Intake runner lengths

[David Sagers]

With port EFI (TPI or any system with the injector just above the intake port of the head) runner length isn't as crutical as with a carb sitting on top of the intake.  Yes the long runners on the early TPI motors really make incrediable torque, but they run out of steam about 4500 rpm on a 5.7 engine and even sooner on a modified engine, especially a 400 small block.

The difference is that the air column in long narrow runners has to move faster to fill the cylinder as compared to air large wide ports, assuming valve timing and everything else is unchanged.

Two of the reasons you wanted the air moving fast is to keep the atomized fuel suspended in the air and to keep a strong vacumn signal at the carb to draw the fuel out of the carb.  With port EFI  both of these concerns go away.

A final concsideration with long narrow runners is that at lower rpm  the comparatively faster moving air column (as compared to large wide open ports) creates a ram effect which provides better cylinder filling and more horse power. At higher rpm the bigger ports have an obvious advantage.  However, on street driven cars, throttle response and low rpm torgue are more important and much funner to drive than an engine that makes 600 hp at 8500 rpm, but is a dog until the rpm reaches 4000.