what afr for max mpg?

[blkaplan]

hey everyone,

with my new found 100 mile commute each day, I have plenty of time to ponder about my tuning map and how it affects my wallet at the pump 2x a week.

normally I have my cruise anywhere from 16-15.5 afr..... Initially i thought a leaner cruise uses less fuel so I must be increasing my mpg correct?

But, if the engine makes less power at a leaner afr instead of somewhere near 14.7, are you using more throttle to maintain the same speed negating your lean tuning?



i am quickly thinking myself into a downward spiral of madness

help a brother out?

[automotivebreath]

...normally I have my cruise anywhere from 16-15.5 afr..... Initially i thought a leaner cruise uses less fuel so I must be increasing my mpg correct?

But, if the engine makes less power at a leaner afr instead of somewhere near 14.7, are you using more throttle to maintain the same speed negating your lean tuning?...

Engine throttling increases pumping losses and thus the mechanical efficiency of the engine decreases. At idle the mechanical efficiency approaches zero.

Engines present high pumping losses at part loads due to intake throttling; this penalizes their fuel efficiency. If the engine is leaned and power is reduced, wider throttle position will be required, reducing pumping losses. This should reduce fuel consumption.

A major problem for lean-mixture is spark ignition. Combustion initialization and flame propagation widely depend on air/fuel ratio. As a result, misfiring and cyclic irregularities occur at lean air/fuel ratios. Therefore extremely lean air/fuel ratios cannot be reached because of flame initialization and propagation limits.

[growley]

No doubt that's a pretty lean mixture. I would think a more stoich mixture would provide better gas mileage.

[blkaplan]

so,,,,, is the theory that yes a leaner afr will give better mpg due to the throttle being farther open but the ignition property of gasoline prevents us from going leaner and getting good combustion?

[740Weapon]

tune for max cruising vacuum.

[barefoot]

Has anybody done some serious playing with EGR to increase economy?

If you're charging your cylinder with half exhaust gas (=~ inert) and half stoichiometric mixture, you're effectively running an engine half the size - so no need to throttle, which introduces pumping losses and reduces effective CR.

tim
crazy-arsed ideas, no real solutions :-D

[740Weapon]

Has anybody done some serious playing with EGR to increase economy?

If you're charging your cylinder with half exhaust gas (=~ inert) and half stoichiometric mixture, you're effectively running an engine half the size - so no need to throttle, which introduces pumping losses and reduces effective CR.

tim
crazy-arsed ideas, no real solutions :-D

but eventually the mixture just wont ignite.

how about 4 cyl displacement on demand... gross.

[barefoot]

how about 4 cyl displacement on demand... gross.
Big difference is that EGR is infinitely adjustable, DoD is either on or off.

And DoD would cause a 4-cyl engine to vibrate like a marital aid.

tim

[automotivebreath]

so,,,,, is the theory that yes a leaner afr will give better mpg due to the throttle being farther open but the ignition property of gasoline prevents us from going leaner and getting good combustion?

That's my take on it. Improve the ignition system or the combustion environment and the limits change.

Every engine is different, the ideal air/fuel ratio must be determined for each engine in the current configuration and operating conditions.

[blkaplan]

what determines the ideal? max torque? max vacuum?

[Unregistered]

what determines the ideal? max torque? max vacuum?

Ideal mixture depends on the objective.

A mixture less than 14.7:1 would be ideal when maximum torque is the goal, fuel economy would suffer at the expense of power. Ideal mixture would be determined for the specific vehicle and weather conditions.

A mixture at or greater than 14.7:1 would be ideal when minimum fuel consumption is the goal, power will suffer at the expense of economy.

At a given speed the engine must deliver X amount of torque to keep the car moving, the objective is to determine what A/F ratio your engine needs to deliver X amount of torque using the least amount of fuel. Conducting several tests at different A/F ratios at identical driving conditions to determine ideal A/F ratio.

Modify the engine and everything changes.

[automotivebreath]

Ideal mixture depends on the objective.

A mixture less than 14.7:1 would be ideal when maximum torque is the goal, fuel economy would suffer at the expense of power. Ideal mixture would be determined for the specific vehicle and weather conditions.

A mixture at or greater than 14.7:1 would be ideal when minimum fuel consumption is the goal, power will suffer at the expense of economy.

