E30 325i Diff / Gear Ratios

[nisaacso]

Hi All,

I'm looking to upgrade my diff for better acceleration. I've heard of e30 M3 guys using a 4.45, but has anyone tried a 4.56 or 4.75. I've got the typical 4.10 diff from and M3 but I'm looking to build one with more aggressive gears. With the 4.10 I'm primarily using only 3rd and 4th gear, and occasionally 5th on tracks with very long straights.

I've run the gear calculators and the 4.75 would give me a top speed of 120 MPH. It would also give me 3 usable gears on the short tracks instead of just 3rd and 4th. I'm very tempted to go with this gear but curious if there are any major drawbacks. Someone commented to me that 5th gear (.81 overdrive) is a weak gear. But I figure that it would still be rarely used, and the really low ratio would compensate for this.

Any advice or shared experience with this would be greatly appreciated.

Thanks
- Nate

[DaveITB1]

Nate,

Depending on the track, we use either the 3.9, 4.10, 4.25, or 4.45. The 4.45 makes 5th very usable, and also makes 3rd and 4th really pull on the hill sections. My calculations put the 4.45 in 5th and 7000RPM at 130mph, running a 14inch Hoosier.

Where are you sourcing the 4.75 from?

Dave

[nisaacso]

Dave,

Great information, thanks for the reply. Do you use the stock LSD lockup, variable, or fixed in the 40-60% range?

The 4.75, well thats something I just made up. Actually, this is supposed to be motorsport gear that is expensive and may not be readily available. I've seen this gear referenced in a couple different places. The first is some old info from Korman (www.kormanfastbmw.com), the second was a forum post from Dan at diffsonline.com.

Here's the quote from Dan regarding exotic gears:

http://forums.bimmerforums.com/forum/archi...p/t-129952.html

how crazy do you want to go?
its about $1000 MORE for a motorsport ring and pinion setup, but i can get them...here is a list, and i responded to your pm
4.18 188
4.27 188
4.36 188
4.45 188
4.56 188
4.67 188
4.75 188
4.88 188
5.00 188
5.13 188
5.28 188
5.44 188

[MIKEGTR]

Isn't 4.45 a bit extreme?

i mean will the car still pull with the same "acceleration" as before... wouldn't a 4.45 take you out of the powerband?

Mike

[DaveITB1]

Dave,

Great information, thanks for the reply. Do you use the stock LSD lockup, variable, or fixed in the 40-60% range?

The 4.75, well thats something I just made up. Actually, this is supposed to be motorsport gear that is expensive and may not be readily available. I've seen this gear referenced in a couple different places. The first is some old info from Korman (www.kormanfastbmw.com), the second was a forum post from Dan at diffsonline.com.

[/b]

We've been running a welded rearend for the past 5 years No problems (except pushing it around the paddock). We upped the rear springs from 500#'s to 700#'s and it took the push out of the front end, keeping the a** end planted.

I've only heard of custom-made gears that high for our cars, but my calculations have the car's speed falling off dramatically above 4.50's. Your shifter arm would be doing a lot of work in the lower gears. But then again, I'm only running a 1.8L 4 banger with 14inch rims, so you 6cyl guys with 15 or 16 inch rims may have more umph to play with.

[nisaacso]

To aggressive??? Well, it depends. From the homework that I've done the consensus is that the 4.27 is the all around gear, good for most tracks. This would be an easy, safe choice.

For me though, 4 of the 5 tracks I race have top speeds of only 110 MPH. This makes the 4.45 very doable. I would still use 3rd and 4th for the majority of the course, and 5th for the 1 or 2 longer straights. the other track, Road America, has speeds in the 120 - 125 MPH range. This would put me right at the rev limiter.

There's only one problem. The 4.45 is a "BMW Motorsport" gear and could cost a minimum of $900, if one is available. This could be the reason most guys end up with a 4.27. I do know of an M3 guy who is running a 4.45 locally, he gives it a thumbs up. Of course, the M3 has an extra 1000 RPM to work with.

I think DaveITB1 might be right, above 4.45 may be counter productive. I think it will make 4th to short and force a shift on the short straights, just in time to down shift for the turn. I've dropped that idea.

[DaveITB1]

Nate,

If you find yourself leaning back toward the 4.45 gear, left me know. I think I can find one at a better price than $900.

I also have a buddy with a junk yard with two crashed 1st gen M3's in it. Those cars has the close-ratio 5 speed in it, where 5th it 1:1. If you are interested in that, i could get a price on that too.

Dave

[MIKEGTR]

To aggressive??? Well, it depends. From the homework that I've done the consensus is that the 4.27 is the all around gear, good for most tracks. This would be an easy, safe choice.

For me though, 4 of the 5 tracks I race have top speeds of only 110 MPH. This makes the 4.45 very doable. I would still use 3rd and 4th for the majority of the course, and 5th for the 1 or 2 longer straights. the other track, Road America, has speeds in the 120 - 125 MPH range. This would put me right at the rev limiter.

There's only one problem. The 4.45 is a "BMW Motorsport" gear and could cost a minimum of $900, if one is available. This could be the reason most guys end up with a 4.27. I do know of an M3 guy who is running a 4.45 locally, he gives it a thumbs up. Of course, the M3 has an extra 1000 RPM to work with.

I think DaveITB1 might be right, above 4.45 may be counter productive. I think it will make 4th to short and force a shift on the short straights, just in time to down shift for the turn. I've dropped that idea.
[/b]

Yeah but an M3 working with a 4.45 is not the same as a 325 working with a 4.45... the s14 will rev much higher than the 325 anyday anytime... (sadly).... on the other hand... after running with a 4.10 i know that a bit lower gearing might do the trick... where, then, can you find a 4.27 lsd for our cars??? that would be great to find?

Mike

[nisaacso]

You can't find a 4.27 in the e30 medium 188 case, unless it came from another racer. You have to make one. The 4.27 (medium 188 size) gear was available in some 735(e32) and 535(e34) automatics. The rebuilders like diffsonline.com and metric mechanics can set you up for around $2000, yikes! If you find the stuff yourself and locate a local builder you could cut this price in half or more.

- Nate

[MIKEGTR]

If i find a 4.27 in a e34 wouldn't i just unbolt the diff from the e34 and slap it on my e30 or is the housing machined differently for the e34 (i.e. different mounting point/axle bolt diameter)???

thx,

Mike

[nisaacso]

The e34 case is identical to the e36 case. The mounting is significantly different.

I think the e30, e23, e24, and e28 had identical cases but the U.S. versions didn't have any of the gears we desire.

[MIKEGTR]

this 4.27 we speak of... the one from an e34. I presume that it is an LSD unit? much like the 4.10 automatic diff for late model 325i/iS e30's???

if so then finding one might not be that hard down here in PR there are a couple of junkers with e34's... odds of them knowing what diff is inside and how much its worth are pretty slim so you could get a good deal!

Mike

[nisaacso]

Mike,

All you need is the Ring and Pinion. They will transfer to an e30 LSD unit. I don't know if the LSD from the e34 case will transfer.

The rear cover should have a tag on one of the bolts near the speedometer sensor indicating the gear ratio in the diff. Check ebay , you'll see photo's of these tags quite often.

- Nate

[MIKEGTR]

ok thx for the info...

much appreciated,

Mike