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Peter,
As you probably know the MFI pump uses a three-dimensional "space cam" to meter the fuel. The two inputs are throttle position and engine speed, and the output is through a pin that rides on the cam surface itself, which in turn moves the rack that controls how much fuel is admitted to the pump's cylinders.
It's critical to match the amount of fuel being delivered to the amount of air being delivered. Obviously, the cam's intake timing and port sizes determine that. But here's the rub: the "S" pump's internal cam is set to deliver a greater amount of fuel, over a different curve, than the "E" internal cam. One would typically use an "E" MFI pump with an "E" engine to have the best possible chance of getting it in tune.
Given the value of "S" pumps (and all things S-associated) today it might be better to sell the "S" pump and buy an "E" to match rather than fight getting the two to match.
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'66 911 #304065 Irischgruen
‘96 993 Carrera 2 Polarsilber
'81 R65
Ex-'71 911 PCA C-Stock Club Racer #806 (Sold 5/15/13)
Ex-'88 Carrera (Sold 3/29/02)
Ex-'91 Carrera 2 Cabriolet (Sold 8/20/04)
Ex-'89 944 Turbo S (Sold 8/21/20)
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