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      01-05-2019, 09:10 PM   #1
EMagnuson
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How can you tell what diff you have.

One of the upgrades listed on my car when I purchased it was that it came with a Dinan 4.10 differential. Is there a way to test out if that is indeed installed without opening it? The person that sold the car showed a picture of the receipt for the differential as proof but seeing as he also owned an E46 M3 I have always questioned which car it was actually installed in. I appreciate any help on this one.

thanks,

E
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      01-05-2019, 09:30 PM   #2
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2006 BMW Z4M  [10.00]
2006 BMW Z4M  [8.50]
2003 BMW Z4 3.0i  [9.00]
Compare your rpm to a stock car in the same gear at the same road speed.
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      01-05-2019, 09:40 PM   #3
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Compare RPM

Quote:
Originally Posted by pokeybritches View Post
Compare your rpm to a stock car in the same gear at the same road speed.
Yeah....what he said.
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      01-06-2019, 05:45 AM   #4
EMagnuson
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Ok, that makes sense. Once I get the battery charged up or replaced I will post some gear/speed data.

thank you!

-E
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      01-11-2019, 03:34 PM   #5
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Pretty simple really, use this calculator and plug in your values to check your RPM

http://www.bokchoys.com/differential...calculator.htm
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      01-11-2019, 09:05 PM   #6
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Quote:
Originally Posted by ANILE8 View Post
Pretty simple really, use this calculator and plug in your values to check your RPM

http://www.bokchoys.com/differential...calculator.htm
This is pretty cool, just one question would you use the front or rear tire width and size on a staggered setup? seems like this does make some difference on the calculator when I change to my front or rears. I hope to get the car out tomorrow for a chance to test this out.

thanks,

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      01-11-2019, 09:34 PM   #7
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You use the rear tire specs in this calculator as they are the driven wheels.

The front tire measurement will be irrelevant for the purpose of this calculation.

Let us know how you go please.

I had 4.10 gears installed last year and this calculator worked out perfectly to double check everything.
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Last edited by ANILE8; 01-11-2019 at 11:55 PM..
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      01-12-2019, 03:45 PM   #8
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Quote:
Originally Posted by ANILE8 View Post
You use the rear tire specs in this calculator as they are the driven wheels.

The front tire measurement will be irrelevant for the purpose of this calculation.

Let us know how you go please.

I had 4.10 gears installed last year and this calculator worked out perfectly to double check everything.
Okay so assuming that I am doing this correctly I believe that I have the 4.1 gearing. Here are the results that I came up with;

Rear Tires are 275 35 18
1st gear redline was about 34 MPH
2nd gear Redline was around 57 MPH

as far as the last item of cruising speed of 70 MPH what gear should you be in for this I am assuming 5th?

In 3rd gear I was doing 4701 RPMs
In 4th gear I was doing 3809 RPMs
In 5th gear I was doing 3244 RPMs
I didnt check it in 6th

One last measurement that I took was that I was in 3rd gear doing 30 miles an hour at 2500 RPM.

ANILE8 thank you for the tool and please let me know if you concur on my findings.

-E
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      01-12-2019, 04:08 PM   #9
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The Cruising Speed is measured in 6th gear which is the easiest way to check.

The way I checked it was to set my cruise control on whatever speed (in this case it was 63mph) in 6th gear and verify speed with phone GPS app. At this speed, my RPM was exactly 3K which equates to 4.10 gearing according to this calculator.

You can redline in each gear if you choose but that is a much harder way of doing it.

From your 1st and 2nd gear redline pulls, yes it looks like you have true 4.10 gears.
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      01-12-2019, 04:13 PM   #10
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You can enter whatever cruising speed you want into the calculator to check.

I know the default is 70mph but you can just change this value to anything you want.

Just remember to always check this speed in your highest gear which in our case is 6th.
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      01-12-2019, 04:34 PM   #11
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you can calculate it easily from your data:

3244 in 5th gear. 5th gear has 1:1 ratio.
With the 4,10 diff your wheels are turning 3244/1/4,10=791,22rpm

Your tire circumference is 80,36", which means 788,45 revs/mile (1 mile is 63360")
So in 1 min you travel 791,22/788,45 mile, so in 1 hour thats (791,22/788,45)*60=60,21mph if you have the 1:4,10 diff

But you're writing you were doing 70.
So 60.21/70*4,10=3.53
So calculated you have a 1:3,54 diff. Thats a heck more closer to the stock diff. (its not even inbetween the stock diff and the 4.10 diff)
Variations in the calculation are your measured speed (did you use gps? or speedo), and tire wear (tyre wear can go up to 8mm or so, so 16mm in diameter)

4th gear is 1,23:1 and 3rd is 1,67:1 so you can now calculate it for those gears.
But I think somethings fishy here, your rpm spacing is not correct I believe? Are you sure your data is correspondant to the gears you give?
If you do the calculations with 4th and 3rd gear you get different values, which is, of course, impossible.
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Last edited by GuidoK; 01-12-2019 at 04:59 PM..
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      01-13-2019, 07:22 AM   #12
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Quote:
Originally Posted by GuidoK View Post
you can calculate it easily from your data:

3244 in 5th gear. 5th gear has 1:1 ratio.
With the 4,10 diff your wheels are turning 3244/1/4,10=791,22rpm

Your tire circumference is 80,36", which means 788,45 revs/mile (1 mile is 63360")
So in 1 min you travel 791,22/788,45 mile, so in 1 hour thats (791,22/788,45)*60=60,21mph if you have the 1:4,10 diff

But you're writing you were doing 70.
So 60.21/70*4,10=3.53
So calculated you have a 1:3,54 diff. Thats a heck more closer to the stock diff. (its not even inbetween the stock diff and the 4.10 diff)
Variations in the calculation are your measured speed (did you use gps? or speedo), and tire wear (tyre wear can go up to 8mm or so, so 16mm in diameter)

4th gear is 1,23:1 and 3rd is 1,67:1 so you can now calculate it for those gears.
But I think somethings fishy here, your rpm spacing is not correct I believe? Are you sure your data is correspondant to the gears you give?
If you do the calculations with 4th and 3rd gear you get different values, which is, of course, impossible.
I like the math behind this, I was driving and trying to give readings to my son in the passenger seat for him to write down. My rear tires are at EOL as they have 2/32nd left on them with plans to replace in the spring so it appears that would affect the calculations to some extent. I was using a bluetooth odbII reader for speed and for RPMs so no I was not using the GPS which could have an effect on the calcs as well. Sounds like I will have to take her out for another spin, always looking for an excuse!!
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      01-13-2019, 10:05 AM   #13
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Quote:
Originally Posted by EMagnuson View Post
I like the math behind this, I was driving and trying to give readings to my son in the passenger seat for him to write down. My rear tires are at EOL as they have 2/32nd left on them with plans to replace in the spring so it appears that would affect the calculations to some extent. I was using a bluetooth odbII reader for speed and for RPMs so no I was not using the GPS which could have an effect on the calcs as well. Sounds like I will have to take her out for another spin, always looking for an excuse!!
Another way involves zero driving. But a helper.
Jack up rear of car.
Place on stands. Chock front wheels
Mark DS with marker of choice.
Rotate wheel one revolution while counting rotations of DS. You will need to relate the last revolution of the DS to a percentage of a full revolution to determine accurately. However if you do indeed have a 4.10 final drive it will be obvious
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