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Old 04-19-2017, 05:46 AM   #1
ciocc
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Lightbulb m20b20 diff swap

Hello everyone, first time posting here, so please correct me if i'm posting in the wrong sub-forum.

My car is an '89 320i with an m20b20 engine. The diff has been humming for quite a while and i decided it is time for a change. My current diff is an open 4.10 and driving at about 100 km/h i get around 3k rpms which is kind of high imo. I was thinking about a 3.91 swap. Is it worth swapping it? Would i get significantly lower rpms at higher speeds? I mainly do city driving but would like to go on longer trips in summer
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Old 04-19-2017, 07:26 AM   #2
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you may like the 3.25 ratio differential. In my e30 at 135kph I am turning 3000rpm. makes the car a 4 speed with a true overdrive feel, much like an old school car
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Old 04-19-2017, 09:07 AM   #3
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you may like the 3.25 ratio differential. In my e30 at 135kph I am turning 3000rpm. makes the car a 4 speed with a true overdrive feel, much like an old school car
Won't the acceleration suffer? It's already kind of slow when starting to move
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Old 04-19-2017, 09:53 AM   #4
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With you having a b20, I would suggest go ahead with the 3.91. You won't lose a lot of acceleration, but you will have better fuel economy and only slightly lower RPM at speed.

Use this calculator to see you various speed/RPM differentials when swapping gears...

http://www.csgnetwork.com/rearendgearcalc.html
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Old 04-20-2017, 08:19 AM   #5
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ForcedFirebird;4771898 I was thinking about a 3.91 swap in my son 1986 325es stick shift want do you think.

http://www.csgnetwork.com/rearendgearcalc.html[/QUOTE]
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Old 04-20-2017, 09:51 AM   #6
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The 86es came with a 2.93, with a 5500rpm red line, it's not going to be fun. You would be better off with a 3.25 for more acceleration, 3.73 is the max I would go.
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Old 04-20-2017, 10:47 AM   #7
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The 86es came with a 2.93, with a 5500rpm red line, it's not going to be fun. You would be better off with a 3.25 for more acceleration, 3.73 is the max I would go.
Thank you for taking your time to answer.
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Old 04-20-2017, 12:27 PM   #8
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Originally Posted by ForcedFirebird View Post
The 86es came with a 2.93, with a 5500rpm red line, it's not going to be fun. You would be better off with a 3.25 for more acceleration, 3.73 is the max I would go.
I'm confused, you're saying that a lower gear ratio (3.25) is better for acceleration?

Higher top speed or lower cruising rpm's = lower gear ratio
Faster acceleration, lower top end with higher rpms = higher gear ratio.

There several different online calculators that can be utilized to find estimated rpm at different speeds. The goods ones will require the input of tire size as well.

OP: I have an excel sheet that can be used if you'd like and it will estimate your RPM across all gears and speeds with tire and gear ratio input.
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Old 04-20-2017, 01:25 PM   #9
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I'm confused, you're saying that a lower gear ratio (3.25) is better for acceleration?

Higher top speed or lower cruising rpm's = lower gear ratio
Faster acceleration, lower top end with higher rpms = higher gear ratio.

There several different online calculators that can be utilized to find estimated rpm at different speeds. The goods ones will require the input of tire size as well.

OP: I have an excel sheet that can be used if you'd like and it will estimate your RPM across all gears and speeds with tire and gear ratio input.
I think he might be comparing the 3.25 to the 2.93 that originally comes with the car.
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Old 04-20-2017, 02:02 PM   #10
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130 mph top speed or lower cruising rpm's = lower gear ratio
The diff's not the top speed limiter in a stock E30 until you get to a 4.10...

t
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Old 04-20-2017, 02:34 PM   #11
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The diff's not the top speed limiter in a stock E30 until you get to a 4.10...

t
Not really much different than the 3.73 and 4.10, both are aero-limited lol.

