Performance Developments Knock Link Sensor
When you absolutely, positively must detect detonation and pre-ignition
Detonation and pre-ignition must be avoided at all costs. Detonation happens when, after the spark plug fires, the remaining unburned air/fuel mix ahead of the flame front is ignited uncontrollably by the pressure of the air and fuel that has already been ignited. It will usually happen on the cylinder's downward power stroke. Pre-ignition happens before the spark plug fires, in which air and fuel is ignited simultaneously by something other than ignition spark. This will usually happen during the upward compression stroke. So as the piston is mechanically moving up, the pressure from the prematurely ignited mixture tries to push the crank in the other direction-not good.
Pre-ignition can cause detonation (and vice-versa if detonation leaves hot spots of a high enough temperature after the exhaust stroke). Both can cause cylinder pressure to increase at such a rapid rate that the head gasket could blow. And that's a best-case scenario. Other possible carnage includes bent rods, spun rod bearings (eventually seizing the motor), damaged piston rings, melted pistons, even holes blown through a piston. Thankfully, knock sensors can detect sonic waves resulting from detonation and pre-ignition.
If a high-performance engine sees relatively high cylinder pressures by way of ignition advance, higher compression, nitrous oxide or increased boost, then a Knock Link could be the answer. With today's knock-sensor equipped vehicles, there's the safety margin of spark retard and fuel enrichment by the vehicle's ECU, be it factory or aftermarket. However, the ECU has to wait to detect some specified voltage of knock sensor activity before it will pull timing. There's a possibility that the damage could already have been done, so why rely solely on the ECU for damage control?
Distributed by Performance Developments, the Knock Link is a fairly simple device using its own knock sensor, which triggers a set of LEDs. The LEDs on the readout show only background noise, which rise exponentially with engine noise (namely torque) and in a way that is easily deciphered. Two green lights indicate normal operating engine noise, while two orange lights urge caution. A sudden rise in engine noise will light up a bright red light (knock), which indicates that something other than normal mechanical engine noise is occurring.
The unit is adjustable for sensitivity, which makes it compatible with any vehicle. And it's easy to set up. Let's say a vehicle reaches peak torque at 4000 rpm. We assume the engine is not detonating at idle (if it is, kill your tuner). Bring the revs up to 4000 rpm and turn the adjustment screw clockwise, increasing sensitivity, until one green light glows. This one green light will represent normal mechanical engine noise. Any extra vibration encountered by the sensor will show up on the LED readout. The goal is to stay green on a full sweep of the rev range.
The Knock Link has been tested on ec's turbo-charged Project E36 M3 for the past year, and it's a great little device. Since the car is controlled by an AEM EMS, the factory knock sensors' activity could be monitored in voltage form, but that requires a laptop in the passenger seat and the hazard of taking your eyes off the road. Also, it's not possible to simply log the run to inspect later-what if the engine detonated through the entire pull without the driver being aware? The Knock Link can be hooked up anywhere, making it as easy to monitor engine noise as it is your speedometer.
Most of the time it's in the M3's glovebox. I refer to it after either an increase in boost, a significant change in weather, or even fueling at a different gas station. Bad things can happen with a bad batch of gas while pushing nearly 170 wheel-hp per liter. On the track, the sensor sits on the dash in plain view.
The Knock Link can be extremely sensitive, yet consistent at the same time. If the car is at part-throttle in any gear, it never fails to display one green light. At the minimum boost of 10 psi (410 wheel-hp), it shows one green light in any loaded gear with 92-octane. Increasing boost to 15 psi (485 wheel-hp) on 92-octane, one or two orange 'caution' lights show up. Just one gallon of 112-octane in a full tank and the readout goes down to one or two green lights, and consistently down to one green light if an additional gallon or two goes in. Instead of adding racing fuel, two green lights glow with 92-octane and boost dropped to 12 psi. The only time I've encountered the red light is going past 16 psi (500 wheel-hp) on pump gas, and that's already at a conservative 10 to 11 degrees of ignition advance. These findings have been repeatable.
The Knock Link comes with a terminated harness, a sensor and a Knock Link LED display for $328. For the more serious tuning shop, Performance Developments also has a Knock Block available for $625, which uses two sensors and a set of headphones to listen for engine knock. -Pablo Mazlumian