Advice for swapping out TH400 for a 700R4...

[C&C Chevy]

Ok, I know this might sound crazy to y'all, but I want to take the 3-speed out and put in a 4-speed trans. The RPO tells me the truck originally came with a TH350 (has provisions for a kickdown cable too) but at some point the PO put in a TH400. Why? Anyway, I don't haul heavy loads or tow trailors. And, I do a lot of highway driving to work everyday. The truck also has a 4.3 v6, and I had thought the TH400's were more for big block, high HP applications? I can just imagine the pain of my lil v6 trying to turn the monster.

Any way, need some advice on pulling off this swap! I've started looking for a good core from a 87-92 locally, and a few have turned up for around $75. Is this a good price for a core?

Next thing I need to do is measure the TH400. Not sure about the crossmember and all that stuff either.

 

[RetroC10Sport]

My truck had a bad TH400 when I got it and I ended up putting in a bad 700r4 which prompted me to get rid of the truck.

I had to change the crossmember, the driveshaft, do some wiring, add a vacuum switch for the TCC, add a newer brake light switch for the TCC, add a new carb and bracket for the kickdown.

Probably would have been easier to go with an aftermarket kit to install a 700, but I wanted everything to look factory.

Many V6 trucks actually had the TH400 long tail factory.

 

[C&C Chevy]

Wow, really? Why would they put a heavy trans like that behind the 4.3? It might be the factory original trans tho. RPO lists it as MX1. I just looked it up and MX2 is the TH350.

Also, I know the TCC will be a pain but I haven't heard of adding a newer brake light switch for it?

And why a new carb? Will the stock qjet not work? I figured that I would need a different throttle bracket for it.

What year of 700R4 did you get? Been on the look out for an 87, since they are supposed to be built better and it's closer to the year for my truck, hopefully reducing any wiring nightmares..

 

[RetroC10Sport]

You need the right bracketry to keep the kickdown adjusted right. If it isn't right, you will burn out the trans in short order. You truck MAY have the correct throttle if the carb is original and you might even have the right brackets already, I have seen some V6 trucks with it.

The TCC needs a switch to turn it off when the brakes are applied, on the factory application it's at the brake light switch.

 

[HotRodPC]

Being yours is a half ton, a V6, I say your pretty much safe with the 700r4, and you will actually benefit from the swap. You'll see much better take from a light, less hp robbed so that 4.3 will actually feel like it has a little pep, lighter weight etc. So I say it's all good. But I will add this, I doubt you really get to take advantage of the OD very often. Maybe with the wind behind you at 80mph you will. It's gonna play hell going back and forth between 3 and OD. If you have a 3.73 or 4.10 rear gear ratio on stock tires, you'd then have a good set up for all the way around, but I have a feeling you have 3.08 or 2.73. Even a 3.42 woudln't be to bad, but 3.73 on down would be best.

 

[C&C Chevy]

Rear end is 2.73. Tires will be 275/60's as soon as I get my 8' rally's painted!

Plan is to replace the v6 with a mild 350 AFTER my daughter drives it for a few years. So, it'll be 5-6 years before the v6 is pulled. Wanting to go ahead and start the tranny swap this fall when the heat subsides, will be gathering parts and knowledge in the mean time..

Took some quick pics of the brackets on truck now, carb is stock qjet as far as I know..

 

[HotRodPC]

Honestly, you're going to notice more pep from a gear change, and you'd be doing that 700r4 a favor. I've your going to buy gears, I'd go with a 4.10s.

Many don't seem to realize, those tall gears can hurt your mpg just as well as hurt. Low gears help city driving, OD compensates for low gears on the highway, so you still get better mpg. In OD with 2.73's, your rpms will be so low, you won't be in the efficiency band of your power band. You're better off driving in 3rd.

 

[oneluckypops]

Honestly, you're going to notice more pep from a gear change, and you'd be doing that 700r4 a favor. I've your going to buy gears, I'd go with a 4.10s.

Many don't seem to realize, those tall gears can hurt your mpg just as well as hurt. Low gears help city driving, OD compensates for low gears on the highway, so you still get better mpg. In OD with 2.73's, your rpms will be so low, you won't be in the efficiency band of your power band. You're better off driving in 3rd.

though I agree with you Hotrod 4.10's would be perfect I would also not wait for a set of them, In the mean time I would look for a 5 lug 10 or 12 bolt that is equipped with ATLEAST 3.73's. We both know 700's HATE high ratios and 2.73's is going to make for a short lived Transmission.

With those gears I almost wonder if the truck originally wasnt equipped with a Manual Transmission.

 

[RetroC10Sport]

With those gears I almost wonder if the truck originally wasnt equipped with a Manual Transmission.

Nope, standard gear ratio for the 4.3/400. Only the 3.08 was optional too.

 

[Stroked]

Honestly, you're going to notice more pep from a gear change, and you'd be doing that 700r4 a favor. I've your going to buy gears, I'd go with a 4.10s.

Many don't seem to realize, those tall gears can hurt your mpg just as well as hurt. Low gears help city driving, OD compensates for low gears on the highway, so you still get better mpg. In OD with 2.73's, your rpms will be so low, you won't be in the efficiency band of your power band. You're better off driving in 3rd.

I've never understood why GM paired 700r4's with such high gears. My 86 had a 305/700r4 and something like 3.42's with 31/10.50r15 tires. It was lugging pretty bad trying to hold overdrive.