At a given speed the engine must deliver X amount of torque to keep the car moving, the objective is to determine what A/F ratio your engine needs to deliver X amount of torque using the least amount of fuel. Conducting several tests at different A/F ratios at identical driving conditions to determine ideal A/F ratio.

Modify the engine and everything changes.


I didn't know I could post with out logging in!

[The Aspirator]

A major problem for lean-mixture is spark ignition. Combustion initialization and flame propagation widely depend on air/fuel ratio. As a result, misfiring and cyclic irregularities occur at lean air/fuel ratios. Therefore extremely lean air/fuel ratios cannot be reached because of flame initialization and propagation limits.Well, with our fancy schmancy megasquirts we can also tune the spark advance quite easily. I've been a fan of tuning it lean until it bucks, richen it up a hair, then advance the ignition till it bucks, then retard it a hair. This is of course only for constant speed cruising, though I'm not sure if it's the most efficient way to do things.

Bne what MPG are you getting? I get 19.5 all the time, 22.5 all highway. I try to tune for 16-17 afr during constant speed cruising, and this new engine of mine requires WAY less ignition advance than anything else I've ever tuned, but still I've got it maxed to the point of almost bucking during cruise.

Also enable the decel fuel cut, it's awesome. Set it above 1500rpm, below 1% throttle, below like 25kpa, and have it activate after 1 second.

[blkaplan]

I am getting a dead consistant 22mpg. When I first MSnS my car, I got 25 mpg on a trip...

[automotivebreath]

... I've been a fan of tuning it lean until it bucks, richen it up a hair...
This finds the limits of spark ignition for your engines combustion environment, do you record lowest fuel consumption when tuning this way?

...this new engine of mine requires WAY less ignition advance than anything else I've ever tuned....
When an engine requires less ignition timing it's likely a result of a better combustion chamber design that provides faster flame travel. I would think running leaner air/fuel mixtures are also possable.

[The Aspirator]

This finds the limits of spark ignition for your engines combustion environment, do you record lowest fuel consumption when tuning this way?
For the most part, yeah. On this engine I've only tracked MPG on like 4 tanks, so it's kinda hard to know exactly what works best, but yes, this method has so far gained me the most MPG. Though I'm still not too impressed with 19.5 city/22.5 hwy.
When an engine requires less ignition timing it's likely a result of a better combustion chamber design that provides faster flame travel. I would think running leaner air/fuel mixtures are also possable. :lol: I know, I love this engine. Light chamber work on an 8v head, lots of bowl work, lots of port matching/smoothing/sanding, it turned out great. 9.5 SCR, .028" squish clearance. Although trying to run this thing on 87 octane sucks cause I've got to yank even mooooore timing, ohh well, drives great.

[automotivebreath]

... 9.5 SCR, .028" squish clearance. Although trying to run this thing on 87 octane sucks cause I've got to yank even mooooore timing...

You have to take timing out because of detonation with 87 octane?

[kyle242gt]

Ben -

Try 16.5:1, 16:1, 15.5:1... etc all the way to stoich. Just set the target AFR and give it plenty of authoritah to make the change. Do the same speed on the same drive on the same day in the same temps (getting me?) and datalog it.

See what duty cycle is for each AFR. That should tell you where you're injecting the least fuel. FPR changes might make a teensy bit of difference, but I doubt there's that much (if any?) difference at 60-80Kpa.

I run 16.1, and get 18 in mixed city/hwy driving with not-infrequent jaunts to 15psi/6500RPM. Highway driving is 24+/-, and that's with the York AC clanking away ;-)

[740Weapon]

i think you guys are playing the extremes too much.

think about it.

LH turbo cars usually get better than 20 mpg. n/a cars in the 23-25 mpg range.

reason: they use middle of the road tuning. which also happens to be mostly optomized for the engine. your going to get your biggest gains by making small changes in the map, a little leaner, a little more timing. they got it mostly right from the factory.

[blkaplan]

to bad there isn't a factory map for a +T

[Unregistered]

i think you guys are playing the extremes too much.

think about it.

LH turbo cars usually get better than 20 mpg. n/a cars in the 23-25 mpg range.

reason: they use middle of the road tuning. which also happens to be mostly optomized for the engine. your going to get your biggest gains by making small changes in the map, a little leaner, a little more timing. they got it mostly right from the factory.

This only applies when you are tuning a unmodified engine. A modified engine will have different air/fuel needs.

[The Aspirator]

You have to take timing out because of detonation with 87 octane?Yes, lots of timing. Probably 10* in most parts of the map compared to when it was tuned for 92.

