I was driving my 71 highboy the other day and the plastic pivot pin for the points plate broke and caused the points to do some funny things. I ended up getting a reman distributor from Napa to throw in there and while I was at it threw a Mr. Gasket 825d spring kit in it. My question is what is the “optimal” timing curve? My tune up tach took a crap in me so I don’t know exactly when the advance is coming in but it is noticeably sooner. Base timing is set a 10°, the mechanical advance is in the 13L slot with a small limiting clip on the post, total timing is 35-36° without vac advance and the vacuum advance pulls an additional 25°. Does this sound acceptable? Should I rn into any detonation issues?

 

"Optimal" can mean different things depending, though generally you're curving the distributor ignition timing to your engine, altitude, fuel quality, gearing, weight, etc. The goal in any internal combustion engine whether it's a grocery getter or a full on performance engine, electronic computerized engine controls, or centrifugal weights and springs, is as much ignition timing advance as it will stand just short of any actual engine knock or ping at all times under all conditions.

So you kind of have to get it in your head to give your engine what it wants, not an arbitrary number like 34° at 2000 RPM or whatever. It might not like that, but at the same time, it might want more, or earlier in. Experiment, it is not a "one and done" kind of deal. Without a distributor machine, you'll have to iterate and gets to be kind of a pain in the you know what.

It isn't really any more complicated than that if pollution emission controls are excluded. As it turns out, optomizing combustion improves power and fuel economy, reduces hydrocarbons and carbon monoxide, but causes a spike in nitrogen oxide pollution, a big contributor to smog. So way back in the day OEMs were forced to do all kinds of stupid things from an engineering perspective. Reduced compression, and retarded camshaft or valve timing, ignition timing. Here's a good primer on how to approach it.

Go Fast For Less - Distributor Ignition

 

Thanks for the pointers. As a general rule how much total timing Is “safe” with a stock 70s 390? Is 36° normally safe? I can’t take the truck out for a test drive till I get the t-case back together and I’m worried I might not be able to hear pinging over the exhaust leak (cheap headers). I just wanna make sure I don’t crack a piston, been there, done that with a 360.

Some of details about the tuck. It’s a 71 highboy with a mid 70s 390, 4 speed, np205, 4.10 gears and 36ish buckshots

 

If you have a stock cam, stick with what the factory said for timing with it. Aftermarket, give a call to their tech support, and they will line you out based on your cam and compression ratio.

 

I guess I don't understand, if you can't take it for a test drive, it won't matter what the timing is set at. How would you know if it's pinging without driving it?

34° BTDC should be safe with most any OHV V8 ever made, and "regular" pump gas. What altitude? Compression? For vacuum advance remember it is tuned separately. Drive around and make some test runs with the vacuum advance disconnected and plugged. 34° or 36 ° would be without vacuum advance.

Find out where it does ping or knock, and back off a couple degrees. Inspect spark plugs often and carefully, detonation will show up as metallic specks or globular spheres on the ceramic. Bad! Small amounts of distributor movement makes big changes in ignition timing. When the centrifugal timing comes in is just as important as how much. Here is where bending the spring arm underneath the breaker plate comes in to play, maybe replacing the heavy spring with a medium.

When you get the mechanical/centrifugal timing where you want it, don't mess with it anymore. Re-connect the vacuum advance and tune only the vacuum can. Usually close to 50° BTDC is what any OHV V8 ever made will run on flat highway at steady cruise. If the mechanical and vacuum advance isn't calibrated correctly the engine will lose about 3 to 4 mpg. The vacuum advance #s you mention are way too much, I run about 54° indicated on the balancer, in the garage, but the actual advance on the highway is probably closer to the mid to high 40s. This is right in line with factory specs, black & white in the shop manual. There is no load on the engine in neutral, so it isn't what the engine actually sees out on the road.

Mechanical advance is a lot easier to measure because it directly responds to RPM, where vacuum advance is load based. Most distributor specs are listed in terms of distributor degrees, not crankshaft degrees, and they assume the distributor is being calibrated on a distributor machine with a vacuum pump or MityVac. So keep that in mind when reading the specs in the manual.