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10-13-2017, 10:10 PM | #1 |
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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.
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10-13-2017, 10:15 PM | #2 |
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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?
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10-13-2017, 10:29 PM | #3 |
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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.
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10-13-2017, 10:30 PM | #4 |
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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.
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10-14-2017, 01:06 AM | #5 |
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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.
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10-14-2017, 01:34 AM | #6 |
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Re: Differential ratio
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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.
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10-14-2017, 12:57 PM | #7 |
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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.
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10-14-2017, 01:05 PM | #8 |
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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 |
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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
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10-15-2017, 02:37 AM | #10 |
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Re: Differential ratio
Justin whats the make of the O/D ?
It is one of Roger Andersons, made locally here in the Bay.
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10-15-2017, 02:41 AM | #11 |
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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.
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10-15-2017, 08:37 AM | #12 | |
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Re: Differential ratio
Quote:
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10-15-2017, 11:30 AM | #13 |
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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%)
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10-15-2017, 09:47 PM | #14 |
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Re: Differential ratio
It's actually 3.05
Jim, what does this figure actually mean? Cheers
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10-15-2017, 09:55 PM | #15 |
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Re: Differential ratio
When in high gear and overdrive, the engine will turn 3.05 revolutions for each revolution of the rear tires.
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10-15-2017, 11:35 PM | #16 |
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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 |
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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.
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10-16-2017, 04:30 PM | #18 | |
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Re: Differential ratio
Quote:
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.
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10-17-2017, 02:03 AM | #19 |
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Re: Differential ratio
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.
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10-17-2017, 10:32 AM | #20 |
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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.
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