383 peak power timing anomoly

[ian_uk1975]

My Gen.1 383 made best power with 34* total timing on the engine dyno. Now the engine's in the car, I took it on a rolling road today and found peak power came with the timing advanced all the way to 43* (and power is down on what it should be). This is very odd and makes me think the excessive advance is a crutch for some other issue. Specifically, I'm wondering about exhaust restriction possibly? Can exhaust restriction affect fuelling and can fuelling that's off (maybe too rich) make the engine like more timing (thinking a rich mixture will burn slower)? Using quite a big cam with quite a bit of overlap. Interested in hearing sound theories on what would make an engine need more timing than it should.

PS. This isn't in an Ultima and I hope you guys don't mind me posting this here... I just know a lot of you run SBCs and thought maybe someone could shed some light.