Boost and Knock Basics
 

Combating Engine Knock in your Forced Induction Engine.

What is Engine Knock?

Engine Knock is also referred to as premature detonation. As the piston in your engine travels up and down, there is a point in it’s travel where it stops and begins to travel in the other direction. Ideally, the force of burning fuel pushes down on the piston at exactly the moment that it reaches the top of its travel and begins to force it back downward (the power stroke of the cycle).

Now, because the fuel does not burn instantly, we change the timing of the spark in the combustion chamber. 0degrees of advance is considered to be this point where the piston turns around and goes the other way. Advance, is when the spark happens before the piston reaches this point. The number of degrees is measured in degrees of crankshaft rotation. So an engine running 14degrees of advance timing (noted by a positive number in scan tools), is creating the spark when the crankshaft is 14 degrees of rotation before each piston reaches this top of travel point. IF, the timing is advanced too far or engine gases ignite prematurly, the force will push down on the piston while it is still traveling upward. This doesn’t seem too bad until you consider the force of this “push”. It is equivalent to hitting your piston with a sledge hammer, and none of us would do THAT intentionally. This is what KNOCK does to our engines. The PCM (power control module) will account for engine knock and reduce timing, and other protective measures depending on the vehicle’s individual PCM program. This lessens the effectiveness of the Power Stroke of your engine. The PCM is protecting your engine from turning itself into scrap iron.

Forced-induction adds another twist to the equation. Boyle’s law states, that if you compress a gas, the gas is heated as a result. We don’t need to go into all the technical details here, but suffice that your supercharger is compressing the air that goes into your motor. Then, the compression stroke of the engine is compressing it further. When we increase the amount of boost to the engine, we also increase the heat of the air as it begins its path into the cylinder where it is compressed even further. Gasoline has a “flashpoint”. This is when the fuel will combust without the introduction of a spark. IF, this temperature is reached in the cylinder there is no more control over the timing of the burning of the fuel. THIS is where we need to take additional precautions when adding more boost to the engine.

There are several ways to combat this.
First, is to reduce the boost on the motor. We don’t want to do this, because if we can increase boost WITHOUT knock, we can increase both HP and Torque. So this option is not valid in this discussion.

Second is to have lower air temperatures going into the system to begin with. This is where a good quality (CAI) cold air intake comes in. The factory intake system is designed for many purposes, the least of which is HP. One of the largest considerations in a factory intake system is the amount of noise it lets out. The factory combats noise by PURPOSELY putting some restriction into the intake. This increases the velocity of the air going into the engine, but decreases the ability to flow large amounts of air in a short time frame. This is why throttle response is improved with a CAI, because the engine can take a GULP of air instead of a long sip like from the factory intake. The closer to true outside air temperature you can get with an intake, the better. This is the reason that many forced induction vehicles also use an intercooler, alcohol injection, Nitrous Oxide and other methods to cool intake air.

Third is to decrease the temperature inside your engine. This can be accomplished a couple ways, or both for even better results. One, is to decrease the temperature rating on the thermostat. This rating sets the temperature when the thermostat opens, and the base operating temperature of the engine. Most stock thermostats are 195 degree thermostats. Remember, “flashpoint” is a fixed number. One degree below the flashpoint and knock won’t happen, so 1 degree of temperature decrease CAN make all the difference in the world. This is why many people that modify forced induction vehicles will use a colder thermostat. 180 or 160 degrees for example. The other way is to decrease the temperature rating or (Heat Range)of the spark plugs.

Spark plugs and temperature ratings simplified:

The temperature rating on a spark plug does not indicate how much heat it creates! or how hot the spark is. What the Heat Range indicates, is how fast the plug body takes heat AWAY from the cylinder. Yes! That is really how it works. That being considered, we want to keep the temperature inside the cylinder down. After the fuel burns, it gets exceptionally HOT. The faster we can take that heat away from the cylinder, the better. NOW, if we take the heat away too fast, the plug will get fouled quickly and not be effective in creating a good solid spark to ignite the fuel. So a balance has to be maintained. Email us and we will be happy to tell you which plug temperature is best for your particular needs based on modifications.

Fourth, we can improve the flow OUT of the combustion chamber. If we add too much boost on the intake side, and don’t account for how the exhaust gets out, boost can pile up. Sometimes referred to as “boost-stacking”. Basically, this means the cylinder is not starting with 0 pressure when it should. There is residual pressure in the chamber, because the exhaust system cannot clear it out fast enough. THIS is where a good exhaust system comes in. Headers can actually DECREASE the amount of boost you apply to your engine. WHY? This is because ALL of the exhaust gas can escape, and the cylinder is starting over with Zero residual pressure. This allows you to step up more boost with yet a smaller pulley, with the same net boost as a result. BUT, with more air volume flowing through the engine. This equates to even more power.

Lastly, The fuel you feed your engine is key. Lower Octane Fuels have a lower flashpoint temparature. This makes low octane fuel very suceptable to knocking. Use the highest octane fuel available to you, and when operating at Wide Open Throttle it is best to use a Higher Octane Racing Fuel, these can be found at Auto Parts Stores and Race Tracks all around the country in 94-109 Octane ratings.

As you can see, it is very important to address these Engine Knock Issues in order to get the most from your engine. A car running too small a pulley for the supporting modifications, will actually run SLOWER than one running a larger pulley with no knock. The PCM will adjust for knock as we stated before and rob your engine of the power you just tried to add. We have been modifying modern, forced-induction vehicles for years. Todays PCM/ECU programing must be understood in order to get the most out of your enigine. This is not new to us. Old School ideas are great but they can only take you so far, todays complex engines must be handled properly in order to get the most out them, while still protecting it. We are happy to help out with a game plan for your vehicle, regardless of where you purchase your products.

