Overdrive options


Active Member
Sep 26, 2017
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New York
what overdrive options do we have for our beloved cbodies?
My 67 wagon has a healthy canned .030 440 in her now along with a .390 gears out back that I absolutely love. It’s a blast to drive. But I’m gonna need to do the tranny soon. And figured now may be a good time to look into options. Looking for direction here. Not at all concerned with originality at all
You can do a A518 with bel housing adaptor and add the OD switch to the dash.
Or add A Gear Vendors overdrive unit to your trans.
Both will require some trans crossmember modifications
The early 518 is just an over drive trans..
The later 518 is over drive and lock-up...
Where I do like the 518 and have a couple in use the
Gear Vendor would be a much better choice for your application...
For any route, you'll want to have a LU converter. Running in OD with a higher-stall converter (I assume you have some stall) can cause excessive trans temp if you're running below the converter's 'rated rpm'.

A518 can go 2 ways - BH adapter Cbarge mentioned, or an adapter plate that fits between the SB trans and BB engine bolt patterns. It's 1/2" thick IIRC, so requires a starter adapter also and anything else required due to 1/2" difference in location.

If you choose a 518, the early ones are non-LU as Xenon said, then LU + OD, then they became electronically governed (avoid that style).

If you go with a GearVendors, your transmission gets significantly longer, so driveline angles may get tricky, you might need to drop the tail of the trans to help that. Or, putting a truck short-tailshaft setup in to a rebuilt 727 could help.

Quicktime makes a bunch of transmission adapters, including ones for SB or BB Chrysler, that will fit a GM 4L60 or 4L80. The 4L60 is a smaller trans with a deeper 1st gear (like a smallblock A999) and the 4L80 is much bigger, stouter, and with ratios similar to a 727. Aftermarket support for these transmissions is **HUGE**, and either one can be built to handle significantly more than stock. There are $750 electronic controllers to make them function, or you could probably install a manual VB in either one with some electrical doo-dads to handle the lockup converter and OD (like you'd need to do to the A518). There are non-electronic versions of both transmissions, but I don't know as much about them. Later versions of the 4L60 have a removable bellhousing also, which I admire. I have often thought about putting a Quicktime + 4L60 in one of my lower-power cars to get the OD and deeper gears.

You could convert to a manual, and then use an 80s truck 4-sp OD - but its 3.09 1st gear, and the ratio drop to 2nd, might not be so good for a healthy-cammed engine, though.

If you go manual, Passon's has an A833 OD with better gear ration spacing, or they were supposed to be coming out with an all-new 5-speed that would fit in the same as an original 4-speed. That was a few years ago, and last I read they had delays. Don't know if they finished it, you'd need to research. (it's probably $4000, too!)

If you want to get really weird, read about Ford's Borg-Warner 3pd-OD they used in some 60s cars and til around 73 in trucks. Supposedly pretty stout, it allows some gear-splitting like the Gear Vendors, and also does free-wheeling when you let off the gas in OD (I think), which also saves fuel. I've read about this trans a bit recently and it seems really cool.

  • For many automatic options you'll need to consider:
  • floorpan mods (most will need it, but the 4L65 might not),
  • crossmember mods,
  • a new-length driveshaft,
  • consideration to running your speedometer,
  • consideration for connecting shifter linkage,
  • and how OD and LU need controlled.
When you add up all the cost of adding an OD: camming down a little and 3.23 gears might be a more economically-attractive option?
You're really only trying to save your engine - you'll never save enough in fuel to justify the trans swap.
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you'll never save enough in fuel to justify the trans swap.

This is abbsooolutly true......
The OD will save on the motor some and make for a quieter drive..
But as far as re-cooping your investment,,,, ain't gonna happen, Bucky.... :D:D

But then,, on these old cars do we ever re-coop what we put into them.....
The GM 4L60 (aka THM700) has a 3.07 (approx.) low gear and a 1.74 2nd gear. Quite a BIG ratio gap, but the trans covers it quite nicely and smoothly (which means "slip" somewhere, either in the converter, gear set apply schedule, or both). All are lockup and need mechanisms to make that happen, electric and vacuum.

The GM 4L80 is the OD version of the prior THM400. It takes more power to run than the other transmissions. Equals "stout" but less power to the wheels.

