Competition Tachs & Gauges - KNS Autosport

Tech Support:
TECHNICAL INFO - Q&A
RFI
A tachometer counts how many times your ignition fires for
each revolution of the crankshaft and. Sounds fairly simple and
bullet-proof, right? NOT REALLY! Radio Frequency Interference
(RFI), also referred to as “electrical noise”, can force your
tachometer to give you false readings. False readings will skew
the accuracy of memory tachometer functions. Stewart Warner
Performance Tachometers are protected from RFI related to
racing, although electrical noise could reach such an excessive
level that the tachometer may see it as legitimate ignition signal
and react accordingly, thus causing an inaccurate readout. So
the question is, “How can I prevent RFI?” Here are some “rules
of thumb”:
1) Be sure there are good electrical grounds throughout
your car, and check to see that: a) your tachometer AND
ignition system both have a good grounds, preferably
at the battery ground post. (The battery acts as a huge
capacitor, sucking up excessive amounts or noise.);
b) the engine block is appropriately grounded to the
battery.
2) Route all tachometer wires as far from ignition system
wires as possible.
3) Use high-quality plug wires, as poor plug wires are a
large source for RFI.
4) Make certain that the spark plug gap is properly set.
5) Check to make sure there is no mismatch of ignition
system and coil—this can also cause excessive RFI.
TACHOMETERS—HOW THEY WORK:
Until recently, tachometers were calibrated based on the number
of cylinders in the engine. Now, there are all types of engine
control modules (ECMs)(on-board computers) and distributorless
ignitions, and the old standard rule—”half the number of
cylinders equals the pulses per revolution (PPR)”—no longer
applies. “Pulses per revolution” relates to how many times the
ignition fires per crankshaft revolution. The tachometer outputs
from the ECMs can range from 1-PPR to 4-PPR for a V-8 engine.
So, the new standard is to refer to PPR instead of the number of
cylinders. Stewart Warner Performance Tachometers work with
most 2-, 3-, 4-, 5-, and 6-PPR, and our memory tachometers
with shift-lights work with 1-PPR as well. This range of pulses
per revolution will cover most 1-, 2-, 4-, 6-, 8-, 10-, and 12-cylinder
engines.
Stewart Warner Performance Tachometers are designed to work
with a wide variety of ignition types, including standard inductive
[coil (-) connection] and any ignition system with a clean
tachometer output signal. The input level can range from TTL 5V
(outputs from newer engine control modules) and 300-400 volt
pulses from coil (-) on most inductive ignition systems. Tachometers
will operate on most traditional capacitive discharge (CD)
ignitions when connected to the ignition primary circuit. However,
for high-output CDs (such as MSD) only connect to the
tach output terminal, never the coil primary. It’s best to consult
our tech support team if you’re unsure, rather than risk damage
to the tach or ignition system.
Our Stewart Warner Performance Team is prepared to help you
with tachometer installations! Call us a call 1-866-SWP-RACE.
Be prepared to provide information about your ignition system.
96 1-800-676-1837 Or 1-866-797-7223