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Old 06-07-2004, 07:10 AM   #10
Scooby Specialist
Member#: 689
Join Date: Dec 1999
Chapter/Region: NESIC
Location: Lebanon, NH, USA
2005 Outback 2.5 XT
Champagne Gold Opal


Originally posted by 8Complex
Well, I have no real proof, but I'm guessing that it's possible for the ECU to see the rpm needed to initiate the cut as it is spraying the injector, and end up shutting off the injector mid-fire. Granted, I can't speak from first hand experience, but I believe it's the main reason most forced induction setups with standalones use ignition cut to rev limit, not fuel cut.
I'm pretty sure the ECU doesn't cut fuel in "mid-fire". Most ECU's use microcontrollers with PWM control registers to set and operate injector duty cycles. It would be pretty stupid for FHI to override the PWM auto-reload registers or gate the transistor output drivers to allow partial lean conditions that can lead to engine failure (and drive up warranty costs), when there are zero-cost ways of doing fuel cut at the rev limit by designing the firmware correctly.

I'd be surprised by a standalone use only ignition cut for a rev limiter. At those high rpms, the last thing you need is fuel (probably at max duty cycle) washing oil off your cylinder walls. I'd stay away from those standalones if I were you. Perhaps you're thinking about the MSD ignition-only products, where they have no control over the fueling?

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