? I beleive the overrun clutches dont operate in 4th? With alot of down shifting is there a way to play with the valving or clutch pressure to help any load shock?

Last..any recommendations on must have parts? I think someone here makes some VB mods for overun in all gears (except 4) ..should i send mine in? Im game for spending some. Money on this being right.

Wait..one more last....anything i should do to the t case or the t case to trans connection to prevent something catastrophic happening (ei off roading and put to much power Down or something)

thanks in advance

Full Power

4L60 +500HP and Towing is disaster waiting to happen. Full stop.
Towing requires LOW GEARS. 4.10, 4.56 are good towing gears.
Towing requires strong transmissions. TH400, 4L80 are good examples.

Mad_Scientist_565

I'm not towing massive weight, or every day. Pretty sure id be pushing it harder in the mud or on a drag strip than i would towing. I just dont want to do a performance mod thats absolutely a deal breaker for towing...should such mod exist. Most ive ever done is about 9000 lbs which is technically over the trucks rating and I've only done it once. I have an f350 when needed.

Full Power

For perspective:
I just pulled OUT a perfectly good running, clean-tight-NO PROBLEM 4L60 and Installed a 4L80 in a half ton suburban.. Not a tow rig specifically, I have a 3/4 ton diesel for HEAVY stuff.
I REMOVED the 4L60 specifically because it was THE weak link. I knew that if the rig ever let me down, or broke down 1000 miles from home, there's a 80% chance it would have been transmission failure.
NOW: Add in 500 horsepower, occasional "light towing" or, in my particular case 12 PSI manifold pressure- and BOOM- 4L80 is the correct answer.

Mad_Scientist_565

I disagree. Specifically in my use case. The one that just went out only has a few upgrades...better clutch pack with more fibers, corvette servo... standard stuff..and it made it 75k miles. And thats with beating the **** out of it and a few severe electrical issues along the way. Now it's only at 450hp-ish right now, but that's also why i upgraded to the stronger 2 peice. If i was a dedicated tow rig, sure... but it's not.

MaroonMonsterLS1

You can easily modify the unit to engage the overruns in 1-2-3 when shifter is in D4
Or you can run the sonnax HD 2-3 shift valve

If you plan to tow regularly, just leave the shifter in 3rd
Don't even get into OD

You'll likely notice the same or better mileage
Improved cooling
And sustained transmission life

As for whether or not your current driving habits hurt the unit...it's possible. If you haven't pulled the unit yet, you can move the shifter to manual 1 or manual 2 and you should get those gears back. Because overruns will be applied and "do the job" of the input sprag

If you have all neutrals, its probably more likely you broke a pump gear or something to that effect. Do a pressure test before you pull the unit to confirm.

Mad_Scientist_565

I was reading elsewhere on this forum and it was suggested to alway blip the throttle when doing a manual 4 - 3. I definitely did not do that all the time. So since you didnt mention anything, Im assuming there is nothing to be done besides leave it in 3rd? When I do tow with it, its usually around 5k-6k lbs, and most of the reason I down shift is because I live in a very hilly area, so engine braking helps alot. A few of the videos I've watched on youtube describe a some timing issues if you make certain changes, so thats what led to my line of questioning.

Any comments on the plan of changing the trans over to the 300 mm shaft in a couple years when I swap the engine?

FranksCustomTrans

As mentioned the mod to make the overrun stay on in d4 is not hard, If you scan though my thread https://ls1tech.com/forums/automatic...-everyone.html you will find it and other mods.
To me for towing in OD or any heavy towing with the 4L60E the overun mods a must. Also if towing in actual 4th gear, WIDE BAND and SONNAX dual piston 4th SERVO a must.
I actually do the overun mod in all my builds from budget to max out as it is some simple and great sprag insurance.

Mad_Scientist_565

Hey Frank,
I was planning on doing the overrun mod. I thought i was understanding it that even though the over run is "on" in D4, that the over runs don't actually function when the trans is in fourth. Is that correct?
That was what led me to the question of how my downshifting might be impacting the transmission if there's no overrun capability when actually down shifting from 4-3. If all that is correct, then is it possible to do something like, for example, unlocking the TC to allow a little bit of slippage until the trans comes up to speed? (taking the weight of the engine rotating assembly and TC off the trans during spin up) Or is it that even necessary? I'm just asking theoretical questions at this point. I am a mechanical engineer but this is my first time diving into a auto trans and I'm just trying to understand exactly how it works. I'm 95% sure the downshifting is what broke it, and I can almost pinpoint the event when my issues started.

FranksCustomTrans

Yes the overrun is off in actual 4th gear of course the sprag is also free wheeling in 4th gear, So not needed, The overrun however does come back on fast during a 4-3 kickdown or pull down even faster if you leave out the checkball at that location so still protecting lessening the impact to the sprag. In the stock tune the converter clutch is never on during any up or downshift and should never be set up to do so.
The overrun mod does pretty much all you can do in that area. Been working on adding a friction to the Overrun to add even more support. Little tricky though as not much space , Forward plate has to have indentation cut for the extra clutch about 1/2 thickness of the clutch and thinner steels which are not available so much be cut and cutting steels on lathe is tedious . so not something i would ever incorporate into all builds . Since they are never holding a heavy load nor really holding in an upshift but rather act as more of a support cushion to the sprag thinner steels is fine.
Now the sonnax overrun hub might be a good addition or hardening the contact to sprag area of the factory one. Another thing I hate about the Borg sprag is the end caps being steel ,Wish they used the brass ones like the GM did I have noted the steels ones wear the hub considerable over time.

MaroonMonsterLS1

I have done the overrun mod for years
It works great

That said...it still takes time to downshift
The exhaust of the band apply in 4th will always beat the overrun apply
You're inevitably going to "hit" the sprag on a 4-3
There's no real way around it

Overrun mod helps lessen it for sure.

But...if you're towing in a hilly area that makes repeated 4-3-4-3-4-3-4-3-4 shifts...just leave it in D3
Or tune the tow/haul mode to not upshift to 4

Trust me...the trans will be happier. The engine will be happier. You'll likely get the same or better mileage
You'll be happier because it doesn't feel like you're riding a bucking bronco

3rd gear locked is great for towing...let the converter unlock and trans stay in 3rd for a good size hill...truck will feel so much better.

Mad_Scientist_565

this is a 96 so it doesnt have built in tow/haul. I do however, have it 411 swapped with the 2001? van tune that allows tow, haul, so I intended to enable it. Not actaully sure what tow haul mode does. I will take your advice and leave it in 3rd when i feel the needs, but Im also going to beef up 4th so I can pull in OD when its flat. Like I said before, its usually not that much weight. Any way to monitor for 4th gear slippage?

I'm glad you guys spent the time figuring all this out. Thanks for sharing your knowledge.