Anything we can do for mileage?

[silverm3bmw]

I am getting a 4 banger for the mileage.  I want to tweak the heck out of it for just that, mileage!  I will be working with the 94 M42 or 96 M44.  I know this is a M42 club, but I am sure you are all well versed with all the 4 bangers.

Going with the M42 twin cam engine as a base, what can I do, if anything, to make sure it gets max mileage?  Drop compression? MAP a lean burn into the computer, maybe with a APEX I ?  

The obvious, keeping my foot out of it does not count as advice, thanks tho. :p

[doitover]

Put as much air in the tires as you can, check the alignment, make sure all the air control stuff is in place, (the pan under the nose, the front spoiler), COP, clean air filter. Drop all the weight you can (delete the mechanical fan, power steering, use a little Odyssey battery). Get the highest (numerically lowest) rear gears you can.

Other than making sure the car is in good tune including alignment and air pressure, I'd be surprised if you can change the mileage appreciably.

[DesktopDave]

I'll bet you could get an easy 20% better mileage without a new map.  I'd assume that the M42 is set up to flow air, so many of the lean burn techniques won't work as well as they might.  Driving like a grandmother is a great start.  Like doitover said, cut weight & raise your diff ratio along with some mods.  I have a 2.93 LSD unit from an e30 325e if you'd like it, works great.

I'd also retain the stock air box, filter and the throttle body heater.  Definitely do the coil-on-plug (COP) conversion and the Mustang injectors too.  Make sure you have no vac leaks and the ICV/CPS/AFM & temp sensors are correct.

Most importantly, keep really good records to see what works and what doesn't.

Other mods I'd suspect would be helpful:
New O2 sensor, maybe a WB unit.
High grade synthetic lubricants with low viscosity, say 0w30 motor oil; ATF in the gear box and 75w90 in the diff.
Avoid any fuel with alcohol, and use the highest octane you can get (I gained ~2mpg with 93 octane for some reason, not sure why).
Mix the coolant with as much water (>66%) as possible to cut pump cavitation & drag and improve heat transfer, then maybe block some of the radiator opening to improve aerodynamics.
Install an 88degC thermostat & fanstat to run the motor a bit warmer.
Install cruise control
Go with tall & thin low rolling resistance (LRR) tires
Make sure the alignment is spot on & tires are well inflated.
Delete the power steering pump
Delete the A/C
Do the fan delete mod
Install a higher capacity battery and keep it charged to cut alternator drag
Pre-heat the intake air from the engine compartment.
Install a full under-body fairing to cut drag especially under the motor and the spare tire/rear lip
New wheel bearings with synthetic grease

[rac3r]

I do know that dropping compression negatively affects engine efficiency. Higher compression ratios are better.

I stripped the interior out of my 318iS, deleted extraneous seats, deleted A/C, deleted power steering, deleted fan, lowered with Eibach Pro-Kit springs and I only have marginally better gas mileage - maybe 1-2 mpg on the highway. I was a bit disappointed. I'm guess it's because aerodynamically, it is a brick and the gearing is relatively short.

Airing the tires up 8 psi above the factory spec on my E36 M3 seems to help mileage by several mpg on the highway, but the gains are offset by abnormal tire wear.

[DesktopDave]

Yeah, it's tough to get gains from the M42.  It's a good compromise for efficiency and power considering it's size right from the factory.  That's why a good chip does so much for the car.  Even great tweaks can only increase mileage by 2-3%.  A difference that small is hard to track with so many other variables as well.

My three best mods for mileage were a 3.73 diff, cruise control, and avoiding high alcohol gas.  As far as driving technique...2nd gear easy starts, cruise control set to 72mph max and drafting semis (@200ft or so) worked well.  Everything else I've done has been only minimally effective.  I had higher hopes for the lubricants I selected, but they're really only better from a maintenance standpoint.

Aerodynamically, the car could definitely use some help.  I'm thinking that a frontal under-body pan and side skirts would help, as would blocking off some of the frontal area.  I don't want to do anything too dramatic, so I'd suspect gains would be modest.

Having said that, I'll be messing with fuel injectors and a WBO2 soon.

The e30 looks difficult to tune aerodynamically; it's boxy and already has a small frontal area.  I've noted a strong vortex pattern on the center of the trunk on my sedan, I'll bet a roof spoiler with strakes would help out a bit.  You'd need a wind tunnel to balance drag against lift effectively.

Kamming the tail with a reverse spoiler or a lower diffuser might help too but it'd look odd.  IMHO the e36 stylists did a great job with that - the rear window is tapered very nicely, even on the sedan, and the rear panel has a good dramatic cut-off.  One more good reason to buy an e36 M3, eh?