auto 4 speed in 72 4x4

[widow_maker]

I am thinking of replacing my th350 trans with either a 700r4 or 200r4 to gain some drivability and gas mileage. I have read in other threads that the 200r4 is almost a direct replacement for the 350 on a 2 wheel drive truck but didn't know if the same was true for a 4x4. I know someone here has already done this change. Any suggestions? 700r4 vs. 200r4, Problems with this change? Can I even run a 200r4 in a 4x4? Not sure if the tailshaft housing from a 350 will fit on the 200. I want to keep my stock NP205 transfer case. I know about the need for a lockup kit for the torque convertor and the specific setting of the tv cable - I have a local trans shop which can set this up. Any insight would be helpful. Thanks. Also, if anyone has photos of this change that would be great or a link to a past thread on this - I'm sure its been discussed before (new here, sorry to the senior members if this is old stuff)

[mrein3]

The right way to get overdrive in an automatic on a 4x4 is to use the 700r4. Advanced Adapters (AA) makes an output shaft that is the correct length and spline count to replace your th350 in front of an NP205. Some people are cutting off output shafts, adding other odds and ends but I wouldn't do that.

Get the output shaft from AA. It uses the factory th350 to np205 adapter you already have on your truck. From here you have decisions to make.

The overall length of this drive line is almost 2 inches longer than the stock one. You can either shorten your rear driveshaft and lengthen your front, or move the engine forward the required distance to keep stock drive shaft lenghts.

I have NOT done this swap. I have the core 700r4 on the floor in the shop right now. When I do this swap I plan to leave the engine where it is and have new driveshafts made.

[super72bb]

Here is a link to the same topic I have had some good help with. I hope it helps.


http://67-72chevytrucks.com/vboard/s....php3?t=145433

[ckhd]

I did this swap in my '72 K10 suburban.

I used a 700r4. You'll need to have a special output shaft installed in the 700r4. I bought a transmission from Bowtie Overdrives, and they put it in when they built it. Once this shaft is in, the tail end of it is the same as the current th350. bolts right up to the existing t-case adapter.

The tranny is 1 3/4" longer than the th350. Your options are to shorten the rear driveshaft and lengthen the front, change your mounting point for your tcase & cut the floor pan for the new t-case shifter location, OR move the engine forward 1 3/4". I chose to move my engine forward, since it was the fastest and cheapest solution.

Bowtie Overdrives also had a shift linkage kit that made the linkage install a piece of cake. You basically remove all existing shift linkage, and replace it with the infinately adjustable linkage that comes with the kit.

Bowtie Overdrives also had the TV cable and brackets for my 'burb, so that was an easy install. Proper TV cable settings are critical with a 700r4, because the tranny won't last 10,000 miles if it's wrong.

I also put a manual converter lock switch on the tranny too.

One thing you need to keep in mind is your engine and the math. What I mean is that if you need 200hp to make your vehicle maintain 65mph going down the road with your th350 in drive, you will need 285hp to do the exact same thing in overdrive. Another way to look at it is that your ouput shaft horsepower is 30% less than with a th350 (30% overdrive means 30% less power in overdrive).

I didn't gain 1 mpg. I got 11mpg hwy with a th350, spent $2500 on a 700r4, and got 11mpg hwy. Why? Well, my Suburban is VERY heavy and I live in a mountainous area. This causes two things to happen:

First, I have to push the gas pedal down farther to maintain the 'cruise' speed on the few flat parts of highway. This puts more fuel in the cylinders, and even though the engine is turning slower, it ends up using the same amount of fuel.

Second, it ALWAYS downshifts when I go up the steep hills that litter my commute.

I honestly believe that I would get better mileage if I had a big block, or if I had port fuel injection. Both would be nice...

If you are looking for a mileage gain for a daily commuter, I suggest you do what I finally did. Get a Metro. 50mpg, 80 miles per day, I save $190 per month.

[widow_maker]

Great stuff! Thanks to all for your replies. ckhd: sorry you didn't gain any mpg - you could buy a lot of gas for $2500. While the terrain in my area isn't as hilly as yours sounds to be, this might require some more thought. Any one else see limited gains in fuel mileage with this change? This isn't my daily commuter though I do drive it regularly in the summer months (stored in winter). I would still appreciate any other input or connections to previous threads on the subject. Thanks again.

