I am looking to educate myself on selecting a torque converter for an LS towing application with boost. Hopefully this will turn into a productive conversation as there are a ton of good resources here that are far more experienced and smarter than I am.

Some background: I recently picked up a 2005 Suburban 2500 with a 6.0/4L80E/4.10s. It's bone stock down to the paper filter. I plan to use it as a tow rig pulling either a 7000# loaded car hauler or a similar weight travel trailer. Of course the itch to add boost is strong as it needs some help up hills. For the sake of the argument let's assume I'm going to run a stock cam and a turbo small enough to spool quickly.

There are two topics I'm curious about - stall speed and lockup strategy. Of course, if you have anything else you think is important to transmission longevity, feel free to add.

Stall speed - I've heard recommendations for keeping a stock stall converter, as using a higher stall can lead to heating the fluid more quickly. That makes sense as the constant throttle inputs will always be churning the fluid, as opposed to a stock stall which will convert more of that energy into driving the wheels. On the other hand, in my own personal experience driving various turbo LS projects, a 2800-3200 works awesome to get the RPMs up, start driving the turbine to bring up boost, and get the car off the line quickly. Why is a stock stall speed beneficial for towing?

Lockup - everyone says to keep the converter locked while cruising to, again, prevent heating up the trans. Makes sense, but with the addition of boost, my concern would be the engine/load generating more torque than the lockup clutch is able to handle. I would think it would be a bad idea to be loaded with a trailer headed up a hill under 5-10 lbs of boost with the stock converter locked up. Granted, a stock lockup clutch has a bigger diameter than an aftermarket single clutch stall converter, but something has to give. Thoughts? Is a triple disc needed?

Pic of Hoss for attention

 

the first thing we mention in TowPig club is GEARS.
Axle Ratio gears optimal for towing will be lower than most factory equipped gas power rigs.
4.56 is reasonable tow gears with 4L80 on 32 to 33 inch diameter tire.
.
Next, you touched on lockup strategy:
definitely want triple disc clutch, and youDEFINITELY will tune FOR solid lockup in 3rd gear in Tow-Haul settings.
will add more later.( Great thread topic, by the way)

 

Gears are always an interesting topic. I have found that converter choice is more important than gear choice, at least in non-towing applications. Keep in mind this only applies to my experience on the street, not at all drag racing.

I had 2.88s in my Jaguar which in paper are garbage but the long ratio with a loose-ish converter gave the engine LESS mechanical advantage, and seemed like it artificially increased stall speed. This helped with spool believe it or not. I'm running 3.54s now and spool is still great but not the same.

In the case of my Suburban I am not crazy about regearing numerically up from the existing 4.10s because I don't tow often enough to justify sacrificing fuel economy when just cruising without a trailer. Not to mention cost of regearing 2 differentials, etc. I am better off going down to a shorter tire to achieve a shorter overall gear ratio, and theoretically gaining MPG due to the lighter tire/wheel combo (currently on 265/75/16 or 31.6" tall, whereas stock size is 245/75/16 or 30.5" tall).

 

I've tuned my dads turbo suburban (stock 5.3 with cam, billet 67/65 turbo, 4l80(built by me) swap, 3.42 gear, stock tire size) for lockup as early as second in tow/haul mode. STOCK "high stall" converter. We run about 7psi with e85, computer calculates 500ftlbs. We stay in lockup all the time for towing. It is fantastic! We can gain speed going uphill with converter locked in overdrive towing 6000lbs enclosed trailer.

Stock lockup clutch is holding just fine. I do unlock it under full throttle. Remember, GM put these transmissions behind turbo diesels, they are plenty stout.
You could get away with a stall converter and towing as long as you can hit lockup to keep it cool. But generally, stall converters are smaller diameter so your lockup clutch is smaller/weaker, and then smaller converters don't seem to put power down as well (towing) in my experience. I just let the turbo do the work.
If youre planning on a triple disc and putting serious power down while in lockup, you will need a better input shaft for sure.

I have done alot of trans tuning to get it right. Many hours behind the keyboard, adjusted tune so it doesn't unlock the converter on decel, temps stay very reasonable.

 

Both your lockup and stall speed points are valid. for a 7000lb payload I would also recommend a stock stall converter and adjust your lockup strategy as needed so it does not slip and does not create unnecessary heat in the transmission. I would like to add that a turbo would not be my first choice for boost if towing is the primary function. with a stock stall converter you would need a turbo small enough to make 5-10lbs of boost at 1800-2000 rpm which will likely limit your power higher in the rpm range with the small turbo. a PD blower while yes it is less efficient higher in the RPM range, will be less restrictive and offer that low end torque you are going to want with a tight converter and towing.

 

That's a fair point about the type of forced induction. I should clarify that I plan to tow about a half dozen times a year, with the rest of it being normal street/highway driving with no trailer attached (read: street hooning). I like the turbo for its power potential and the fact that you don't have to deal with belt slip, heat management (W2A), etc. It's also nice not to have to deal with the parasitic loss of the blower always spinning. The plan is a baby 67/73 or a 67/65 or a 66/73 on a T4 housing which should spool pretty quickly on a 6.0. I am not really concerned with high rpm power production as I don't plan to rev it over 5000 (it is a Suburban after all...).

 

In that case you are on the right track. What is your planned towing distance/time for when you make those half a dozen trips a year? any steep grades along the way?

 

a 7875 t4 billet wheel with a .96 back housing would be insta spool. We have the same turbo on my sons 5.3 and it spools almost instantly. On a 6.0 it would spool even faster. I built a turbo kit (same turbo) for a 6 liter gto and it was instant spool.

be careful with convertor choice. We quickly wrecked an FTI 3200 truck convertor rated for 550 hp, our own fault trying to stall it for too long while building boost on the footbrake. Then (my son) He towed with a yank 3800 single disc, smoke it in one tow. I told him not to tow his side by side with it. Now we have a 3600 circle D triple disc. I'm no longer a yank fanboy to say the least. Much better convertor than the yank. The yank sucked at street driving. It was waaaay too loose even though it was custom built to out combo. Yea he smoked it, his own fault, but it didn't work well either for daily driving. The circle D, omg. The difference is night and day. Drives like stock, starts moving when you take your foot off the brake, will stall 3000 on the footbrake without trying hard. He bought it so he could tow with. At this point, I don't think theres a better convertor for the street. I'm now wanting one for my car.

Back to yours. Look into getting a HD stock replacement. I'm pretty sure Dacco makes them, I've had good luck with Dacco reman units on my stock trans builds.

 

Is the Circle D converter a 258mm?

 

yes, 258. For some reason I thought it was 252.