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Old 09-22-2018, 10:25 AM   #1
amarsh48
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Default 4:11 Rear end & No overdrive???

at 24 I have just purchased my first Model A 1930 Tudor and live in one of the hilliest parts of Pittsburgh Pa. I was having rear end trouble and need to rebuild the differential. I was told by a local Model A enthusiast that I should run a 4:11 rear end to compensate for the hills and any compression loss the engine may have. I know the motor does have some blowby, but is 4:11 a little overkill for this car? Unfortunately because of where I live in order to go just about anywhere I need to take a 4 lane bridge (usually 1/4-3/4 of a mile) and the speed limits on those bridges are usually 50 mph!!?? Can a 4:11 keep up for the short duration?

Thanks in advance?
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Old 09-22-2018, 10:48 AM   #2
Jim Brierley
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Default Re: 4:11 Rear end & No overdrive???

I would stay with the 3.78:1 which is probably what is in your tudor now. 4.11's will limit your cruising speed and the tudor is light enough that it should pull the hills OK as is.
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Old 09-22-2018, 10:49 AM   #3
Tom Wesenberg
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Default Re: 4:11 Rear end & No overdrive???

I wouldn't go to 4.11 unless you also add an overdrive.
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Old 09-22-2018, 11:57 AM   #4
heneste
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Default Re: 4:11 Rear end & No overdrive???

I drive a stock station wagon that was equipped with the 4.11 drive and no modifications to the driveline (no overdrive etc.). It was ok on the hills but the engine was really working under load. I would tend to go with one of the other gears such as 3.54 or 3.78 so you still have hill climbing ability but better top end drive speed with lower rpms.
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Old 09-22-2018, 12:51 PM   #5
Charlie Stephens
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Default Re: 4:11 Rear end & No overdrive???

I have a 3.54 rear end in my '31 RDPU and is comfortable to drive at 55 thus making your bridge crossing requirement. It will go faster if I want but due to aerodynamics I like 55. I don't think you would like the 4.11 gears, too low. Try finding a a few more local automotive enthusiasts and asking them. Maybe join a club?

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Old 09-22-2018, 02:25 PM   #6
MAG
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Default Re: 4:11 Rear end & No overdrive???

I have a 30 Town Sedan with a 4:11 and 27% OD It pulls hills fine out of OD but on straight and level it really needs the OD when speed is around 40. Engine has had some extra work inside.
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Old 09-22-2018, 04:41 PM   #7
Synchro909
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Default Re: 4:11 Rear end & No overdrive???

If you are concerned now about the condition of the engine, it is going to need attention a long way before the new diff does. The lower ratio will also shorten what life is left in the engine. I think the best solution would be rebuild the engine, put in either the standard 3.78 ratio or 3.54 with an O/D. That takes dollars and if you don't have them at the moment, maybe you should consider maybe another rear end (good for parts later) or another way of delaying the spending of money on the rear end now. That's just my opinion and I'm sure some will differ, some will agree.
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Old 09-22-2018, 09:31 PM   #8
Pete
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Default Re: 4:11 Rear end & No overdrive???

With NO overdrive, stick with the 3.78's.
Down the road, if you plan on an overdrive, 4:11 is the better choice.
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Old 09-22-2018, 09:55 PM   #9
Richard in Anaheim CA
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Default Re: 4:11 Rear end & No overdrive???

Well, it seems nobody else is going to mention a high compression head so I will.

A 5.5 Snyders head will most likely solve the hill climbing problem and crossing the bridge problem without changing the rear end ratio which would be an enormous cost compared with changing the head.

To answer the first rebuttal before it takes place,no it will not wipe out your babbit or cause other catastrophic damage unless your engine is on its last legs already. 5.5 compression is really not "high compression even in the time period of the Model A Ford
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Old 09-23-2018, 11:21 AM   #10
Jim Brierley
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Default Re: 4:11 Rear end & No overdrive???

Richard is right about compression not hurting babbitt. I ran a 7.4:1 Cyclone head for years with no problems.
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