67 rear end swap or regear? - Impala Tech
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post #1 of 8 (permalink) Old 10-20-2017, 10:50 AM Thread Starter
 
 
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67 rear end swap or regear?

So ive got a 67 4 door (almost the supernatural car), but its running a little high on the rpms for me. Its got a 283/th350 and according to the axle code, 3.36 gears. Running on my 215/75r14s, the thing sounds like it needs to shift into another gear at 45mph.. so what would be cheaper/easier? Finding a rearend with 2.73 or taller gears, or just having a shop swap the gears? Also, its already slow off the line, will 2.73s make it unbearable?

Note: im not building a race car, but i dont want to be a rolling hazzard when i pull out into the road.

Also ive heard some caddies had 2.23s, so how would that be?
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post #2 of 8 (permalink) Old 10-20-2017, 01:56 PM
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You can not change to a gear ratio lower than a 2.73 because that is a thin a gear as will fit on your three series carrier. You have a choice of a 3.08, 3.36, 3.55 ratios originally from the factory with a three series carrier. To get a 2.73 rear gear ratio you would have to install a two series carrier in place of your threes series.

In the aftermarket you can add these ratios of 3.73, 3.90, 411, or a 4.56:1 ratio with a three series carrier without a spacer.

Rather than change the rear gear change your transmission to include an over drive if you can not live with the current final drive which was installed to get a heavy car to move with a tiny engine with little torque and a PowerGlide transmission (A PG is a broken TH350 because a TH350's second gear ratio is the same ratio as the low range of a PG, leaving in second all the time instead of first gear).

Big Dave
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post #3 of 8 (permalink) Old 10-21-2017, 01:28 AM Thread Starter
 
 
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I figured the most cost effective while giving me a good look is getting taller wheels for the back. But anyways, does 3k rpms at 70mph sound about right in its stock form? Im just worried of overworking the engine if i go on a road trip.
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post #4 of 8 (permalink) Old 10-21-2017, 09:45 AM
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Originally Posted by Mrcorps16 View Post
I figured the most cost effective while giving me a good look is getting taller wheels for the back. But anyways, does 3k rpms at 70mph sound about right in its stock form? Im just worried of overworking the engine if i go on a road trip.
Sounds about right and same as my '63 with 4 speed Muncie manual and 3.36 rear. I would not change anything.

I'm not the least concerned about the rpm. I spent MANY miles in the '60s on a similar set up. Cruised at 70 mph w/o any long or short term problems. It will run all day and night at that rpm. The engine likes that a lot better than the 25 mph urban stop and go driving.
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post #5 of 8 (permalink) Old 10-21-2017, 10:58 AM
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Originally Posted by Mrcorps16 View Post
... But anyways, does 3k rpms at 70mph sound about right in its stock form? Im just worried of overworking the engine if i go on a road trip.
No that is what your engine would spin if you had a 4.11 rear gear in your rear end, not a 3.31 ring and pinion set. This is calculated with a 28 inch tall tire (you can look up your tire height by plugging in your tire dimensions on line in many web sites). If you had a shorter (smaller) 26 inch tall tire you would have a 3.90 rear gear.

If you have a 3.31 rear gear which was placed in the back according to the stamped numbers then your transmission is slipping badlly, because it worn out. You won't know what the actual rear gear is unless you pop the rear inspection plate off and count the teeth on the ring and the pinion gear.

The 283 is a short stroke engine that will spin to 10,000 RPM with a solid roller cam. It can run all day at 3,000 RPM. The longer the stroke the more torque you make but the higher the piston speed which increases engine wear. I have not calculated your piston speed but I would wager it is nearly half the piston speed of a 383 small block Chevy.

Big Dave
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post #6 of 8 (permalink) Old 10-21-2017, 02:47 PM
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Quote:
Originally Posted by Big Dave View Post
No that is what your engine would spin if you had a 4.11 rear gear in your rear end, not a 3.31 ring and pinion set. This is calculated with a 28 inch tall tire (you can look up your tire height by plugging in your tire dimensions on line in many web sites). If you had a shorter (smaller) 26 inch tall tire you would have a 3.90 rear gear.

If you have a 3.31 rear gear which was placed in the back according to the stamped numbers then your transmission is slipping badlly, because it worn out. You won't know what the actual rear gear is unless you pop the rear inspection plate off and count the teeth on the ring and the pinion gear.

The 283 is a short stroke engine that will spin to 10,000 RPM with a solid roller cam. It can run all day at 3,000 RPM. The longer the stroke the more torque you make but the higher the piston speed which increases engine wear. I have not calculated your piston speed but I would wager it is nearly half the piston speed of a 383 small block Chevy.

Big Dave
Big Dave,

Here's one calculator:

https://www.blocklayer.com/Rpm-GearEng.aspx

I entered the gear ratios from an Muncie M20 (final/4th gear ratio = 1.0), a 3.36 rear ratio, and the diameter of a 7.50x14 tire (26.7"). At 3000 rpm THIS calculator says 71 mph. That jives with both my old memories from the '60s and my present '63 Impala w/Muncie M20, 3.36 rear and approx 26.7" diameter tires.

I'm NOT arguing with you but is it possible you erred entering data into the calculator you used?

You are an engineer and so am I so we both are capable of doing the calcs ourselves. I am too lazy

My goal is simply to provide good info to the poster. If I'm wrong, it is not intentional.
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post #7 of 8 (permalink) Old 10-21-2017, 03:04 PM Thread Starter
 
 
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If i go fron 26.7" tires to 30" tires, what should i expect in performance differences other than a 300 rpm drop at highway speeds?
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post #8 of 8 (permalink) Old 10-30-2017, 04:32 PM
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My '65 has a 283 and powerglide with 3.08 gears. It runs just fine and I get 15 mpg but I don't drive over 70 mph. I would think with the added lower gear you have, 2.73 would probably work fine.
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