Author Topic: m42 with 2.93lsd? (Read 2997 times)

cyborgben · « **on:** April 28, 2012, 08:01:51 PM »

i was wondering how incredibly slow this would be... but also how good would the mpg be? maybe just a chip. i pulled a 27K mile 2.93lsd diff today and it just got me thinking...

bmwman91 · « **Reply #1 on:** April 30, 2012, 02:11:39 AM »

It's a little hard to say. Yeah, the car would be sloooooow with that rear end. I am also skeptical that the gas mileage would improve much, either. The cars were optimized for maximum MPG in 5th gear at ~55mph. Given that we all go 65-70mph on the highway, it stands to reason that a 3.47:1 rear end would put the engine RPM in the intended sweet spot at those speeds. If you did 65mph in 4th gear, this 2.93 rear end would end up in the ballpark of where you would want to be in terms of RPM at 65-70mph.

Lower RPM doesn't mean better MPG in all cases. There is an optimal RPM range where internal friction and heat loss through cylinder walls is balanced. Any faster and internal friction eats more power. Any slower and the combustion cycle takes long enough to lose too much energy as heat through the cylinder walls. ~2600RPM seems to be that "sweet spot" on the M42 where MPG is best.

And of course, the aerodynamic losses, which become very significant above 65mph, will remain the same.

Personally, I would look for something between 3.5:1 and 3.9:1 if you want to boost MPG and not have the car be a total slug.

cyborgben · « **Reply #2 on:** April 30, 2012, 01:35:01 PM »

thanks for the detailed thoughts. I guess I will pass on the 2.93 to someone with a high hp turbo 325 who could use it... i will need the money, since i just pulled the valve cover to see cam sprockets with HORRIBLY POINTY TEETH urrrrrgh! will this ever get on the road?

follow up thought - if the timing chain is tight and pretty quiet (assuming then that guides are still intact and tensioner is working well,) how long do i have before the pointy cam sprockets become a problem? could i make a month until summer really starts and i have some time? how big is the risk?

bmwman91 · « **Reply #3 on:** April 30, 2012, 04:37:25 PM »

As long as the tensioner piston & rail are in good shape, you will be largely OK driving it with pointy cam sprockets. Just don't redline it or anything. A couple of months should be no problem at all.