Actually direct injection creates more power and better fuel milage than port injection. Period.I feel pretty confident that Ford didn't add port injection to save you from deposits on your valves. Very likely it was due to emissions and/or fuel mileage. Seems to be the driver for most OE's nowadays. JMHO, your mileage may vary.
At the time Ford introduced the change to the 2.7 from 100% direct injection to the combination of direct and port they were having longevity/deposit issues with the valves on the 2.7. It is my assumption these were correlated.
It does make sense since Top Tier gasolines were agreed upon in the 90s between ASAE and the petroleum distributers to add certain amounts of cleaners and detergents to get the "Top Tier" seal These cleaners/detergents were specifically designed to target deposits on the valves and related components as a means of increasing durability/longevity.
Direct injection does not allow any of those cleaners/detergents to flow across any components (other than injectors) prior to injection into the cylinder.
Again I am making assumptions but based on somewhat of a knowledge base.
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