I have had both to terminal velocity at Daytona 5th gear flat out, these bricks can't do much over 135mph (on the GPS, 150 indicated) with a stock-ish m20's. The 4.10 will just get you there quicker, and you only lose about 500rpm on shifts.
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Old 04-20-2017, 02:40 PM   #12
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With you having a b20, I would suggest go ahead with the 3.91. You won't lose a lot of acceleration, but you will have better fuel economy and only slightly lower RPM at speed.

Use this calculator to see you various speed/RPM differentials when swapping gears...

http://www.csgnetwork.com/rearendgearcalc.html
Quote:
Originally Posted by sgtskid View Post
I'm confused, you're saying that a lower gear ratio (3.25) is better for acceleration?

Higher top speed or lower cruising rpm's = lower gear ratio
Faster acceleration, lower top end with higher rpms = higher gear ratio.

There several different online calculators that can be utilized to find estimated rpm at different speeds. The goods ones will require the input of tire size as well.

OP: I have an excel sheet that can be used if you'd like and it will estimate your RPM across all gears and speeds with tire and gear ratio input.
Check out the link I posted earlier, or do you have one you made for the getrag already?

I am saying that a 3.25 will accelerate quicker than a 2.93. You have it backwards, the lower the number, the higher the speed since it's a ratio to 1.

Engine spins 3.25 times for one wheel rotation vs 2.93 (in 4th gear).
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Old 04-20-2017, 04:01 PM   #13
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the 3.25 acceleration is not THAT slow. people talk up and down like your car is going to be a slug but its not. If you suck at shifting, or if your car has a super heavy flywheel for delayed shift times (cough cough m42) then you may be about the same to 60 since you dont have an extra up-shift. Taking second gear to 70mph is alot of fun and having 4th gear to buzz along at 60mph is a nice kind of speed limiter.

My m42 gutted sedan with the 3.25lsd is faster than a bone stock e30 325is. m42 power!

You cant have an all in one diff. I love my tall gears - and with my prosthetic leg the versatility works out very nicely. The sweet spot for me is my 3.46 diff that I am building and will be installing soon
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Old 04-21-2017, 05:57 AM   #14
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Originally Posted by youngmineeh View Post
I think he might be comparing the 3.25 to the 2.93 that originally comes with the car.
Read it again. That makes sense.

Quote:
Originally Posted by ForcedFirebird View Post
Check out the link I posted earlier, or do you have one you made for the getrag already?

I am saying that a 3.25 will accelerate quicker than a 2.93. You have it backwards, the lower the number, the higher the speed since it's a ratio to 1.

Engine spins 3.25 times for one wheel rotation vs 2.93 (in 4th gear).

The sheet I have is actually set up for a T-56 but the gear ratios can be changed easily. It's a basic sheet but I created it so I could see everything at once. The sheet works two ways: 1. input speed, diff ratio, tire size, and desired gear and it'll output the estimated RPM for that gear and speed. 2. using previous input information and setting a rev limit will allow the user to see at what speed, in each gear, the engine would red line.

https://drive.google.com/file/d/0B2e...ew?usp=sharing

Let's see if the above link works.
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Last edited by sgtskid; 04-21-2017 at 06:07 AM.
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Old 04-22-2017, 01:57 AM   #15
Madhatter
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Originally Posted by 5Toes View Post
the 3.25 acceleration is not THAT slow. people talk up and down like your car is going to be a slug but its not. If you suck at shifting, or if your car has a super heavy flywheel for delayed shift times (cough cough m42) then you may be about the same to 60 since you dont have an extra up-shift. Taking second gear to 70mph is alot of fun and having 4th gear to buzz along at 60mph is a nice kind of speed limiter.

My m42 gutted sedan with the 3.25lsd is faster than a bone stock e30 325is. m42 power!

You cant have an all in one diff. I love my tall gears - and with my prosthetic leg the versatility works out very nicely. The sweet spot for me is my 3.46 diff that I am building and will be installing soon
Driven a 2L M20 before? there is a reason why they came from the factory with 4.10 and 4.27 rear ends. Think closer to M40 power and torque kind of figures.

You won't notice the difference swapping to a 3.91, its barely going to do anything for your revs. So just put in another stock 320i diff and call it a day.
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