What engine swap for improved fuel economy?

[Andy.B]

I currently have a 2.7i in my car, running a BimmerHeads 272 cam, a 93 octane tune, and 3.73 rear end. I get about 20-22 mpg. It's not much lower than I got with the stock m20b27/3.25 rear end I was previously running.

I moved a few months ago, and my daily commute increased to about 35 miles each way. I love my car, but between the pretty lousy economy, the extra cost of 93 octane, and the not terribly comfortable ride on coilovers, I've started to get a bit tired of some aspects of it, and begun considering alternatives. That said, I don't want to ditch my e30, so I'm considering making changes so it can meet my needs.

The suspension is easy enough... I'll probably sell off the coilovers and swap in OEM style coil springs (h&r or similar) this summer.

The fuel economy is a harder fix. My fiance has an m30b35 swap that actually does about the same, if not better, than my car. That probably comes down to a combination of the stock tune, and gentler driving. But it's got me thinking...

What swaps out there could help me gain fuel economy? I don't want to go backwards in terms of power, so I'm not going to swap in a stock m10, but I don't want or need v8 power either. Being a newer and more efficient engine, does an m50 swap improve fuel economy? Are there specific variants I should look for? Is this an impossible goal?

[Swanny]

I'd say m50tu

[lukeADE335i]

M51 diesel - cheap engine, plenty of torque & good diesel economy!

[Andy.B]

Starting to seriously consider the m50tu. Thanks swanny. Trying to make sense of ews/obd/etc variables to make sure I know what I'd be getting into.

[Kershaw]

stock m20b27, 325e status, 30mpg. m42b18, 30mpg. m10b18, 30mpg. m21b24, 38mpg.

m51 diesel is probably a good option, but good luck finding that in freedomland.

Also, try to calculate the amount of money you'll spend to improve your gas mileage against how much you'll actually save. The easiest, most cost effective option, might be just going back to a stock tune and doing nothing else.

[dnguyen1963]

stock m20b27, 325e status, 30mpg. m42b18, 30mpg. m10b18, 30mpg. m21b24, 38mpg.

m51 diesel is probably a good option, but good luck finding that in freedomland.

Also, try to calculate the amount of money you'll spend to improve your gas mileage against how much you'll actually save. The easiest, most cost effective option, might be just going back to a stock tune and doing nothing else.

Yeah, similar argument for spending $70k on an Audi diesel to get a few mpg improvement.

[chadthestampede]

http://cleveland.craigslist.org/ptd/5402531757.html

That being said I want your coilovers when you get rid of them.

[McGyver]

http://cleveland.craigslist.org/ptd/5402531757.html

That being said I want your coilovers when you get rid of them.

Some info about that M51, looks to be an M51D25 OL. https://en.wikipedia.org/wiki/BMW_M51

[Kershaw]

Hahah, that's exactly what I'm talking about. Spending $3500 to bump your gas mileage up. That's crazy. It would take years to make that return back.

12,000 miles / 22mpg = 545 gallons of gas * $1.90 = $1036

12,000 miles / 35mpg = 342 gallons of diesel * $2.15 = $737

1036-737= $299.

You'd spend $3500 to save $299 a year. It would take you nearly 12 years to split even. And that's not even considering the extra cost associated with finding parts for a non standard engine.

Honestly, your MOST ECONOMICALLY VIABLE OPTION is to find someone with a 318 that wants a power upgrade and swap engines with them.

Or, just suck it up and realize you'd probably spend $300 on your e30 to get 40 more horsepower anyway. I know I would. Hell, I'd do it twice if I could.

[TobyB]

M42. The inital buy- is will be very cheap,
so the extra few mpg will actually pay for it.

Never said it would be FUN, tho, did I?

The M42's in mine get just about 30 mpg in mixed commuting.

...since no- one else said it...

t

[Andy.B]

I don't want to go backwards in terms of power, and I know that is hypocritical.
As for cost, well, I have a well equipped garage. I've parted out cars before and managed to come out ahead. I did my fiance's m30 swap for under 1k out of pocket after all was said and done ( $750 of that was the custom driveshaft and mounts, neither of which are needed for the m50). I fully understand that it makes no sense to spend 3k to save a few dollars on fuel... But I I don't plan to spend 3k. I should be able to get a decent amount just by parting out my current engine. If I sell off my current engine goodies, find a donor e36, strip off what I need, sell off some more parts, and scrap the shell, well, I can probably break even. Not to mention that my current bottom end is over 240k miles and increasing fast. It's going to need some attention sooner or later, so I might as well start looking at options now.

I don't want to make a huge hit in power, so an m10 is out. Rolling back from the 2.7i to a fresh m20b27 would be nice, but doesn't really seem worth the money/effort. An m42 is interesting, but if it's the same amount (or possibly more) work than an m50 doesn't make much sense either.

[Andy.B]

I never got close to 30 mpg, even with the stock m20b27. Granted, the car had over 200k when I got it, so that's probably part of it, but I guess the point is the same. It's not necessarily a choice of doing nothing, or spending money to "save" on gas. SOMETHING has to be done, because at 240k, this engine is not going to get any better.

[chadthestampede]

A 2.93 diff would probably be the most economical way to gain a few mpgs

[e30jake]

A 2.93 diff would probably be the most economical way to gain a few mpgs

This is by far the best option. Or a 2.79 if you can find one from an early eta. When I did my 2.7i (stock cam, stock i ecu) I got 30-34 mpg depending on speed on the highway with the stock 2.93. Once I switched to a 3.46 it dropped into the mid 20s.

[Kershaw]

I never got close to 30 mpg, even with the stock m20b27. Granted, the car had over 200k when I got it

Something was wrong with your motor. My 325es consistently got 29-31mpg. 230k miles.