7.25 to 8.25 swap? /6 to 318 swap?

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WoodenNickle

WoodenNickle

Just Damn.....
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I have a 72 Swinger HT with a /6, 904, and 7.25 rear-end. My son has an 84 D150 Prospector with a 318, 727, and a 8.25 rear-end. He moved out and left the truck with me. It got me to thinking about cannibalization. I got to thinking about taking my drive-train out and putting his in the Swinger. But I'm not really all that likely too since both run good. (My drive-train is coming out anyway so I can slap the body on a rotisserie for work/paint.) Besides my moral dilemma of cannibalizing a perfectly good contraption, I started to wonder about all the retrofitting involved. I'm pretty sure I won't, but what would I be looking at if I did?

Would his 8.25 even fit in the Swinger? Is there any 8.25 that would bolt in, in place of the 7.25? What Make/ Model would I need to find as a donor? If there's not a direct bolt in, what would be the best suited 8.25 with minimal mods?

Would an 8.25 improve ANYTHING behind a /6? Or would I be wasting my time without putting in a 318? I'm no race-car driver, so I'm not hoping to blow anybody's doors off, but I'd like to be able to get up to a highway speed in the Swinger without people blowing MY doors off.

Thoughts? Suggestions? I have access to some decent pull-a-part yards and I'm not pressed for time.
 
Yes there is a bolt in A Body 8.25 rear. You can find them in the 73+ BBP cars. A neat trick is with the 8.25 rear is you can quite easily install rear disc brakes. Just find a 96 and older Jeep Grand Cherokee and take the rear disc brake setup of and it will be a direct bolt in to the A Body 8.25 rear. You can also take the ring and pinion & limited slip unit and get better rear gear ratio into the A Body 8.25 rear. All for dirt cheap money from junk yard parts. That said though, when you find the A Body 8.25 rear make sure it has the 2.94(or higher number wise) because the 2.71(or lower) uses a different housing.

I personally have no problem with swapping motors that said a junkyard LA roller Block (late 80s/early 90s vans and trucks) or mag block is what I'd use. Which can be had at a junkyard on the cheap. If you do decide to swap/or just install a V8 you'll either need conversion mounts or swap the K frame (not that hard to do and a great to rebuild/replace worn out bushing/parts), if swap k frames which is what I'd do, try the get your hands on the 73+ spool mount version because it's a much better design.

As far as the transmission goes I believe you can use a truck 727 tranny in an A Body but don't quote me on that. Personally I'd use a 904 over a 727 because the 904 is smaller lighter and uses less power to operate than a 727 and a stock 318 with black holes for pistons could use all the power savings it can get, but that's just me.

Not sure if this helped or hindered you but just giving you ideas etc... but I'd go V8 over a slant 6 anyday of the week! Oh and before I forget, ditching the 7.25 for a 8.25 (or bigger) is ALWAYS a good idea! The 7.25 rear is the biggest POS ever and even a bare bone stock slant 6 can break a 7.25 rear, just sayin.
 
So if;
(1) I just go get the 8.25 and steal me a limited slip and disc setup, would that improve anything behind a slant?

(2) If I do the above AND put this 318 in front of the 904, what would that do for me?

(3) Would I have to mod the driveshaft for either scenario?
 
If you use a 904 and go to a 8 1/4 you'll need to shorten the driveshaft you already have that's it.
 
So if;
(1) I just go get the 8.25 and steal me a limited slip and disc setup, would that improve anything behind a slant?

Besides not worrying about breaking the thing not much (also wayyyyyyyyy easier to find parts for). The 8.25 housing you need/want comes from an A Body and nothing else. The disc brake and limited slip and gears can come from the jeep. The disc are a hell of a lot easier to change pads vs shoes and way easier to check for abnormal wear. Also disc brakes cool down a lot faster than drums which means you can stand on the brakes longer/harder but you know that already. Basically a 4 wheel disc brake setup is better than a disc/drum or 4 wheel drum brakes (I assumed you had front disc already and if not a front disc setup is a hell of an improvement over front drums regardless of what engine you are using). Just remember that there are lots of a-holes on the road so being able to stop quickly and safely is sooooo much more than power. But to answer your question, you won't see much of an improvement with slant but start adding more power and you get the idea.

