Want to wake up my 73 Newport Custom

[badvs3vil]

Hello everyone,

Over this winter I am thinking about a project to do on my 73 Newport Custom. Its has a 400BB with a 4B Holley. It has the 3 speed 727 TF.

My thoughts were around the rear end. Currently its the factory 8.25 rear diff at 2.71 GR.

A few people have said that I should change that out for something with a higher gear ratio. What should I look at getting? Should I just go around to a salvage yard and find one?

Thanks in advanced.

 

[413]

Yes rear end gears make a big difference. And 2.7 will hamper performance. Same with tire height, it affects the gear ratio. How tall are those rear tires? They look tall.

And you will want lower gears, which have a larger number.

 

[mrfury68]

If you are going to do a lot of cruising on the open road or even around town, a 3.23 ratio would suit you well. It would have a little more jump at your lower speeds but wouldn't give you much higher rpms at highway speeds. If you are doing mostly city driving with a little open road cruising you could opt for a 3.55 gear ratio. Quicker out of the hole but increased rpms on the open road. Tire height comes into play as mentioned earlier so that is a consideration as well. If you are looking to wake up that 400 have you considered dual exhaust? A nice set of 2 1/4 inch dual pipes would help that motor breathe a lot better. I did that to my Fury with a 383 2bbl and it made a difference, even with the stock manifolds. Hope this info helps. Good luck.

 

[badvs3vil]

If you are going to do a lot of cruising on the open road or even around town, a 3.23 ratio would suit you well. It would have a little more jump at your lower speeds but wouldn't give you much higher rpms at highway speeds. If you are doing mostly city driving with a little open road cruising you could opt for a 3.55 gear ratio. Quicker out of the hole but increased rpms on the open road. Tire height comes into play as mentioned earlier so that is a consideration as well. If you are looking to wake up that 400 have you considered dual exhaust? A nice set of 2 1/4 inch dual pipes would help that motor breathe a lot better. I did that to my Fury with a 383 2bbl and it made a difference, even with the stock manifolds. Hope this info helps. Good luck.

I already have duals with short tube headers running right now. Thanks for the info. I don't really do any open road driving, if I do, its short.

 

[badvs3vil]

Yes rear end gears make a big difference. And 2.7 will hamper performance. Same with tire height, it affects the gear ratio. How tall are those rear tires? They look tall.

And you will want lower gears, which have a larger number.

Those in that pic are 22"s but that was only for messing around. I normally run American Racing 17's, you can see in my other reply.

 

[badvs3vil]

I guess side question, if I wanted to change gear ratio to 3.23 or 3.55, do I need to change out the whole rear end or can I just change gears?

 

[413]

Those in that pic are 22"s but that was only for messing around. I normally run American Racing 17's, you can see in my other reply.

The overall tire diameter is the key number you want to be concerned with, not the wheel diameter.

 

[Big_John]

I guess side question, if I wanted to change gear ratio to 3.23 or 3.55, do I need to change out the whole rear end or can I just change gears?

You'd probably be time and money ahead if you found an 8 3/4 rear and center section. That's what I would look at.

 

[badvs3vil]

The overall tire diameter is the key number you want to be concerned with, not the wheel diameter.

I run 235/55/R17

 

['66 Fury I]

As Big_John suggested, I would agree that a swap to an 8-3/4 rear end would be a plus. I'm running 235-60/14's and a 3.23 rear. This suits me fine with good highway revs. Since you are running 235-55/17's, and you say you do very little highway driving, you mighe like 3.55 gears. This is only a suggestion and others may add thier suggestions. Lindsay

 

[CBODY67]

The referenced 17" tire size is pretty close to the G78-15 size the BASE non-a/c tires the Newports came with back then.

Which Holley 4bbl and intake manifold?

Personally, I like to drive faster and get better fuel economy, which the 2.71s are very good for, with the torque peak (with the factory cam) being at close to 80mph. With a 3.23, it would be more like 70mph, with it being lower still with 3.55s. Which is why the 3.23 ratio would be so good, to me. PLUS, with some looking, you might be able to find a complete 3.23 or 3.21 rear axle assy with SureGrip, maybe? That way, no messing with rear axle lube and getting the gears set-up to be quiet.

With any rear axle ratio change, do NOT forget about the speedometer drive/driven gears in the tailshaft of the trans, too.

Just some thoughts,
CBODY67

 

[rd92west]

Wake up your Newport. Humm.
3.23 are good all around gears. 3.55 will liven things up a little.
If you are just driving around town and very seldom out on highway you might consider 3.91 gears
Great fun. Burns lots more fuel because you wont be able to resist pushing the gas pedal down.
But the Newport will be fully awake.
And I agree a swap to 8.75 housing will pay dividends.

 

[bnz84]

Check out this an other calculators to get an idea of what the result of changes will be. The good news is there is no shortage of opinions on all the *body sites. Just search for a gear ratio and a ton of things pop up. I have a rear tire 2" taller than yours (29" total) and absolutely had to get a taller gear than the stock 2.76. The 3.55 I changed to, for my tire size, still is almost a highway gear. Maybe 2700-2900RPM at 65-70. With your shorter 27.2" diameter I would start at 3.55 for more lively driving and , as others said, consider 3.91.

 

[badvs3vil]

Check out this an other calculators to get an idea of what the result of changes will be. The good news is there is no shortage of opinions on all the *body sites. Just search for a gear ratio and a ton of things pop up. I have a rear tire 2" taller than yours (29" total) and absolutely had to get a taller gear than the stock 2.76. The 3.55 I changed to, for my tire size, still is almost a highway gear. Maybe 2700-2900RPM at 65-70. With your shorter 27.2" diameter I would start at 3.55 for more lively driving and , as others said, consider 3.91.

