**3.91 Diff Swap: Everything you need to know!!!**

Originally Posted by Shinobi

Dude, read his original post. He gave you the link to all the info there, and its a link that gets posted in alot of the differential threads here. I'll even give it to you here:

http://www.bokchoys.com/differential

Never saw that website. But thanks bro. Its all a learning process.



and I learnt something: To not mess with my 3:46 differential

Originally Posted by Shinobi

Yikes...good thing you're not supercharged. You wouldn't be in 1st long at all. I did think about going with a lower ratio, but because I autox, I want to still be in 2nd over 60mph because courses are *usually* designed to be ~60 max. From what I hear, STI and EVO Guys hate having to shift to 3rd during their runs. I'd be curious what impact it has on your 1/4 mile times. You wouldn't happen to have a G-Tech Pro or something would ya?

I don't have a G-Tech Pro, but the next time Gainesville Raceway has an open track night, I'll take the car out there. I think they're closed until the end of January for winter and Gatornationals.

Originally Posted by mek206

Sweet thread. Hey Kpeng, I have a 328i. What kind of differential do i have? and what choices do i have if i wanted to go for something better?

As Albert said, bokchoys.com/differential will give you all the info you need.

Originally Posted by BMWPower06

3.91 seems WAY too low Ken, i mean im doin almost 70 in second gear right now. I mean if your happy with it thats all that counts Id def go 3.64 at most.

Keep in mind I have an auto, it's different for me.

Originally Posted by ari00

i don't get it. didn't he say he already took it for a drive and was at 3k rpm's at 90? even if it's 3200, that's not too bad.

and the E36 M3 you referenced...was that a manual? if so, the numbers Ken's using seem accurate...he's got a step tranny. i have a 6sp manual with a 3.46 and i'm running about 3200rpm's at 80mph.

Good job Ken on taking this project on. Wish i could do a 3.64 on my car but i do lots of highway driving and it would kill my mileage.

As Ashley said, I was cruising at 3K RPM at 90 before the swap. I've yet to find out what I do now. I will take it on the highway tomorrow. I drove around town tonight, and i managed better than I thought earlier. At 50 mph, I was only at 2000 RPM.

Originally Posted by shady3one3

^an e36 m3 w/3.91 and same tires cruises at 92 in 5th gear @ 4750rpm, i dont think Ken will see 90mph @ 3 grand.

the E36 M3 has a different transmission gearing. can't compare.

Originally Posted by ari00

Yeah, the revs will definitely take a hit on the highway in that case. Either way, it's cool that he took the initiative to be the guinea pig on this one.

in his first post he said he is not the first to do this. he's not even the first on this site to do it. but he is the first on this site to do the write-up.

Originally Posted by kpeng

As Ashley said, I was cruising at 3K RPM at 90 before the swap. I've yet to find out what I do now. I will take it on the highway tomorrow. I drove around town tonight, and i managed better than I thought earlier. At 50 mph, I was only at 2000 RPM.

you'll do 14.5% more revs.

nice write-up.

if the diff cover bolt pattern is the same, you might want to consider putting the finned diff cover on and using some redline fluid. even the 3.64 diff that the auto ZHPs use have the finned diff cover. not to mention it looks very high performance sitting back there. that small diff is going to get hot.

Originally Posted by Alex323Ci

if the diff cover bolt pattern is the same, you might want to consider putting the finned diff cover on and using some redline fluid. even the 3.64 diff that the auto ZHPs use have the finned diff cover. not to mention it looks very high performance sitting back there. that small diff is going to get hot.

Hey Alex,

I actually considered doing the finned diff cover, but my funds are limited at this point. The next time I need to change diff fluid, I'll do it. It's definitely a good idea to do it. I'm running Redline 75w90 diff. oil in it right now

Originally Posted by Alex323Ci

the E36 M3 has a different transmission gearing. can't compare.

in his first post he said he is not the first to do this. he's not even the first on this site to do it. but he is the first on this site to do the write-up.


you'll do 14.5% more revs.

nice write-up.

if the diff cover bolt pattern is the same, you might want to consider putting the finned diff cover on and using some redline fluid. even the 3.64 diff that the auto ZHPs use have the finned diff cover. not to mention it looks very high performance sitting back there. that small diff is going to get hot.

I don't think the cover is absolutely necessary, seeing as it runs fine in the x3, but it does look pretty cool. I wish I was 6spd

Originally Posted by Activ3

I don't think the cover is absolutely necessary, seeing as it runs fine in the x3, but it does look pretty cool. I wish I was 6spd

It wouldn't hurt though, I just don't have the cash to do it right now.

And David, just get the 3.91 anyways, haha.

Originally Posted by Alex323Ci

the E36 M3 has a different transmission gearing. can't compare.

the math is spot-on compared to reality with my car, it won't be too far off for his

Alrighty then.

Glad to see everything got worked out!

Originally Posted by ALPIN3

Glad to see everything got worked out!

Hey Chris, thanks a bunch for your help!

Nice job Ken..

Just took the car out for another spin. Was cruising 75 mph at 2900 RPM.

Originally Posted by kpeng

Just took the car out for another spin. Was cruising 75 mph at 2900 RPM.

Thats not bad at all...mine is pretty much right at the same spot around 75mph. Its nice having the extra revs to work off of when you need to pass (or just have some fun). How fast have you taken it since the install? I think our top speed may be back around the stock top speed (128 or so) but I haven't gone that fast since installing mine. I need to go to a track and test it out one of these days.

Yeah, I'll take it to the track whenever they open up again. I've only taken it up to about 80. I'll have a true test coming up when I make my 320 mile trek down to South Florida.

Nice job. good to know there's other Diffs readily available for swaps.

Originally Posted by sycE46

Nice job. good to know there's other Diffs readily available for swaps.

Should add that info to your website

Your site has helped me a million times, haha. Great stuff.

Originally Posted by kpeng

Should add that info to your website

Your site has helped me a million times, haha. Great stuff.

Yah, I'm thinking about it. I haven't updated my website in ages. Can I use your pictures? I'll let people know its from you.

Nice job Ken...keep the mods comin'!!! Now if only there was a solution for Xi's!

Originally Posted by sycE46

Yah, I'm thinking about it. I haven't updated my website in ages. Can I use your pictures? I'll let people know its from you.

you need to add the 3.64 for the auto 330 ZHP too

Originally Posted by Alex323Ci

you need to add the 3.64 for the auto 330 ZHP too

Yah, I gotta do that to. When I first wrote this up, the ZHP only came in Stick.