I sent this out once, but it never made it to the FOT list for some
reason. I've got a JK Jackson OD gearbox. As near as I can tell, it's got
a late TR-6 gearset (steel bushing in 3rd gear), but the early A-type 22%
overdrive. The effect is a 7 speed gearbox. Shifts from 2 to 2OD, 3, 3OD,
4, 4OD at 6000 rpm result in 700 to 1000 rpm drop. I would think that with
a close ratio gearset that the 2OD and 3OD would be close enough to 3rd and
4th to not make much difference.
I love the added drama that 2 methods of shifting give me, but when the OD
conks out due to some sort of electrical glitch I find my lap times don't
really change.
- Tony Drews
At 09:53 AM 1/12/2006, Henry Frye wrote:
>I do agree with all Bill Babcock and Glen Effinger have already said.
>PROPERLY USED, hanging an OD on the back of a close ratio gearbox can be a
>good thing. IMPROPERLY used, or learning to rely on it and then it breaks at
>a bad time, can be bad and lead to very expensive consequences.
>
>When I started racing, I ran a race prepared OD with a stock TR4 gearset. As
>I was using all my mental capacity to just keep the shiny side up, there was
>no way I used the OD to it's fullest potential. What I found is I was unable
>to mentally process what to do with that switch, plus the clutch and
>shifter. Ken Gillanders suggested ditching the OD box and building a close
>ratio box without an OD. Lap times decreased instantly.
>
>Now that I have been doing this for a while, I find I encounter places where
>I run out of revs but think I will lose time if I upshift just to
>immediately downshift again. These are the times I really want my OD back.
>But I keep asking myself if this will really decrease lap times? Not sure,
>but it would save some revs on my motor. On race day I find I never grab the
>next gear, I hold the loud pedal to the floor and have been rewarded with
>some outstanding numbers on my telltale!
>
>Has someone looked at the math using the commonly available close ratio
>gearset with an OD? I know 3-OD is taller than 4th, so you would never use
>that accelerating up a straight. Where does 2-OD fall in relation to 3rd?
>Probably pretty darn close, again making it not something you would use for
>faster straight line acceleration.
>
>JK Jackson has figured out the ultimate gearset to use with an OD. Gives 7
>well spaced gears. THAT makes sense, but makes for a pretty busy cockpit.
>
>Tony Drews has a bunch of seat time in both the JK box and a straight close
>ratio box. Care to comment?
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