Intake runner lengths

[Bruce Plecan]

-----Original Message-----
From: David Sagers <dsagers at ci.west-valley.ut.us>
To: diy_efi at efi332.eng.ohio-state.edu <diy_efi at efi332.eng.ohio-state.edu>
Date: Friday, December 11, 1998 11:40 AM
Subject: Intake runner lengths


>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.

The lenghts are just as important.

  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.

Is this due to the runner or the fact that is when the fuel/timing
calibrations begin turning of the performance?.
>
>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.
>
Seems to be ignoring the pressue wave in the runner as the valve closes.
Bruce