Differential substitutions for higher speed

AA Ford Discussion Group relating to the repair and restoration of your AA Ford.
wild4679
Posts: 47
Joined: July 8th, 2017, 7:14 am
Body Type: 82B
Model Year: 1931

Differential substitutions for higher speed

Post by wild4679 » October 22nd, 2018, 9:42 am

I confess this is a rookie question. Has anyone found a substitute differential/axle that can replace the stock 30-31 differential/axle to increase the highway speed? And, do with without "extreme" modification of rear suspension, torque tube, etc.

Or even better, is there a substitute for the differential itself (ring and pinion gears) that would fit inside the stock case of the 30-31?

Comment: I spent an afternoon inside the shop of a hot rod builder, and those guys really think outside the box, while I look for bolt on replacement NOS type stuff. Perhaps an AA restorer has come up with an idea to increase highway speed.

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Chris Haynes
Posts: 2005
Joined: September 7th, 2003, 5:18 pm
Body Type: 82A
Model Year: 1930
Location: North Hills, CA/ Pine Grove, CA

Re: Differential substitutions for higher speed

Post by Chris Haynes » October 22nd, 2018, 11:14 am

There are two different gear ratios for the AA. How to easily find out which you have depends on whether yous is a worm drive or ring and pinion. You mention ring and pinion so I will assume that is what you have. There might be a metal tag bolted towards the top of your case. If not remove the speedometer drive from the drive line. The ratio will be stamped on the drive gear. The "High Speed" gears used on the '30/'31 AA were also used on a WWII White Scout Car. These gears can sometimes be found at dealers of WWII truck parts.

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Chris Haynes
Posts: 2005
Joined: September 7th, 2003, 5:18 pm
Body Type: 82A
Model Year: 1930
Location: North Hills, CA/ Pine Grove, CA

Re: Differential substitutions for higher speed

Post by Chris Haynes » October 22nd, 2018, 11:15 am

There are two different gear ratios for the AA. How to easily find out which you have depends on whether yous is a worm drive or ring and pinion. You mention ring and pinion so I will assume that is what you have. There might be a metal tag bolted towards the top of your case. If not remove the speedometer drive from the drive line. The ratio will be stamped on the drive gear. The "High Speed" gears used on the '30/'31 AA were also used on a WWII White Scout Car. These gears can sometimes be found at dealers of WWII truck parts.

ModelAkid
Posts: 468
Joined: June 3rd, 2013, 6:15 pm
Body Type: 186-B stake
Model Year: 1931
Location: SoCal
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Re: Differential substitutions for higher speed

Post by ModelAkid » October 25th, 2018, 10:24 am

I have had a couple high speed '30-31 differentials. One had the ratio tag on one of the bolts near the bottom. Another just had the ratio stamped on the case.

vonheine
Posts: 43
Joined: September 30th, 2009, 8:02 pm
Body Type: none
Model Year: 1930
Location: Massachusetts

Re: Differential substitutions for higher speed

Post by vonheine » October 29th, 2018, 5:03 pm

It would take some research to find the right custom gear company to make brand new ring and pinion sets, but it's not impossible. As discussed, the Ford AA high speed ratio is 5:14 to 1 , (7 teeth on the pinion, 36 teeth on the ring gear). A custom gear manufacturer could copy the bearing diameters and other essential dimensions from a good Ford set, so that it fits properly in the case, but make a new ratio such as 3:85 to 1, (7 teeth on the pinion and 27 on the ring gear.) The truck would need a strong engine, more than 40 hp to pull properly at low speeds, but it would have a great top-end, probably hit 60 mph. A Stipe camshaft, a good distributor set-up, a downdraft carb, and a 6:1 head would probably do the trick to provide enough power to cover all the bases.

How many people would order a set to get the price down? Could the shop quote 12, 25, and 50 sets?

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emf
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Joined: March 21st, 2016, 5:21 am
Body Type: Firetruck
Model Year: 1928
Location: Hudson, NY

Re: Differential substitutions for higher speed

Post by emf » October 30th, 2018, 4:51 am

I would almost guarantee that custom gears would cost more than a Mitchell. Why re-invent the wheel. Since I put a Mitchell on my truck with high speed rear, I can easily do 45 to 50 on the highway.

