The Ford Barn

Bick in New Zealand · 10-13-2017, 10:10 PM

I have the diff out pf my coupe at present trying to diagnose why the rear end seized up. I thought while it is out I would go through everything to make sure I don't have to take it out again for a while. The teeth on both c/w and pinion look quite worn, and I am planning on replacing them. I have the option of 3:78 or 4:11. I am running a Mitchell transmission and 26% overdrive, what are the pros and cons of the 4:11? It presently has a 3:78.

Mike Peters · 10-13-2017, 10:15 PM

Question: If you have been using the 3.78 with a 26% Mitchel, how was it working for you? Curious, why did the differential seize up?

TerryH · 10-13-2017, 10:29 PM

Unless you are in very hilly or mountainous country, I can't imagine why you would want the 4.11 gears with the Mitchell; I don't what you are gaining. The 26% Mitchell paired with the stock 3.78 rear end is ideal for most applications.

daveymc29 · 10-13-2017, 10:30 PM

I have a 3:54 with the Mitchell tranny and 26% O/D. IF I HAD IT TO DO OVER I WOULD HAVE NOT GOTTEN THE HIGHER GEARS IN FIRST AND SECOND IN THE TRANNY, THE REST OF IT WORKS VERY WELL FOR ME. I ALSO HAVE A COUPE WITH THE 33% BW O/D AND THE STOCK TRANNY AND 3;78 REAR. WHAT WORKS WELL ALSO. IF I HAD MY DRUTHERS, I'D GO WITH 33% O/D AND 4:11 REAREND. THAT WOULD WORK BETTER FOR ME WITH THE MITCHELL TRANNY I HAVE.

Synchro909 · 10-14-2017, 01:06 AM

3.78/4.11? A big factor in that decision lies in your motor. Assuming it is not stock, what has been done to it? Another factor is the body weight - what body is on the car? Neither of these things have been mentioned.
I run a 26% Mitchell with a 3.78 rear end on all 3 of my Model As. Phaeton, Tudor and CCPU. They run well and cruise effortlessly at 50 mph, even towing a camper weighing close to a tonne. Unless you are dragging a heavy body up hill and down dale, I would recommend the 3.78 ratio.

Bick in New Zealand · 10-14-2017, 01:34 AM

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The 3:78 was fine, it just seemed that from what I had seen that quite a few were running 4:11s with their overdrive.

It would appear that the locking up problem occurred in the overdrive. I haven't taken it apart yet, but everything else has checked out ok.

Jim Brierley · 10-14-2017, 12:57 PM

I find the 4.11 works best with the 26% OD, gets better gas mileage, pulls the hills better, etc. You end up with a 2.96:1 over-all ratio, plenty tall for cruising! Colombia rear ends were recommended to run the 4.11 R&P back in the day, they had a 26% OD ratio.

Dave in MN · 10-14-2017, 01:05 PM

I also have a 4.11 behind my 26% Mitchell. Cruising at 60 has my engine at it's sweet spot. With the 4:11 gears the car can pull pretty good grades without having to shift down. I have a touring engine for what its worth to the comment...maybe a bit tired though as it has over 90,000 miles on it.
Good Day!

quickchange · 10-14-2017, 03:52 PM

Justin whats the make of the O/D ? I run 3.78 in the delivery with the Michell O/D its stock apart from B carb & cast iron higher comp head, Never a problem , that's why it has a gear box, However if running a warmed up engine then go for the 3.54, besides they are cheaper, Have in stock , Cheers Derek

Bick in New Zealand · 10-15-2017, 02:37 AM

Justin whats the make of the O/D ?

It is one of Roger Andersons, made locally here in the Bay.

Bick in New Zealand · 10-15-2017, 02:41 AM

Just started to take the overdrive apart, and we discovered that the front bearing had seized. This was a sealed self lubricating one that had done probably less than 700 miles.

old31 · 10-15-2017, 08:37 AM

Quote:

Originally Posted by Jim Brierley

I find the 4.11 works best with the 26% OD, gets better gas mileage, pulls the hills better, etc. You end up with a 2.96:1 over-all ratio, plenty tall for cruising! Colombia rear ends were recommended to run the 4.11 R&P back in the day, they had a 26% OD ratio.

Jim, please help me with the math or provide a calculator. How do you end up with 2.96 with a 4.11 and 26%. I tried every way I could think of and the math does not come out right.

Jim Brierley · 10-15-2017, 11:30 AM

Old 31, I was going from memory, so you know what that does! It's actually 3.05. 4.11x.74=3.05 (74+26=100%)

Bick in New Zealand · 10-15-2017, 09:47 PM

It's actually 3.05

Jim, what does this figure actually mean?