Jay T. Sandvick
Modbug Performance
Co-Owner/Technical Supervisor

Van Ward
Modbug Performance
President/Managing Partner

that is an awesome discription! many people on this board, if they take the time, will learn a ton from what was written. Thanks for educating us all

Yep pretty much goes like this.

Supporting Mods: Colder Plugs, Colder Thermostat, Cold Air Induction, Premium Fuel, Larger Cat-back exhaust.
Intermediate Supporting Mods: Headers, High Ratio Rockers and/or Cam, Ported Blower, Ported Throttle Bodies, PCM Tuning, Larger Injectors etc.
Big Dod Mods: Larger Blower, Extensive Head Modifications, More Aggressive Cams, etc.

But as you can see every step along the way is either an airflow issue, cooling issue, or fuel issue. Your engine is a Pump the more Air/Fuel you can move through it, the more power you generate.

awesome, just awesome. feel like i just went to superchargers 101! what products are u offering or is it services?

Great little article, helped me fill in some gaps. :)

Thanks for posting, I learned a few things from that article :)

Is there any way we can make this a sticky?? Because almost every newbie thinks that putting high octane fuel in their base Cobalt will make it run better, which of course isn't true at all. Not to mention all the valuable info in the article for us more experienced guys, like heat dissipation, spark plug heat ranges, air intake temps, ect.

I think it would be very beneficial to have this at the top of the page in this forum ;)

I think we should sticky it too!

Modbug,

What would you recomend as min mod's to do before we put a smaller pully?

We are trying very hard to track down somw products for CobaltSS and Ion Redline owners. At this time given that the CobaltSS is intercooled, this is a great help. We recommend using NGK BKR7E Plugs with a 3.25 pulley. Those are one heat range colder than stock. This will help dissipate heat and fight pre-ignition. We also recommend using only higher octane fuels. Most states only offer 91 octane as a premium fuel. If you can find higher than 91 thats great. Some places have 93 and 94 octane pump gas. We are working on getting 180 and 160 degree thermostats also. Those will help keep the engine cool. Basically that is a good start. A good scan tool is handy to track your engine performance. Scanning O2 and KR thats Oxygen and Knock Retard. Those parameters will tell you allot about how your engine is performing. Remember that Knock will cause the PCM to take timing away and reduce power. KR is measured in degrees of Timing needed to keep your engine safe. Ideally you want 0 degrees of KR. 2-3 degrees is acceptable if its only occasionally. A Scan Tool is one of the best purchases you can make as a tuner, that gives you the information you need to make the proper adjustments.

Also as you drop smaller and smaller in pulley size you will need to add more mods to the car. Headers, Catbacks, Colder and Colder Plugs, Ported Blowers etc. Everycar is different and there is no cookie cutter approach that works for every car. Climate, Elevation, etc have alot to do with performance.

I am glad you have enjoyed the reading.

THANK YOU! This board really needs vendors like this. however, i highly doubt that you would gain anything from a ported m62, at least the current gen ones. Ive seen some pics of the underside of the blower, and it looks perfect. The triangular shape that it is supposed to have is there, and unless there is a step between the blower and the TB, then there is not reason to port it. According to some pics the Zinner posted on the TB, that thing is massive. According to my calc. its about 75-78 mm TB. GTP owners use a 78mm TB as an upgrade. A stock LS1 TB is 78mm, just to give you an idea.

Only time and research will tell on that front. I am sure we will hear all the same arguements that the w-body community brought up back in 1997. About how the blower is at peak performance with a stock pulley and you won't get gains by porting them, and so on. We gotta get dirty and see what works and what doesn't, but our experience with the 3.8 SC L67 motors will hive us a solid foundation to start from.

Just as a note, the SS is already equipped with a 180deg thermostat from the factory, not a 190 like most cars. You could maybe get away with a 160, but you certainly wouldn't want to go any lower than that, as it'd keep you out of closed-loop. And I'd bet 160 will cost you some noticeable fuel economy.

i agree on the Wbody sc thing. However, the Gen 5 m90 looks almost identical to the m62's underside. There is a few pics on the redline forums on a stock m62 sc and it looks very nice. Id be worried about losing effiececy if you port it. Just my $0.02 on the subject, but it looks like you will be a vendor good vendor for this site, as long as you dont turn out to be another intense company.

As far as porting on the M-series blowers goes, most of the work is done to the inlet side of the blower assembly. You are correct in that the outlet side is very well done in its current form.

The inlet side, however; can use some minor tweaking to make sure all surfaces are "gasket matched" and the smoother the transition between the surfaces, the better the air-flow. Although performance gains will be minimal on this type of modification on a stock engine, once the "envelope" is pushed on the performance of the engine-the sum of the supporting modifications is greater than the individual gains added together.

Opinions on blower designs have varied throughout the years, but realize that blower design is a very touchy place to dabble. Each change in design will effect changes in different areas of the blower's charicteristics. ie, porting a blower in one way will yield better performance at high boost levels, but lose lower speed boost charicteristics and visa-versa. There are plenty of other modifications that should be made first to make room for gains from a ported blower assembly. A stock SS, will see only small gains from a ported blower alone.

Remember that your engine is essentially and air pump, cold air in-burned, hot air out. Flow must be must be considered on BOTH sides of the engine. You can make it flow in easily, but if the exhaust can't get out of the way; the new air cannot get in.

Depending on your climate, level of mods, and driving habits (ALL factors in fuel economy)-an engine with a 160* stat can actually give more power with no sacrifice in mileage. Fuel economy is FAR more effected by driving habits and environmental conditions than on base operating temperature of the engine. Not to say that it is not a factor, but a small one by comparison to driving habits and environment.

Questions are always welcome.
Jay

Good read ModBug, I learned a few things from it.