transmission swap

[Stoke]

I am going to do an engine and transmission swap and wanted to know if I was in for any clearance problems. I have a 86 GMC and I am swapping the 305/turbo350 out for a fuel injected 350 with a 700r4 transmission. I know that there wont be any clearance problems with the motor but didnt know about the transmission. If anyone has made this change I would appreciate any info you could share with me.

[slug]

no problem at all, the only problem you might have is the tranny mount lining up with the crossmember

[babyJay]

You'll have to re-position the crossmember about an inch forward??? if I remember correctly. Just have to do some measuring and drill new holes in the frame. No biggie. You'll also need to make provisions for a TV cable and TC lockup!

[BowTieOgre]

Is the 350/700r a mated pair? The only other thing I can think to add is you might have to address the driveshaft length depending on your tailhousing length on the th350.

[MylilBowTie]

If its a LWB the driveshaft will work. If its a SWB the it will need to be cut down.

[Pyrotechnic]

Originally Posted by MylilBowTie

If its a LWB the driveshaft will work. If its a SWB the it will need to be cut down.

Just curious, no thread jacking intended, but I have a 77 LWB with a turbo 350 and a 1 piece drive shaft. Why do SWB's need to be cut down and LWB's don't ?

[MylilBowTie]

I'm no expert or anything. But all the SWB trucks I have seen that have th350 are the short tail (6in) transmission. Same with the LWB th350s those are the long tail (9in) transmission.

The 700r4 and the long tail (9in) th350 are pretty close match in length. So if his truck has a th350 and its a short tail (6in). His driveshaft will be longer do the short tail.

One piece and two are a toss up on LWB trucks. Seems to me the majority of the two piece is the 3/4 and 1 ton LWB trucks. Though I have seen a small few 1/2 tons have them also.

Clear anything up or make it worse?

[Pyrotechnic]

Makes sense now. I didn't know SWB trucks got the 6 inch tailshaft version. At least that will save me some money when I finally get a 700R4.

I think the Big 10 of Heavy Half LWB trucks got the 2 piece and regular half tons got the 1 piece.

[rfmaster]

Originally Posted by MylilBowTie

The 700r4 and the long tail (9in) th350 are pretty close match in length. So if his truck has a th350 and its a short tail (6in). His driveshaft will be longer do the short tail.

The 9" tail THM-350 and standard 700R4 are for all practicable purpose are identical in length. You'll have to move cross member and drill new mounting holes in frame rails.

[Stoke]

The 350/700r4 are a mated pair. I am dropping in a tbi 350/700r4 from an 88 pickup into my 86 pickup. They are both longbeds so I am hoping the driveshaft will work like everyone has said. If the motor and tranny are a matched pair do I still need to worry about the TV cable and TC lockup issue that BABYJAY mentioned? Also since I am putting this TBI motor in my 86 is it easier to order an aftermarket wiring harness for the conversion or just use the wiring from the 88 truck?

[babyJay]

Since they are already a mated pair, the adjustable throttle-valve cable should already be there, where you normally have a standard kick-down cable. The lockup can be done with a toggle switch. Search the boards, I know I've read about it on the site.

[Stoke]

Since I will be installing the 350/700r4 together with the onboard computer that came out of the 88 truck will I still need a lockup switch? I thought that the computer controlled the lockup of the transmission?

[rfmaster]

Stoke

ECM (1227747) controls 700R4 lock-up via three wire interface (white four pin connector), but you'll also need to wire brake pedal switch for TCC release function. It is a very simple two wire circuit.
After market harnesses are nice - wires are higher quality and connectors are not broken. Otherwise you'll need to exercise care when removing harness from '88 donor. BTW if IIRC I used 2-1/8 52mm GreenLee tools hole punch for the firewall though hole (just because I had it around). Worked like a charm.

//RF

[shrtbx]

another thing people overlook ( I myself have done the same), is the rear end gearing. 3:08s and 2:73s are no good for the 700 r4. The Overdrive gear wont function right. This is the second swap i have done as far as engines and im finding out you really have to pay attention to the small things. You might want to get a new gear selector too for the dash, not a big deal but they are different.

[Pyrotechnic]

As I guideline, I say that 28" and under tires are good with at least 3.42 or higher, and larger than 28" tires need 3.73's or 4.11 depending on how tall your tires are. Although thats just a guideline. The big thing is that you aren't lugging the motor in overdrive or having it downshift to 3rd often.

[gchemist]

All you need to know!!
http://transmissioncenter.net/700R4_Swap_Info.htm

Other good info:
http://www.geocities.com/crosley_az/700spd.htm
http://www.73-87.com/7387garage/drivetrain/tvcable.htm

A performance lockup torque converter can be used to remove the electrical lockup.

[MylilBowTie]

Originally Posted by shrtbx

You might want to get a new gear selector too for the dash, not a big deal but they are different.

The steering column for the three speed th350 is different than the one for a 700r4.

[CavScoutsC10]

that's what i have on my chassis now. a 350 lt1 with a 700r4. i didnt have to make any changes thus far. i do know that i have to use a different driveshaft.

[rfmaster]

Cav

Are you going to run mechanical spedo???? You may need to invest into external VSS and old style driven gear case from older 700R4.

How big are those fuel lines feeding your injection system (supply/return)???

//RF

[CavScoutsC10]

i haven't gotten to that part of the build yet. but, i've been looking at getting the manual speedometer so i can keep the original instrument panel and keep VSS.

the other option is to keep it electronic and get a digital panel. but that panel is about 550. that's a little steep.

the fuel lines are for a 94-97 camaro. so it should be enough.

[rfmaster]

Cav

If you are going to get external VSS to work with conventional mechanical speedometer make sure that you know how many pulses your ECM or PCM is expecting. Older C3 ECM which were used with TBI expect VSS to produce 2000 square pulses per mile. You probably know this but later PCM expects VSS signal input to function properly. According to JTR a 40 pulse per driveshaft revolution speed sensor was used on 1993 and newer trucks with automatic transmission, 1994 and newer rear drive cars with the automatic transmission. An external VSS is a lot cheaper than new digital dash.

That should be a sweet running truck when it is all done.

//RF