Transom Soot vs Octane
 

Background..... 496HO with Whipple Stage 2 tune.

The Whipple stage 2 maximizes the power output of the stock engine by running 91 octane or better, increasing the maximum spark advance, increased ignition power from the individual coils, denser air charge from the low temp thermostat and maximized air-fuel ratio in each individual cylinder. Calibration increases torque levels throughout the entire rpm curve while making more peak hp for increased speeds. Customers report far cleaner transoms (less soot) while increased throttle response.
Key features:
• Hi-performance PCM calibration
• Low-temp thermostat
• 5450 RPM limit
• 5% - 8% power increase over stock
• Increased acceleration with increased torque output

Dustin said this tune runs 12.5-1 A/F ratio at WOT. 13.0-1 at part throttle. With my scan tool the stock Merc tune had 24* max timing. The Whipple tune has 34* midrange and 32* WOT.
I've noticed when running 93 octane my transom is horribly covered in exhaust soot. Pain in the a** to keep clean. Running 89 octane results in hardly no soot. Drastic difference. No difference in speed with either octane. This will be my 3rd year running the tune so I am not worried about hurting the engine running 89 oct. Plugs look good, no signs of detonation.

Why the big difference between octane's? Can we interpret how well an engine is running by how much soot we see?