Cheers

Tom Wesenberg · 10-15-2017, 09:55 PM

When in high gear and overdrive, the engine will turn 3.05 revolutions for each revolution of the rear tires.

updraught · 10-15-2017, 11:35 PM

So, by my calculation(?):

4.11 + 36% is 2.63
3.78 + 26% is 2.79 ratio
4.11 + 26% is 3.05 (which is equiv to 3.78 + 19.5%)

3.78+13.5% is 3.27
3.78 + 6 % is 3.54

3.78 is 3.78
3.78 - 9% is 4.11

Jim Brierley · 10-16-2017, 11:03 AM

Tom is correct. Looking at updraught's figures, 2.63 or even 2.79 ratios are way too high for the Model A engine to pull, and will likely damage the engine, IMO.

Synchro909 · 10-16-2017, 04:30 PM

Quote:

Originally Posted by Jim Brierley

Tom is correct. Looking at updraught's figures, 2.63 or even 2.79 ratios are way too high for the Model A engine to pull, and will likely damage the engine, IMO.

I hold Jim's advice in high regard and I do not want to start an argument but I calculate that 2.79 is the figure I get with a 26% OD and a 3.78 diff ratio. There have been times when I have forgotten to take the car out of O/D when I stopped on our outback trips. There was very little fuss taking off again with this ratio while pulling a camper trailer weighing about a tonne. (one tonne is 2,200 pounds) Sure, the higher ratio is noticeable but not hard to manage.
BTW, with this setup, we cruise at 50 mph and return up to 21 mpg, in the old measure.
All of that said, the engine is not standard. I wholeheartedly agree with Jim if the motor is standard.

Bick in New Zealand · 10-17-2017, 02:03 AM

Some great discussion guys, thank you for the input.
We pressed the shaft out today and here is what it looked like. According to the supplier the bearings were high quality, SKF needle bearings.

Jim Brierley · 10-17-2017, 10:32 AM

I used to drive my widebed to Las Vegas every year on the Winfield Weekend, with the Colombia and 4.11's. At first I had 32" rear tires, it ran just fine. Later I changed to 28" rear tires and got better gas mileage, ran cooler and pulled the hills easier. There some long steep hills on the I-15, and even in May, the weather is quite hot.

10-13-2017, 10:10 PM	#1
Bick in New Zealand Senior Member Join Date: May 2010 Location: Napier New Zealand Posts: 931	Differential ratio I have the diff out pf my coupe at present trying to diagnose why the rear end seized up. I thought while it is out I would go through everything to make sure I don't have to take it out again for a while. The teeth on both c/w and pinion look quite worn, and I am planning on replacing them. I have the option of 3:78 or 4:11. I am running a Mitchell transmission and 26% overdrive, what are the pros and cons of the 4:11? It presently has a 3:78. __________________ 1928 Roadster (CA67) 1931 Deluxe Coupe RHD 1931 Victoria RHD Model A's don't leak oil they just mark their territory.

10-13-2017, 10:15 PM	#2
Mike Peters Senior Member Join Date: Aug 2012 Location: South East Wisconsin Posts: 1,279	Re: Differential ratio Question: If you have been using the 3.78 with a 26% Mitchel, how was it working for you? Curious, why did the differential seize up?

10-13-2017, 10:29 PM	#3
TerryH Senior Member Join Date: May 2010 Location: Fountain Valley, Calif. Posts: 937	Re: Differential ratio Unless you are in very hilly or mountainous country, I can't imagine why you would want the 4.11 gears with the Mitchell; I don't what you are gaining. The 26% Mitchell paired with the stock 3.78 rear end is ideal for most applications.

10-13-2017, 10:30 PM	#4
daveymc29 Senior Member Join Date: Sep 2010 Location: Danville, CA Posts: 1,554	Re: Differential ratio I have a 3:54 with the Mitchell tranny and 26% O/D. IF I HAD IT TO DO OVER I WOULD HAVE NOT GOTTEN THE HIGHER GEARS IN FIRST AND SECOND IN THE TRANNY, THE REST OF IT WORKS VERY WELL FOR ME. I ALSO HAVE A COUPE WITH THE 33% BW O/D AND THE STOCK TRANNY AND 3;78 REAR. WHAT WORKS WELL ALSO. IF I HAD MY DRUTHERS, I'D GO WITH 33% O/D AND 4:11 REAREND. THAT WOULD WORK BETTER FOR ME WITH THE MITCHELL TRANNY I HAVE.

10-14-2017, 01:06 AM	#5
Synchro909 Senior Member Join Date: Jun 2014 Location: Melbourne, Australia Posts: 7,496	Re: Differential ratio 3.78/4.11? A big factor in that decision lies in your motor. Assuming it is not stock, what has been done to it? Another factor is the body weight - what body is on the car? Neither of these things have been mentioned. I run a 26% Mitchell with a 3.78 rear end on all 3 of my Model As. Phaeton, Tudor and CCPU. They run well and cruise effortlessly at 50 mph, even towing a camper weighing close to a tonne. Unless you are dragging a heavy body up hill and down dale, I would recommend the 3.78 ratio. __________________ I'm part of the only ever generation with an analogue childhood and a digital adulthood.

