40 Coupe Diff Ratio Question

4

47Merc

Senior Member
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?
 

Attachments

  • 40 coupe September 2017 009.jpg
    40 coupe September 2017 009.jpg
    58.8 KB · Views: 87
  • 40 coupe September 2017 006.jpg
    40 coupe September 2017 006.jpg
    45.9 KB · Views: 94
  • 40 coupe September 2017 002.jpg
    40 coupe September 2017 002.jpg
    63.4 KB · Views: 459
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
 
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.
 
37 Cab said:
Your gear shifter appears to be way rearward from stock... How does that work?
Click to expand...

The gear stick will come through the floor at the rear of the removable floor panel and therefore will be a couple of inches in front of of the stock seat in its most forward position. This wont worry me as I drive with the seat right back.
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.
 
You expected the driveshaft to turn 3-3/4 turns or so. that's because you are losing 1/2 of the motion through the differential. It is not easy to turn both rear wheels 1 rev, unless you have an assistant.

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.
 
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 :cool:
 
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
 
Mart said:
You expected the driveshaft to turn 3-3/4 turns or so. that's because you are losing 1/2 of the motion through the differential. It is not easy to turn both rear wheels 1 rev, unless you have an assistant.

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.
Click to expand...

Of course.. You are right on that one Mart.. Guess I am just too used to working with everything overhead on a lift.. Spoiled in my old age..
 
Someone may correct me , do you need the forward diff brace when converting to open drive, or is that with parallel springs???
 
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.
 
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.
 
JM 35 Sedan said:
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.
Click to expand...

Ring gear teeth / Pinion teeth gear sets for early V8:
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.
 
You must log in or register to reply here.
Back
Top