[Matt Dupuis]

As Automotivebreath said, there is a point of diminishing, and even negative, returns when going leaner and leaner. You can provide the engine with less fuel, which requires more throttle to run, and that should reduce pumping losses and therefore increase economy. However, that combination will require a different optimal spark timing for peak efficiency, and that ignition timing might be substantially more advanced, which means more negative work being done on the piston, wasting power internally. Setting the AFR alone doesn't give you the best results, and neither does trying to go as lean as possible without stumbling/bucking.

The difference between 16.0 and 14.5:1 AFRs is that 10% more fuel is being sprayed at 14.5:1 AFR. However, the engine is probably making 5-10% more torque at the richer mixture, which reduces the economic gains of trying to go leaner.

It's generally accepted that an engine is most efficient at pumping air when it's at it's torque peak. Coincidentally, the Brake Specific Fuel Consumption is lowest where the engine is most efficient at pumping air, which suggests that the engine uses the least amount of fuel per horsepower developed at the torque peak.

However, for a gasoline engine where manifold vacuum alters the tuning resonance of the intake manifold, alters the scavenging of the exhaust, and therefore alters the cylinder filling of the engine, peak efficiency shifts around all over the RPM map for different throttle openings. Also goes a long way towards explaining why diesels get such good economy on the highway - the gear ratios are chosen to keep the engine near it's torque peak, and that peak doesn't change much at all under varying loads.

So one engine might be tuned for lowest AFRs and the widest throttle opening, but the cruise RPM could be so far away from the torque peak at that vacuum reading that it'll use more fuel just trying to keep the car moving. You could also tune the engine for more vacuum, which means richening up the mixture and advancing the timing, which might put the torque peak closer to the cruise speed, giving better results.

Also explains why some newer vehicles sip fuel gently at 65 mph, but at 75 mph you can watch the gas gauge move. They're tuned for advertisable, labaratory-clean fuel economy at a given speed/load, but alter that speed/load and it throws everything out of whack.

I generally avoid trying to get the car to run lean for cruise, 'cause I've never really seen the benefits to outweigh the constant "lean spot" you go thru every time you accelerate from a light.

[kyle242gt]

I generally avoid trying to get the car to run lean for cruise, 'cause I've never really seen the benefits to outweigh the constant "lean spot" you go thru every time you accelerate from a light.
Get a turbo that spools at 2000RPM. Voila, you're into boost before you even notice you're running lean.

[blkaplan]

Update!!!

I have had really good luck tuning my car to 14.9:1 for cruising. This has provided me with the best results. On an all highway tank I average 28mpg.

One key thing I did was enabling the AFR corrections to fine tune for 14.9 on the highway, this helped ensure consistency.

[740ATL]

afr corrections = afr target?

28mpg, hell yeah!

[blkaplan]

afr corrections = afr target?

28mpg, hell yeah!

Yes AFR corrections are setting up the AFR target table and allowing megasquirt to tune everyday based on your 02 feedback.

This is crucial for a daily driver.

[740ATL]

Ben, can you datalog with the corrections given by the target afr table, and use these values to tune via megalogviewer or equivalent? or is it more of a once you shut it off it's gone type thing?

[benflynn]

auto tune will do it for you, probably have cruise worked out in one long ride to work

[740ATL]

Ben have you used autotune much? Do you have a recommended set of settings?

[benflynn]

this ben has not, i never drive the car in a matter that will not scew autotune, it needs to be cruise, calm driving

[Dauntless]

28mpg, hell yeah!

I'm averaging 25 mpg on K-Jet with a B23E, H cam, M45, and 3.73's. That's an overall average of suburban and open road driving, on 10% ethanol blend fuel. You should be able to do better on a B230 with programmable EFI.

[stylngle2003]

I'm averaging 25 mpg on K-Jet with a B23E, H cam, M45, and 3.73's. That's an overall average of suburban and open road driving, on 10% ethanol blend fuel. You should be able to do better on a B230 with programmable EFI.

First off, it's a B230f+t (and in Mike's case, a B234+t) with 3.73s that gets driven VERY hard. Plus, is your 25mpg US gallons, or Imperial (had to ask, i'm not sure)? Ben is running ~12psi and uses boost (and his blowoff valve) quite a bit. This makes for not so good mileage, and on a turbo car, at least here in the US, anything above ~24mpg is fantastic

[linuxman51]

its not hard. A fair amount of timing at an afr of around 15.0-15.5 got me close to or over 30 with the 16vT.

