Lean cruise tune for MPG.

[DJ19885.0]

I have quite a good tune right now for my 1988 mustang that is basically stock as of now with the usual bolt-ons. I have read that you can tune your VE table values when cruising and low load to be very lean for better mileage. Does this mean I can lean cruise out until it is basically stumbling and running rough, and then add just enough to keep it running well for best mileage? Will this hurt anything? I know that anytime you are under acceleration or moderate load, it needs be be reading "rich" on a NB o2. I am also in the process of converting to run E-85 by upping the fuel system. Can I use the same idea of running a very lean cruise after I convert to E-85?

Thanks!

PS: I would also like to thank everyone involved with megasquirt ie: producing it for so cheap, having awesome documentation, software, etc! I expected it to be way more involved assembling the board and tuning than it actually was! I am having a ton of fun messing with it, and having the laptop in the car is SO addicting, hah! I have managed to get a lot more noticeable power out of my mustang with only basic bolt-ons and cant wait for future mods!

[rocklizzard91]

i'd like to know how lean i can go too.
i drive a 16v VW, but im sure the idea is the same
i hate terrible mpg's
22 usually

[Matt Cramer]

Sure, under light throttle (I'd say 50 kPa or below), there's no harm to the engine if you back off the fueling and then add just enough to make it not stumble. I had a Honda that would run 18.0:1 or leaner that way.

[Bernard Fife]

DJ19885.0,

As Matt says, you can run lean at cruise without hurting anything. There's not enough heat in the combustion chamber to hurt anything at low loads. At high loads, a lean mixture is a potential disaster, but not at low loads.

However, the optimal amount of lean is as lean as possible, as long as you are not getting misfires (which throw all of that cylinder's fuel out the exhaust - wasting it completely). When you are driving, you will feel these mis-fires as 'surging' under steady throttle (like something is lightly pulling the car backwards for a second or so then letting go, then repeating).

There's more on tuning for economy here: http://www.megamanual.com/ms2/tune.htm#economy

Lance.

[DJ19885.0]

Wow, thanks for the quick responses! I'll see If I can beat my previous best of 26mpg highway with the stock computer. That is If I can ever keep my foot off the floor =).

[4T08]

My 450whp 408 windsor gets 25 MPG on the highway, cruising around 16-16.5:1. I'm looking forward to switching to megasquirt 3 in the next couple months to convert to sequential injection and COP on all 8 cylinders. I don't think it will pick up much more power but i'm hoping to get some more drivability and maybe with some more aggressive tuning, hit 30MPG on the highway

[rocklizzard91]

Sure, under light throttle (I'd say 50 kPa or below), there's no harm to the engine if you back off the fueling and then add just enough to make it not stumble. I had a Honda that would run 18.0:1 or leaner that way.

wow! really?! 18.0:1?!
i was thinking that over 16 was bad!
my tuner said anything over 17 is bad bad bad.
ive got a VW 16v which is slighly similar to some honda engines.
are all motors ok to go as lean as possible with no misfires?

i'll have to check out that link for fuel econ as well
i'll join the OP in saying thanks for info guys

[Matt Cramer]

How lean you can go under light throttle depends on the engine, but as a general rule for light throttle tuning, if it doesn't ping, it doesn't misfire, and it doesn't count as a pollution controlled vehicle, you're not running it too lean. I wouldn't try this above 50 kPa or so, and my Honda may have taken particularly well to lean mixtures (Supposedly the Civic VX goes even leaner on cruising, on the stock ECU!). It seems just how much you can get away with depends on the combustion chamber / piston top design.

[cygnus x-1]

How lean you can go under light throttle depends on the engine, but as a general rule for light throttle tuning, if it doesn't ping, it doesn't misfire, and it doesn't count as a pollution controlled vehicle, you're not running it too lean. I wouldn't try this above 50 kPa or so, and my Honda may have taken particularly well to lean mixtures (Supposedly the Civic VX goes even leaner on cruising, on the stock ECU!). It seems just how much you can get away with depends on the combustion chamber / piston top design.

It absolutely depends on the combustion chamber design and piston top shape. I have an old Honda (iron block!) with more dome style chambers that can barely do 16:1 before it starts to misfire. The modern 4 valve/cylinder pent-roof design (like the newer Civics) is far better in terms of combustion efficiency than older designs.


C|

[rocklizzard91]

How lean you can go under light throttle depends on the engine, but as a general rule for light throttle tuning, if it doesn't ping, it doesn't misfire, and it doesn't count as a pollution controlled vehicle, you're not running it too lean. I wouldn't try this above 50 kPa or so, and my Honda may have taken particularly well to lean mixtures (Supposedly the Civic VX goes even leaner on cruising, on the stock ECU!). It seems just how much you can get away with depends on the combustion chamber / piston top design.

ok cool i will have to play with that.
is there a general rule to identify ping?
you said misfire is where it feels like pushing/pulling.
i want to make sure i know what ping feels/sounds like before i mess around too much.

i will listen to the "dont try above 50kpa" but just wondering....what happends if i had it lean above that?
im still very new to this tuning concept, so it'd be nice to know.

[4T08]

The higher the load when you go lean, the more likely you are to get detonation, and the more likely that detonation will cause damage.

At lower loads, there is very little cylinder pressure, so even if you DO get detonation, it's pretty much harmless.