Bizarre Rear End Gear Ratio or bad test method?

[MoparGuy68]

While under my Super Bee measuring the width of its 8 3/4 rear, I thought I would perform a test to determine the ring and pinion gear ratio.

I did two tests, that gave the same result.

First I marked the drive shaft (using chalk) with marks every 90 degrees labeled 1, 2, 3, 4.

Then marked a starting point on the outside of the brake drum with chalk. Next, I rotated the driveshaft counting how many turns it made for one revolution of the brake drum.

The results were shocking! One revolution of the brake drum = 1.625 revolutions of the driveshaft!! WTF?? This seem like such an incredibly low numerical ratio number it is hard to believe! That’s one complete revolution of the driveshaft, plus stopping midway between Mark 3 & 4 during the second revolution, not even completing the second revolution..

I did the test again, this this time rotating the brake drum one full revolution and watching the driveshaft marks as it turns. Same result 1.625 turns of the driveshaft (It should be the same result whether rotating the drum or the driveshaft).

The drivetrain of this car gets even more bizarre. When driving at about 60 mph (in 3rd gear 727 automatic), the rpms on the factory tach are around 2800 RPMs. Excessively high!

I’m currently having all the hydraulic brake parts rebuilt, which has taken 3 months, so far, at Karps. They are not finished yet.

In my opinion, the front disc brake calipers were dragging excessively on the rotors. The front wheels were not as free spinning as I think they should be when your foot is OFF the brake pedal. The passenger side caliper dragged more than the drive side.

This aspect, I would think, would raise the rpms at any given speed, as the engine has to work harder to overcome the caliper drag. This would also explain why the engine would get too hot while driving, but temp stayed normal when running in neutral or idling.

The caliper drag does NOT explain a 1.625 ring and pinion ratio..

Is the correct test ONE turn of the drum/wheel and count the number of turns of driveshaft, or TWO turns of wheel? If I go by TWO turns that gives me a more sane result of 3.25 turns (probably 3.23 gears).

1.625 ratio is impossible.. That ratio doesn’t exist, except maybe for Bonneville salt flat runs at 300 mph?

 

[dadsbee]

3.23 open rear diff... as you're only turning one wheel by the sounds of things. 1.625 x 2

 

[Don Frelier]

2,800 is the sweet spot for me

 

[ruffcut]

2800 RPM is not excessive. ruffcut

 

[Big Bad Dad]

Is the wheel on the other side turning backwards when you do the test? Hold it stationary and try again...

 

[beanhead]

Sounds like you have an open differential...those pesky spiders can transfer the motion from side to side without a direct turn of the driveline. Usually with a peg-leg you test with one tire planted on the ground and turn the free tire twice and count the # of turns at the yoke.
And we'd need to know the rear tire size to know if 2800rpm @60mph is indicating excessive slippage.

 

[beanhead]

Check it out
http://wallaceracing.com/calc-gear-tire-rpm-mph.php

 

[MoparLeo]

Here is a link to most any kind of calculator you could want.
http://www.wallaceracing.com/Calculators.htm

 

[33 IMP]

Your technique is wrong. You can check out you tube videos to do it right.
But as others have said above, you've got a 3.23 open rear.

 

[MoparGuy68]

Yes, I was only turning ONE wheel. The passenger side rear wheel. The driver side rear wheel doesn’t always turn with the driveshaft. When grabbing the driveshaft and turning clockwise while facing the rear of the car, the passenger side wheel is always turning forward. The driver side wheel sometimes turns forward or doesn’t move at all. This was with both wheels up in the air, off the ground. Doing this by myself, everything has to be done by myself. Nobody around to help.

Ace chief mechanic (cough), at Brakemasters, who claims he works on old Challengers, told me I have a Sure-grip..

It surely acts, during these “tests”, like an open rear, though an open rear that leaves two black marks on the road when punching the throttle.. Which I witnessed shortly after buying this difficult car.

So 3.23 ratio it must be. RPMs seem MUCH higher than my 71 Challenger that had 2.76 ratio. Challenger had a rear end that nobody EVER touched, 100% as it left the factory. That’s why it was quiet and worked so well, unlike this noisy pile in the Super Bee. Not really too sure which is the louder the engine or the rear end..

 

[33 IMP]

Even a wasted cone sure grip that wont put down two stripes, will still turn both wheels the same direction when they are both in the air.
You have an open rear, and its sound like a noisy one to boot.

 

[MoparGuy68]

The rear end starts making noise at around 30mph, peaking at it’s loudest around 40mph. The faster you go above 40, the quieter it gets.

Noise is present only when the engine is under load. The moment I take my foot off the throttle, even at 40mph, the noise stops immediately.

Because of this, I try to drive either below or above 40mph. Haven’t driven since mid July, been waiting a long time on brake part rebuilds.

After the brake work, will come the rear end work.

 

[dadsbee]

Much to my surprise, I found my "new" acquisition is an open 3.23, despite the fact it can smoke both Cobra P275's !

 

[72sunroof]

The rear end starts making noise at around 30mph, peaking at it’s loudest around 40mph. The faster you go above 40, the quieter it gets.

Noise is present only when the engine is under load. The moment I take my foot off the throttle, even at 40mph, the noise stops immediately.

Because of this, I try to drive either below or above 40mph. Haven’t driven since mid July, been waiting a long time on brake part rebuilds.

After the brake work, will come the rear end work.

Sounds like the front bearings are bad.
Gearset is probably toast also.
Someone probably crushed the crush sleave and then backed it off.

 

[hunt2elk]

2800 rpm at highway speed is pretty normal for 3.23's and not excessive by any means. Sounds like you need to make a call to Cass aka Dr. Diff for your rearend issues.