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A customer just bought last night an `84 corvette with the crossfire (puke) fuel injection and 4 speed. On the console it reads 4+3 overdrive. Before he bought it he was telling me it was a 7 speed, so what gives? How do you shift into the 3 overdrive gears? Just appears to be a 4 speed shifter and gear knob just shows the 4 gears and reverse. I see a rocker switch labled overdrive on the console but it doesn't appear to do anything after 4th. Is there anything we should know on how to get those 3 overdrive gears?? Do you have to be going so fast before it works?? Or flip the switch and change gears with the shifter?? We just don't know. :willy:
 

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Before he bought it he was telling me it was a 7 speed, so what gives? How do you shift into the 3 overdrive gears? Just appears to be a 4 speed shifter and gear knob just shows the 4 gears and reverse. I see a rocker switch labled overdrive on the console but it doesn't appear to do anything after 4th. Is there anything we should know on how to get those 3 overdrive gears?? Do you have to be going so fast before it works?? Or flip the switch and change gears with the shifter?? We just don't know. :willy:
This is a rather old fashioned way of getting longer legs on a car. Overdirves were once much favoured by British car makers to rpovide a more relaxed cruise, though they have been largely replaced by five or six speed gearboxes

Basically what you have is a small epicyclic gear box added to the rear of the stardard 4 speed box which gives you about a 2/3 reduction in revs in 2,3and 4 for any given road speed.

On an '84 Corvette this overdrive unit is tied into the engine management systems if memory serves, though the switch you describe may be intended to bypass the management system.

Check the operation by accelerating to a steady cruise in top and then flick the switch to engage the overdrive - you should see an an immediate reduction in revs at the same speed if everything is working. There shoudl also be a lamp that comes on when the OD is engaged

If the OD is not working there are several posibilities - the switch could be bad and I think there is a relay in the system that can go kaput and prevent the overdrive from engaging as well as a whole host of possible mechanical reasons it won't come on.
 

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Discussion Starter #5
Awe, figured it out. You need to use the rocker switch to enable OD operation, then starting in second, you can push in the center on top of the shifter (the part that shows were the 4 gear's pattern are and reverse) and it will get the OD. Then push it again and shift to third, then push the shifter button and OD in 3rd, etc. Kinda cool, almost like a split rear end, but not meant for racing that's for sure!!
Thanks everyone for the responses!!
 

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Gear Vendors over/under drive!

Made by Gear Vendors they are guranteed to 1,000hp & on an auto car they can knock considerable time off your 1/4 mile times, if you know how to use it.
I've got one behind a C-4 & on a 2.5 mile raceway, we knocked almost 6 seconds off our track time, instead of that big RPM drop between gears, it allows you to keep the motor in its sweet spot all the time.
cost - 2500 & up
 

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i stand corrected

Nash's unit, eh, almost exactly the same, wouldn;t be surprised if they;re made by GV, but, your right, Sold & serviced by Doug.
GV guarantees their units to 1200hp & on the dyno shows just how efficent the unit is, driveline loss - 4 hp per 100hp!
 
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