[super72bb]

I must admit after reading that... it kinda takes the wind out of my 700 R4 sails. I guess the only thing is, My truck is a bit lighter than the Burb however I just have the stock 350 under the hood.

My whole reason for wanting to change it over was just to make it a bit nicer to drive. With the 4 speed it seems like I am driving a 5 ton delivery truck and it isn't very practical for around the city.

[mrein3]

Originally Posted by ckhd

I did this swap in my '72 K10 suburban.

...Stuff Deleted...

One thing you need to keep in mind is your engine and the math. What I mean is that if you need 200hp to make your vehicle maintain 65mph going down the road with your th350 in drive, you will need 285hp to do the exact same thing in overdrive. Another way to look at it is that your ouput shaft horsepower is 30% less than with a th350 (30% overdrive means 30% less power in overdrive).

You have that exactly backwards. Lets say you are going 65mph with a th350. Your engine is spinning at x rpms. For arguement sake and ease in math lets assume x = 3000 rpms. (Which probably isn't far from the truth with a th350 and 3.73 or 4.11 rear end gears).

Now in the same vehicle, going down the same piece of blacktop that was doing 3000 rpms with the th350 would be doing 2100 rpms with a 700r4. (30% lower rpms because of the .7 to 1 final in a 700r4 verses 1 to 1 final in a th350.)

You need LESS energy to push that same vehicle down the road with a 700r4 verses the th350.

Originally Posted by ckhd

I didn't gain 1 mpg. I got 11mpg hwy with a th350, spent $2500 on a 700r4, and got 11mpg hwy. Why? Well, my Suburban is VERY heavy and I live in a mountainous area. This causes two things to happen:

First, I have to push the gas pedal down farther to maintain the 'cruise' speed on the few flat parts of highway. This puts more fuel in the cylinders, and even though the engine is turning slower, it ends up using the same amount of fuel.

Second, it ALWAYS downshifts when I go up the steep hills that litter my commute.

I honestly believe that I would get better mileage if I had a big block, or if I had port fuel injection. Both would be nice...

I found my rpm calculator. I ran some real numbers making some assumptions.
Assuming you have 4.11 rear end gears, 28.75" tires:
3000 rpms = 62.44mph (th350, highway speed)
2100 rpms = 62.44mph (700r4, highway speed)
Same rpms, 28.75" tire, 3.73 gears:
3000 rpms = 68.8mph (th350)
2100 rpms = 68.8mph (700r4)
Same speed, 28.75"tire, 3.07 gears:
2100 rpms = 58.51mph (th350)
1470 rpms = 58.51mph (700r4)

What may be happening is you are putting your engine into an rpm range BELOW optimal. If you are running 3.07 rear end gears I think 1470 rpms is too low.

Originally Posted by ckhd

If you are looking for a mileage gain for a daily commuter, I suggest you do what I finally did. Get a Metro. 50mpg, 80 miles per day, I save $190 per month.

This is a VERY good idea. I didn't get a Metro, I got an Escort. I've put 225,000 miles on it since new and never logged a tank under 30mpg. With all those miles it still get 32-34mpg going 80 on the interstate and 60 on the us highway. If I take it on a trip off the interstate and keep it in the 55-60mph range the mileage goes up to 36-37. If I knew in 1994 what I know now I would have ordered two.

[widow_maker]

ckhd: when you moved your engine forward, did you fabricate your own engine mounts or does someone like advanced adapters manufacturer them? I run an electric fan setup in my truck so I have the room to move and the additional room around my HEI distributor would be nice. This might be the best solution for my proposed change. Thanks for the feedback.

[gator2511]

I was thinking about the same thing but I found out the 700r4 won't tow my travel trailer. Lots of info and prices at bowtie overdrives and eatmyshift.com Good luck.

[CraigS]

You need LESS energy to push that same vehicle down the road with a 700r4 verses the th350.

It takes the same amount of energy to push the truck down the hwy at a given speed, no matter what the gearing is. This is torque. HP is how fast you can apply the torque.

If you needed less entergy with higher gears, why not go with a 99% overdrive and require almost NO energy?
A 5 hp Briggs & Stratton should do it.......