Fast 30400-KIT-EZ-EF Instructions Manual - page 32

32 

RPM Module 

EZ-EFI

®

 is an advanced fuel injection system. Engine speed is one of the fundamental elements of its 

fueling calculations. When not controlling ignition timing, it relies on the vehicle’s existing ignition 

system for a steady, reliable RPM signal. An inconsistent or noisy RPM signal can appear to the ECU 

as erratic engine speed. Spikes, dropouts and general instability of the RPM signal fed into the ECU 

can have an adverse effect on the running of the engine. It can also interfere with proper learning or 

prevent learning altogether. 

There are two basic options for supplying the EZ-EFI

®

 ECU with an RPM input when relying on the 

vehicle’s existing ignition system. 

1. Aftermarket capacitive discharge (CD) ignition −  An aftermarket capacitive discharge

(CD) ignition box is recommended. This form of ignition is beneficial to just about any

application. Having a known good, strong spark prevents a host of potential running issues.

This type of ignition is also well suited for use with EZ-EFI

®

 because it typically has a

“Tach” output which is a clean digital signal. This is the preferred RPM source for the EZ-

EFI

®

 system when it is not controlling timing.

Other aftermarket ignition products besides ignition boxes may also supply a clean digital 

tach signal.  For example, an aftermarket HEI module with a dedicated tach output (not to be 

confused with the “TACH” terminal in the cap of a standard HEI distributor) can be a good 

RPM source for the EZ-EFI

®

 system.

If using one of these ignitions, the “RPM INPUT” wire in the EZ-EFI

®

 wiring harness is 

connected directly to the “Tach” output from the ignition box. The RPM Module included 

with the kit is not used in this case. And no part of the EZ-EFI

®

 system is connected to the 

coil. 

NOTE: Some aftermarket ignitions have a feature for displaying their programmed rev limit. 

This is accomplished by briefly outputting an RPM signal on their tach output wire soon after 

key-on. The intent is to have the vehicle’s tachometer display the rev limit setting. This 

feature is not well suited for some fuel injection systems – including EZ-EFI

®

 – as it appears 

the engine is actually running at that speed. This can lead to the ECU rapidly pulsing the 

injectors (injecting a large quantity of fuel) and flooding the engine. Contact your ignition 

vendor to verify whether or not your ignition product has this feature and for details on