21. We used a Ready-To-Run MSD Pro Billet distributor to control the spark. Timing was set at 18 initial and 35 total for the first few runs. Then we moved it up to 21 initial, 38 total, and the engine liked the change. But we later found out that the cheap distributor hold-down clamp I used was letting the distributor rotate, thus retarding the timing. AFR has tested the 195cc heads on a lot of SBC applications and found that 34-35 degrees total timing is usually where the engine makes best power.