BKLOCO
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| posted on 2/8/06 at 01:50 PM |
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Fuel cut on overrun.
I'm going to post this question on the MegaSquirt forums but I thought I would post it here as well to see if anyone had any thoughts.
I am lead to believe that most modern fuel inj systems have a fuel cut on overrun algorithm built into the program.
MegaSquirt (as far as I know) doesn't have this facility.
I would like to incorporate this function in the interests of economy.
My thoughts are as follows:
My engine runs with MAP setting from 25 kpa. to 100 kpa and idles around the 30 kpa region.
It only ever dips below 25 Kpa on overrun.
I figure that if I add a row of bins at 15 Kpa and enter a value of zero into all these bins then that will efectively give me fuel cut on overrun.
Can anyone who has experience with MegaSquirt mapping see any problems with this.
The only one that I can see is that I will be running the map 12 X 11 rather than 12 X 12 due to "wasting" one row of MAP values.
I don't think this will cause undue problems as many ECUs run 8 X 8 anyway.
I think it will simply make my map a little "coarser"
The only other thing that occurs to me is: Do you need fuel on overrun to lubricate the bores?
I figure that if you have no fuel injected then the engine is simply acting as an air pump.
Constructive comments please.
Experience is what you get when you don't get what you want!!! 
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nre
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| posted on 2/8/06 at 02:01 PM |
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The emerald ecu has a setting for fuel cutoff, I've found it quite tricky to get it to work without affecting jerkiness as you go back onto the
throttle, and fuel economy gains seem to be minimal.
You basically specify a throttle pot position below which fuel is cut off and a lower rpm limit (so that the engine doesn't stall when you dip
the clutch as you come to a standstill).
I don't see any issues with doing damage to the engine, as no fuel = no combustion. Excess fuel will wash oil from the bores which is a bad
thing...
Try out your idea, I don't see it doing any damage, but not sure if you'll find a satisfactory solution...
Neil
Sylva Mojo Zetec on GSXR750 throttle bodies...
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Doofus
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| posted on 2/8/06 at 02:45 PM |
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There's a fuel cut option on the MSnS-E code that lets you cut fuel when over a certain RPM, lower than a certain KPa and when the tps is shut
that means you don't sacrifice a row of bins.
I've tried using the fuel cut and like Neil I get a judder and a pop out the exhaust every time the fuel cut turns on and off. It's very
annoying at best and at worst causes a huge bang out of the exhaust. I think it's down to the engine pumping oxygen into the hot exhaust where
there is unburned fuel. A leaky exhaust makes it worse on low MAP bins.
I turned the fuel cut off and now I just get a burble on overrun which I thought was safer since I was doing a track day with strict noise limits.
To get the best from fuel cut I’d try keeping the min RPM setting higher than what you’d drive in traffic at (to try and stop it popping when you
drive to the shops) and set the min KPa just under 25. Then use the TPS calibration tool to see what the raw input is when the throttle’s shut and add
5 to it.
The best bit is you can play with stuff till you get it right =)
Paul
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