1980 Dodge Diplomat question!

[SlantSixSullivan]

Hey everybody, I'm a new member to this site, and I'm new to M bodies as well. Previously, I have owned the good ol' A bodies, Darts and Valiants. All with slant sixes in them. For daily use, I was never dissapointed or let down by the slant. I am purchasing a 1980 Diplomat which, of course, has a slant six in it. I had planned to swap a Valiant slant with a 360 with a 727 trans and 8.25" rear, for the sole purpose of gaining power. I had since totalled the car at the fault of a texting driver, and found this Diplomat as a replacement. Here is my question: Just how much more power will I get out of this stock 360 with auto trans? I'm not looking for high performance, just something a little bit more fun than what I'd been used to so far. I understand the M bodies are roughly the same size and weight as the A bodies, so this setup will be close to what I had been planning anyway. It seems the slant in this Dippy is a tad weak, which I partially blame on the emissions nonsense they started in the mid-70s. If I do the swap and remove all emissions, I hope to notice a great difference in power. Any questions or comments are most certainly welcome!

 

[Justwondering]

Nice looking ride!
I'm a car novice but there are many great wrenchers here that will chime in with opinions...

 

[SlantSixSullivan]

Nice looking ride!
I'm a car novice but there are many great wrenchers here that will chime in with opinions...

I'm no expert myself, but I do know that in the case of these old engines, 2 more cylinders equals more power. Just how much more? I'll find out!

 

[LSM360]

The A bodies were a little lighter than the M bodies, especially the early A's. What 360 are you going to use? It's certainly going to make more power than a slanty, but they suffered from the emissions regulations too, e.g. low compression. Do you already have one in mind? Are you going to build it? A 360/727/8.25 can be a nice combo. The 8.25 is lighter than the 8.75, more readily available, less expensive and pretty reliable if you're not going to build something with monster h.p.

 

[SlantSixSullivan]

The A bodies were a little lighter than the M bodies, especially the early A's. What 360 are you going to use? It's certainly going to make more power than a slanty, but they suffered from the emissions regulations too, e.g. low compression. Do you already have one in mind? Are you going to build it? A 360/727/8.25 can be a nice combo. The 8.25 is lighter than the 8.75, more readily available, less expensive and pretty reliable if you're not going to build something with monster h.p.

The 360 I have, I believe was built in the early 70s, pre-emissions, as far as I can tell. I don't plan on building it at all, as I am using this car as an almost daily driver. I am just doing all new seals and gaskets, and some black paint. Absolutely nothing special, just more cubes than the slant. I can check the block for the year it was made.

 

[slant6billy]

So does the dip have a 7 1/4 now or did it come with an 8 1/4? Most six coupes would be 7 1/4. Note, some 7 1/4 have 10 bolt housing on the back as well 9 bolt. The 8 1/4 will be ok and you can get a sure grip unit for it. One legger open rear will stress the driverside axle. I've broken one leggers with slants, but I'm a special kind of abusive idiot. And just use regular tires...... Drag radials are the devil and weak leaf springs are common on FMJs... Don't do as I have done to an 8 1/4

 

[SlantSixSullivan]

So does the dip have a 7 1/4 now or did it come with an 8 1/4? Most six coupes would be 7 1/4. Note, some 7 1/4 have 10 bolt housing on the back as well 9 bolt. The 8 1/4 will be ok and you can get a sure grip unit for it. One legger open rear will stress the driverside axle. I've broken one leggers with slants, but I'm a special kind of abusive idiot. And just use regular tires...... Drag radials are the devil and weak leaf springs are common on FMJs... Don't do as I have done to an 8 1/4 View attachment 18171

Ouch! I like the car, by the way! Yes, the Dippy has the ol' peg leg 7 1/4, but it will be changed with the motor and trans to handle the abuse.

 

[slant6billy]

Find a wagon FMJ for the 8 1/4 or perhaps some fellow Ohioian could provide you one for a reasonable price?

A good number of Ohio folks on here. They are some good folks too!

 

[SlantSixSullivan]

Find a wagon FMJ for the 8 1/4 or perhaps some fellow Ohioian could provide you one for a reasonable price?

A good number of Ohio folks on here. They are some good folks too!

I definitely don't need anything bigger than 8 1/4, for what I'm doing. Just normal use to and from work, errands, and well...yeah, some mischief around town. So, that being said, I am left now with choosing the correct gear ratio to use. I am mainly interested in two things: acceleration and economy. I understand that with a carbureted small block, I won't be getting the fuel economy I get in my S-10. That's fine, I'll sacrifice that to have a cool ride. So I'm left with acceleration, not top speed. I may use the car on the highway on rare occasions, with the possibility of a road trip or two every year. My understanding of gear ratios is limited. I know that it is a relationship between the driveshaft and the rear end, that's about it. So which available ratio is best for my build?

 

[slant6billy]

I definitely don't need anything bigger than 8 1/4, for what I'm doing. Just normal use to and from work, errands, and well...yeah, some mischief around town. So, that being said, I am left now with choosing the correct gear ratio to use. I am mainly interested in two things: acceleration and economy. I understand that with a carbureted small block, I won't be getting the fuel economy I get in my S-10. That's fine, I'll sacrifice that to have a cool ride. So I'm left with acceleration, not top speed. I may use the car on the highway on rare occasions, with the possibility of a road trip or two every year. My understanding of gear ratios is limited. I know that it is a relationship between the driveshaft and the rear end, that's about it. So which available ratio is best for my build?

