Steve Crowe
10-13-2007, 02:58 PM
Over the course of time I've repaired or rebuilt many of these but I've got something running around in my mind I can't quite sort out.
Yes, this is about the OLD TH350 3-speed automatics and it isn't in an LS-anything.
OK, here's some background. Earlier this year I ran into a deal I couldn't turn down. 1978 Chevy G30 1 ton 125" wheelbase van, 400 4 bbl small block, TH350 and the 14 bolt rear end. Only 63,000 original miles for $700 and running great.
There were of course a few fine tuning items to be done but rebuilding the old Quadrajet and replacing the power steering hoses was cheap, topping off the R-12 Freon charge was a bit expensive as it has a rear AC unit as well.
Anyhow I'm not really happy about the rear end gearing and/or the shift points of the transmission. According to the books it should have come with a TH400 but best I can tell the one that's in it is original.
Changed the fluid/filter even though it was still bright red. Changed the vacuum modulator as preventative and it's dialed in at 14 psi but here's the deal. The 1-2 shifts seem to occur just fine, but it tends to think a bit longer than I'd like to go from 2 - 3. I'm familiar with changing the modulator springs/weights to raise shift points but I'd like to lower the 2-3 and the only springs out there are for raising them.
I haven't measured it but the rear end is really low, typical for a big vehicle from the age when the speed limit was 55 so you can imagine I'd rather the engine wasn't revving so high to go 80 - which it will do in a heartbeat. :-)
If I change the rear end gearing it will obviously be going faster when the governor commands an upshift but I'm still afraid that 2-3 is going to still be late.
Vacuum level is fine, used the red stripe modulator as this thing has no EGR, absolutely -0- emissions stuff except for the PCV, big trucks were exempt at the time.
Anyone have experience at making lowering the 2-3 shift point without bothering the 1-2 much at all?
Yes, this is about the OLD TH350 3-speed automatics and it isn't in an LS-anything.
OK, here's some background. Earlier this year I ran into a deal I couldn't turn down. 1978 Chevy G30 1 ton 125" wheelbase van, 400 4 bbl small block, TH350 and the 14 bolt rear end. Only 63,000 original miles for $700 and running great.
There were of course a few fine tuning items to be done but rebuilding the old Quadrajet and replacing the power steering hoses was cheap, topping off the R-12 Freon charge was a bit expensive as it has a rear AC unit as well.
Anyhow I'm not really happy about the rear end gearing and/or the shift points of the transmission. According to the books it should have come with a TH400 but best I can tell the one that's in it is original.
Changed the fluid/filter even though it was still bright red. Changed the vacuum modulator as preventative and it's dialed in at 14 psi but here's the deal. The 1-2 shifts seem to occur just fine, but it tends to think a bit longer than I'd like to go from 2 - 3. I'm familiar with changing the modulator springs/weights to raise shift points but I'd like to lower the 2-3 and the only springs out there are for raising them.
I haven't measured it but the rear end is really low, typical for a big vehicle from the age when the speed limit was 55 so you can imagine I'd rather the engine wasn't revving so high to go 80 - which it will do in a heartbeat. :-)
If I change the rear end gearing it will obviously be going faster when the governor commands an upshift but I'm still afraid that 2-3 is going to still be late.
Vacuum level is fine, used the red stripe modulator as this thing has no EGR, absolutely -0- emissions stuff except for the PCV, big trucks were exempt at the time.
Anyone have experience at making lowering the 2-3 shift point without bothering the 1-2 much at all?