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Wed Aug 26, 2015 1:45 pm

Ok, what I am saying is it's not for nothing that the line for best fuel economy is at leaner than 16:1, not at 18:1, in the graph posted above. Not a VW? doesn't matter, same principle applies.

I'm sure you've seen it before, put in larger idle jets in some combos, and mileage goes up. I can't be the only one who's seen this, shirley?

Wed Aug 26, 2015 2:18 pm

optimal ignition timing is different for different A/Fs. So was the timing optimized for the richer mixture or the leaner one? Likely it wasn't changed AT ALL. Most folks adjust the jetting to match their timing, but almost never adjust the timing to match their jetting.

And if you lean it out to the point of misfire, MPG goes down.

Wed Aug 26, 2015 2:47 pm

I think this is what I'm talking about. See how steep that curve gets?

http://www.mummbrothers.com/SRF_Stuff/Secrets/Driveline/Air_Fuel.htm

You lose a lot more power going from 17:1 to 18:1 than you would going, say, from 14:1 to 15:1. Someplace on that curve you simply hit diminishing returns, it's not for nothing they say here that best fuel economy was at 15.something :1.

I may be wrong, but I'm pretty sure that the relation between A/F and mileage is not linear.

Wed Aug 26, 2015 2:48 pm

of course it's not linear. The best MPG is in the 15.5-16.5 range. Go to 25:1 and MPG goes to 0.

Wed Aug 26, 2015 3:01 pm

OK, I looked up some things, and it seems it IS mostly linear, but only at full throttle. At 1/4 throttle, its much more pronounced.

Wed Aug 26, 2015 3:04 pm

I think we agree.

Wed Aug 26, 2015 5:26 pm

Good points FreeBug and John!

FreeBug wrote: I think leaning it out beyond 16:1, you will hit the law of diminishing returns, as efficiency drops off even faster than fuel, past that point.

Not necessarily. Yes, 16:1 is peak efficiency, but leaning out below that to 18:1 or leaner doesn't automatically cause the efficiency to drop like a shoe, as long as it doesn't misfire. Take the diesel cycle for an example. At cruise they normally run way lean of peak efficiency (sometimes as lean as 40:1), yet diesel engines still get great fuel mileage. The Honda CVCC engine and other stratisfied charge engines are another example of how extreme lean burning (as low as mid 20's:1) doesn't effect fuel mileage all that much, and in reality is much more efficient than running rich. Efficiency in a gasoline engine will only start to drop drastically once your engine can't keep up with a solid ignition and combustion.

FreeBug wrote: Also, that what you might gain through lean burning, you will lose from having to open the throttle more.

Actually opening up the throttle more is advantageous to efficiency. If the AFR were the same, efficiency would be best at WOT. Remember that at idle the engine is 0% efficient and increases as the throttle opens. The reason running around at WOT usually lowers fuel mileage is because 1) gasoline engines usually enrich themselves at high load which lowers efficiency, and 2) because this kind of driving usually causes higher vehicle speeds that cause greater exponential energy losses due to aerodynamic drag and braking.

But the increase in efficiency in running a more open throttle, granted the AFR still being lean, is very well proven and documented. This is part of the actual dynamic compression ratio, because if you stretch out the air by means of a partially closed throttle and then compress it again your overall dynamic compression ratio is going to be lower and less efficient than if the throttle were all the way open. Again, the diesel cycle clearly demonstrates this principal. For another an example, many newer vehicles get better fuel mileage with a functional EGR valve because since the induction charge is diluted with inert exhaust gas the throttle must be held open more in order to achieve the same power to cruise. Running leaner than 16:1 does exactly the same thing as EGR since the extra air without fuel is inert, just like exhaust gas. Smaller engines and lower gearing also increase fuel mileage because they also cause the throttle to be held more open and therefore increase the manifold pressure and therefore total dynamic compression ratio.

FreeBug wrote: I'm sure you've seen it before, put in larger idle jets in some combos, and mileage goes up. I can't be the only one who's seen this, shirley?

Quite true. But that is true mainly due to two other factors. One is whether or not the engine can efficiently ignite the lean mixture and whether or not the ignition timing has been adjusted (advanced) to correspond to the slower flame speed typical of lean AFRs. CDI systems don't light lean mixes very well. Points with lower voltage coils and smaller spark gaps don't do much better either. Leaning out from stoichiometric with these systems will quickly start to produce misfires and inefficient combustion.