Sent from my iPhone 4s using Tapatalk

 

[HotRodPC]

though I agree with you Hotrod 4.10's would be perfect I would also not wait for a set of them, In the mean time I would look for a 5 lug 10 or 12 bolt that is equipped with ATLEAST 3.73's. We both know 700's HATE high ratios and 2.73's is going to make for a short lived Transmission.

With those gears I almost wonder if the truck originally wasnt equipped with a Manual Transmission.

Yep, 3.73 drum to drum 10 or 12 bolts are out there for $100-$150 ready to bolt in. That $150 could potentially save a $1500 transmission. Never know, you just might do OK with the 3.73's too and just keep it that way.

Nope, standard gear ratio for the 4.3/400. Only the 3.08 was optional too.

I guess they did that cuz of no OD, but that's a horrible thing to do to a 4.3 making it take off with a Th400. High hp absorption and ****** tall first gear make the Th400 a poor choice behind a 4.3 and then a tall ratio on top of that.

 

[HotRodPC]

I've never understood why GM paired 700r4's with such high gears. My 86 had a 305/700r4 and something like 3.42's with 31/10.50r15 tires. It was lugging pretty bad trying to hold overdrive.


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It had to do with economics and emissions. Even though the fuel mpg didn't happen, it didn't hurt it, but Feds lowered emission standards. So less rpms meant less pollution.

 

[C&C Chevy]

*adding gear swap to the list..

between 50 and 70 mph the truck drives really well on highway. I can pass with no problem. It's getting on the highway that's nerve rattling at times! from 30-50mph there is absolutely no acceleration. I had an 87 with a 350TBI, 700r4 and 3.08's and acceleration was no problem. I could easily be doing 70-75mph to merge from the on-ramp into traffic. With this truck, i can floor it and only be doing 55-60mph, a little bit scarier when you have a semi blowing down your neck. Of course I knew the 4.3 would be lacking in power, and that's the reason I'm leaving it in for my daughter.

So, for now I need a 700R4 and a rear end with say 3.42's or 3.73's? Keep in mind that I will be replacing the lil v6 for a mild FI 350 or 5.3 down the road..

btw, where is the best power band in a 4.3? Just curious. I've read that 1800-2000rpm for a 350 is optimal on highway for mpg's..

 

[oneluckypops]

*adding gear swap to the list..

between 50 and 70 mph the truck drives really well on highway. I can pass with no problem. It's getting on the highway that's nerve rattling at times! from 30-50mph there is absolutely no acceleration. I had an 87 with a 350TBI, 700r4 and 3.08's and acceleration was no problem. I could easily be doing 70-75mph to merge from the on-ramp into traffic. With this truck, i can floor it and only be doing 55-60mph, a little bit scarier when you have a semi blowing down your neck. Of course I knew the 4.3 would be lacking in power, and that's the reason I'm leaving it in for my daughter.

So, for now I need a 700R4 and a rear end with say 3.42's or 3.73's? Keep in mind that I will be replacing the lil v6 for a mild FI 350 or 5.3 down the road..

btw, where is the best power band in a 4.3? Just curious. I've read that 1800-2000rpm for a 350 is optimal on highway for mpg's..

I wouldnt reccomend 3.42's 3.73 would be the highest ratio i wouldd reccomend. But at the same time they would defintly be better then wussy 2.73;s.

 

[HotRodPC]

*adding gear swap to the list..

between 50 and 70 mph the truck drives really well on highway. I can pass with no problem. It's getting on the highway that's nerve rattling at times! from 30-50mph there is absolutely no acceleration. I had an 87 with a 350TBI, 700r4 and 3.08's and acceleration was no problem. I could easily be doing 70-75mph to merge from the on-ramp into traffic. With this truck, i can floor it and only be doing 55-60mph, a little bit scarier when you have a semi blowing down your neck. Of course I knew the 4.3 would be lacking in power, and that's the reason I'm leaving it in for my daughter.

So, for now I need a 700R4 and a rear end with say 3.42's or 3.73's? Keep in mind that I will be replacing the lil v6 for a mild FI 350 or 5.3 down the road..

btw, where is the best power band in a 4.3? Just curious. I've read that 1800-2000rpm for a 350 is optimal on highway for mpg's..

Bear in mind, the suggestions we're making is to keep your transmission alive. I'll assure you, with your trans kicking in and out of 3rd and 4th and in and out of lock up, you won't do well on mpg and your trans will die a short life. I'd be looking for more of a 2200-2400 rpm. Depending on your power band, 2200-2300 shouldn't be much different in mpg if any at all, could even be better. Just don't get yourself confused in BS you hear from others.
You'll hear all the time, I get 2mpg better @ 65mph than I do @75mph. So then they do a gear change to be at the same rpm @75 as they were at @65, and now they are pist because they get the same mileage on the highway and lost city mpg. How is this??? Makes perfect sense. With the taller gear, you're putting a bigger strain on the motor to get the truck to leave a red light, so you have to give it more gas to get it rolling. Same also goes for the highway rpm, it's a bit more strain to hold the same speed. People don't realize you will ALWAYS get better mpg @65 than 75 because you're increasing wind drag the faster you go. Higher wind drag is going to mean less mpg, and you actually have a better chance of fighting that increased wind drag with a lower gear. Your motor and trans is going to appreciate the break too.