So after talking about it in MRfred's tuning note thread, I decided to see what the envelope for lean cruise misfires are. I learned a few very interesting things as a result.

Here is a copy of my current timing map and I also included the dynamic advance table that we thought only functioned when in closed loop:



What I found was that cruising at 47* of timing and 17.5:1 AFR was a sure way to make it misfire. Richening it up to 17:1 and leaving the timing or dropping the timing to 45* and keeping the AFR was enough to stave it off however. On this same roadtrip I wasnt getting my normal 26-27 that I would on gasoline but I did manage to get almost 23 on E98 with the above cruise setup. I ended up cruising at 17:1 and 45* timing to do this.

Okay so keep in mind that I have my open loop load and throttle tables 100% zero'd so the car is in open loop all the time, however this what I was logging:

 

Awesome. 17:1 AFR has been working for me, but I have not touched that dynamic advance table at all, I believe I'm running a sea of 38's on my timing table and still using open loop threshold of about 30%, so the car does richen up from time to time. I still see about 41* of timing with EGR fluctuation. I'm tempted to try a bit more, but the car runs so good as-is I feel as though I'm pushing the limits on the AFR as it is!

24mpg is easy on this setup, even with FIC 1050's at 70% peak IDC and lots and lots of WOT time

Edit: sorry, I thought this was posted in the ECUflash forum. My comments are on gasoline, NOT E85. Sorry

 

No worries

Nice to know that the 17:1 works for you as well. I have found on gas (though your results may differ) anything leaner than 16.2 or so seems to require more throttle and less vacuum (i.e. more load and realistically more fuel) to maintain speed. If you live on really flat ground this might not be an issue but here in the NW lots of elevation changes dont really permit leaner than that.

 

Does E85(98) 17:1 take more throttle to keep the car moving? I noticed that when I ran my closed loop AFR cruise patch at 15.5:1 with gasoline, power was definitely off a bit. Is there also some loss of power when going leaner with E85?

 

17:1 is about the limit but yes the same phenomenon happens on e85/e98 as well. Beyond a certain point you have to use more throttle (thusly more vacuum and load) to maintain the same speed.

 

I've tried leaning the car out in open loop only (using the min coolant temp adjustment) but found the AFR would bounce around too much under normal driving. I'd target 16.5 and see anything from 15.0-17.0.

How's your result as far as the AFR bouncing around?

 

Mine is pretty stable but I have the openloop threshhold adjusted to 10 load to get in open loop. Alot of that is relative to injector size and scaing I think. The bigger the injector the less control you end up with. Mine are pretty consistent though even being 160# injectors.

 

Isn't it a benefit to actually need more throttle at cruise? By increasing the throttle, you should be reducing your pumping losses. Ideal sistuation would be getting the motor to run in boost under cruise so that you are getting energy recovery from the turbocharger. If you could get the A/F ratio lean enough to make so little power that you were able to mainatain a crusie speed while under boost, you would likely dramatically improve fuel economy because you would be greatly reducing pumping losses by using the energy recovered from the turbo.

Energy is energy and what you are after is improved efficiency. Just because you have to open up the trottle doesn't mean you are necessarily delivering more fuel to get a given energy output. If total system efficiency increases by opening up the throttle, then you will effectively reduce fuel consumption.

Obviously there are limits to what can be done, but the ideal sistuation is a motor that is using an energy recovery device such as a turbo to reduce pumping losses while still being able to use so little fuel energy that it could maintain a given cruise speed.

Combine BMW's throttle less cam system with Honda's lean burn direct injection technology and a gas/electric hybrid drivetrain with a turbo and E98. Best of the best currently available that I can think of.

 

In a turbo car, this may not be universally true. 0 psi of boost at say, 1/4 throttle, is because the turbo is pressurizing the intake track, and that takes energy. There's probably a balance between pumping losses and energy wasted making the turbo do work. An ideal setup would probably be one where the turbo spins just fast enough to not act like a restriction in the intake.

 

I see what you are saying, but since the turbo is also extracting energy from entropy in the exhaust and not strictly mechanical (pressure and volume), it seems like being able to run under boost to some extent would have an overall net gain because you are using energy that would be other wise wasted to atmosphere.