Quote:
Originally Posted by hexdmy
I feel I need to add to this, while it's nice to be able to tune to MBT, many engines will reach their knock threshold before they get to MBT with pump fuel. This is why knock monitoring is so important. Also, if you carefully monitor knock and creep up on timing, you are generally not going to kill a street engine if it just begins to ping. In practice, on engines that reach their knock threshold before MBT, you cannot tune the engine safely unless you reach the knock threshold, otherwise you would not know the limit of the motor. With a set of knock headphones, it is possible to hear knock just as it begins, well before you would hear it with just your ears . Of course, the more power you make with a given engine, the higher the risk of damaging it with knock. I think of it like a blow torch, you can pass a blow torch over a piece of metal without affecting it one bit, if you hold it there you will start melting things.
- Chris
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Hi Chris
We use active knock detection such as Det cans and we have a knock box but I have never heard knock before reaching correct timing in each cell and achieving knock...at least yet on a Nissan motor. This is an AGE old tuning debate that was actually discussed even this weekend at the EFI University session we had with Seth from EFI University. The debate and EFI University ended up being a conclusion that tuning off MBT and not being sole focused on knock precautions. Most knock sensors are not even properly configured for the motors they are running on and most oem knock sensors are not designed to pick up the minor knock you are mentioning.
Part of the way to avoid knock while tuning on PUMP gas to begin with is to move towards more rich AFRs than you would commonly expect, especially with the quality of fuels being lower, to actually achieve the same energy out of many of the pump gases today a richer mixture must be used. I honestly have to disagree that most motors will see knock before you reach max best torque. When watching an OEM knock sensor that is actually configured and in working condition for the correct vehicle, at least with Nissan applications, this has not been the case in my experience. With Knock sensors that are aftermarket and not properly calibrated specifically for the voltage range of true inaudible knock this becomes the opposite and it becomes harder and harder to tell if you are actually picking up real knock or supersonic engine air vibration or some other engine condition.
It might also be good to note here that knock IS NOT detonation / extreme engine pressure or typical preignition referred to by most people referring to it...Knock is also not flame fronts meeting in the center of the quench zone of the combustion chamber...
Knock is literally the sound waves of the air inside of the combustion chamber reaching such a pitch and velocity that they explode the fuel air mixture milliseconds prior to TDC during a piston stroke. This vibration is what will actually damage your internals. And also note that on most nissan motors inaudible knock will first start to effect ROD bearings, not pistons in most cases. This is another misnomer.
I think you present an excellent point in that MBT will not always tell you if a motor is tuned in a way to avoid all knock. However tuning with det cans or after market knock senors is an even more risky way to tune off of, based on the fact that most aftermarket knock sensors are extremely hard to properly calibrate due to each individual engine having a different voltage threshold to properly identify actual engine knock and not some other vibration or cam noise in the engine.
In a perfect world you could hook up det cans, knock boxes and an external 3rd party knock sensor and hear knock that would normally be inaudible, see knock on the 3rd party sensor easily without having to spend thousands of dollars on equipment and development to properly configure the sensor to see REAL knock and not some other vibration from engine to engine and also be 100% clear of any detonation before TDC while tuning for MBT. It's an endless debate and I think that honestly it has more to do with engine to engine, vs. tuning method to tuning method. Some engines are much more prone to knock and inaudible det than others. For example many 4G and Subaru engines often see these conditions and their users are very involved in the development or more and more knock feedback and knock based tuning. Age old debate and I hope I shed some light as well