A/F ratios effecting MPG

[Mach1Owner]

I'm very good at car ECU tuning but never through about how A/F ratio effects MPG. I know that with a better Intake/leaning it out more/adding timing will increase power and improve MPG.

I never thought about how to use my tuning to focus on just MPG before. Normally I tune for 91 or 93 oct and power, with MPG gains being a nice side effect. I'd like to tune my Nissan to be tuned for 87 or 89 Oct to make the best power I can make on it while also making best MPG.

Does anyone know how I can take MAF and A/F ratio readings and work that into MPG estimates?


Thanks.

 

[CurtisB]

A while back I was reading on MAF sensor tuning and I happened upon this article. http://www.fuelsaver-mpg.com/tuning-for-mileage. I'm not sure how you could estimate your MPG from a tune, being that gearing affects MPG and that would be pretty tough to factor in. I think you're just going to have to take scan readings while the vehicle operates and the rest is a little bit of trial and error. Good luck.

 

[Cujet]

First, you will need a wideband sensor. You really can't rely on the information provided by a conventional O2 narrowband sensor when operating extremely lean.

Lean operation can provide better BSFC , under most conditions. (Brake Specific Fuel Consumption) (often noted as Lb's fuel per HP per Hour) (or Grams fuel per KW per Hour)

Second, you will need to take a guess at the "optimal" Fuel/Air ratio for best BSFC, for your engine and the conditions involved. (from past experience, most engines will tolerate a A/F ratio of 17/1 without lean misfire, under modest load conditions. Some will easily tolerate 18/1 and the very best engines 22/1 ) Remember, engines that can run smoothly at 22/1 are designed to do so.

Also, note that such lean mixtures burn very slowly. (yes, I know this goes against what we've been told, but we are dealing in facts here, not hearsay) Note: A normal mixture of 14.64 to 1, under high load conditions is the MOST likely mixture to detonate. So, under high loads a normal mixture is RICH. Leaning it from that point is heading towards stoic and detonation results. This is INCORRECTLY called lean. When in fact the engine is likely still RICH OF STOIC.

So, to effectively operate an engine leaner than stoic, you must open the throttle more (for a given HP) and you may need to advance the timing significantly.

Interestingly, with throttle by wire vehicles, the additional throttle plate opening can be done without the driver noticing, and feeling like the pedal is "down to the floor" and the engine is barely making power.

 

[Clevy]

To get max hp my tuner enriches the a/f to 11.5 and 12 to 1. Stoic is 14.7/1.
Lean mixtures burn a whole lot hotter and they burn less fuel however they aren't ideal for max power.
A common o2 sensor is a narrow band,you need a wideband to get any form of accuracy.
Exactly what kind of software are you using as far as tuning and are you just reading a data log or are you live tuning on a dyno.
I tunes my mustangs with the sct pro racers package I got for the mother of all sweet deals.
Are you saying your tuning with just a handheld. If so what brand. Although they aren't really what I'd call tuning they will modify the tune enough to get into trouble with pre ignition.

 

[Cujet]

Originally Posted By: Clevy
... Lean mixtures burn a whole lot hotter...


Not true in the case of 17/1 operation. In fact, cylinder head temps will go down with such lean operations. More air, less fuel, much slower combustion.

In fact, detonation is very unlikely using such lean mixtures.

What you may be thinking about is the possibility of a poorly tuned "lean" engine to turn exhaust pipes red hot. This is due to improper timing advance and the slow combustion of a lean mixture, combined (possibly) with high load operations and high RPM.

I neglected to add in my earlier post that fuel economy gains of 25% have been possible on some vehicles when ultra lean operations are utilized, via proper design. That design would include a lightly loaded engine (not a big vehicle with 4 cylinders) low RPM at cruise speeds, specifically designed combustion chambers and so on. One such engine that responds well to lean operation is the Ford 5.4L V8 with 3 valve cylinder heads.

 

[Cujet]

I wonder if the OP will ever show back up in this thread...

 

[Mach1Owner]

Sorry I had completely forgotten I made the post here and not on another site.