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Old 06-21-2008, 08:49 PM   #1
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ECUs with MAP, driveability programming and P+G.

Hi folks,

In a throttle body discussion I stumbled on the other day, it came to light that MAP sensors do not react very quickly to changes in throttle position. Hence ECUs with MAP metering systems use a delta value from the throttle position sensor (rate of change) to guess how much extra fuel to squirt in, anticipating a wider than current throttle opening. This stops the vehicle from stumbling when the throttle is suddenly opened, due to the MAP sensor lagging slightly.

As you might guess, this behavior looks like having negative effects on F-E should you not be aware of it. This would be because if you pulse the throttle too rapidly, you get a richer mixture than you need. So, it appears best practice during a pulse, thinking here of a brisk pulse to get the BSFC benefit, would be to feed in the throttle smoothly, rather than to stab it to a pre-determined/learned position. To some drivers smoothness is second nature, but others have to think about it.

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Road Warrior
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Old 06-21-2008, 09:39 PM   #2
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Thanks for the tip. Smoothness will improve FE on any car. My car doesn't use a MAP sensor, it has a MAF and there is still an accelleration enrichment feature programmed into the ecu. I've watched the O2 volts on my datalogger and have seen a jump in fuel during rapid throttle changes, both opening *and* closing. Yep, decel enrichment too. Luckily the effect tapers off within a second or two.

Even carbureted cars have a throttle pump to add fuel when the throttle is opened quickly. I used to have a 1988 Festiva that didn't have it, and it would stumble badly if I opened the throttle too quick. I learned to be slow in that car, and I learned that I didn't like it.
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Old 06-21-2008, 11:15 PM   #3
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"Guess" isn't the right term. "Anticipate" is more like it. The goal of the accel pump is to keep the correct mixture instead of having it spike lean with sudden throttle increases (I think that you're assuming the goal is to run rich while accelerating). It squirts fuel at the same time as you hit the throttle instead of measuring the mixture and then reacting to it. That's like playing catch-up. That's what causes hesitation or even stalling. Accel doesn't hurt your mileage, though I suspect that cheap cars and domestic cars are all tuned half-assed and probably just dump a ton of fuel. I'm willing to bet that you're better off stabbing the throttle on a pulse rather than gradually increasing the throttle. You have one quick accel burst as opposed to one long accel burst.

Alpha-N based maps have excellent throttle response but they suck for driveability because they don't adapt to changes in altitude and weather. Alpha-N with load based corrections seems like the logical fix, but it turns out that load based maps with throttle correction works best.
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Old 06-22-2008, 06:09 AM   #4
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Quote:
Originally Posted by DRW View Post
Smoothness will improve FE on any car.
In my experience with my VW, slamming the pedal to the floor instead of smoothly pushing it has actually helped. I can't be 100% sure of that, but I am getting great results. By doing so, less time is spent at part-throttle with its higher pumping losses and BSFC. With modern accurate fuel injection systems there's none of the extra enrichment, so that part isn't an issue.
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Old 06-22-2008, 02:17 PM   #5
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I'd imagine that VW has a MAF system not a MAP.
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Old 06-22-2008, 05:00 PM   #6
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Quote:
Originally Posted by RoadWarrior View Post
I'd imagine that VW has a MAF system not a MAP.
Yes, it does, and I was thinking it but failed to include it in my post. The statement I responded to said "any car", though...
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