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Old 08-12-2014, 11:04 AM   #26 (permalink)
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Originally Posted by serialk11r View Post
An older car wouldn't really adapt to the piggyback, with narrowband O2 sensors, no EGT sensors, and open/closed loop based off rpm the ECU is easily fooled.

For fuel economy increasing I don't think the tuning would be that difficult especially if you can use a switchable map (not that I'm saying this is easy to get in most cases) and use a wideband input. You can erase some of the emissions safety margin built into the top of the powerband by leaning it out to a more reasonable 0.85 lambda instead of the 0.75-0.8 on a lot of stock cars, and then lean out the cells in your cruising range to 1.2 lambda, and tweak the spark advance just a bit (since you're changing the low load cells there's already a lot of advance).

If you drive a lot of highway miles like many people in the US, the 5-10% fuel you'd save doing that could pay off the piggyback in under 2 years. A Greddy EMU is what, $800-900 with a harness? Unichips are less but I don't know if they're as capable. I think the EMU can run the injectors independently using a wideband input, and you can use a narrowband emulator to get the stock ECU to cooperate, since the stock ECU would then have no idea what the injectors are doing at all. A gas guzzler SUV/truck/luxury land yacht burns through several k in fuel in a year easily.
At 30,000 miles, an increase of 10% from 15 to 16.5 mpg will save $636 at $3.50 a gallon. We are also assuming that a good tune can get 10% more economy. I'd bet that figure would probably be more realistic at less than 5%, if there is any gain at all.

But, remember, not everyone can and should tune their own car. A bad tune can destroy an engine quickly. Tuning and dyno time cost money, so you can probably add another $500-$1000 to the $800-$900 tuning hardware. Now we are up to a $1,300-$1,900+ payback of almost 100,000 miles using the above numbers.

The manufacturers do a very good job of tuning and keeping engines safe. Leaning them out and changing the timing gives them less of a safety margin if there are weather changes, bad gas (lower octane), etc.
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