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If you inspect the spark map on the previous page, this will make sense. The spark advance values at the specific load point (1.04 g/cyl) and two closest engine speeds (6,000 and 6,400 RPM) are 25 and 26 respectively. The ECM interpolates between the two closest points, so Commanded Spark Advance is approximately 25.5 as no advance is being pulled due to other modifiers such as ECT or IAT. When we subtract the 3.4 degrees of knock retard from the Spark Advance table value of approximately 25.5 degrees (interpolated), we get 22 degrees of spark advance which is exactly what our data shows in the third row of the Graph in white font. So the math all adds up. To address the knock you see, you can reduce the spark advance values at those specific load and engine speed points by entering lower values into the spark table. However, truth be told, I always do multiple dyno runs to see if the knock is truly occurring or whether it's just a false knock reading so long as it's a modest amount of knock retard, typically under 4 degrees. Anything more is cause for concern and caution. If you perform several pulls and the knock retard is consistent at the engine speed and load points, then definitely do something about it and reduce the spark advance values by at least the same amount as the knock retard value in the data log. You can try adding some advance back and re-testing, but if it doesn't take and knock retard occurs again, reduce spark advance. If, on the other hand, the knock retard is inconsistent and never more than a few degrees worth, you don't necessarily have to pull timing as it's likely false knock. With bigger cam lobes comes more valvetrain noise and this can sometimes "fool" the knock sensors. Also, running a lighter oil, such as the factory-recommended 5w30 can cause enough valvetrain noise to occur with a good size cam to trigger false knock readings. On LS7 track engines, we have had the best luck using 15w40 oil in the winter (here in South Florida) and 20w50 in the summer. This reduces valvetrain noise and thus false knock. It also keeps the engine better protected and lowers overall engine temperature when it's being beaten on around the track. Just don't go ape-sh** with the throttle until the engine oil is up to proper operating temperature and pressure looks normal. When using a heavier weight oil, wait until the engine oil is up to temp, then go hand someone their backside. So what's all this effort worth? How does over 70 hp at the rear wheels sound, or about 90 hp at the crank? We started off with 484 hp and 475 lbs/ft at the rear wheels and ended up with 556 hp and 509 lbs/ft of torque! Taking into account the Corvette's 17% driveline loss, that's 670 hp at the crankshaft with no loss of torque whatsoever above 2,000 RPM. And yes, it does break the 345/30/20 street tires loose in 3rd gear as it revs past 5,000 RPM. And the sound!!! Check out the dyno video below, and then go order your cam. Foreplay is over; time to get busy!
The screenshot below shows a dyno run under load in 4th gear. Notice the Commanded AFR (bottom left of the Chart) is 13.05. The actual AFR according to the wideband O2 (in the Graph, second row, "WB NGK (afr)" in white) is 12.96. Pretty close, right? So we know that the airflow modelling is accurate and our MAF table is correct. If you look in the third row of the Graph, you'll see that Knock Retard (KR, in red) is 3.4. Th ECM is pulling 3.4 degrees of spark advance due to knock activity. The engine speed is 6,318 RPM and the Cylinder Airmass is 1.04 g/cyl.