The gear ratio is most common for fuel economy. 2.76 / 3.23 ???? possible? You can get a Richmond gear from Summit or Jegs with a 4:11. Might be too aggressive if you dont run a decently tall tire on the back. Jeeps use the 8 1/4, but in 96 the splines changed. You have some options, but the speedo gear is unique on our cars with a plastic snapspring housing and no available gears. Switch it over to an earlier A body style aluminum house and a body speedo cable

 

[LSM360]

I have a 8.25 in my M with a 3.21 Yukon ring and pinion from Randys ring and pinion and a Mopar Sure Grip (which you really don't need with a mild 360 if you want to save some money). The 3.21 is a pretty nice gear for around town with the occasional highway trip. Personally, that's the highest I'd go because of fuel economy and annoying revs, for a daily driver. Sounds like you have a real good plan to me. Nice 360, 727, and an 8.25. Great combo for a daily driver without going nuts. You can still have a nice smooth idle, a little rumble without being obnoxious, some power without needing 93 octane, and still get 15 mpg or so.

 

[SlantSixSullivan]

I have a 8.25 in my M with a 3.21 Yukon ring and pinion from Randys ring and pinion and a Mopar Sure Grip (which you really don't need with a mild 360 if you want to save some money). The 3.21 is a pretty nice gear for around town with the occasional highway trip. Personally, that's the highest I'd go because of fuel economy and annoying revs, for a daily driver. Sounds like you have a real good plan to me. Nice 360, 727, and an 8.25. Great combo for a daily driver without going nuts. You can still have a nice smooth idle, a little rumble without being obnoxious, some power without needing 93 octane, and still get 15 mpg or so.

Thanks for your input; I believe I found this most helpful, after providing some information. That's all I want, something stock to drive around and have fun with. I just needed a little bit of knowledge before proceeding. Of course this project won't be until spring, I'll enjoy the slant six for a few months!

 

[AJ/FormS]

that old 360 by adding just a 4bbl and headers, will about double your power. But the better deal is the huge bottom end torque increase.
And that is exactly what these cars need.All aspects of performance will be increased,and if you promise not to peg-leg the little rear, it will survive a good long while. With a small number rear gear,something in the 2s, you might be surprised by the fuel mileage.
And no, the 4bbl and headers are not a requirement for your application, I just used them as an illustration to make the 'about double' statement. I think the slantys were rated 140hp and the stock 2bbl 360s mightabin 240hp. But the 360 has off-idle mega-torque compared to the slanty. But just so you know; the 4bbl (especially the small TQ), and headers cost nothing in terms of fuel mileage, but will add a good amount of both torque and power, and if you stay out of the secondaries (most of the time at least), it should also do a little better on gas.There are no downsides to a spread-bore 4bbl, and headers.

 

[Mr C]

So a 70's 360 will get you up to 245 hp in stock 4 bbl trim. The 1978 225 super six (2bbl...but you probably already know that!) made 110 hp. An 8.25 will be totally fine in stock trim...even the 7.25 does fine in 80's Lean burn 318 powered cars.

 

[LSM360]

Thanks for your input; I believe I found this most helpful, after providing some information. That's all I want, something stock to drive around and have fun with. I just needed a little bit of knowledge before proceeding. Of course this project won't be until spring, I'll enjoy the slant six for a few months!

Heck ya, enjoy that slanty for the time being! Half the fun (at least to me) is the anticipation of whats to come, and taking your time in planning it out and having everything ready so that it goes as smooth as possible.

 

[Justwondering]

Doh! (Head slap).... plan things in advance?
I'm still in 'react to the next crisis' mode....

My goal is to actually get ahead of this curve and get to the planning stage...

 

[LSM360]

Doh! (Head slap).... plan things in advance?
I'm still in 'react to the next crisis' mode....

My goal is to actually get ahead of this curve and get to the planning stage...

Well I have found no matter how much planning and gathering of parts for projects at least two crisis always arise! I guess it just goes with the territory.

 

[SlantSixSullivan]

I was doing some thinking last night.. Instead of a big 727, why not use a 904? It would take a LOT less power to turn one, making it easier to get the wheels moving. I plan on doing a B&M shift kit anyway, so I can find out which parts may need "beefed up" to handle the abuse. I found a rear end, an 8 3/4 from a B body with 3.23 gears. The dimensions are 1/2 inch different from the 7 1/4 in my M body, resulting in a 1/4 inch difference on each wheel, no problem. I haven't even begun my project, and plans are already changing!

 

[LSM360]

I wouldn't want to have to do the fabricating nor have the weight of the heavier 8.75 rear for what your building. Don't get me wrong, it's a great rear end and stronger than the 8.25, but the 8.25 is a direct fit and plenty strong for your build, lighter, and less costly. But hey, if you can swing it, you'll have a good rear under the car. And a 904 or variant built a bit should handle the power just fine.

 

[SlantSixSullivan]

I wouldn't want to have to do the fabricating nor have the weight of the heavier 8.75 rear for what your building. Don't get me wrong, it's a great rear end and stronger than the 8.25, but the 8.25 is a direct fit and plenty strong for your build, lighter, and less costly. But hey, if you can swing it, you'll have a good rear under the car. And a 904 or variant built a bit should handle the power just fine.

I never gave the fab work a second thought.. being that the 8 3/4 is much different from an 8 1/4, I'd have to shorten the driveshaft. I will have to continue my search