m20b20 diff swap

[ciocc]

It'd have to be REALLY bad, with smoke and stuff...

t

Oh, it's not that bad, it just hums rather loud. Thank you for the answer

[TobyB]

It'd have to be REALLY bad, with smoke and stuff...

t

[ciocc]

Thank you everyone for your answers, I have decided to stick with the stock 4.10. Also I have another question - can a bad diff increase fuel consumption?

[ThatOneEuroE30]

My 320i had a 3.91 when i bought it. I wouldnt go any lower. That fucker was SLOW

[Madhatter]

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.

[sgtskid]

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.

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/0B2egNF1Z1NQqWmIySWZpSEFFR28/view?usp=sharing

Let's see if the above link works.

[5Toes]

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 :)

[ForcedFirebird]

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

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).

[ForcedFirebird]

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.

[TobyB]

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

[youngmineeh]

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.

[sgtskid]

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.

[franan]

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.

[ForcedFirebird]

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.

[franan]

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]