Mystery Knock Retard (KR)

Dec 22, 2002

When should you not have any KR? A problem that has plagued many GP’s has been the mystery KR. Some GP’s have always been a little slower than they should be. Some GP’s are haunted by KR that just won’t go away, or they get Knock Retard much sooner than they should. The problem can be frustrating and seemingly impossible to fix. However, we have some new light to shed on the subject.

Head gaskets After tearing down many engines, we have learned that not all of the head gaskets get aligned the same from the factory. In fact, many of them are perfect while others are horrible. With a round bore, it is unclear why the factory head gaskets are not circular but for now this is something we are forced to live with. On most GP’s the gasket hangs into the combustion chamber at least a little. The metal is very thin on the edge of the gasket and will glow red-hot. This can cause pre-ignition and detonation. Even one cylinder having this issue can cause excessive KR.

The second issue comes from the design of the head. When the exhaust valve opens, to a lift around a half an inch or so, it reaches the same plane as the gasket surface. When the gasket overhangs the cylinder wall at this location you are closing off part of the flow channel. This causes a drop in flow that would not be detectable using a standard flow bench with a 3800 cylinder head. Using a flow bench we know that stock heads tend to quit performing better at lifts around .500, in fact some tests have shown that they tend to flow worse. Adding this knowledge to the fact that the stock gaskets overhang, we can assume that cams with extreme lifts are not going to perform well unless measures have been taken to avoid the overhang problem and the head has been reworked to provide gains over .500 lift.

The main problem with this information is that there is no way of knowing how bad your overhang is without removing the head. You can assume though that there is a 90% chance that you have some type of overhang. We suggest replacing your head gaskets with something other than stock if you get the chance.

Injectors  We sent in about 50 stock Delco injectors to get flowed and cleaned and the results were quite revealing. The injectors all seemed to vary a few percentages. Some of them ended up being as much as 6% off! We also had some Lucas 42.5’s flowed and found 2 of them to flow closer to 40lbs than the 42.5’s they were supposed to be. This information is very helpful in helping us to answer many yet unexplained questions. For example, lets say you had a weak L67 injector on cylinder 3 from the factory. In this scenario you are going to be prone to excessive KR and run the risk of blowing up your engine from running that cylinder lean even if your O2’s look ok on the Autotap.

The other problem with having different flowing injectors on random cylinders is that you are going to have to run with a richer mixture than someone with matched injectors. This will cost you power and never let you realize the potential of your car. Many people have told us that they see the best power with O2’s higher than what we recommend. This may be because of the injectors not being matched properly for your cylinders. ZZPerformance recommends getting your injectors professionally flowed from a company that will give you flow ratings of each one. After you have this information, place them as such: L67 – place the 2 weakest injectors on cylinders 5 and 6 If you have a DHP PCM version 1.0 for the L67 then your #3 and #4 cylinders are adjusted to run with higher pulse widths than the others. In this case we have a few recommended options

  • Buy a DHP PCM tuned to ZZP specs and install matched injectors
  • Get a custom tuned PCM with fuel adjusted to your injector needs
  • Place the two weakest injectors on cylinders 3&4; install the strongest 2 injectors on cylinders 1&2.

On the L36 injector placement can be just as important. Some NA 3800’s seem to suffer from KR just as bad as the supercharged guys. We recommend: L36 – place the 2 weakest injectors on cylinders 1 and 2 If you have a DHP 1.0 PCM for the GT the above still holds true. If you have a DHP PCM tuned to ZZP specs for the L36 then we recommend running matched injectors.