|
Sounds like load induced knock. Load can be defined as actual cylinder fill rate versus theoretical maximum fill rate (Volumentric Efficiency/VE). If at 2200RPM the VE is 90% than the 2.5L QR25 (if memory serves me right) engine in your sentra is actually flowing 2.25L worth of air every 2 revolutions of the crank. (2.5*.9). At part throttle the fill rate is lower since you are restricting incoming air flow with the throttle plate, which is why a vacuum gauge reads vacuum. The engine is trying to flow that full 2.25L of air, but you are restricting it. High Load (in my book) would be defined as using 85-100% of the available VE.
High load at low RPM produces high cylinder fill rates, and since the burn rate of 2.25L of fuel mixed with 0.173L of fuel (assuming 13:1 AFR at WOT) is fixed, we adjust ignition timing to produce maximum torque. The piston velocity changes as RPMs increase, but the burn rate remains relatively constant. This is why ignition timing generally advances as RPMs increase. This is not always true since VE also changes as RPMs increase, so the timing does not ever ramp up at a fixed rate because the VE generally drops off as RPMs rise.
Too much load at low RPMs will require very little ignition timing advance since the piston is moving so slowly. The factory tune on your car may have a little too much timing at those RPM/Load points which is causing slight knock. Lets face it, if you want to get somewhere faster, you don't floor it at 2000RPM in sixth, you downshift a couple gears to increase your torque multiplication and put the engine in a more efficient power producing part of the rev band.
I would venture to say, that if you are putting enough load on the engine at 2000RPM to induce knock, you should downshift at least one gear. You will probably accelerate quicker with less throttle input and as a result, use less fuel. A fuel consumtion calculator like scangauge or MPGuino will help prove this point.
|