68 390 auto to an overdrive conversion

[James H.]

Good Day
I am retiring my '67 high milage coupe and bought a '68 390 auto for my sportsman road car.
Can not find a 2:73 rear gear, so would like to learn more about converting the 400 auto to an overdrive auto.
Please respond with some suggestions.
Thank you, Chris Henry (61744),

[Mark E.]

Re: 68 390 auto to an overdrive conversion

James,

If you just want to drive it with little fuss and cost, retain your TH400 and change rear end gearing. You didn't say what you have now, but my '70 454 turns a comfortable 2,500 RPM at 65 MPH with 3.36 gearing. 3.08 and 2.73 were also available in the day, and although you mention otherwise, I would be surprised if not readily available today.

Adding overdrive or swapping transmissions is not only expensive and possibly detrimental to the car's value, but also an exercise in engineering a myriad of details- speedo cable, column lock cable, throttle position sensor, controller box, wiring harness, TCS connection, shifter position in the console, shift indicator, adapting the torque converter lock, cooling lines, driveshaft length, dipstick location, exhaust clearance, ground clearance...

There are probably kits that address some of these, but the list and price tag gets longer as the project proceeds. And you can expect considerable downtime while all this is worked through.

[Duke W.]

Re: 68 390 auto to an overdrive conversion

Revs @ 60 = tire revs/mile (axle ratio), so for your '70 with OE revs/mile tires that would be 775 x 3.36 = 2604.

Unless one does a lot of interstate driving at 70-80 MPH I don't see the point of spending perhaps thousands to swap in a four speed OD automatic in place of of a TH400 with up to 2:1 torque multiplication, and a starting gear greater than 10:1 on a big block is ridiculous unless you need to haul a 10,000 pound boat trailer out of the water at a boat launching ramp.

... just my opinion.

Duke

[James H.]

Re: 68 390 auto to an overdrive conversion

Mark and Duke
Thank you for your input.
Yes I do put 10,000 or so miles on the old corvette a year.. My 300 HP '67 has a powerglide with a 2:73 gear and does very good.
To restate, the '68 is a lot more conformable to drive.

SO Can anyone help me find a 2:73 to go in the '68 ?? I look on ebay every week but have had "0" luck.
Thank you James (61744)

[Leif A.]

Re: 68 390 auto to an overdrive conversion

Mark and Duke
Thank you for your input.
Yes I do put 10,000 or so miles on the old corvette a year.. My 300 HP '67 has a powerglide with a 2:73 gear and does very good.
To restate, the '68 is a lot more conformable to drive.

SO Can anyone help me find a 2:73 to go in the '68 ?? I look on ebay every week but have had "0" luck.
Thank you James (61744)

James,
Have you checked with Gary Ramadie??

[Duke W.]

Re: 68 390 auto to an overdrive conversion

A 2.73 axle was an optional "economy" ratio for hydraulic lifter big blocks with TH 400 in '68 and ALL big blocks with TH 400 in '69 including L-71 and L-88, but doesn't appear in the powertrain charts after '69.

The NCRS Corvette Spec. Guide doesn't breakout axle ratio counts, but I imagine that there were damned few ordered during those two years, so finding one in serviceable condition or even a rebuildable core with a good ring and pinion is going to be tough.

I don't know if a 2.73 gear is available in the aftermarket, but many aftermarket gear sets are said to be noisy.

So your bet bet is what Leif said to check with all the known axle rebuilders and add Corvette Salvage yards.

Duke

[James H.]

Re: 68 390 auto to an overdrive conversion

Leif
I do not know Gary Ramadie. Can you help with a contact?
I have called ZIP Bair's Gevo Gear Tracy's
Can you think of any more contacts, to help.
I could change out the '67 to the '68, but the '67 is the one I drive to every National.
A nice man at Gevo looked up his suppliers and said they do not offer any 2 series Eaton gears.
With help I will keep looking.

Thank you
James

[Terry D.]

Re: 68 390 auto to an overdrive conversion

What gear do you have now?

[James H.]

Re: 68 390 auto to an overdrive conversion

210,000 mile 308 very noisy. The rebuilt 390 seems not to like 2300 or so rpms. I have always likes to keep highway rpms around 2000

[Leif A.]

Re: 68 390 auto to an overdrive conversion

Leif
I do not know Gary Ramadie. Can you help with a contact?
I have called ZIP Bair's Gevo Gear Tracy's
Can you think of any more contacts, to help.
I could change out the '67 to the '68, but the '67 is the one I drive to every National.
A nice man at Gevo looked up his suppliers and said they do not offer any 2 series Eaton gears.
With help I will keep looking.

Thank you
James

James,

Gary Ramadei of Hartford CT....gary.ramadei@radiall.com
203-776-2813

[Mark E.]

Re: 68 390 auto to an overdrive conversion

210,000 mile 308 very noisy. The rebuilt 390 seems not to like 2300 or so rpms. I have always likes to keep highway rpms around 2000

3.08 is very highway friendly. Could the real issue be the noisy gears or that the engine "seems to not like [a certain speed]"?

The tune of an engine affects how it behaves at a certain RPM- specifically ignition timing. Playing with initial, centrifugal and vacuum advance will make a significant difference in engine behavior.

Or maybe what you're sensing is exhaust noise. Does the exhaust resonate (drone) at cruising RPM? Changing this involves modifying the exhaust (or more likely, reverting to a stock exhaust).

And of course fixing the noisy rear-end will make a big difference in comfort.

I'd address these before assuming you need overdrive. Prior to the '80s (and overdrive) virtually all cars, even luxury sedans, turned well over 2,000 RPM at highway speeds while delivering a quiet comfortable ride.

[James H.]

Re: 68 390 auto to an overdrive conversion

Thank you too all--
I really appreciate all the input and want to reach out, again, to anyone that knows where I can get the 2:73 Eaton gear for my "68.
Zip's Automatic Conversion looks exciting, but maybe too complicated for the end game.
James (61744)

[Leif A.]

Re: 68 390 auto to an overdrive conversion

Thank you too all--
I really appreciate all the input and want to reach out, again, to anyone that knows where I can get the 2:73 Eaton gear for my "68.
Zip's Automatic Conversion looks exciting, but maybe too complicated for the end game.
James (61744)

Did you read post #11? Did you call Gary??

[James H.]

Re: 68 390 auto to an overdrive conversion

Leif
Yes called Gary, left message but no call back yet.

Somewhere out there is a 2:73. With help maybe one will turn up. Like I have said, I got one in the "67 with powerglide and enjoy keeping up with traffic at 2200 RPM. Thank you again for your help.
James