Eliminating KR

Oct 13, 2011
There are many performance mods available for our cars that take different approaches to making hp but they all have one thing in common: Airflow. To make hp you have to have airflow and lots of it. Since the SC forces everything into the motor I have found that no matter what I do to go faster I am always somewhat limited by the size of pulley that I run. The smaller the pulley the more flow meaning the more hp. But as soon as you get to about 10lbs you run into a KR problem with the L67. Reason being is that the more you compress a flammable mixture the more explosive it becomes. So with this highly compressed air/fuel mixture wanting to explode too fast the car has to retard the timing or back off the timing advance, each of which will cause you to lose more power than you gained with the increased airflow. There are a few possible solutions to this problem.
  • Lower the compression of the motor.
Without rebuilding my motor or at least taking off the heads I can’t change the compression so that’s out.
  • Lower the boost level.

While this is a better option than water injection I don’t want to decrease the airflow by running a larger pulley to lower boost because then I would lose power.

  • Decrease the explosiveness of the mixture.
To do this you can
1.Run high octane fuel but anything over 93 is expensive and hard to find.
2.Use octane booster but that is bad for your motor, expensive and not very effective.
3.Cool down the intake charge, or cool down the combustion chamber but most people can’t afford an intercooler, don’t want to tackle the difficult installation and aren’t willing to lose boost from the restrictiveness of the IC. Another way to cool the intake charge is to change to a centrifugal SC which is more efficient but that has the problem of being an expensive custom install.

Having looked at the 3 fixes above it would appear that we are out of practical options, but here is what I did to help eliminate my KR:

  • Free flowing exhaust.
The exhaust is very important because the more air that you can get out of the combustion chamber quickly the less is left over to cause you power robbing problems. The exhaust gas is extremely hot and any left over in the cylinder will cause the chamber and the mixture to stay hot thus contributing to detonation and finally KR. Also with less exhaust in there, there is room for more fresh mixture and this will help prevent boost stacking. It will lower the manifold boosts slightly as well, which in this case is good.

I believe the biggest restrictions in the factory system are the downpipe and the manifolds. I had the ‘99’s downpipe replaced with 3″ tubing with the factory flex left in place. This goes into the factory cat, which is gutted. I want to add here that testing before and after gutting the cat produced results showing that the cat was not a restriction even while running 13.2’s. Even though I don’t think that the factory cat is much of a restriction the mod is free so hey it can’t hurt. Just make sure you have an O2 sim before doing this. From the cat I have 2 ½” mandrel bent pipe that goes into an SLP Y and from there into the factory mufflers. I kept these because I don’t see them as being any kind of a restriction and I wanted to keep the car quiet. Up front I upgraded the factory exhaust manifolds as shown here:
ZZP Exhaust Manifold Porting. On the heads I removed all of the imperfections and burs right at the exit with a dremel. I didn’t want to take them off of the car so this was really all I could do.
  • Fuel.
You must run the best gas that you can find, period. I only use Shell but any top brand gas 93 or higher should do. Running rich will also help you but only to the extent that you don’t start losing power from being so rich. In our GTPs .91 is about the perfect balance of removing KR vs. losing power from running too rich. Anything above .92 and you really start to slow down from the motor basically drowning in fuel. With race gas you want to be running around .88 depending on your boost & the fuel octane and with an IC or WI even lower .82-.86. The only time that you do need to be excessively rich .92-.94 is while you have the NOS spraying. To adjust the A/F ratio you can send in your MAF to have us mod it, get an FPR, or use a MAFT.
  • Intake charge temp.
I learned that heat is created from compressing air. Think of it like this: A given volume of air has heat in it. Say you take a gallon of 70 deg air and compress it down to an ounce. Well now you have a gallon of ‘air heat’ in the space of an ounce. The ounce will now be 1000’s of degrees! There is no way around this and when applying this principle to our cars – 14.9lbs of boost you are cutting the space that the air occupies in half! So you now have very hot air in the intake manifold. But that’s not all there is also heat that the SC makes just from being inefficient added to this. There is another physics principal at work here, something that we can do something about. The outlet temp of the SC can be determined by boost. But to calculate it you don’t just take your boost as it applies to atmospheric pressure, you have to take the difference of the SC outlet to the intake vacuum. So if you have a restrictive intake you might have some vacuum in the intake causing the SC outlet temps to be higher than they need to be. Not only will the vacuum cause your outlet temps to be higher it will also mean more hp robbed from the motor to turn the charger. This is why I started the throttle body project and spent hundreds of hours studying the intake design of our 3800 engines. Everyone has a decent air box but the TB is a restriction even with the stock pulley and becomes even more so as you start to run the smaller pulleys. If I had to guess I would say that having a mod’d TB is like getting an extra lb of boost for free (as it applies to KR). Not only that but most people don’t realize that the TB to SC gasket hangs in the airflow just a little bit causing more restriction. Cutting this gasket out 2mm bigger is a free mod.
  • Combustion chamber temp.
The hotter that the combustion chamber is, the faster the explosion is going to take place and the less timing you can run. To cool down the combustion chamber without an intercooler or water injection you need to cool down the whole motor. To do this I removed the engine cover, removed the weather stripping on the back of the engine compartment, removed the thermal blanket on the underside of the hood, wrapped the exhaust manifolds, put in a modified stat, made a fan switch so that I can turn on both fans at any time from in the car and replaced ½ of the Dexcool with distilled water. Water while not having the heat range of coolant has much better thermal properties to cool the engine. For some people running the engine too cold will set an SES light. To get around this you can put a 16-20 k ohm resistor in place of the air intake temp sensor (Email me if you need one sent to you for free). I didn’t need to do this but some people with finicky computers will. As far as I’m concerned there is no way around running a very cold engine if you want to make power, period. It’s either doing these things, get an intercooler, or do a lot aof internal motor work. The nonsense I hear about engine wear and oil sludge is just that, nonsense. I have the temp management down so good on my car that I can just about do laps at the drag strip and never have it heat up. Here is a pic of my temp gauge after driving 90 minutes to the track in the summer.

All of these little tricks do have their limits though. I can only run the * 3.1″ pulley with the outside temps being under 75 or so and then I start to get some decent KR.

Hope this helps with some of the questions that I’ve been getting.

* Update, with a modified blower on the car producing more boost than normally possible with a given pulley and the new info just coming into light about the PCM learning timing I have since switched to using the 3.4 and the 3.25 for daily driving.