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09-03-2017, 06:45 AM | #1 |
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40 Coupe Diff Ratio Question
I have just pulled the old engine in my 40 coupe and are going to install a 59A merc motor with a 5 speed Toyota Supra gearbox so have had to change the drive shaft to an open type.
It appears to have the original diff fitted and I have fitted a Speedway drive shaft conversion kit and am trying to work out what the diff ratio is. With the old 3 speed it would run quite well at around 60 mph on the open road and freeways but was starting to get a bit excited at 65 - 70 so I thought it would have about a 3.75 to 1 ratio. When I jack up 1 back wheel and turn the drive shaft the wheel turns 1 rev plus about 5 degrees for every 2 turns of the drive shaft or to put it another way for every 1 rev of the wheel the drive shaft turns 1 and 7/8 revs. I expected the drive shaft to turn 3 3/4 to 4 turns for every 1 turn of the wheel so I was wrong. can anyone tell me what they think the ratio is or how to calculate it? |
09-03-2017, 07:03 AM | #2 |
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Re: 40 Coupe Diff Ratio Question
You need to jack up both wheels and turn them both 1 rev and count the driveshaft turns. At least that is the way I have always done it. Should get roughly 3 3/4 turns on the shaft for a 3.78 gear. Just over 4 turns for a 4.10
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09-03-2017, 07:05 AM | #3 |
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Re: 40 Coupe Diff Ratio Question
Your gear shifter appears to be way rearward from stock... How does that work?
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09-03-2017, 07:59 AM | #4 |
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Re: 40 Coupe Diff Ratio Question
With only one wheel jacked up the differential will half the ratio. Do 2 turn of the wheel and you will see 3-3/4 turns of the drive shaft. This will mean that you have the fairly common 3.78 gear ratio.
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09-03-2017, 08:56 AM | #5 | |
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Re: 40 Coupe Diff Ratio Question
Quote:
I may just straighten the stick out or turn it 90 degrees and face the bend forward. I will have to see how it is when it all goes back together. Worst case scenario is to install buckets but I'd rather not. |
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09-03-2017, 09:06 AM | #6 |
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Re: 40 Coupe Diff Ratio Question
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Easiest method, is what you have already done, except turn the one wheel TWICE. This compensates for the other wheel turning zero times. The pinion will then turn (in your case) just over 3-3/4 times, as you have a 3.78 axle. You turned it once and got 1-7/8 turns. Double that (for two turns) and you will get the 3.78 number you want. On Youtube search "Early Ford axle ratio check" and you will see and hear me winging my way through the technique. Mart. |
09-03-2017, 02:21 PM | #7 |
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Re: 40 Coupe Diff Ratio Question
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09-03-2017, 02:57 PM | #8 |
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Re: 40 Coupe Diff Ratio Question
You can also remove the diff oil plug and count the number of teeth.
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09-03-2017, 03:03 PM | #9 |
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Re: 40 Coupe Diff Ratio Question
47 Merc... where did you get the rear trailing arms & shock kit to do the open driveshaft swap.. I am considering doing so on my 47 to put in an automatic trans.. thanks
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09-03-2017, 03:57 PM | #10 |
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Re: 40 Coupe Diff Ratio Question
If you have the original rear end the ratio will be stamped on the bottom flange. Like maybe 34 and 9 divide the 34 by 9 or whatever the no. are and that will give you the ratio. Al
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09-03-2017, 04:55 PM | #11 |
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Re: 40 Coupe Diff Ratio Question
Could put a seat in the trunk and have someone shift it from there.
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09-03-2017, 08:45 PM | #12 | |
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Re: 40 Coupe Diff Ratio Question
Quote:
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09-03-2017, 11:29 PM | #13 |
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Re: 40 Coupe Diff Ratio Question
Someone may correct me , do you need the forward diff brace when converting to open drive, or is that with parallel springs???
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09-04-2017, 09:15 AM | #14 |
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Re: 40 Coupe Diff Ratio Question
Not sure what a "forward diff brace" is, but you need something to keep the axle from rotating. When the torque tube is removed some type of additional radius rod(s) must be added to provide that function.
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09-04-2017, 10:08 AM | #15 |
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Re: 40 Coupe Diff Ratio Question
If you do a search you will find posts that explain the best procedure for checking rear gear ratio, and IMO that is by removing the rear plug in the center section of the banjo housing, and counting the number of teeth on the ring gear.
Sometimes going by the numbers stamped on the botton rib of banjo housing can be false info if gears were changed by a PO, and the housing numbers were not changed.
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09-04-2017, 03:37 PM | #16 |
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Re: 40 Coupe Diff Ratio Question
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09-04-2017, 04:42 PM | #17 |
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Re: 40 Coupe Diff Ratio Question
Dual springs will solve the need for a torque arm and twin radius rods but it might ride kind of stiff without the right kind of springs. The Hot Rod works has a kit that is illustrated in their on line web site. It's not as strong as I would like it but it would likely be OK for normal use. Here is a link. http://www.hotrodworks.com/product/1...orque-arm-kit/
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09-05-2017, 02:32 PM | #18 | |
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Re: 40 Coupe Diff Ratio Question
Quote:
40/9 = 4.44 39/11 = 3.54 (Common) 39/12 = 3.25 (Uncommon, but available from Vanpelt) 37/9 = 4.11 34/9 = 3.78 So, there is some ambiguity if you count 39 teeth, but 99% chance it is 39/11. |
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