[Cliff R]

Couple of comments in the FWIW department.

Right to start with the cam is too small and LSA too tight for a 469cid engine with 10.4 to 1 compression. It's going to be sensitive to total timing, especially at light throttle cruising.

This will cause several things to happen. First, the engine will not like or want a lot of timing at cruise, so ignore the PM's from the "part timers" and forget the nonsense about trying to find a "sweet spot" by adding in 22 degrees of vacuum advance at idle speed then ending up with nearly 55 degrees of timing at light throttle cruising.

Second, there isn't one ounce of difference once you move the throttle an 1/8th of an inch running ported vs manifold vacuum to the advance provided you are using the ported source in the main casting. Both sources are FULLY uncovered and providing the same exact vacuum to the distributor off idle, light throttle cruising, and any heavy or full throttle.

The ONLY difference is at idle speed. If your engine wants, likes or requires a LOT of timing at idle speed then use manifold vacuum to the advance.

Most of the "quirky" symptoms you are getting from the engine combination is the cam choice. It's simply too small and LSA too tight for that much engine with that much compression. I've tuned a number of them and the symptoms you are getting are pretty much on par with what I've seen here, the XE274 camshaft is a piss-poor choice for a 455 build right to start with. It leaves too much power on the table and builds too much cylinder pressure too early in the rpm range. It might work OK at 8.5 to 9 to 1 compression, but will not work well, or at least not nearly as well as a bigger cam on a wider LSA over 10 to 1 compression.

These days I usually just sit back and watch these threads run, because telling the truth quickly gets me labeled as the Comp XE cam "basher".

Anyhow, in lieu of changing the camshaft, do the following things instead.

Unhook the vacuum advance and time the engine to where it makes absolutely the most power everyplace when you are running it hard, heavy and WOT throttle. Play with the advance curve and initial and total timing. A dyno or drag strip would be the best place to do this, but you can get close on the street. Use the fuel/octane that you plan on running all the time, nail down the mechanical curve, initial and total timing and be done with it.

Next, reduce the travel of the vacuum advance unit to about 10-15 degrees and start tuning with it. MVA or ported really doesn't matter, if it idles cleaner on MVA then that's fine, but try both places and fine tune the idle mixture screws accordingly.

Next, remove the set screw over the APT and raise it up slightly, about 1/8th turn CCW at a time and dial in the part throttle A/F for smoothest operation at light part throttle/normal driving. To really nail down the part throttle A/F and tune I would also make changes to the VA unit and vary timing added between about 10 and 15 degrees till you find the "sweet spot" for light part throttle operation. Keep in mind here that ported or MVA is exactly the same when doing this, the only difference is that MVA adds timing at idle and when coasting, otherwise no difference.

Give this stuff a try and let us know what happens. I really wished you were closer, I could nail all this down in less time than it took me to type this!.......Cliff