Overdrive Transmission Options

67 440 vert

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Has anyone changed out the 904/727 setup for a later model overdrive. There are many options out there and I want the easiest route for a 318 LA engine
 

Greg Ducato

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ANY automatic overdrive conversion will require some floor or tunnel modification, crossmember fabrication, driveshaft modification or replacement, linkage mods, cooler line mods or replacement, some form of electronics control, tv cable rod or linkage and modifications etc etc. If your car has a reasonably stock 318 and rear gearing in the 2.7 to 2.9 range then you are better off keeping your 3 speed automatic and saving a lot of money and hassle.
 

moonman29

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I agree with Greg. Leave it. From a financial point of view, how long will it take to payback what you spent by only getting 2 or 3 mile per gallon more than what you currently get? Is this your daily driver? Do you put 15,000 to 20,000 miles a year on it? If you were to drive 15,000 miles per year @15mpg and @$2.50/gal, it will cost you $2500 in gas. Same conditions @18mpg, it will cost you $2082.50, saving only $417.50 per year, not worth it in my opinion. My dad wanted to do the same conversion to his Dart and he only drives it 3,000 to 4,000 per year and once I showed him this comparison, he said never, he'd rather do something else to the car. Just my opinion.
 

Dennis H

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Looking for cruise rpm drop or mileage? Gearvendors?
 

SDCoronet

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My Coronet with stock drive and gearing (318, 727), makes driving my car at freeways speeds a miserable experience. At 65mph, the engine is screaming along at RPMs that scare me – It isn’t fun at all.

Within a year, I hope to be able to swap the 727 with an A500. In my opinion, the hassle and cost of the swap will be more than worth it to make my car an enjoyable cruiser.

Here is thread that convinced me that it’s the right move:

https://www.forabodiesonly.com/mopar/threads/42rh-a-500-in-a-duster-with-pictures.178459/
 

Greg Ducato

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It would depend on your final drive ratio but for example a car with a 2.94 rear end ratio and 26.5 inch tall tire will be at 2600 rpm at 70mph plus a small amount of converter slip. If you have a mechanical fan with no clutch it can really sound like the engine is screaming even at that rpm but it isn't. In fact, using an overdrive with a .70 4th gear ratio will drop rpm at 70mph with this same combination to 1800 rpm. While that looks good on paper, consider the aerodynamics of our old cars and the fact that a small engine does not produce much torque or horsepower at such a low rpm. Your engine will be struggling at that rpm to move the car through the wind at that speed, it may end up using more fuel at this rate of operation than it would have stock.
 

RamRunner

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I swapped in an A518 with a standalone controller. I have 3.73 gears, and cruise 70 mph @ 2250 rpm. If your looking for the easier route, a gearvendors will be the way to go. That means almost no floor surgery, less wiring, and you keep your trans installed. However the 518 has a greater overdrive at 31%, may be cheaper, and the standalone controller is $350.00.
 

Greg Ducato

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That's a good choice of product, how did you accomplish the TPS and TV cable on your carb/throttle body?
 

RamRunner

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That's a good choice of product, how did you accomplish the TPS and TV cable on your carb/throttle body?
Ram runner has a 5.9 liter magnum swap, with EFI. The engine and trans came from the same totaled Ram truck, so I spliced into the TPS wiring. The EFI has since been upgraded to FAST EZ EFI, and I again spliced into the TPS wiring, and bought the FAST throttle return/cable bracket along with the 700r4 TV cable bracket. Works great.
 

Kern Dog

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I agree with Greg. Leave it. From a financial point of view, how long will it take to payback what you spent by only getting 2 or 3 mile per gallon more than what you currently get? Is this your daily driver? Do you put 15,000 to 20,000 miles a year on it? If you were to drive 15,000 miles per year @15mpg and @$2.50/gal, it will cost you $2500 in gas. Same conditions @18mpg, it will cost you $2082.50, saving only $417.50 per year, not worth it in my opinion. My dad wanted to do the same conversion to his Dart and he only drives it 3,000 to 4,000 per year and once I showed him this comparison, he said never, he'd rather do something else to the car. Just my opinion.

If the goal is only fuel savings, I agree completely.
If the goalS, plural, include extended driving range between fill-ups, a quieter ride, less vibration at speed, longer engine life and overall increase in driving pleasure, I say the switch is probably worth it to many people.
I am getting close to buying a 5 speed manual kit for my Charger. I have been stuck on an automatic OD option until recently.
 

mopar 3 B

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It would depend on your final drive ratio but for example a car with a 2.94 rear end ratio and 26.5 inch tall tire will be at 2600 rpm at 70mph plus a small amount of converter slip. If you have a mechanical fan with no clutch it can really sound like the engine is screaming even at that rpm but it isn't. In fact, using an overdrive with a .70 4th gear ratio will drop rpm at 70mph with this same combination to 1800 rpm. While that looks good on paper, consider the aerodynamics of our old cars and the fact that a small engine does not produce much torque or horsepower at such a low rpm. Your engine will be struggling at that rpm to move the car through the wind at that speed, it may end up using more fuel at this rate of operation than it would have stock.
Your calculator is way off. You better review some math.
 

Greg Ducato

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2.94ratio x 70mph x 336=69,148.8 divided by 26.5 tire diameter = 2609rpm multiplied by .70 overdrive ratio = 1826rpm

I'm no Einstein so please help me to see where my calculations are off
 
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