Improving Throttle Body Performance (Knife edging and flush fitting)

[Phochem]

Readers of enthusiast performance magazines will probably recall seeing advertisements for "oversized throttle bodies". The claims expound on improvements in engine breathing that follow from fitting a larger-than-stock throttle body to an automobile engine. What the sales pitches do not explain, however, is that the effective diameter of the intake system is only as large as its smallest opening -- generally the inlet of the intake manifold!

 

 

IMPROVING WHAT YOU'VE GOT

Although the size of the intake manifold inlet will eventually limit airflow, there are a number of simple throttle body modifications that will improve engine breathing at marginal cost. Two such methods are "knife edging the throttle plate" and "flush-fitting throttle plate screws".

 

 

KNIFE EDGING

The volume and velocity of air passing through the throttle body will be determined by the aerodynamic properties of the throttle plate. Performance engine builders know this all too well and often fit complicated sliding throttle mechanisms to regulate airflow into their competition motors. Mainstream automobile manufacturers, however, often overlook the humble throttle plate when tuning an engine for optimum power.

 

Such is the case with Volvos where throttle plates are simply "punched" from thick brass sheet.

 

Punched throttle plates are characterized by their blunt leading edges that promote airflow separation at high velocities. The result is a throttle plate that will effectively regulate airflow, but fails to promote maximum breathing as engine speed rises.

 

 

By carefully removing the throttle plate and beveling its leading edge, airflow through the throttle body will more closely resemble the theoretical limit afforded by the bore. Note that a trailing edge bevel is not required as the throttle shaft introduces virtually unavoidable turbulence at the throttle plate's mid-point.

 

 

 

Readers will note that the edge of the leading edge of the "after" picture is reduced, but not entirely eliminated as sufficient material will be required to effectively seal the throttle body at idle!

 

Flow across the throttle plate will be improved after careful modification. The result is greater power -- particularly at high rpm when airflow is at its greatest!

 

 

 

FLUSH FITTING SCREWS

The next "quick and dirty" little trick to improving throttle body performance is to flush fit the screws that retain the throttle plate to the shaft. In most cases, this is very straight-forward with the elimination of the thread locking tabs that prevent the screws from backing out. In situations where button-head screws protrude above the throttle shaft, however, an appropriate flat-head screw in carefully beveled seat will be required.

 

With the locking tabs removed, however, an appropriate thread sealer will be required to prevent the screws from vibrating free. One of the better products for this task is Loctite