Join Date: Apr 2011
Location: Edmonton, Alberta Canada
Re: 67 C4 slams into Reverse from P or N
Ok, not sure if this will help... but here goes...
What happens when you select reverse in a C4?
2 things have to happen, the high/reverse clutch has to apply, and the low/reverse band must apply.
Here's the deal...
Those 2 things happen "almost" at the exact same time.
BUT... If the high/reverse clutch applies first, that gets the mass of the gear train spinning.
A fraction of a second later... the low/reverse band grabs and stops the spinning gear train. WHAM!
The trick is to get the L/R band to apply FIRST. this holds the gear train.
Then bring on the high/reverse clutch, hopefully in a gentle manner.
If clearance is tight in the high/reverse clutch, and the low/reverse band is adjusted on the loose side,
you will get a harsh reverse, even on a stock trans.
When I build a C4, or any trans with a L/R band, I adjust the band fairly snug.
Sometimes as tight as 1/2 turn.
I believe the factory adjustment is 4 turns. Waaaaay too much.
I also fuss with the high/reverse clutch clearance.
Generally going for as loose as I feel comfortable with.
Usually around 0.070" to 0.080" clearance.
"Normal" rule of thumb on clutch pack clearance is 0.010" per clutch.
Soooo, with a loose Hi/Rev clutch clearance, and tight L/R band clearance,
What happens is the band applies first (very important) and then the clutch comes on, giving a nice feel into reverse.
With a modified trans (shift kit) all bets are off.
It depends on the guy that installed the kit,
and whether or not he did any "tweaks" or "tricks"... or even if he installed it correctly.
Plus the afore mentioned clutch and band clearances. How did he set it up?
A good test to try...
You can try tightening the L/R band adjustment really snug so it locks the band "on" at all times and see how reverse engages.
DO NOT DRIVE THE CAR!!!! ONLY DO THIS AS A TEST!!!!
Basically, you are locking the band "ON" and then the only thing applying in reverse, is the Hi/Rev clutch.
If it is still harsh, you need to get out a pressure gauge,
and see what mainline pressure is in reverse, BEFORE going into the valve body.
Knowing the pressure ahead of time lets you make an informed decision about what to modify in the valve body,
instead of just guessing about what to do.