Knock Retard and You...
What is knock retard?
What is knock retard? Settle down... We're not questioning your mental ineptitude; rather we're discussing the common ailment found on modified GM 3.8L V6s. Knock Retard, or "KR" as it's commonly referred to, is a reference to the number of degrees your PCM retards your timing to eliminate knock. So then you might be wondering what knock is? Our rocket scientists spent all day coming up with an answer to that one! To understand what knock retard is one must first understand what knock is.
What is knock?
Knock describes what happens when the air/fuel mixture in your cylinder flash ignites prior to the spark plug igniting it. You got that right... plenty of bang without the spark. At first this doesn't seem like such a tragic event but this phenomenon actually happens prior to the piston reaching the top of it's compression stroke which means that when knock is experienced that particular piston at that particular moment is being pushed down by the combustion when it's trying to push up. What gives? Well, usually the piston. Typically knock results in a chunk of the piston being digested by the motor.
Back to "knock retard"
So we come back to knock retard. GM has placed 2 knock sensors in your 3800. These determine knock by sensing certain frequencies that are present when knock exists. When the PCM senses knock it retards timing (or turns it back) until it is eliminated. If you have 1.5 degrees of knock retard this means that the PCM has to retard your timing 1.5 degrees to eliminate the knock it's picking up.
What causes knock?
Heat. Heat. Heat. Heat is the sole cause of knock. When there is excessive heat in the cylinder the air/fuel mixture will pre-detonate. In the supercharged versions of the motor this comes in several different fashions:
- Cylinder Pressure - High cylinder pressure (poor flowing exhaust, higher amounts of boost) results in higher heat.
- Engine Temperature - High engine temperatures lead to higher cylinder temperatures and thus knock.
What eliminates knock and why?
Essentially any way you can cool the cylinder and/or the entering air/fuel mixture will reduce knock. Here are some specific examples of modifications that will eliminate knock and how they achieve this:
- Performance Downpipe/U-bend replacement - reduces cylinder pressure by allowing spent exhaust gases to leave quicker. Reduces back pressure thus reducing cylinder pressure, thus reducing heat.
- Ported Manifolds/Headers - These reduce knock on the same premise that the downpipe does though their effect is generally more noticed especially when combined with a performance downpipe.
- Cams/Rockers - Cams are a great way to eliminate some knock quick. In fact, our prototype cam allows a stock car to run a 3.0" pulley knock free. Other manufacturers cams offer similar results. Cams increase the amount that the valves open as well as increase how long the valves stay open. Both result in lower cylinder pressure and less heat.
- Intercoolers - Intercoolers are by far one of the most effective knock eliminators available. Intercoolers cool the air leaving the supercharger reducing cylinder temperatures. The cooler air is also denser which reduces cylinder pressure. "Half size" intercooler kits are great for eliminating knock on your existing pulley while the full size kits will allow you to drop multiple pulley sizes with no knock.
- Ported Superchargers - These work on the same premise as the intercooler. Porting the blower allows it to work more efficiently which reduces the amount of power it costs the engine to run it (lowering load) and cools the outlet air temperature.
How can I tell if I have knock?
Scantools! Scantools are priceless when it comes to modifying your car. When approached by a customer with a stock car asking what their first modifications to their car should be we always recommend a scanner. There are various scanners on the market most of which scan knock as well as countless other variables which are priceless to the home mechanic.
Here are a couple scanners available:
- Aeroforce Interceptor - It's a gauge, it's a scanner, it fights crime on it's off days! Ok while the Aeroforce Interceptor gauge might not be pulling Fluffy out of a tree anytime soon it is a very powerful scanner and datalogger and a pretty nifty gadget at that!
- LS1M Scantool - For the organized or the unorganized trying to get a grip on their life, LS1M (www.ls1m.com) offers a Palm Pilot based scanner. The LS1M scanner scans, gives you horsepower estimates, and lets you put your grocery list all in the same unit. Cost: $170 if you have a Palm Pilot; $245 w/ a Palm IIIXe PDA
- DHP PowrTuner(Discontinued) - Last but certainly not least is DHP's offering: the PowrTuner. The PowrTuner scans, datalogs, and even has a neat feature that will set your laptop's screen into a flashing frenzy should knock go above a pre-determined amount. If that's not enough, the PowrTuner also gives you complete freedom to change countless perimeters in your PCM. Too much timing? Dial it back. Shifts too soft? Firm them up! The PowrTuner does it all. Cost: $389.99
So now that you're an expert on knock what do you do with your new found knowledge? Use it! As much as we love supplying you with parts we hate hearing story after story of people blowing their motors when it could have been avoided. A heavily modified engine with no knock will be nearly as reliable as a stock engine. A daily driven 12 second car with a back seat is a reality.
So you think you're a professional now? You want to question our superior knowledge? Fine but before you do that make sure you read these frequently asked questions about knock. And if you do defy the laws of physics and thermodynamics and find something we omitted, email it to us! firstname.lastname@example.org We'll be sure to credit you as long as our team of committed researchers and scientists verify your information first! And remember, when you want to take the KR out of your setup, shop W Body Store. Our researchers have confirmed we're the best and we pay them extra not to be biased!
Q. If the PCM retards timing to eliminate knock why should I worry about it?
A. This is a common question and misconception. The thought is that since the PCM is retarding timing to eliminate knock that the engine is no longer experiencing any knock. There's one fatal flaw to this logic however... The knock sensors have to sense knock in order to retard timing to eliminate it. The knock retard function is not one that is in place to allow you to run the smallest pulley without blowing your motor; it's in place to save the motor in the worst case scenario. Keep in mind: the smaller the pulley, the less room for error. Remember, GM engineers didn't take into account your need for speed.
Q. Is any amount of knock ok? I only have 1 - 2 degrees...
A. In a perfect world we would experience no knock. Unfortunately our world is far from perfect and you might be getting knock despite all your efforts to eliminate it. You still have options... Go up a pulley size. You will generally put down the same amount of power by going up .10" in pulley size with no knock than retaining your current pulley size with a couple of degrees of knock. The same power and peace of mind? How can you go wrong!
Q. What effect does knock play as I continue to modify my vehicle?
A. The importance of monitoring knock is only amplified when you continue to modify your vehicle. While 1 - 2 degrees of knock on a lightly modded (3.4 pulley) vehicle might not be devastating a more aggresive setup (2.8 pulley) won't stand up long to that same 2 degrees. An important rule of thumb is that the more aggresive the setup the less tolerance to knock it will have.