Also all your typical aftermarket distributor timing curves are not set for best efficiency in a lean burning engine running at part throttle. If anything, they are setup for best power running rich at full throttle. Since running lean and at part throttle both cause the flame to burn slower more advance is needed in order to maintain and increase efficiency. This would merit at the very least a distributor with vacuum advance. It's been quite proven that all our mechanically advanced distributors don't run efficiently on stock carburation without enriching the jets. Not that mechanically advanced distibutors are bad. They're great for applications where all out power is the goal. But fall short of being optimal for fuel efficient applications.

Thu Aug 27, 2015 1:35 am

> [email protected] wrote: this stuff is all covered in the wideband tuning thread.

It may be but it's a rather tedious 61 page read to get information that could probably be consolidated into 4 or 5 pages.

I do feel for you having to repeat yourself so many times. Seems like some people just can't get the simple concept of setting up the progression circuits before you go after the main circuit.

Scott Novak

Thu Aug 27, 2015 10:06 am

Or saying something is really important to them, but too lazy to read a 61 page thread to get a good grasp on the concepts.

Actually the concepts are covered in the first 10 pages, then regurgitated for the thick headed that simply skimmed the first 10 pages, or thought their situation was somehow different and special. The concepts apply to all applications.

Fri Aug 28, 2015 3:31 am

I still found additional useful information after page 10. it was long but it was worth the read.

Do VW exhausts typically get hot enough that a copper heat sink is needed for the O2 sensor? I've got the O2 bung welded about 4" from the collector on the exhaust pipe before the muffler.

I just did my front brakes and I'm ready to hook up the LM-1.

It will be interesting to see just how fast my Beetle will go just on the progression circuits.

Scott Novak

Fri Aug 28, 2015 5:01 pm

You just have to get A/F close. The big difference is ignition timing from what I found.

I tuned my old N/A combo via knock sensor and managed 39.98 miles per gallon on a 70 mile drive holding 70 miles per hour. Done with GPS, not the stock speedometer. That car ran low 12's.

Currently with my low 10 second turbo combo, my datalogging is showing mid to high 30's at 65 mph, but I haven't done the "real world" test with it yet to see what it really is getting. I also don't have my knock sensor wired in anymore, so I can't tune it that way anyway anymore.

Sun Aug 30, 2015 10:08 pm

Eric,

Did you use a Safeguard knock sensor? If so where did you mount the sensor on your engine case?

Sun Aug 30, 2015 11:00 pm

74 Thing wrote: Eric,

Did you use a Safeguard knock sensor? If so where did you mount the sensor on your engine case?

I'd be interested too.

And yes, ignition timing is more important than AFRs when it comes to efficiency. As long as you have a good burn at stoich and leaner you are converting all your fuel into heat. But will that heat be turned into power? Not if it happens when the piston is now half way down the cylinder.

Sun Aug 30, 2015 11:24 pm

Scott Novak wrote: The Air Fuel ratio for maximum horsepower for gasoline appears to be about 12.7:1. The AFR for maximum mileage appears to be approximately 18:1.

What percentage of horsepower is lost using an 18:1 AFR versus a 12.7 AFR?

I'm wondering if it is even practical to jet a carb for 18:1 AFR throughout it's entire range for maximum fuel mileage.

What are your thoughts?

Scott Novak
this was studies in depth in the 80's. how lean the engine will accept depends heavily on the engine design. Past the point of diminishing returns you will find intermittent misfire and/or the burn will be so slow the heat is going out the tailpipe more than driving the piston. Find out experimentally is the only way to know, and you might make a change to the engine and find quite a bit of difference. It could be anywhere from 15-18, some could go to 20 with the right combination.

Tue Sep 01, 2015 3:20 pm

I used a Knocksense wired into my megasquirt. It retarded timing when it detected knock. I adjusted my ignition map so it was maxed out without any knock.

The downside is, at heavy engine loads, it showed knock no matter what, and if valves weren't adjusted well, and rocker side play wasn't tight, it false triggered knock as well.

But for light cruising, it was tuneable and I got great results.