I have found just the opposite than what is described below. Until the Walter
Mitty Challenge this past April , I had always raced with an 'A' Type
overdrive. When the tranny that was supposed to be installed did not get
delivered , Barry Rosenberg and I had to come up with something quick. I had
already built a 74> TR6 O/D 'J' Type gearbox to put on ebay. It was installed
in the TR4A at the last moment and once on the track I found it to be very
smooth and quick in and out. The overdrive unit came from a Volvo 245 and was
not modified internally. Time will tell if it will hold up to racing
Just my experience...
From: Randall Young <email@example.com>
Sent: Sep 21, 2005 2:43 PM
Subject: RE: overdrive question
> I've never gotten into the innards of the various types of
> overdrives. Most people I know who race their TR's with an overdrive
> use the "A" type.
> Is the "A" superior to the "J" type for racing?
I've never raced with either, but I've driven both on the street. I would
guess the main advantage is the faster shifting of the A-type, due to the
presence of the accumulator. The A-type in my TR3A shifted much faster than
I could with a clutch, while the J-type in my Stag takes substantially
longer. Seems like the A almost reads my mind, just a touch of the switch
and the nose of the car is already jumping up. By contrast the J takes at
least 3 beats to engage, sometimes longer. The A is also more consistent
shifting, seems like the J varies depending on temperature, phase of the
moon, etc. And the J lacks the "kick in the pants" of the A ... the TR3A
literally jumps visibly forward (during impromptu drag races) while the
Both ODs were rebuilt by Herman, so presumably wear is not an issue. No
doubt the characteristics of the J-type could be improved with suitable
modifications, but I think the only way to get to the almost instantaneous
engagement of the A-type is going to be to add an accumulator. Otherwise
the pump has to displace enough oil to move the main valve and the pistons,
even after the solenoid (which is a slower-moving balanced pressure design)
closes off the pressure relief.