I 'estimated' using a light, but not a dial back (and just learned my HF unit is off 10%) without isolating vacuum/mech advance - and eyeballed it as the balancer mark moved past ticks on cover. Ended up I was way off... instead of 6/20 mech/vac advance, it's 12/16.
I compared timing specs from 67 and 69 manuals (damaged engine was 69 428 with 62 heads) to measurements with this 75 HEI. Books didn't give maximum centrifugal advance for 67 and 69 (only up to 2300, but maybe that was supposed to be max) - I got 3 more degrees after 2000 rpm with the 75 dist.
BIG difference is 10 degrees max vac advance with 67 and 69 points distributors, and 16 max with the 75 HEI. Curves don't look much different though 67 starts steeper.
The 428 with '75 HEI would detonate steadily when I'd accelerate lightly on the highway at 70 mph (2200 RPM in this car) - it had little load but 16 degrees of vac adv would kick in. I didn't accelerate hard from 70 because it rattled like crazy and power dropped off. Looking at the diagram (I think I got it right...), the 428 had too much vac advance with 75 HEI - a vac adv limiter might have fixed the detonation problem. I knew nothing about detonation damage before. Attached are photos of what it did to bearings (maybe you can tell... is all main bearing damage from detonation? I was thinking the front bearing was out of alignment - engine ran strong, but the shop did a bad job rebuilding that engine).
Back to this car... Set init timing to 10 BTDC spec and hooked to manifold vacuum (as was in those days). Car seemed "lighter" driving low speed through neighborhood (than with ported vac), but not much different with highway cruising. I'd set it to 12 (may have heard a couple knocks at WOT on hwy), so set back to 10 - it was also harder to start at 12 when hot. At 10 with manifold vac, I was surprised after 20 mi drive, getting off highway, then a few blocks to the house, the engine was still about 140F (had tested thermostat in boiling water - it opened, then closed), with no pressure in cooling system. Is good in a way, but engine not at proper operating temp (has no AC, 4-row copper radiator).
Got sidetracked when master cylinder finally gave out.. was having to pump brakes, so got more confident with good brakes. Takes off pretty good (can break tire loose now) and chirps into 2nd gear - haven't tried to do a "power braking burnout". On highway, punched it from 70 mph, it kicked down and took off pretty good - let off gas a bit so it'd shift into 3rd (close to 95/4500rpm) and felt the torque pulling... got scared and let off (I'm older now). No detonation. I'd filled up with 91 octane gas. Figure it's running about like it should, but it's not hot summer so there will likely be problems with detonation later. Should note this engine has "8-eyebrow" KB silv-o-lites, so want to avoid detonation (I know these are much less than ideal but I just wanted to get this car running again). Doesn't mean so much, but I think this is running at least as strong as the 73 Catalina 400 - that car weighed 4190 lbs, this car is 3553 (69 GTO hdtp is 3080), and I have lower rear axle ratio. Is good enough for now. I checked butterfly on Qjet (is 7040263 so okay for this) and spring is 5/8 turn from zero (okay) - there's a little lag when secondaries open (not really a bog.. feels like linkage sticks/pops loose).
You're right about the cam. Wallace showed 066 in this ZX engine (was a CA A.I.R 2-BBL), but that cam was also used in 67 and 69 10.5:1 325 and 350 HP 400 engines (I'm confused about this). I'm running closed chamber 670s now, and not sure if I'll buy much with the 13 heads (75cc OC) - static CR will drop to 9.29 from 9.55. I have a Melling SPC-7 cam (068 clone?) and probably should have put it in (was going to keep cam/lifters/matched to bores, but 3 lifters seemed stuck hard so I put in new set of Summits), but wanted to see how this would run with a stock 400 - and to get this distributor issue figured out.
That 428 was a mind-blower - with points distributor, 10.25:1, Crane HMV-272-2-NC cam, timing at 9 BTC, 62 heads, 2" (small) exhaust pipes, stock int/exh manifolds, Holley DP (don't know CFM), TH400, and premium pump gas 20 years ago, it'd burn tire (no posi) from 35 mph punch, shift out of first at 60 mph, twist the car a bit, shift out of 2nd at 95, while still increasing acceleration - I backed off when speedo showed 90 (just calculated that this was 130mph) and it was still accelerating/pulling strongly (I thought if this convertible flips, I'm dead. Suspension was tight/good, but still...).
This engine is tight. No oil blowing when I run it like this. Both the 428 and a 67 GP 400 (with Edelbrock upgrades) had too much crankcase pressure and would blow oil from breathers with kickdowns (~4500-5000 rpm). The rings never seated in those engines. Did a leakdown test on this block/heads and it was fine (5%?). The only problem I've seen was just did compression test and all cylinders are 155-160 except #4
at 180. Not sure what to make of this - I've got one lifter intermittent ticks in cyl 2-4 area so need to adjust while running as you wrote in recent post (not sure if loose valve adj might increase compression???). Put in a fresh set of R43S Delcos gapped at 0.040" - was using old plugs and think I had a miss on right bank - it's running smoother.
I just read a thread where a guy swapped 455 #64
87cc heads with #12
72cc heads on a '70 400 so he can run "full advance curve" on pump gas and it runs better (CR Drop from about 9.5 to 8.4:1) - I've much to learn about all this.