Is there budget friendly 4 speed auto conversion?

[CrazyCanadian]

I have a 1975 C20, the engine was replaced with a 350 crate motor a few years ago but now the reverse has gone on the transmission. Am trying ro research a suitable 4speed automatic or even 5speed if such a conversion does exist. Mainly looking for better fuel mileage and lower RPM on the freeway.

Any ideas are appreciated!

[mongocanfly]

700r4

[andrewmp6]

700r4 would be the easiest swap since it came in these trucks factory,Anything newer will take more stuff to swap it like a pricey controller which are a grand up alone.

[ratskin]

Count on shortening your driveshaft about 3 inches. About $100.

[Jrgunn5150]

You can pick up a 4L80 for under 500 bucks pretty easy, a 400.00 microsquirt will run it, and a driveshaft will be about 300 bucks.

So call it 1400 all in. It will lower RPMs, and save fuel, but take a long time to pay for itself in fuel.

[GRADYS Performance]

First, What is your intended use for the vehicle? If it is just a driver with no heavy loads the 700R4/4L60 is the least expensive swap. If you are carrying heavy loads of towing heavy loads frequently the the 4L80E is the transmission you need.

The 700R4 only requires a lock up kit to control converter lock up, a rear transmission mount relocation and the driveshaft will have to be shortened. As well as the transmission and converter of your choice. If you carry or tow heavy loads you will have to use 3rd gear only no overdrive.

The 4L80E requires a computer to operate, a TPS sensor for the computer if column shift the frame mount has to be relocated rearward. Cooler line relocation. Transmission mount moved rearward. Driveshaft shortened and a Mechanical speedometer conversion if you have cruise control or wish to continue to use the original speedometer.

Here is a 4L80E swap in a similar vehicle. http://67-72chevytrucks.com/vboard/s...YS+Performance

[daddyjeep]

I am certainly not a transmission expert, but aren't there things that can be done to the 700R4 to make it handle more abuse? What might those things be? I have a K30 that I wouldn't mind getting a 4 speed in as well and I tow about 6K with it.

[Jrgunn5150]

Originally Posted by daddyjeep

I am certainly not a transmission expert, but aren't there things that can be done to the 700R4 to make it handle more abuse? What might those things be? I have a K30 that I wouldn't mind getting a 4 speed in as well and I tow about 6K with it.

There are, better planetarys, increased clutch counts, etc etc. By the time you have one of those beefy units built, that may or may not hold up in a one ton, you're at least 2k into it minus converter.

IMO, 4L80E is where it's at, guy's put Transgo kits in the stock unit and proceed to run 9's with them.

[GRADYS Performance]

Yes, the 700R4 can be upgraded but still not near the strength of the TH400/4L80E series of transmissions. A 700R4/4L60 transmission is comparable to a TH350 transmission in strength.

With the load you are carrying a 700R4/4L60 transmission would be marginal in durability and longevity even with all the upgrades available for it.

[daddyjeep]

That's what I needed to know. Thanks guys.

[D13]

Easiest way is to by an 86 truck and take all the parts - carb with linkage, trans, radiator, lines, and correct length drive shaft. New seals, new mount, and a conversion U joint (12 bolt to 10 bolt) and off you go. Now you have everything for a weekend swap. It actually is better to use an 87 (some improvements in the trans) but you will still need the cab connection for the cable.

BTW if you do the math it takes a lot of miles to pay back the cost of a conversion.
You don't say what trans is in it now. If it's a TH350, a 200-4R is almost a drop in (need the TV cable and a switch, that's it). If it's a TH400 or a manual trans, no direct drop in for overdrive.

[Reggler]

What about a gearvendors over/underdrive?

[GRADYS Performance]

Originally Posted by Reggler

What about a gearvendors over/underdrive?

In round numbers that is a $3000.00 upgrade. And still no Lock Up torque converter.

[sweetk30]

4L80-e and this guy here www.usshift.com

[Super_Dave]

Originally Posted by ratskin

Count on shortening your driveshaft about 3 inches. About $100.

Actually, it depends on what transmission came in your C20. A TH350, Short Tailshaft TH400 or Long Tail Shaft TH400.

My '74 C20 had a 350 CID with a TH350 and it's a bolt-in swap. No drive line shortening needed.

I have been told that the short tails haft TH400 is also a bolt-in but didn't have to deal with a TH400, so can't confirm.

In my case, I cruise at 60MPH around 1800+ RPM with 3:73 gears in a 14 Bolt OEM Rear End. My tire and wheel combo is +/- 1/2" from OEM wheels and tires.

[GRADYS Performance]

If you swap a TH700R4/4L60 transmission in place of a TH350 Check your driveshaft length to verify you have the proper travel. There are at least 2 different lengths of TH350 transmissions, None of which are the same length as a TH700R4/4L60 transmission.

[Super_Dave]

Originally Posted by GRADYS Performance

If you swap a TH700R4/4L60 transmission in place of a TH350 Check your driveshaft length to verify you have the proper travel. There are at least 2 different lengths of TH350 transmissions, None of which are the same length as a TH700R4/4L60 transmission.

When the performance shop installed mine, the 700r4 length was within .25" of the TCI TH350 I as replacing. We didn't have to change anything, including the cross member. Maybe I was lucky somehow. All I know is what we did and how it turned out.

[wilkin250r]

Originally Posted by D13

BTW if you do the math it takes a lot of miles to pay back the cost of a conversion.

Several people have posted something similar to this, but I think you guys may be forgetting, his transmission is toast right now. He has no reverse. So it's not just a matter of comparing conversion costs vs fuel savings, you've got to factor in the cost of repairing his current transmission if he decides NOT to do the swap.

I'm thinking a swap is only going to be a few hundred bucks more...

[barry1982]

Just throwing my 2 cents in.....what is the gear ratio in your diff? A buddy of mine did similar swap, and his truck ended up burning more fuel because it was now "lugging" in overdrive.....