[benflynn]

i have gotten 24+ going to the beach w/no OD, but i guess it is mainly downhill to the beach, was the only time i looked, but w/o a new tune for the larger injectors i am always rich

[The Aspirator]

Best I've ever gotten with MS in the last 3yrs is 25-26mpg highway, in town I usually average 20. I keep it fairly advanced in cruise, and usually around 15.5-16.5 AFR.

[fengler]

so,,,,, is the theory that yes a leaner afr will give better mpg due to the throttle being farther open but the ignition property of gasoline prevents us from going leaner and getting good combustion?

the dsm guy who bought my motorcycle pants showed me his engine tuner system. He said he programmed the computer to go into closed loop LEAN mode when cruising and then the standard open loop ~12:1 rich mixture under boost. He said he got 35-40mpg on the freeway by doing this. Im not sure what his afr was/is, but if you want i can ask him.

[blkaplan]

Ben, can you datalog with the corrections given by the target afr table, and use these values to tune via megalogviewer or equivalent? or is it more of a once you shut it off it's gone type thing?

auto tune will do it for you, probably have cruise worked out in one long ride to work

Ben have you used autotune much? Do you have a recommended set of settings?

You can datalog the correction factor.

What I did is used vexme to get my tune as close as possible to what I wanted without an AFR correction.

When I was happy I turned AFR correction on to make sure it would stay as close as possible to my initial ideal settings

[blkaplan]

I'm averaging 25 mpg on K-Jet with a B23E, H cam, M45, and 3.73's. That's an overall average of suburban and open road driving, on 10% ethanol blend fuel. You should be able to do better on a B230 with programmable EFI.

Car is 3100 lbs dryweight, still has A/C, power steering, power windows, power locks, and full interior.

The 225 45 17's helped with the gearing but I bet some lighter wheels (i have eikers) could help me .3mpg or more...

and the turbo sitting in the exhaust stream isn't the best for cruising efficiency.

I am very happy with 28

[The Aspirator]

As you should be too! Man your car is a pig too, my 244 weighs 2900 without me, that's with a spare, power windows, PS, no AC and full stock interior.

[blkaplan]

As you should be too! Man your car is a pig too, my 244 weighs 2900 without me, that's with a spare, power windows, PS, no AC and full stock interior.


Its not that bad, that was with ac and an auto trans. probably 3/4 tank?

[tjts1]

Has anybody done some serious playing with EGR to increase economy?

If you're charging your cylinder with half exhaust gas (=~ inert) and half stoichiometric mixture, you're effectively running an engine half the size - so no need to throttle, which introduces pumping losses and reduces effective CR.

tim
crazy-arsed ideas, no real solutions :-D

The EGR found on our cars does absolutely nothing for fuel economy. Its only there to reduce NOx. In 1989 Volvo sold the 240s with or without EGR side by side in California and the rest of the country. As you can see fuel economy was identical across the board except on the highway for 5 speed cars. EGR equipped 5 speeds lost 1mpg on the highway.
http://www.fueleconomy.gov/feg/bymodel/1989_Volvo_240.shtml
By 93 they eliminated EGR even on California cars.

EGR equipped 740s lose 1-2mpg on the highway.
http://www.fueleconomy.gov/feg/bymodel/1989_Volvo_740.shtml

I realize EGR is used to coax more fuel economy out of modern engines but the system found on our engines is just too ON/OFF. I just want to block it off.

[blarf]

I realize EGR is used to coax more fuel economy out of modern engines but the system found on our engines is just too ON/OFF. I just want to block it off.

Careful with your words there. Volvo used three or four types of EGR setups on the 700s depending on the market.

[tjts1]

Unless the EGR valve has an electric motor directly controlling degree to which it opens, its still just a glorified on/off switch. It might be a smoother, slower, better controlled on/off switch but... we you get the picture. You can only go so far with vacuum control.

[blarf]

Sweden, Switzerland, and Australia got so-called stepless EGR setups on some engines. I suspect that this would yield a pretty significant amount of control over the on/off style that we got in the US.

However my data only goes up to the '88 model year 700s, so nothing about LH with EGR.

[fengler]

egr prevents part throttle detonation

[tjts1]

The engine has a knock sensor. It knows how to use it.

