Fuel Economy: AFR vs. RPM? - Page 2 - Miata Turbo Forum - Boost cars, acquire cats.

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Old 09-01-2012, 05:23 PM   #21
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Yeah, I follow. I just brought it up since Tina Turner (or whoever the hell that is in Palmtree's new "let's taunt the angry, drunk moderator by changing our avatar from something really disturbing to something even more disturbing" avatar) had specifically mentioned operating at different RPMs in the original post.
It's Rupaul betch
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Old 09-01-2012, 05:24 PM   #22
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didn't you get crazy-good fuel economy? Wtih a 4.30 and 5-speed, you should be pretty ******* happy with my badass tuning at 34mpg.
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Old 09-01-2012, 05:27 PM   #23
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didn't you get crazy-good fuel economy? Wtih a 4.30 and 5-speed, you should be pretty ******* happy with my badass tuning at 34mpg.
Yeah I did. You're tune is sex. I was just curious for my own understanding. This was a scenario, not what my car is actually doing.
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Old 09-01-2012, 05:34 PM   #24
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Yeah I did. You're tune is sex. I was just curious for my own understanding. This was a scenario, not what my car is actually doing.
You need VVTuner.
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Old 09-01-2012, 05:42 PM   #25
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You need VVTuner.
I'll take yours off your hands
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Old 09-01-2012, 05:52 PM   #26
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You gotta look at BSFC. (brake specific fuel consumption) That is a measure of how much fuel is converted into energy.

Back to the original question, I will guess that best BSFC is at a lowish RPM, where the motor is run at stoich or leaner than stoich, where timing is at MBT, and the throttle is open wide enough so there is no pressure drop across it (e.g. 105 kPa in the manifold and the intercooler, at 2500 RPM).

Here is what BSFC plots look like:
https://www.google.com/search?num=10...w=1535&bih=796

Note that best BSFC is at lower RPM, heavyish throttle.
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Old 09-01-2012, 07:35 PM   #27
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So, from a practical standpoint, how does someone like myself or the OP go about measuring the actual BMEP of our engines using the tools and equipment which we have available to us?
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Old 09-01-2012, 08:20 PM   #28
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You need a dyno that calcs BSFC using an input from the (corrected) duty cycle from the ECU.
No easy way.

But the typical plots show the general trend:
- lowish RPM
- stoich or leaner
- MBT
- no pressure drop across throttle (except for higher RPM operation where it goes rich)

As to the orig Q: I will guess that a bit higher RPM while a bit leaner, will have higher BSFC than lower RPM but richer, all else being equal. My guess is based on the typical plots, showing fact that BSFC is a weaker function of RPM than of throttle pressure drop. The sudden worsening of BSFC in those plots at heavy throttle, higher RPM, is due to richening in order to control EGT. I also am basing my guess on the operation of the mid 90s lean burn Civic HX. It only goes into lean burn below a certain RPM, but also IIRC below a certain MAP (can't do it at heavy throttle), possibly because EGT's will be too high. But then again, cruise actually has lower BSFC than low RPM acceleration, and improving cruise BSFC with lean-burn will have a bigger impact on MPG.

The high BSFC of low RPM heavy throttle is why "pulse and glide" hypermiling works.
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Old 09-01-2012, 08:31 PM   #29
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Quote:
Originally Posted by JasonC SBB View Post
You need a dyno that calcs BSFC using an input from the (corrected) duty cycle from the ECU.
No easy way.
Oh, ok. So it wasn't really relevant to the thread, you just figured that Palmtree would enjoy learning about a method to solve his dilemna which he cannot implement.

Thanks.
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Old 09-01-2012, 09:23 PM   #30
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For his very specific 2 scenarios, he can log 4th vs 5th gear, on the same exact flat stretch of road, in the same direction, using cruise control at the exact same speed, doing it at least 4x (to check for consistency), alternating between 4th and 5th. Look at the average duty cycle from point A to point B. Note that the duty cycle must include a correction for dead time (i.e. not be simply electrical pulse duty cycle). I didn't bring this up because I thought it was too obvious.

I brought up BSFC maps because the *concept* is important in this discussion.

When AEM makes a version of their propshaft torque transducer for the miata, then BSFC can be calculated from datalogs.
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Old 09-01-2012, 09:40 PM   #31
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Well, "math crew" is here.
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