I need a little help from the switch pitch gurus. I have a few questions on setting up the pump in the switch pitch: 1. Did GM set the switch pitch pumps up to run at lower pressure for some reason? 2. Is the Sonnax 400-LB1 kit a good idea for a switch pitch behind a 400 rwhp big block? 3. Is it a good idea to restrict the converter feed down to 0.160’ in the switch pitch pump? If you want the long story…. I have had a switch pitch TH400 in my wagon for over a year now and I really like the way it works. When I first installed I tested the line pressure (drive, no vacuum to modulator, on brakes/0 mph, 1200 rpm; tested at the plug of the side of the case) and it was only 125 psi. I had looked at the pump before I put it in and the spring wasn’t one of the blue/orange/yellow shift kit springs and it still had the horseshoe shim so I am thinking it should be the original GM calibration. At 125 psi this is lower than my old TH400 (140 psi on a 73-BC trans, stock pump) and below the GM spec recommended range of 135 to 165 psi for the TH400. The transmission works perfectly so I didn't have a reason to pull it out even with the pressure on the edge like that. I am setting up another switch pitch TH400 now and I took a look at the donor pump. The stock spring (with one horseshoe shim again) in this one is easy to compress with my finger and looks to be really weak too. I have some TransGo shift kit springs around here and they are way stiffer than this original pump spring. Did GM set the switch pitch pumps up to run at lower pressure for some reason? Does it have anything to do with the thinner pump gears used in the 1966 and earlier TH400s? (.726” ’67 and up vs .625 ’66 and earlier). I was thinking of replacing the weak spring on this one with something a little stiffer, but I don’t need one of those stiff TransGo “race only” shift kit springs. The spring in the Sonnax 400-LB1 Line Pressure Boost Kit is supposed to be only 10% stiffer than the stock spring. I have the kit and the squeeze test confirms this Sonnax spring is somewhere between the squishy stock spring and the TransGo race springs. This kit also has a new boost valve in it that sounds interesting too. Is the 400-LB1 kit a good idea for a switch pitch behind a 400 rwhp big block? Is the replacement boost valve a good idea too or should I just use the spring? Finally, I was looking at doing the converter feed restriction in the pump and I noticed the switch pitch pump casting already has a much smaller port (about 0.199”) than the later fixed pitch casting (about 0.259”). Rebuilders recommend restricting this down to 0.160 in fixed pitch converters, especially with higher pressure regulator springs. Is it a good idea to restrict this feed down to 0.160 in the switch pitch pump as well?
I know this is an old thread but suspect the reason why the pressure is low is due to the pump using the earlier aluminium smaller diameter boost valve
