Fiero Secrets

Reprinted with permission of Matt Gruber, editor of Fiero Secrets from 1984-1991, newsletter of the World Wide Fiero Club.

Adjustable Fuel Regulator on Holly TBI increases Performance of Modified Engines (4 cylinder)

Unlike the fuel pressure regulator on the O.E. TBI unit (which is set at the factory), the fuel regulator on the replacement Holly Model 3739 for the 84-86 4 cylinder is adjustable. This feature allows you to match a particular engine’s breathing capability with an optimum fuel pressure setting. The result: peak performance. By increasing the fuel pressure, you increase the amount of fuel flowing through the injector for a given amount of time that the injector is open. The Holley TBI can be adjusted from 10PSI to 17PSI.

For instance, if you plan on using the Holly Model 3739 TBI with an aftermarket manifold and header, any deviation from the factory set 12PSI level won’t be necessary. On the other hand, if you are planning extensive engine modifications including: head work, porting, increasing the cubic inch displacement or installing a more radical camshaft, increasing the fuel pressure will be necessary to prevent an overly lean air/fuel ratio (modifications recommended for off-road use only).

Since modifying your engine allows for more airflow, a higher fuel pressure setting is needed for a corresponding increase in fuel flow. At the stock fuel pressure settings, it may not be possible for the injector to flow enough fuel because it can only stay open so long.

With the modified engine, the O2 sensor will signal the ECM (Electronic control module) that more air is flowing through the engine. The ECM automatically responds by modifying fuel flow to achieve the optimum 14.7 to 1 air/fuel ratio to increase fuel flow. When more air and fuel passes through the engine’s induction system, it inevitably improves performance.

Basically the Holley regulator consists of 2 chambers, separated by a diaphragm. On the fuel side of the diaphragm a valve is employed to increase or decrease fuel flow as system pressure fluctuates. The spring side contains a calibrated spring with an adjustment screw that can be set for optimum system pressure.

Other features of the Holley adjustable regulator include: a hardened steel ball and valve assembly, and a stainless steel regulator spring that resists corrosion.

The stock throttle body bore is 1.68”. The Holley throttle body bore measures 2.00”. This means more fuel/air flow and more responsive acceleration.

(Ed note: an accompanying chart to this article showed that on a dynamometer, power to the rear wheels with the OE unit measured 79HP at 4400 RPM’s. With the Holley throttle body, the result was 100HP at 4400 RPM’s. Also, note that this replacement throttle body is still available from the Fiero Store as their article #52510, listed at $359.00.)