Re: Thinking of going from MSD to HEi
"How does one limit timing w/HEI ?"
Now, that has started full on wars from people not knowing, nor caring about how the curves were designed in the first place. Jamming a screw in, ripping, tearing, grenade attack, full on nuclear detonation, welding/brazing, hacking, hammering, dynamiting, are not the right way to set the mechanical curve on a large coil in cap HEI. We designed these distributors to use a myriad of combinations of center curvatures, and weight profiles, to set the start and limit points,and degrees of curve, with just at 437 different combo's of weights/center. Getting the right curve for yur application, doing it the right way, may well take decades of test, change, test, change. And, the after market "Curve Kits" are NOT the answer for a street driven mechanical curve, because those curve kit weight and center combos are not designed for street use, they are for drag racing only. Those kits make it workable to run 18 to 20 degrees initial, no vacuum advance, and give 14 to 16 degrees of mechanical advance, for drag racing.
The ONLY real "performance" curve ever used in a large cap HEI from GM came, comes in carbureted engine ZZ series crate engines. 41 weights, 375 center. This combo is also used on late 70's/early 80's 350/454 Suburban, and truck engines.
As far as the vacuum advances, more "top tuners", "Dyno Hero's", and really bad "techs" usually have NO CLUE as to how to set one up right, so, they tell us to leave it disconnected, for various bogus reasons. A properly set up vacuum advance, even a stock one, can really help an engine cool better, drive better, get better fuel mileage, and just plain help with carburetion and performance. There is one topic on a Pontiac board that has a person fighting poor performance and heating issues on is stock 389, and he takes it bck to the mechanic, whom insists the vacuum advance won't help anything. Once the car oener did the actuel vacuum advance mods to set the degrees delivered, and plugged it into the base of the carb, the engine cooled right off, and woke right up.
He now doesn't take his car back to that "mechanic", it runs too well now for him to let it slide back into problems from the top tuner mechanic he had. Some other really great people try to help as well, nice folks, but they too have been indoctrinated to not use the vacuum advance, let alone correctly. Not bad people, they just never got the right info to try and prove to themselves works.
There are people out there that have listened to a few very knowledgeable people, done it right, and saw the fixes that work. Just have to hit on the right person.
Curving a large HEI, not too tricky, but, the right weight and center combo, along with the right adjustable vacuum advance kit, and the whole mess can turn into a right nicely curved ignition.