In my opinion , at 15 degrees your motor is very restricted and not happy producing alot of heat that is unnecessary . Do you have a EGT gage hooked up?
Tiger heads liked 15* on a 200 shot but compression was 12:1 and I was on C16 fuel. Now I am on E85 and 9.6:1 compression and am almost totally in the dark on what this motor will want for timing. The 2 fuels have different characteristics so I figured to start safe. If I had e-heads or iron heads maybe I'd start higher??? Even with 15* on nitrous I still torched the block and pass. side head and lifted ringlands. Strange thing is, I had local nitrous gurus inspect the plugs and they all said the tune was as good as it could be!
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67 Firebird 3440 lbs, Pump gas 537 IA2, tiger heads, 4.86 rear, 32 x 17.5's, full interior, 4 mufflers, street shape 1.36 60', 6.11@113.42 1/8 mi. 5.67@123 on small shot.
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