Awesome info, thanks. Any advise on where I should look? Just look for a 8 3/4 rear end with at least 3.55GR? I would assume just look at some junk yards and just rebuild it?

 

[bnz84]

i looked around Maryland for a long time for a 3.55 Sure grip. Found some but all had to be rebuilt or if they were in better condition had shorter gears. Exactly like my 2.76 that I removed! I did find some on ebay but the price was not far from a complete build. So that is what i did, got one built from Gevos I met at Carlisle. Keep asking on all the A,B,C body sights. Also perhaps sights like Pro-touring or drag racing sites where they are getting rid of their lame 3.55 or 3.91 gears for kinda cheap.

 

[bnz84]

I think sometimes a local trans rebuild place will have a guy that does rears. Or at least "knows a guy". If you go that route at least u will saving shipping.

 

[badvs3vil]

Sorry for all of these newbie questions all.

Can I put higher gear ratio in my current 8 1/4?

 

[CBODY67]

As far as complete rear axles go, as C-bodies usually came with 2.71 or 3.21 (in your model year in 8.25" rear axles), that's pretty much what might be in the salvage yards. Due to how the rear leaf springs are isolated from the rear axle housing, the model years available would be 1970-1973, typically. 3.55 or 3.91 ratios were options on B & E-bodies in the earlier 1970s and not in a C-body. The B & E body axle assys are too narrow for a C-body.

If you desire to play "stop light drags", due to their weight, a C-body CAN be made into a decent player in that game, but what it might take to make it competitive can also highly ciompromise any other use of the car. Especially considering that almost EVERY small car has a gazillion-speed automatic with a deep low gear in it! No way to compete against that without installing one of the Chrysler 8-speeds or transplant a GM automatic where the TF currently is. When the competitor has someting like a 4.5-4.8 low gear ratio, no way a TF's 2.45 low gear ratio will be able to compete without something like a 4.50 rear axle ratio. AND that just gets you off the line. OR you replace the current 400 with a 500+ cid stroker motor. Either way, expect single-digit fuel economy. With the needed torque multiplication, reinforcement of the rear suspension will also be needed . . . too.

In many respects, you've already got a pretty decent ride. Seeking to "improve it" can easily become a "One thing leads to many more" sort of expensive exercise. Including a stronger rear axle assy, somewhere in the mix.

Fine tune and tweak the existing equipment to possibly improve off-idle throttle response. Chase a 3.2 rear axle if desired. Enjoy the car for what it is! Don't try to make it something that its not, unless you might emulate MEV and his stroker '66 NewYorker. At what cost?

Just 3 cents-worth of thoughts,
CBODY67

 

[CBODY67]

Sorry for all of these newbie questions all.

Can I put higher gear ratio in my current 8 1/4?

If you use "factory gears", you'll need the complete gear set and what the ring gear bolts to. If using aftermarket gears, they usually have a spacer to put between the ring gear and the "differential carrier". Generally, the carriers have a given gear ratio spread that will fit them, using factory gears. Typically, once you get into "3-series" gears, you can go all the way to 3.91 from 3.23, but not starting from a 2.7 gear ratio . . . at least in other brand rear axles.

Some terminology . . . "Higher numerical gear" = Lower gear ratio (as in 3.91). "Lower numerical gear" = Higher gear ratio (as in 2.7 or 2.4). "Higher gear ratio" = more mph/1000 engine rpm. "Lower gear ratio" = fewer mph/1000 engine rpm or "higher numerical gear ratio".

Enjoy!
CBODY67

 

[badvs3vil]

As far as complete rear axles go, as C-bodies usually came with 2.71 or 3.21 (in your model year in 8.25" rear axles), that's pretty much what might be in the salvage yards. Due to how the rear leaf springs are isolated from the rear axle housing, the model years available would be 1970-1973, typically. 3.55 or 3.91 ratios were options on B & E-bodies in the earlier 1970s and not in a C-body. The B & E body axle assys are too narrow for a C-body.

If you desire to play "stop light drags", due to their weight, a C-body CAN be made into a decent player in that game, but what it might take to make it competitive can also highly ciompromise any other use of the car. Especially considering that almost EVERY small car has a gazillion-speed automatic with a deep low gear in it! No way to compete against that without installing one of the Chrysler 8-speeds or transplant a GM automatic where the TF currently is. When the competitor has someting like a 4.5-4.8 low gear ratio, no way a TF's 2.45 low gear ratio will be able to compete without something like a 4.50 rear axle ratio. AND that just gets you off the line. OR you replace the current 400 with a 500+ cid stroker motor. Either way, expect single-digit fuel economy. With the needed torque multiplication, reinforcement of the rear suspension will also be needed . . . too.

In many respects, you've already got a pretty decent ride. Seeking to "improve it" can easily become a "One thing leads to many more" sort of expensive exercise. Including a stronger rear axle assy, somewhere in the mix.

Fine tune and tweak the existing equipment to possibly improve off-idle throttle response. Chase a 3.2 rear axle if desired. Enjoy the car for what it is! Don't try to make it something that its not, unless you might emulate MEV and his stroker '66 NewYorker. At what cost?

Just 3 cents-worth of thoughts,
CBODY67

That's some major advise there my friend. Thank you.

Like you said, I am just looking for a bit more power off the line. Just to play. I am not looking to race anyone. I've already had the rear end out when I replaced the spring and shocks, so I was looking for an "easy" job that I might want to tackle over winter.

I guess I have other options as well. I have not touch the insides of the engine. I replaced the intake and carb. I guess I could look at throwing a cam in it or something.