(2) If I do the above AND put this 318 in front of the 904, what would that do for me?

The 904 uses far less power to operate than a 727. Just get a good rebuild on the 904 and you're good to go. If I'm not mistaken the 904 takes about 15 Hp to power were as a 727 takes about 40HP to power it (my numbers may be off a bit but you get the idea) with a small hp motor any savings you can get is the way to go because the more you save from drivetrain loss the more you can put to the ground. That said if just swap the stock pistons to a flat top version in a 318, watch that motor wake the heck up! A good cam wouldn't hard as well as a 4 bbl intake (a performer from Edelbrock is and excellent upgrade and can be had for just a few bucks).

(3) Would I have to mod the driveshaft for either scenario?[/quote]
Just by installing the 8.25 rear, you'll need to have the driveshaft shortened. Not a lot but it will need shorting (even more so if you use the 727).
 
I appreciate all the info folks. Never had the opportunity to mix and match before, usually I went for whatever would fix my vehicles pronto. I had 4 kids, myself, and my wife in separate vehicles that had to run. Now I have more vehicles than people and some time on my hands finally to do some car work on what I want to, not what I have to.

So I'll probably go get me another 318 and an 8.25 and do the Dart up right from the start. It has the 904, so I won't have to get that. Would I need to change the converter?
I'll just tweak the truck and give it some paint. Maybe I'll even let my son have it back. HA!
 
I don't mean to thread jack your post but I've got a question! 1970duster I am putting together a 72 swinger I just picked up its a factory power front disc brake car and the thought of rear disc interests me greatly the previous owner had already swaped in a 8.25 so that's not a problem my question though is when you do the rear disc swap do you have to run a adjustable prop valve for the rear????? Sorry again OP I had to ask!
 
Myswinger said:
I don't mean to thread jack your post but I've got a question! 1970duster I am putting together a 72 swinger I just picked up its a factory power front disc brake car and the thought of rear disc interests me greatly the previous owner had already swaped in a 8.25 so that's not a problem my question though is when you do the rear disc swap do you have to run a adjustable prop valve for the rear????? Sorry again OP I had to ask!
Click to expand...



No complaints from me! I learn from everything I read and your question may be something I didn't know that I should have asked as well. Carry on sir.:coffee2:
 
Myswinger said:
I don't mean to thread jack your post but I've got a question! 1970duster I am putting together a 72 swinger I just picked up its a factory power front disc brake car and the thought of rear disc interests me greatly the previous owner had already swaped in a 8.25 so that's not a problem my question though is when you do the rear disc swap do you have to run a adjustable prop valve for the rear????? Sorry again OP I had to ask!
Click to expand...

Okay I got a question for you. Do you have the single piston calipers or 4 piston calipers? The single would mean you (should) have the same bolt pattern front and rear 5x4.5. If you have the 4 piston caliper it would mean you have the 5x4 bolt pattern. I ask this because the 8.25 rear only came as a 5x4.5 big bolt pattern.

To answer your question there are two ways to do it 1. Install an adjustable prop valve (highly recommended and the way I'd do it), 2. Use a prop valve from a 4 wheel drum brake car (while this will work I personally wouldn't do it that).
 
WoodenNickle said:
I appreciate all the info folks. Never had the opportunity to mix and match before, usually I went for whatever would fix my vehicles pronto. I had 4 kids, myself, and my wife in separate vehicles that had to run. Now I have more vehicles than people and some time on my hands finally to do some car work on what I want to, not what I have to.

So I'll probably go get me another 318 and an 8.25 and do the Dart up right from the start. It has the 904, so I won't have to get that. Would I need to change the converter?
I'll just tweak the truck and give it some paint. Maybe I'll even let my son have it back. HA!
Click to expand...

Just to let you know that a slant 6 transmission has a different bolt than a small block transmission so you'll need to acquire a small block 904 transmission (not that hard).
 
1970Duster yes the previous owner also swaped the front to the single piston front calipers also, and thank you I now know why he did that instead of keeping the 4 piston ones. This is my first mopar. I agree with the use of a prop valve but I figured I would ask you never know there might be a mopar prop valve out there that would work well and look factory!
 
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