They do to a point and then start falling again.

http://www.s14.net/photopost/data/50...r-co-chart.gif

I had thoughts of putting a big injector right infront of the turbo to heat up the fuel charge using that injector only to feed the engine while trying to lean it out, be for cruise only of course, ideas ideas

I set mine up for lean cruise 15.5:1 but I had issues at low gear low rpms with stumbling.

Lean burn engine and Miata don't mix. Although I have been able to get mine to cruise around at 16.5:1, the throttle response is terrible even with big asynch pulses. My guess is that the engine is just really dry, and when you throttle in hard it takes ALOT of fuel to wake it up.

May require another table for enrichment. Or an asynch specific to the ranges your running super lean. Even still best theoretical fuel mileage is at 15.2:1-15.4:1. Then I don't think the next fuel economy hump comes until after 20:1. Best to shoot for 15.2:1 and save yourself some time! I made a chart for tuning practices around here somere... Aha! Attached!

Where did you get the "best theoretical fuel mileage is at 15.2-15.4" value?

It's high peak temperatures in the combustion cycle, not high EGT's that causes high NOx.

VTEC-E disables one intake valve below a certain RPM (around 2300 RPM IIRC), inducing a strong swirl and turbulence in the cylinder to help homogenize the mixture. That's how they were getting away with such high AFRs.

Another lean-burn idea shot down by the EPA is/was VW's SFI engines, where a "pocket" of fuel was stratified in the cylinder and ignited. Also has high NOx so it's not used in the US.

Thermal dynamics book on otto-cycle engines. The actual burn quality goes way down as you go above 15.5:1. So while your not using as much gas per metered air, the energy your extracting per gas volume is disproportionately lower. This goes back up for some reason at 20:1. I would guess that is because all the engines they've been able to run at 20:1 have special modifications for lean burn such as direct injection.

In the cylinder itself there is a stratification of gasoline charging. At the bottom of the cylinder there maybe 9:1 air, while at the top 22:1. When you change the overall AFR your reshuffling this whole column, and the burn characteristics are different all the way through. These burn characteristics may be less and less ideal to the point where the burn doesn't produce the proper conditions to fully combust the fuel.

Engine designers have tried for decades to increase the ability of the engine to burn leaner and leaner charges. EGR was the first of such inventions. This system forced already exhausted air that was mostly inert into the chamber to reduce intake pumping losses and to increase the pressure in the chamber to combust more the fuel. Likewise, cylinder head designs began incorporating quench regions into the cylinder head to break down the stratification of charge in the chamber so they could run leaner and burn the mixture faster. Now we have the next generation technologies such as special piston designs, direct injection, and so on to push the envelope farther!

With our current technology level we are forced into using the AFR's the engines were designed for which is around 15.3 for economy and 12.7 for power. There isn't any easy way to make that range wider unless we fit some of the newer technologies. Some people even remove some of the technologies on their engine like EGR! I've also seen people rounding the quench regions on the cylinder head to "reduce sharp edges" which also reduces turbelence/charge mixing. I don't know which has the harder effect, but I wouldn't go mucking about in the chamber without a really good reason! I'd also keep the EGR on if possible. Good for mileage, emissions, and longevity. Even if it is against the racing pop culture ;)

/end rant

I'm at 15.4 in cruise and getting 28mpg with a 400lb tire trailer at 85-90mph on road trips, lol. Jeff did not appreciate that in his car with the 5-speed/4.30, and I'm certain he appreciated my cat-less exhaust.

I saw 31mpg once with the trailer and it's an aero brick. My daily can't break past 26mpg on road trips, without the trailer.

Well there ya go, you learn something every day. I can go home now.

I am of the racing pop culture that dislikes EGR

I wonder if there's some useful way to use the VTCS and VVT to create a leaner/lower gas consuming mixture.

1. decrease port cross section and increase tumble by closing the VTCS butterflies
2. midly advance the intake cam to increase overlap a little and reduce pumping losses
3. require more throttle position to further reduce pumping losses

Sounds a little like an Atkinson cycle, y8s.

does the turbo make it a miller?

I have a 65% probable plan to make a mpg efficiency experiment using a B6 or BP (whatever's cheaper at the time), removing 2 cyinders, throwing my ol' spare 1544 and seeing what happens. Smaller engine would require more throttle so less pumping loss there, plus less internal friction, etc. If I do a BP and can find someone who f'ed up a VVT head...that may be cool. But I also figure I'll try only using one intake valve on the 2 cylinders to increase swirl (find a spare cam and grind off the appropriate lobe).

Just read through the articles about that smokey guy,
increasing the airtemp with hot cooling water then exhaust gases is indeed the correct way to go for a better efficiency (all thermodynamics guys probably know that)
for not knocking just lean it out... (at my work we test natural gas engines, and sometimes we have to do a knocking test to evaluate new knock software... this is achieved by going from lambda 1.9 down to 1.7 YES we put in MORE FUEL to produce knocking!!)
when I put some mods to my engine, i'll let you know ;-)

EDIT: and the natural gas can be added before the turbos or after them --> the injection isn't the problem

Holy cow!! Talk about pop culture engineering. What a load.

Exhaust Gas Recirculation is the same thing as eating shit. From a thermodynamic standpoint (not "Thermal Dynamics"), it is just about the worst thing you can do. It was invented in the 70's to lower NOx emmissions to try to comply with the 1970 Clean Air Act. That was about the same time those wonderful big block V-8s went from 10:1 to 8:1 compression -- again, not good from a thermodynamic standpoint. EGR was definitely NOT about trying to accomplish lean burn or increasing engine efficiency.

The other stuff is mostly good information. Note that cylinder head quench regions -- which is a technology that allows you to run higher compression without detonation -- is also pretty old. Widely used in the mid-60s but fell out of favor because, again, high NOx. The change to big-block MOPAR cylinder heads is probably the most obvious example.

Wonderful head engineering these days to introduce swirl while still preserving flow. Charge stratification is the enemy (unless you do it on purpose to preserve a combustible mixture in a specific area near the spark source -- which is what a lot of the discussion above is about).

EGR . . . 100% bad. Rule of thumb . . . don't eat shit.

where'd you read that EGR is bad for cruise? Heywood?

EGR is fine in cruise . . . at least the way it is currently implemented with ECUs (it was god-awful in the 70s). But I certainly take issue with the assertion that EGR improves thermodynamic efficiency. That is total BS.

I think the asertion is that EGR makes the engine produce less power per stroke thus requiring you to open the throttle more. increasing throttle angle reduces pumping losses and improves volumetric efficiency.

you effectively limit your fresh air intake while reducing the restriction of the intake path.

at WOT, the above is meaningless because you can't open the throttle more to reduce pumping losses.

Agree. There are a couple of other things that are done with modern EGR systems:
1. Because a mixture with non-combustible exhaust gas burns more slowly, spark advance can be increased.
2. Because a mixture with non-combustible exhaust gas burns more cooly, there is less conduction head transfer into the engine block or waste heat out the exhaust pipe.

I think I'm just overgeeking and focusing on the thermodynamics rather than on the real world need to reduce NOx while utilizing ECU algorithms to keep efficiency as high as possible. Certainly, if you are running a stock ECU you would be foolish to disable EGR.