which tranny would you use....

[CG]

if you were building a 72 C20 big block truck, and most of the heavy duty use would be pulling a car trailer or camping trailer etc? it has the turbo 400 now. would you even bother going to a newer style trans with overdrive? gear venders overdrive unit is out of my price range. looking for suggestions, thanks

[Longhorn Man]

The only overdrive I would run in that aplication would be a 4l80e... the gear vendors would be cheaper if youw anted to retain the automatic feature. You can get a manual valve body now for the 4l80, but it's a full manual, so you'd be shifting on your own. (I'm considering this for after my 500 gets built up) However, it'd hold up, has overdrive, and in doing it this way, it'll be (somewhat) cheap.

[CG]

hmmm i just figured the gear vendors would be out of line....maybe not? as far as shifting manually...i hate it. maybe in a hotrod or something it would be ok, otherwise i hate having to shift...lol, yes im a lazy butt.

edit: been doing some reading...boy there are alot of ways to go with this, especially if you have alot of cash (wish i did)

[4x4Poet]

Are you in the middle of rebuilding the BBC? Still a 402?

[CG]

basically whats happening is this...i have this extra cheyenne super thats sitting in limbo. a friend of mine was looking at it today and was trying to justify to himself taking it off my hands. he was thinking gas mileage would burry him. i told him to stop whinning about it, this is a truck built for hauling and towing not a lil commuter car...but, i told him i would ask what kind of later model tranny would work in it. we looked at the gear vendors unit online and its nice but its a very expensive add on. so after that long winded explanation, the 402 will be rebuilt but probably to pretty much stock specs (if anything happens at all that is).

[4x4Poet]

There is an expensive way to have you cake and eat it too:

There are two lower gear sets available for the TH400. Both are very expensive, yet they retain the high gear 1:1 ratio:
http://www.mikestransmission.com/catalog4.html

Look for this info down the page:
*LOW RATIO GEAR SETS
Gives a lower first and second gear but retains 1:1 high gear. * This increased torque results in quicker launches and more responsive acceleration but does not hurt top end performance.

MT 2775...TH400 2.75 Low Gear LIST: $690.00 RACERNET: $595.00

MT 2776...TH400 3.00 Low Gear 1.57 Second LIST: $1098.00 RACERNET: $ 995.00*

The gear set alone is $600! Maybe cheaper vendors exist, BUT (big one), if you have to rebuild the TH400 anyway, the cost of the gearset would be no more than having to source a stand-alone 4L80 or a Gear Vendors overdrive. And, keeping the lower geared TH400 has several advantages:

1. Keeping the TH400 would retain the same driveshaft length. A 4L80 or overdrive would necessitate DS length changes. Maybe a yoke change, too.
2. Sourcing the TH400 gearset and professional rebuild would be easier than sourcing a stand-alone 4L80. For instance, that link above does it all.
3. Re-installing the upgraded TH400 would be easier than installing a 4L80 or overdrive.
4. The upgraded TH400 with an improved shift kit would be tougher than the 4L80.
5. Retaining the TH400 avoids a tranny mount change & crossmember relocation.
6. Here's the biggie: With the upgraded TH400's deeper first gear, you could swap in a higher R&P ratio so that the TH400's 1:1 third gear thru a higher R&P provides near a 4-speed transmission's OD gearing. Yet, first gear final ratio would be back to stock for no compromises on the street. The cost of a R&P swap (<$500) would bring the costs back up, though still with no hassles.
I did forget to ask what rear axle you have. If it's an Eaton, this may be a deal killer since few ratios are available and are expensive to purchase and have installed.


Using a gear calculator:

Stock TH400:
2.48 first gear x 4.11= 10.19 "crawl ratio"
Upgraded TH400:
2.75 first gear x 4.11= 11.3
2.75 first gear x 3.73= 10.26

So, the lower 2.75 first gear enables moving from 4.11 R&P to 3.73 R&P with a resulting slightly lower first gear final ratio through the R&P. Obviously, the same could be said for moving from 3.73s to 3.55s.

With 30" tires and 3.73 R&P, rpms @75mph would be 3133.
The same with 3.55 R&P would give 2982 rpms @75mph.
3.25 R&P would give 2730rpms @75mph.
3.08 R&P - 2587

A little taller tire would reduce the rpms further within each ratio. All while retaining a fine low first gear in the TH400. Going with the super expensive TH400 3:1 first gear set (with lower 1.57:1 2nd gear) would enable the use of the higher R&P ratios while retaining near stock first gear final ratio throught the R&P. So, some fuel savings would recoup the costs a little. Maybe a lot if you keep the truck longterm.

I'm no auto fan, but I'd upgrade the TH400's first gear ratio for the desirable advantages I listed even if doing so were more expensive. I like upgrading stock components for a sleeper look, too. Don't consider this a suggestion as much as enjoyable personal research on my part. I luv this stuff.

[4x4Poet]

Originally Posted by CG

...so after that long winded explanation, the 402 will be rebuilt but probably to pretty much stock specs (if anything happens at all that is).

Ok. On the engine side, I'd upgrade to retro-fit roller lifters to gain power while enabling the use of a high mileage, torquer cam. Cam longevity would be a side benefit. Use hyper pistions for light weight. Go TBI/EFI or use a fully rebuilt Q-Jet (including re-done throttle bores, for example) and mileage would seem pretty good for a BBC if not for today's gas prices.

[Longhorn Man]

towing with 3 gears, and such a large space between second and third would suck once you hit a hill, and if the load is real big, just the shift from 2 to 3 on flat land would bog the motor real bad.

[4x4Poet]

Longhorn Man, with the cheaper of the two lower first gear sets for the TH400, second gear remains the stock ratio (1.48:1). The more expensive 3:1 first gear set uses a 1.57:1 second gear. I don't know if that would be enough of a change to make a difference you posted. Unless you're talking about the TH400 in general. I wasn't aware that 2nd to third was a big jump while towing up hill.

[Longhorn Man]

I was refering to the added gear sets you were talking about... good point on the not too much a difference

[CG]

holy cow poet...thats more than i can wrap my pea sized brain around...lol. i forwarded that off to my buddy. thanks for all the thought that went into that. ive read alot about the 4l80e longhornman that sounds like a viable way to go also. if he goes for it off goes the cheyenne, if not, back in truck limbo.