Author Topic: 3.91 lsd (Read 4576 times)

dbgrubbs · « **on:** February 04, 2009, 11:17:04 AM »

I just picked up a 3.91 lsd out of an '84 318i. I'm planning on installing it when gas goes back up over $3 gal. Just wondering if anyone else had used this on their 318is? Looking for impressions on acceleration and gas mileage compared to the 4.10. I know it won't be anything earth shattering.

Thanks

tjts1 · « **Reply #1 on:** February 05, 2009, 03:53:51 PM »

I'm using a 3.64 from an 84 or 85 318i. No problems here after 15k miles. You will have to swap the speedometer sensor from your existing diff to the new one. The plug on the back of the sensor changed.

dbgrubbs · « **Reply #2 on:** February 07, 2009, 09:51:21 AM »

A 3.64? That had to have really hurt your acceleration. I wanted something to give me slightly better mileage but wouldn't hurt my acceleration too much. I've been searching the salvage yards for several years and only recently came across the 3.91lsd. Pretty rare.

tjts1 · « **Reply #3 on:** February 07, 2009, 10:53:34 AM »

Quote from: dbgrubbs;66054

A 3.64? That had to have really hurt your acceleration. I wanted something to give me slightly better mileage but wouldn't hurt my acceleration too much.

Not really.
http://www.m42club.com/forums/showpost.php?p=64009&postcount=7
The taller differential allows me to get to 60mph in second and complete the quarter in 3rd as oopposed to 1 extra shift with the stock 4.1.

OKTAY · « **Reply #4 on:** February 07, 2009, 11:43:02 AM »

Wow,
7,8 sec to 60mph
13,15 to 80mph and all with 3,64 differential? Very impressive and your 135whp is impressive as well.
I wonder what the results would be with 4,10 though? Or with 3,91? I personally think 3,91 is the best after some modifications but, once again your numbers are very interesting... Well done tjts, no wonder m42's are still that popular small gems...

tjts1 · « **Reply #5 on:** February 07, 2009, 01:15:13 PM »

In my opinion, 3.73lsd would be ideal diff with a the stock 6500rpm redline. 3.91lsd would work best with a 7000 or 7200rpm redline. My 3.64 is an open diff but I've been keeping an eye out for a 3.73 lsd or a 3.91 lsd. With the chip, I can run the engine up to 7200rpm and with gas being cheap, I would be happy to swap either ratio as soon as I find it in the JY.
cheers
Justin

xwill112x · « **Reply #6 on:** February 07, 2009, 08:46:38 PM »

arent you scared of blowing it up by holding it at such high rpms?

tjts1 · « **Reply #7 on:** February 07, 2009, 09:33:35 PM »

I don't hold it at that RPM. That's just peak for a split second before shifting if I was driving balls out with nothing to lose. The other 99% of the time I'm just a normal driver like everybody else. The maximum continuous engine speed according to BMW is 6300rpm.
http://www.motortraders.net.au/groups/group.asp?group=1&menu=89

xwill112x · « **Reply #8 on:** February 07, 2009, 10:05:06 PM »

Quote from: tjts1;66098

I don't hold it at that RPM. That's just peak for a split second before shifting if I was driving balls out with nothing to lose. The other 99% of the time I'm just a normal driver like everybody else. The maximum continuous engine speed according to BMW is 6300rpm.
http://www.motortraders.net.au/groups/group.asp?group=1&menu=89

someone told me that they bent a rocker twisting it up, thats why im holding back on a chip..
would it hold together???

tjts1 · « **Reply #9 on:** February 07, 2009, 10:47:59 PM »

It couldn't have been an M42 there aren't any rocker arms in this engine. There is a lot of good info on the M42 in the link I posted. This engine was built to live at high RPM. If your timing chain and tensioner are in good shape, you don't have anything to worry about. Plus swapping differentials to something taller than the stock 4.1 will only reduce RPMs for the majority of your driving. An aftermarket chip (even the fleabay one I have) will improve throttle response an power at all RPMs. You don't have to revs the nuts off of it see the benefits.
I don't rev the engine into the stratosphere every day but I do borderline abuse every chance I get. It has 160k miles and its running like a top. I fully expect it to last at least until 200k miles or when I get bored with it. Just feed it a steady diet of synthetic motor oil, keep the vacuum leaks in check and it will last a very long time. This engine was overbuilt from the factory.

xwill112x · « **Reply #10 on:** February 08, 2009, 10:04:03 AM »

well that helps me alot on my decision..

ive always been weary of redlining my engine, scared it will explode.
but my chain, cam sprockets, and tensioner all look brand new (the tensioner is new- less than 3k miles on it)

and basicly, your saying my engine wont blow...even with the tits reved off of it intill 250k?