10-14-2017, 01:34 AM	#6
Bick in New Zealand Senior Member Join Date: May 2010 Location: Napier New Zealand Posts: 931	Re: Differential ratio Sponsored Links (Register now to hide all advertisements) The 3:78 was fine, it just seemed that from what I had seen that quite a few were running 4:11s with their overdrive. It would appear that the locking up problem occurred in the overdrive. I haven't taken it apart yet, but everything else has checked out ok. __________________ 1928 Roadster (CA67) 1931 Deluxe Coupe RHD 1931 Victoria RHD Model A's don't leak oil they just mark their territory.

10-14-2017, 12:57 PM	#7
Jim Brierley Senior Member Join Date: Jun 2010 Location: Temecula, CA Posts: 4,088	Re: Differential ratio I find the 4.11 works best with the 26% OD, gets better gas mileage, pulls the hills better, etc. You end up with a 2.96:1 over-all ratio, plenty tall for cruising! Colombia rear ends were recommended to run the 4.11 R&P back in the day, they had a 26% OD ratio.

10-14-2017, 01:05 PM	#8
Dave in MN Senior Member Join Date: May 2010 Location: Jordan, MN Posts: 1,411	Re: Differential ratio I also have a 4.11 behind my 26% Mitchell. Cruising at 60 has my engine at it's sweet spot. With the 4:11 gears the car can pull pretty good grades without having to shift down. I have a touring engine for what its worth to the comment...maybe a bit tired though as it has over 90,000 miles on it. Good Day! Last edited by Dave in MN; 10-17-2017 at 10:47 AM.

10-14-2017, 03:52 PM	#9
quickchange Senior Member Join Date: May 2010 Location: Nelson. New Zealand Posts: 2,009	Re: Differential ratio Justin whats the make of the O/D ? I run 3.78 in the delivery with the Michell O/D its stock apart from B carb & cast iron higher comp head, Never a problem , that's why it has a gear box, However if running a warmed up engine then go for the 3.54, besides they are cheaper, Have in stock , Cheers Derek

10-15-2017, 02:37 AM	#10
Bick in New Zealand Senior Member Join Date: May 2010 Location: Napier New Zealand Posts: 931	Re: Differential ratio Justin whats the make of the O/D ? It is one of Roger Andersons, made locally here in the Bay. __________________ 1928 Roadster (CA67) 1931 Deluxe Coupe RHD 1931 Victoria RHD Model A's don't leak oil they just mark their territory.

10-15-2017, 02:41 AM	#11
Bick in New Zealand Senior Member Join Date: May 2010 Location: Napier New Zealand Posts: 931	Re: Differential ratio Just started to take the overdrive apart, and we discovered that the front bearing had seized. This was a sealed self lubricating one that had done probably less than 700 miles. __________________ 1928 Roadster (CA67) 1931 Deluxe Coupe RHD 1931 Victoria RHD Model A's don't leak oil they just mark their territory.

10-15-2017, 11:30 AM	#13
Jim Brierley Senior Member Join Date: Jun 2010 Location: Temecula, CA Posts: 4,088	Re: Differential ratio Old 31, I was going from memory, so you know what that does! It's actually 3.05. 4.11x.74=3.05 (74+26=100%)

10-15-2017, 09:47 PM	#14
Bick in New Zealand Senior Member Join Date: May 2010 Location: Napier New Zealand Posts: 931	Re: Differential ratio It's actually 3.05 Jim, what does this figure actually mean? Cheers __________________ 1928 Roadster (CA67) 1931 Deluxe Coupe RHD 1931 Victoria RHD Model A's don't leak oil they just mark their territory.

10-15-2017, 09:55 PM	#15
Tom Wesenberg Senior Member Join Date: May 2010 Location: Mpls, MN Posts: 27,582	Re: Differential ratio When in high gear and overdrive, the engine will turn 3.05 revolutions for each revolution of the rear tires.

10-15-2017, 11:35 PM	#16
updraught Senior Member Join Date: Feb 2015 Location: Australia Posts: 1,970	Re: Differential ratio So, by my calculation(?): 4.11 + 36% is 2.63 3.78 + 26% is 2.79 ratio 4.11 + 26% is 3.05 (which is equiv to 3.78 + 19.5%) 3.78+13.5% is 3.27 3.78 + 6 % is 3.54 3.78 is 3.78 3.78 - 9% is 4.11 Last edited by updraught; 10-16-2017 at 06:36 PM. Reason: adjusted 3.27 percentage

10-16-2017, 11:03 AM	#17
Jim Brierley Senior Member Join Date: Jun 2010 Location: Temecula, CA Posts: 4,088	Re: Differential ratio Tom is correct. Looking at updraught's figures, 2.63 or even 2.79 ratios are way too high for the Model A engine to pull, and will likely damage the engine, IMO.

10-17-2017, 10:32 AM	#20
Jim Brierley Senior Member Join Date: Jun 2010 Location: Temecula, CA Posts: 4,088	Re: Differential ratio I used to drive my widebed to Las Vegas every year on the Winfield Weekend, with the Colombia and 4.11's. At first I had 32" rear tires, it ran just fine. Later I changed to 28" rear tires and got better gas mileage, ran cooler and pulled the hills easier. There some long steep hills on the I-15, and even in May, the weather is quite hot.