[linuxman51]

Car is 3100 lbs dryweight, still has A/C, power steering, power windows, power locks, and full interior.

The 225 45 17's helped with the gearing but I bet some lighter wheels (i have eikers) could help me .3mpg or more...

and the turbo sitting in the exhaust stream isn't the best for cruising efficiency.

I am very happy with 28

yeah thats pretty good, what was towery getting mpg-wise before he swapped motors?
my dimmest memories of the NA 8v ms'd were that it was substantially "quicker" than with the stock stuff (hell it busted off a low 17 with a dead autobox), and the fuel gauge rarely moved, but I didnt really waste any time trying to tune the car since I had the 531, turbo, and other coolness slated to go in.


After the fact, I got the highway mpg's up around 28-29 @ 80-85mph.

[Dauntless]

On an all highway tank I average 28mpg.

First off, it's a B230f+t (and in Mike's case, a B234+t) with 3.73s that gets driven VERY hard. Plus, is your 25mpg US gallons, or Imperial (had to ask, i'm not sure)? Ben is running ~12psi and uses boost (and his blowoff valve) quite a bit. This makes for not so good mileage, and on a turbo car, at least here in the US, anything above ~24mpg is fantastic

US gallons. I've never driven a good turbo car, but I doubt you'd get into boost much on the highway, except for going up hills. I agree that it's very good fuel economy. I'm just saying that if you're tuning for economy, you can probably get a a bit more out of it yet. Btw, my fuel economy with my car is unheard of, most people are lucky to get 18 mpg with K-Jet here.

Apparently the car is much heavier than mine, which would detract from the economy a bit.

[blkaplan]

US gallons. I've never driven a good turbo car, but I doubt you'd get into boost much on the highway, except for going up hills. I agree that it's very good fuel economy. I'm just saying that if you're tuning for economy, you can probably get a a bit more out of it yet. Btw, my fuel economy with my car is unheard of, most people are lucky to get 18 mpg with K-Jet here.

Apparently the car is much heavier than mine, which would detract from the economy a bit.

I am sure I could peak 30 mpg if I maintained my speed a 55-60 mph but we were averaging around 73-78mph. Also a non hydraulic overdrive would probably pay dividends for cruising efficiency.

On an average all city tank with lots of boost runs and just being a general jack ass I get about 22-23mpg. Still very good IMO

[towerymt]

I am sure I could peak 30 mpg if I maintained my speed a 55-60 mph but we were averaging around 73-78mph. Also a non hydraulic overdrive would probably pay dividends for cruising efficiency.

On an average all city tank with lots of boost runs and just being a general jack ass I get about 22-23mpg. Still very good IMO

try staying in the same lane for more than 500 yards, too

[blkaplan]

try staying in the same lane for more than 500 yards, too

what ever, the left lane is the passing lane retard!!! anyone who cruises there needs to be shot.

[stylngle2003]

we were always passing :rofl:

[blarf]

Don't forget that a B23E is a higher compression motor than a B230FT (or a B230F). :e-shrug:

I've seen 27-28mpg in fairly aggressive highway driving in a non-turbo 850.

[blkaplan]

Don't forget that a B23E is a higher compression motor than a B230FT (or a B230F). :e-shrug:

I've seen 27-28mpg in fairly aggressive highway driving in a non-turbo 850.

Non turbo ..... that is a big difference, (also was it a euro 2.0 liter 850?) its a b230f+T which means its 9.8

[towerymt]

what ever, the left lane is the passing lane retard!!! anyone who cruises there needs to be shot.
I'll remember that the next time you're holding someone up in the left lane.

if you could only see you drive....

[blkaplan]

I'll remember that the next time you're holding someone up in the left lane.

if you could only see you drive....


What ever, you just mad cause your motor gots a case of the runs.

[towerymt]

What ever, you just mad cause your motor gots a case of the runs.
haha. watch it, mr. graham cracker rods.

All right! But hear me and hear me well - The day will come. Oh yes, mark my words, Seinfeld - your day of reckoning is coming. When an evil wind will blow through your little playworld, and wipe that smug smile off your face. And I'll be there, in all my glory, watching - watching as it all comes crumbling down.

[Dauntless]

Don't forget that a B23E is a higher compression motor than a B230FT (or a B230F). :e-shrug:

Yeah I know, but B23E's are known for having quite bad economy, probably due to the K-Jet Basic. Anyway Ben, quite good economy, good work there.