My 68 400 is pretty much stock, other than a slightly hotter cam than an HO. I run a mix of 93 Unleaded and 110 Racing fuel for a typical 98 octane. For best perfomance what's the best timing 9 - 11 BTDC?

You will need to know your total mechanical advance. Do you have a dial back timing light? What heads do you have? Tranny, stall, rear gears, etc. What is it set at now with vac adv unhooked at idle? You could just increase at 2 degree increments until you hear rattling under load and then back off until it goes away. Best way is to experiment at a dyno or the track. All engines will be different.

I agree. Timing by ear is the way to go. It's the only way to get everything your engine has to offer. Your timing light will only get you close.
Jim

I've been timing my Firebird by ear since the late 70's. It's the only way to time them.

I have a similar setup to yours on my 68 400 and the timing at idle with vacuum plugged, fully warmed up, is about 11-12 degrees. I fiddled with it for days and that seemed to be the best setting. All done by ear and gas pedal of course.

Thanks. My car's engine block, heads, exhaust manifolds, intake and Carb are all original. Only the Cam is slightly hotter. The rear is a 355 posi; trans is the original 400 turbo. The distributor is also the original dual diaphram (advance/retard) type. I'll try the 11 - 12 degrees and see if it pings. I appreciate the advice.