Looking for some Differential advice

72DartGuy

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Hey, thanks for any help! Here’s my deal. I have the Dart in my pic. Right now it has a 225 and a 904 trans with a weak 7 1/4 rear end. It bogs out climbing hills. I have a whole new drivetrain for it. Here’s my question. Would I get anything by putting my 8 3/4 sure grip 3:73 rear end in first? Right now it’s a beautiful slug.
Oh, wtf happened to our site?
 

33IMP

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The chances are good that you might have a two series gear in your 7 1/4. So a 3.73 gear should make your slant quite a bit more lively.
 

str12-340

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some better gears would certainly help your slant, but you might want to say what you are going to do with the car. 3.73s are pretty low if you get out on the highway at all. You'll find yourself running out of 3rd gear and winding up that slant at 70 mph. I'd pick 3.23s for a run around drive it anywhere car.
 

72DartGuy

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Thanks guys, I’m going to put a built 727 and a 408 Stroker in her down the road. I have all the parts. Just want to know if I’d save myself a lot of time by swapping the stock 7 1/4” rear end for my 8 3/4” 3:73 sure grip rear end?
 

MidnightSwinger

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I’d swap it. Although I must say a stock 72 slant six should have no problem pulling a steep grade. It ain’t gonna win any races but they have all of there torque available by 1200-1500 rpm.

The 3.73’s won’t hurt, as stated previously, you are going to be above 2500 rpm at highway speeds. Nothing the slant can’t handle.
 

RustyRatRod

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All this advice is worthless since you left out some key points. One of the biggest is, what size tires do you have and what size do you plan to change to, if you change? Tire height has a huge effect on final gear ratio. On my 64 for instance, I am still running the stock height 13" tires and I have 3.23 gears behind a slant 6 and it is very peppy. I am extremely happy with it.
 

str12-340

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remember the driveshaft issue. I think you will need a shorter shaft for a 904 and 8 3/4, and then an even shorter one for 727 and 8 3/4.
 

413

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3.73 is too much gear for a stock /6. It’s too much for a stock 318.

it will limit the top speed and make freeway trips impossible.
 

RustyRatRod

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3.73 is too much gear for a stock /6. It’s too much for a stock 318.

it will limit the top speed and make freeway trips impossible.
I had a 3.90 behind mine for a short time. It was a lot of fun, but WAY too much.
 

MidnightSwinger

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3.73 is too much gear for a stock /6. It’s too much for a stock 318.

it will limit the top speed and make freeway trips impossible.
What about trucks? Slants and 318’s with 3.91”s and 4.10’s.

Only about a 450 rpm difference between 3.23 and 3.73 @ 65mph with a 25” tire which is on the short end. I run a 26” and plan to go to a 28” when my 3.73’s go it.

What kind of top speed are we talking about with a stock slant six anyway? 90 mph?

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72DartGuy

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All this advice is worthless since you left out some key points. One of the biggest is, what size tires do you have and what size do you plan to change to, if you change? Tire height has a huge effect on final gear ratio. On my 64 for instance, I am still running the stock height 13" tires and I have 3.23 gears behind a slant 6 and it is very peppy. I am extremely happy with it.
I’ve got 205-70-14’s on the front and 265-60-14’s on the rear. Not changing them. I have an offenheiser? Intake and a 2bbl Holley now. It’s a dog. It’ll do maybe 50 on the freeway going up hill. When I downshift it just bogs out. I have a 408 Stroker and a built 727 to run that rear end with. I just don’t want my car to sit for another year.
 

RustyRatRod

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I’ve got 205-70-14’s on the front and 265-60-14’s on the rear. Not changing them. I have an offenheiser? Intake and a 2bbl Holley now. It’s a dog. It’ll do maybe 50 on the freeway going up hill. When I downshift it just bogs out. I have a 408 Stroker and a built 727 to run that rear end with. I just don’t want my car to sit for another year.
That's a 26.5" tall tire. Yeah, it's gonna be a dog with the highway gears I'm sure it has, but 50 up hill? Sounds like it has some tuning issues, too.
 

