Hey guys. I have a '71 Dart. Currently it has a 904 in it which was rebuilt about ten years ago and is beginning to show signs of age. I just threw a rebuilt, very peppy, early (small register crankshaft) slant in it that I am ultimately intending to turbocharge. This car is my daily driver and it is already reasonably quick and fun as hell to drive, but I am putting about 80 miles a day on it for my commute and maintaining ~85 mph on the interstate has it wound up pretty high all the time. I am not exactly sure what my rear gear ratio is. I feel like overdrive would really save some wear and fuel. I have been eyeballing a 4-speed swap, namely an A833OD, and currently have the opportunity to snag one for $250 locally. I am aware that installing it in the car will require some intensive fab work.

My main concern is that I've read different opinions on the result. Is this worth doing? I don't fully understand some of what I've read. I have done a couple of junkyard T5 swaps in older cars so I can handle the pedals and tunnel mods and all that, but the gear ratio math makes my brain melt. All I care about is- will I lose or hamper the ability to turn rubber into smoke? Will it be tangibly any better or worse on the highway? It seems self evident that an OD gear would cruise better on the freeway but some people are saying otherwise. Would appreciate any insight, preferably translated to dummy...

If you can get a complete 833OD for reasonable money then I would say go for it. Are the gear ratios optimal? No, but that .73 od does allow for some gearing in the rear axle. I wouldn't call it "extensive" modifications. Basically a pedal assembly, some linkage, and a driveshaft in addition to the obvious bellhousing/trans/clutch. Of course it get a hold cut for a shifter, but it is easier to cut a hole than to fill a hole.

For years I debated what to do with my 65 valiant. It is a 904 and the complete lack of feasible trans swaps led me into the same answer. I drive a manual trans as my daily driver so I will be T5 swapping it in the future.

If money isn't a factor going straight to a T5 swap is likely a better option to the old 833OD units, but the factory OD is definately more plug and play since it is a factory designed and installed combination.


The question on the gearing can't be answered without knowing your current rear gear. Also the horsepower/powerband of your current engine will play a factor since some engines will lug at rpms that other engines enjoy. The 904 gearing is not remarkable different to the 833 for first gear and top gear, with both being in the 2.4(1st) and 1.0(top gear) ballpark. The 833OD gets you a 27% reduction in cruising RPM IF you have the torque to drive at the RPM you are at. The 833 OD would be a poor choice in a heavy vehicle with a 225 and 2.73 rear gears, but would be quite nice in a 3000 lb car with 3.23-4.10 gears.

You did say it is a small crank register engine. Is it a 225 or 170? I can't recall if 198 had a small register variant if so those rods are very desired.

I looked it up. My memory was wrong. Most 833 trans have around a 2.6 first gear, but he 833OD is 3.09. This means that off the line acceleration will be improved compared to a normal build 904.

The 833OD would likely work well with your car/engine, assuming you have a relatively wide torque band. I suggest 3.23 or 3.55 rear gear. MPG should increase by 10-15% relative to a 904. You will have to get in the habit of revving the engine more (in 1st and 2nd especially) to not lug the engine after your upshift. Very economical way to go. T5 is better due to closer gear spacings, but MUCH more fab work and expense. 1st gear will give plenty of opportunity to make tire smoke, but shifting to 2nd with the big RPM drop will not keep the tires spinning. I have 833OD in my 62 Valiant with 250 HP engine and it works well (3.73 rear gear), but have another car with T5 and similar engine and that is more fun, but as I said much more work and $$.

I say go for it!

Lou

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Home of Slant6-powered fun machines since 1988

I have an 833 O.D. and 3:23 gears in my Valiant wagon. I love it. I do 70 miles a day with 3/4 of it freeway @75 mph. I am averaging 22 mpg.

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I make rusty parts go fast. Norm
71 Road Runner
65 Valiant SW
79 Lil Red Express
78 D-300 Club Cab

The OD 833 works really nice with 3.91's-4.30 or even 4.56 gears. My sons 72 Dart with 4.30 was sweet to drive with 4.30's and got over 20 MPG on the highway on the way to Elk Creek dragstrip one year. My truck (which weighs only a couple hundred pound more than your Dart) worked really nicely with the 3.91's it came from the factory with.

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2 Mopars come with Spark plug tubes. One is a world class, racing machine. The other is a 426 CI. boat anchor!

12.70 @ 104.6