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Injectors above or below Throttle Plates

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David Ferguson made a comment on another ITB post that got me thinking.

"If your injectors spray below the throttle, then you want to reference your fuel pressure regulator to manifold pressure. This will just cause errors in trims if you don't and you will just end up "baking in" those errors. Personally, I prefer to get a 10% fuel increase if that's requested at all manifold pressures.

If the injector spray to atmosphere, (ie, above the throttle), then it is OK to reference the fuel pressure regulator to atmosphere."

I am running Borla EightStack which is modeled after the Weber 48IDA's - the injectors are situated above the throttle plates. I have had two similar systems and ran both of them with MAP compensation. When the injector is above the plate, is this setting incorrect? What implications would it have?



The injectors are characterized with "differential fuel pressure". i.e. the Fuel Pressure in the fuel rail minus the fuel pressure that they spray into. So at higher RPM throttle nearly closed (lower MAP), the differential fuel pressure might change unless the fuel pressure regulator is referenced to wherever the injector sprays.

Your MAP compensation is not there to deal with differential fuel pressure changes, but to compensate for the load the engine sees and getting the mixture correct in all situations.

Here is an article from Paul Yaw at Injector Dynamics regarding this topic:


Thanks, David. That article finally turned on the light switch.

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