72DartGuy

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That's a 26.5" tall tire. Yeah, it's gonna be a dog with the highway gears I'm sure it has, but 50 up hill? Sounds like it has some tuning issues, too.
Very likely, thing is, it fires right up, runs around town no problem. It won’t smoke the tires, but it runs great. You’ve seen me go through all the bs with this car over the last year. All of the interior and wiring is done now. Next is the new motor trans and rear end. I just want to drive it for a while I have it. What happened to the site? I’ve been gone for a while
 

MidnightSwinger

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Rusty is right, snap a pic of your carb and intake for us. You are at 1800 rpm in 3rd @50 mph, you should be able to downshift and accelerate up any grade in San Diego (I’m from north county)

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72DartGuy

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Rusty is right, snap a pic of your carb and intake for us. You are at 1800 rpm in 3rd @50 mph, you should be able to downshift and accelerate up any grade in San Diego (I’m from north county)

View attachment 1716015667
Thanks! I’ll do that tomorrow. I’m in La Mesa. Driving back from Lakeside is pretty much all uphill. My car wants to stall at 2000 rpm and doesn’t want to do any more. It only wants to go 45-50.
 

MidnightSwinger

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Can you shift it to second gear? Do you have kickdown linkage hooked up? I daily drove a 1bbl slant down there for years with no issue.

If the slant is hurt, you might want to wait on the rear end swap and focus on the motor.

Like rusty said, you might be over carbed, tall gears and tall tires. Worst possible combo. We need more info on the setup though.
 

RustyRatRod

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Very likely, thing is, it fires right up, runs around town no problem. It won’t smoke the tires, but it runs great. You’ve seen me go through all the bs with this car over the last year. All of the interior and wiring is done now. Next is the new motor trans and rear end. I just want to drive it for a while I have it. What happened to the site? I’ve been gone for a while
It got a server upgrade.
 

Mopar44134

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Ran into a /6 barracuda once with 4.10s. Jumped me off the line, scared me. who wants to be beat by a /6. Caught him by the end of 1st. I had 3.23s. he was going to put a 440 in it soon.
 

AJ/FormS

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La Mesa is around 550 ft above sealevel, so your slanty, if a 225, should have plenty of climbing torque with whatever gear is back there. I agree with those that are steering you towards a tune-up.
The things I would jump on are;
>making sure the throttle valve is opening all the way with the foot-feeder,
>a stall-analysis,
>a restricted exhaust,
> a compression test,
>a full timing curve analysis
not necessarily in that order, lol..

The stroker will not need 3.73s, so in all likelihood, you would be pulling them out in the future, for less gear, to hit the hiway with.
The 72Dart has a large rear overhang, that tends to make it an evil-handling beotch, even with upgraded suspension. The stroker will need the tallest and fattest tires you can stuff under the back. With factory tubs, and to fit period type wheels, this will require a spring relocation, and the rearend will need to be narrowed. Mini-tubs should be considered. She is however, a great 318car.

Bigger number rear gears help in three ways;
1) the car leaves the line easier and quicker
2) if you put second gear in the right place, you get better hiway passing power
3) at WOT, the engine will spend less time in each gear, so the zero to 60mph will decrease

3.73s will make a great city-car, but yes it is a bit much
With 3.73s and 26" tires; 32mph would be about 2250cruising in Second/4100WOT in First. The rpm of the peak power, of the stock 225, is pretty low. At 32mph=4100 in First gear, the engine is already fast approaching up-shift rpm. So when you nail it at 32mph, it may already be wheezing thru the single exhaust, and she will be up-shifting in a heartbeat. That's how I see it.
IMO, a higher stall TC would be better option, but the exhaust system better be up to the task. You can easily get 50% more off-the-line power with a lil more stall; perhaps even double. Back in about 1976, I installed a 2800TC behind a fresh 69spec 225 with an A904/2.76s, into my 69 Barracuda fb, and was very pleased with the result. Once you get the Rs up, 2.76s are 32=3100WOT@10% slip in First gear, and that gear will pull to 45mph@4400; so you got a nice window there.
 
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413

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The small carb, small cam and valve springs will be your problems. It won’t make any power with its tongue hanging out on the freeway. Mileage goes way down.

I have a friend did 3.91’s to a stock 1970 318. Was fine on local roads. Took them out pretty soon after driving it on the freeway.

I bought some 3.91’s and put them on a stock 273-2bbl driver car to see if they were in working order. 50 mph was the limit. Didn’t want to go on the freeway unless the foot was way down on the gas.
 

j par

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The small carb, small cam and valve springs will be your problems. It won’t make any power with its tongue hanging out on the freeway. Mileage goes way down.

I have a friend did 3.91’s to a stock 1970 318. Was fine on local roads. Took them out pretty soon after driving it on the freeway.

I bought some 3.91’s and put them on a stock 273-2bbl driver car to see if they were in working order. 50 mph was the limit. Didn’t want to go on the freeway unless the foot was way down on the gas.
Impossible??...
 
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