Frank

Kaboom
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Joined: September 24th, 2018, 9:47 am
Body Type: Firetruck
Model Year: 1929

Re: Differential substitutions for higher speed

Post by Kaboom » November 7th, 2018, 8:11 pm

Emf can you expand more on the "Mitchell" I too have some Interest about my 29 3 speed w direct/under drive. I'm not sure which gear ratio in my work gear rearend.

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emf
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Model Year: 1928
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Re: Differential substitutions for higher speed

Post by emf » November 8th, 2018, 8:10 am

My truck is an early with 5:1 rear. I put a 36% Mitchell overdrive in and could not be happier. I can cruise at 45-50 all day long just jumping from 3rd to overdrive and back. The install was easy and all the parts fit as advertised. You just have to anticipate your stops a little more in advance... :wink:

Frank

Here is a link to the earlier discussion: viewtopic.php?f=1&t=5898

Kaboom
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Joined: September 24th, 2018, 9:47 am
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Re: Differential substitutions for higher speed

Post by Kaboom » November 8th, 2018, 6:28 pm

Thank you EMF

31 SWFL
Posts: 42
Joined: May 19th, 2009, 7:19 pm
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Model Year: 1931
Location: Bonita springs Fl

Re: Differential substitutions for higher speed

Post by 31 SWFL » November 11th, 2018, 6:05 pm

65mph@ 2600rpm.built engine,BRAKES,Mitchell Overdrive .ran 50/55 today 60miles one way with the ac on today it can be done.have around 2000miles so far and run that speed all the time.but the devil is in the details
Build it to make you happy NOT other peoples opinions.

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blgitn
Posts: 183
Joined: July 6th, 2013, 6:36 am
Body Type: 275-A
Model Year: 1931
Location: West Point, VA 23181

Re: Differential substitutions for higher speed

Post by blgitn » November 12th, 2018, 4:39 am

31SWFL,

You have my attention; how about some details?! Maybe some pictures?
R/ Roger.

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blgitn
Posts: 183
Joined: July 6th, 2013, 6:36 am
Body Type: 275-A
Model Year: 1931
Location: West Point, VA 23181

Re: Differential substitutions for higher speed

Post by blgitn » November 12th, 2018, 4:41 am

Beg pardon. Just saw them in the other thread!!
Very nice!
R/ Roger.

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Chris Haynes
Posts: 2005
Joined: September 7th, 2003, 5:18 pm
Body Type: 82A
Model Year: 1930
Location: North Hills, CA/ Pine Grove, CA

Re: Differential substitutions for higher speed

Post by Chris Haynes » November 12th, 2018, 12:32 pm

emf wrote:
October 30th, 2018, 4:51 am
I would almost guarantee that custom gears would cost more than a Mitchell. Why re-invent the wheel. Since I put a Mitchell on my truck with high speed rear, I can easily do 45 to 50 on the highway.

Frank
I can do 50 with high speed gearing alone.

31 SWFL
Posts: 42
Joined: May 19th, 2009, 7:19 pm
Body Type: 82 B ?
Model Year: 1931
Location: Bonita springs Fl

Re: Differential substitutions for higher speed

Post by 31 SWFL » November 12th, 2018, 4:21 pm

No doubt you can do 50 with5:14 and tires but why twist the engine that hard when you don’t have to.my truck with me in is very close to 3900#’s (38 and change depending on witch holiday :mrgreen: )and one other note i’m Not running a bushing u joint,i’m Running needle bearings and there is pictures of it already posted some time ago.my main reason for doing what I did was the majority of people said it can’t be done :lol: and I lost track of smiles per mile
Build it to make you happy NOT other peoples opinions.

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emf
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Model Year: 1928
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Re: Differential substitutions for higher speed

Post by emf » November 13th, 2018, 5:51 am

Chris Haynes wrote:
November 12th, 2018, 12:32 pm
emf wrote:
October 30th, 2018, 4:51 am
I would almost guarantee that custom gears would cost more than a Mitchell. Why re-invent the wheel. Since I put a Mitchell on my truck with high speed rear, I can easily do 45 to 50 on the highway.

Frank
I can do 50 with high speed gearing alone.
I guess when I mentioned the speeds I achieved, I implied that my engine was running at a speed that would keep it together. With standard engine components for the A, your engine must be running over 3000 rpm. Go for it...

Frank

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