IF you've really got enough cam to need a 3.90 gear, it'll probably be too much to be able to really use the OD at lower cruising speeds. Effectively a 2.73 axle ratio in OD. Which means a 2000rpm cruise at about 57mph. So you'll see how that plays into the torque range of the engine and cruise rpm level, even with a 440. No need having it kick in and out of OD on the highway, when a hill is encountered.

I concur that less cam and a 3.23 gear might be a better choice. Then meter the carb for fuel economy on the road rather than big dyno power numbers. Think "more torque" in the cam selection rather than big hp numbers. "Torque" happens in the mid-range rpm, which is where the car will normally drive. "BIG HP" only happens at 5000rpm+, which is only a very short time and under WOT conditions. Torque spins the tires. Horserpower is mostly bragged about.

In general, what are the specs (cam, heads, intake, carb) now on the engine? Just curious why it might need a 3.90 gear?

As for a torque converter, aim for a 6-cyl converter as it will yield a higher stall speed than the normal converter. BUT Chrysler had "low stall" and "high stall" 10.75" torque converters, too. By about 2000rpm cruise, they are mostly "locked-up" anyway, unless you stand on it, by observation. Maybe 2500rpm? Most heat is built when the converter is trying to slow down the vehicle, rather than accelerate it, by observation.

Just some thoughts,
All good points. Another point to consider is OD ratio.

4L80 is a 25% OD
Gear Vendors is a 29% OD
518 is a 31% OD

Depending on the speeds you drive, you might find the “hunting” in and out of OD mentioned above.

Gear Vendors is upwards of $2500 plus any other mods and the price of a transmission rebuild. Can you make the PATC 518 conversion work for less? A 4L80E with a stand alone controller?

I think (Think! ) someone makes a fuel injection system that includes a 4L80E controller. That might be about the same price as the Gear Vendors and you would have electronically controlled FI, timing and trans.
The Flyin' Whale has a Gear Vendors Overdrive Unit and I think it is worth every penny that the builder of the car spent for it. I know that you are building a "hot rod" and unless you want to go with a manual transmission I think that the Gear Vendors Unit is your best bet as it will handle high HP engines with no reliability issues.
My buddy Polaraco did a 5.9L and 518 swap in a 72 Polara.
Had had to swap out the 3.23 for 3.55 so the engine and trans would work better. The highway gears were too much.
Ahhh, Polaraco! :thumbsup::thumbsup::thumbsup:

While I absolutely abhor many of 'virtues' (a very loosely-applied word) of social media, webforums like these have allow Average Joes to make a mark and have a mini-legacy with their accomplishments.

Mag162 -
What's your budget, what's your skill level, will you do the work yourself, how long do you want the car to be down for mods?
In MANY cases, seeking to keep in the "OEM Application" orientation with ODAutomatic transmission swaps can be the least expensive, all things considered. Still might be some adaptations of equipment, NOT including the driveshaft shortening issues or rear axle change costs.

There are some threads in here about using the later TF from a Dodge diesel pickup. But that needs and expensive bell housing adapter, then various pressure switches to make things work the related internal electronics. Pretty easy, once you get it all understood, but still expensive, which can make starting with a LA engine better than a B/RB, it seems.

AND, there might be some floorpan clearance issues with the 518s?

Back in the later '70s, when fuel economy ratings were important, Chrysler published a pamphlet on fuel economy and how various vehicle specs affected it. Seems like their research indicated that it took a 3% rear axle ratio change to make a 1% difference in highway cruise economy? A VERY interesting little publication which our local dealer got for several consecutive model years.

The generally lower cruise rpm of an OD situation becomes VERY dependent upon cam selection and when the cam give the engine its best "cylinder pressure" (rpm band). Kind of like they used to use a rule of thumb on axle ratios for trailering . . . gear the car for max torque rpm at the speed limit in the state you'll be driving in. OTR trucks can be geared similarly, which is why many fleets had to "re-gear" for the lowered 55mph national speed limit in the 1970s.

As most OD ratios are in the .70 range, 30% drop in gear = 10% better fuel economy. Starting with 20mpg, then another 10% starts to become significant. Less so with lower mpg levels.

