1986 C-10 Silverado TH400 to 700R4 ?

[mcintyrederek]

Hi Group,
I have acquired a 1986 C-10 Silverado.  The info inside the glove box compartment says it's a 4.3L with a TH400 transmission.  Someone along the way dropped a Goodwrench 350 in there.  It runs great but only 3 gears, and the RPM's on HWY speed is pretty high.  I'm seeing quite a bit of 700R4's with locking converters floating around and am getting interested.  What's the chances of that bolting up with minimal problems?  ie, driveshaft length, number of input splines, crossmember position, etc.  I'll modify anything I need to.  This is not a collector's item truck, I'm actively using it as a service truck and looking for better economy.  Thanks in advance.
Derek McIntyre

[zieg85]

Welcome from NW Indiana.  Drive shaft length, splines, crossmember position, TV Cable to the carb are all things that will be different.  What rear end gear is in there now?  It is easy enough to do the swap but you should be aware of the differences.  The TV cable is the most important thing to get perfectly adjusted.  You can roast a brand new 700R4 rather quickly if it is not...

[Jason S]

Do you pull a trailer very much?

If not and if your current rear axle ratio is something like 3.73 or 4.10, you may be money ahead by swapping out the rear end to a numerically lower (higher geared) rear axle. Something like 3.42 or lower.

[mcintyrederek]

Thanks for both responses.  I take back the initial post of having a TH400.  Now I am not sure.  I need to look at the numbers on the transmission oil pan I guess to try and identify.  All I know is it's a 3 speed.  All the info in the glove box says is "3 speed auto."  As for the rear end, it says "REAL AXLE RATIO 2.73"  So dropping a 700R4 on there would probably mean I'd be running 800 RPM going 60 MPH which would be ridiculous.  Maybe the engine is not running as fast as I suspect.  I probably need to install an aftermarket tach just to see.  By the sound of it, I'm guessing 2600 at 60 MPH?  I am just used to running 1700 at 60 MPH with my '85 Monte Carlo with the 200 4R and locking converter.

And yes, I do pull a trailer often, not heavily loaded, but heavy enough that running the 700R4 even with a 3.73 rear end may not be the right thing to do.  Maybe I should leave it alone, it's a good running truck but just eats a lot of gas.  Carb is a Quadrajet, my timing may be a little off which could affect the fuel economy, too.
Derek.

[Captkaos]

You should be able to identify you transmission from the pan:  http://forum.73-87chevytrucks.com/smforum/index.php?topic=11124.msg81608#msg81608
If it still has a 2.73 gear adding overdrive isn't going to help.
how much mileage are you hoping out of it.  You are going to see tops 17mpg on the highway out of a 2wd truck.

[mcintyrederek]

It's a TH400.  Looks like that's a pretty darn good GM transmission.  Seems pretty tough and popular.

I'm getting about 15 MPG for average driving.  My Ram 1500 with 4.7 gets about 16.5 and it's a bigger truck.  I guess the lower RPM in 4th gear plus the fact it's a 2005 with fuel injection means it should get a little better.  I drove a V6 2005 Ford F150 for over 230K miles doing service work and got 18 MPG with 500 lbs of tools on it.  Overall I guess the Silverado is getting what it should given the engine and carb.  I'm a very easy driver and hardly ever tack over 1800, unless of course, I'm in 3rd and driving 55. 

I got this old Silverado from a friend.  It has been sitting in the woods with small trees growing up through it for nearly 10 years.  I have no idea how many miles is on it, the odometer starts with a zero so either I've got 108K or 208K on it (not sure?)  Anyway being a Chevy fan, I rescued it and cleaned it up enough to drive it for a nice farm truck.  Only paid $1000 for it and the engine only has 20K miles on it.  Goodwrench crate motor.

Anyway, thanks for the conversation and help.
Derek.

[rich weyand]

Fuel injection helps mileage a lot.  Fuel injection corrects the mixture in real time so that it is 100% spot on.  Carburetion is by necessity an approximation, since it is not a closed-loop system.  An engine will run when very rich, but run it too lean and the fire goes out, so the error in carburetion is set up always to the rich side, and that reduces mileage.  Figure 2-3 mpg better with fuel injection.

[zieg85]

In 86 the TH350 went away and the TH400 or 700R4 was your automatic choices.  I haven't driven a 700R4 that I liked, my vote would be to stick with the TH400 especially with a 2.73 highway gear.

[Jason S]

15 mpg doesn't sound real bad.   

The glove box tag says 2.73, do you think the rear end had been swapped out at some time?  Gearing can also be too little, especially for a loaded work truck.

How much have you looked into the ignition system, carb tuning, etc.? 

As for the distributor, if you haven't, check the simple stuff like cap and rotor for excess wear. You may gain some benefit with a re-curve kit to bring full advance in earlier.   

Other items to consider include: tuning the carb and ensuring the secondaries are correctly adjusted, adjusting idle and checking metering rods. 

[Captkaos]

15 is pretty respectable for that truck.  You might try doing stuff that Jason suggested and see if you can squeeze more out of it.

[mcintyrederek]

I am nearly certain the rear end hasn't been changed.  I went through the engine this weekend, adjusted the timing and the carb just to satisfy myself.  No real improvements on performance, and I don't expect to see much in fuel economy.  The combination of the 350 crate engine, the transmission and that rear end really works very well.  There's plenty of power and it just runs good all together.  I never even open the secondaries, the thing has enough take off just putting along with the front two. 

Thanks for the suggestions and replies, I think I'll just keep it like it is unless something tears up, then to save a few RPM's I'll try finding something like a 3.73 rear and coupled to the 700R4.  But then again, adding the TV cable and shortening the driveshaft doesn't sound like much fun, maybe I'll just stick with the TH400.  decisions, decisions..