[Blown353]

15 degrees is VERY low timing. I would keep bumping it up and keep a close eye on the power numbers and the plugs. I'd be willing to bet it will run best somewhere between 24 and 30 degrees under boost on 91 octane. You're probably leaving 75-100hp on the table running only 15 degrees of timing under boost.

What heatrange plugs are you running? Keep an eye on that too as you bump up the timing. If your heads use 3/4 reach gasketed plugs I'd say you will probably end up with something equivalent to an NGK BKR6ES, that should be about right for that compression and 9 psi of boost. Maybe even one step hotter at 5ES, but start with the colder ones.

With my blowthrough carb setup on 91 octane I was running the distributor locked at 36 degrees, plus another 14 degrees possible with vacuum advance. The BTM box pulled it down to 27 degrees at 15 psi of boost. That's with 15 psi of boost, no intercooler, 9.25:1 compression, and water injection.

Another thing, don't trust the knob on the BTM. Start the engine and run it and use a hand pump with an accurate gauge to apply pressure to the BTM at your maximum boost level, then adjust the knob to your desired timing number while watching the pointer with a timing light. Obviously, do this with your vacuum advance disconnected and at a high enough RPM so the distributor is "all in" if you have the centrifugal advance hooked up. The numbers on the knob aren't really that accurate. In the ballpark, yes, but not exact. I did this on my BTM and made new tick marks on the knob every 0.5 degrees per PSI of *actual* retard as verified by the hand pump and timing light.