Get the engine tuned so it's operating best in the 3000rpm range, for what you've now got. Make it "sound happy", which means it usually is. Worried about engine wear? Get some quality synthetic motor oil and change it when the TBN gets toward 2.0, with a quality oil filter. Tune the carb for 14.2 air/fuel ratio at cruise, but also make sure the power mixture is where it needs to be for good power (12.7, as I recall?). Keep engine temps at cruise between 180 and 210 degrees F. That's WITH what you have now. THEN, learn to drive at more steady speeds, especially if fuel injection is involved, but also realize that getting to "cruising speed" is best done briskly rather than slowly (getting to the most efficient speeds sooner rather than later). Tire pressures and front end alignment are important, too.

I know that having a relaxed cruising rpm is good. Which is why I like 2.76 axle ratios, for road use, but with a little less power at lower speeds. Always did like 2-3 WOT upshifts at the 90mph range.

In the '90s, some dealers ordered their OD Automatic pickups with "highway" gears for allegedly better fuel economy. I got calls about their trans "always shifting" on the highway. After getting their explanation, I told them everything was normal for what they were experiencing. That was then verified when I tracked their rear axle ratio! At that time, a used rear axle assy was over $1000.00, plus labor and swapping out/rebuilding the brakes as needed. A new gearset was about $500.00+ (at that time), plus labor. End result . . . not cost effective to do for the benefits. My suggestion was, knowing that the trans was doing what it was supposed to be doing, be sure they got a pickup, next time, with something like a 3.42 rear axle ratio than a 2.73.

Certainly, there were some aftermarket means of decreasing the "hunting for gears", with some sensor swaps, but at the dealership level, I couldn't advocate that. Some aftermarket shift kits claimed to do that, too.

When we first got involved with the aftermarket OD situation, it was for HD2500 6.2L diesel pickups. With the diesel's lower governed rpm, if you geared them to pull, little "road speed". Of you geared the for the road, decreased pulling power. So an external OD unit was the answer to better use available rpm. That was with manual transmissions, rather than automatics.

GearVendors was one of the earlier add-on OD units, so since back then, they could be one of the better units on the market, I suspect.

Personally, I think it'd be nice to upgrade to a Chrysler 8-speed, a good EFI, and such. But no electronics to run it just yet. In the OEM use, they do work SLICK! Much better and smoother than similar GM uses, by observation! Perhaps an aftermarket software upgrade might make the GM 8-speeds work better. GM had stand-alone controllers for their GMPerformance Parts OD automatics. NOT cheap, by any means, BUT OEM-level items.

Y'all enjoy!
There is a controller for the 8spd now through sound German.
The trans is huge though, 9" tall at the rear mount, major tunnel reconstruction to make it fit.
I have a modified a500 behind my turbo efi 383.
People are right, you need plenty of gear if you have any cam. I have a .484 in the 440 in my charger and the tko 5spd with a .69 5th and the car wants to be going 70+ before 5th really works.
Ahhh, Polaraco! :thumbsup::thumbsup::thumbsup:

While I absolutely abhor many of 'virtues' (a very loosely-applied word) of social media, webforums like these have allow Average Joes to make a mark and have a mini-legacy with their accomplishments.

Mag162 -
What's your budget, what's your skill level, will you do the work yourself, how long do you want the car to be down for mods?
I don't really have a budget but I don't wanna drop 5k on this idea either. I am more of a straight swap kinda guy. I don't think I can do this swap without help. Im not set up at home to do this kinda of work.
I was wondering if there was something that was a straight forward swap. I didn't realize how involved this would be.
I read an article the other day stating that in 77, Chrysler transmissions recieved a lock up torque converter. Correct me if I'm wrong, but that should drop your rpms by about 200 at highway speeds.
You can do a A518 with bel housing adaptor and add the OD switch to the dash.
Or add A Gear Vendors overdrive unit to your trans.
Both will require some trans crossmember modifications
Gear Vendors doesn't require any crossmember modifications. There is one spot where the GV is close to a support in the transmission tunnel. That just needs a little massaging with a BFH.

Probably not a problem for the OP, but the A518 is not compatible with a console shifter. The tailstock is so much larger that it would interfere with the shift linkage.
A500 was too big around as well, I have console shifter with oem pieces top and bottom and custom built a cable shift lever on the oem shifter