I would think it would want more than 8 degrees; where you had it sounds about right. Did you get any pinging with it the way it was before? Having your timing retarded can make your engine run hotter and generally decreases performance.
FWIW the chiltons manual for my 71 k-20 says to set the timing at 0.
Unless it's an original distributor that still has the original amount of mechanical advance for your particular engine, factory specs don't mean anything. Even if it looks like a stock distributor, chances are good that it's been replaced with a rebuilt, one from another engine, or modified. I'd set total to 36º and let initial fall where it may. If you want something different, recurve the distributor.
Forget the "numbers". Give the engine what it wants. If you do that, it rewards you by running better. When any adjustment you make makes it run worse, then you have it the best it can be, no matter what the "number" is.
Since you say it has a mild cam its likely an aftermarket perf cam ,so most aftermarket perf cams like approx 16-18 deg base timing,not 8 for stock gm cam or the fairly retarded 14 deg it was just prior to backing the base timing off to the stock 8 deg setting its currently at.
Retartded base timing results in watsed fuel,poor fuel mileage,poor throttle responce,lost power,hotter running motor,just how bad these symtoms are is determined by how much the base timing is retarded.
Set to 18 base and if total is iver 36-38 max you will need to send dist out to have the mech adv curevd for 18 deg all in by 2800 rpms.
Then 18 base + 18 mech in dist = 36 total,USE 93 FUEL to start with ,then try 91 later to see if the 93 doesnt ping.
But dont just set to 36 total now as suggested and leave it be . That's because doing that usually leaves the base timing retarded/below 16-18 deg if the dist has stock mech adv curve that an be 25+ deg . In that case setting total to 36 deg with 25 deg mech adv results in 11 deg base timing which leaves the base timing 7 deg retarded which is alot.
So if total is over 36-38 deg max when running 18 deg base timing you need to send the dist out to have the mech adv recurved for18 d4eg max all in by 2800 rpms.
Vac advance, to manifold vacuum source. HEI or points (or aftermarket) won't impact what timing your motor likes. But an electronic ignition will be more reliable and is capable of delivering hotter spark.
Forget the "number" business, if you haven't verified your timing mark by some independent means such as a piston stop. Timing marks are notoriously inaccurate.
People seem to like to get all hung up on making their timing mark read some particular "number" at idle, and seem to like to look it up in a book of stock "specs". This is not productive. The engine does not whup out a timing light, read the "number" on the tab while idling, and decide accordingly how to run. It responds to the actual timing itself, not to what the timing tab says.
Every engine is different. Every one will require some different static setting, some different centrifugal (RPM-based) advance curve, and some different vacuum (low-load, high-speed such as cruising) advance. However, nearly all naturally aspirated engines running on gasoline, will run best with their "total" (static plus centrifugal, but NOT including vacuum) advance somewhere around 34-38°, as SWheaton described. A SB with Vortec or similar design heads will want about 34°, one with traditional heads will want about 36°, and most BBs want about 38°; all of which are a function of flame proagation speed through the combustion chamber, according to its shape. It is nearly impossible to get a reliable indication of this without a timing tape on the balancer, or other hard physical indicator of that type. A "dial-back" light is WAY too prone to errrors.
It is easy to find the ideal "total" timing. Put the car on a chassis dyno, run it at 3500 RPM or so at full load, and twist the distributor for max power. You'll find, if you measure it with a timing tape, that it will be about 36°. Make a mark on your balancer that lines up with the "zero" on your tab, so you can put it back. Calibrate your other curves from there.
The only thing that knowing a "number" is good for, is putting the distributor back if you disturb it.
The engine does not care whether the spark comes from a mechanical set of contacts, or from an electronic circuit (points vs HEI). Gasoline burns exactly the same no matter what circuitry is in the ignition system, so therefore the timing will need to be the same. All that the engine cares, is that the spark occurs when it wants it to. Like Zeke said though, electronic distributors are more stable, meaning they stay the same over time, meaning you're not having to continuously chase the timing and put it back where the engine wants it.
Very true that some ballencers /timing tabs etc are off a few deg (2-4) here or there but giving #s like 18 deg base & 36-38 total gives someone a good starting place to begin dialing in thier particlular setup. But i have found its not very often that they are way out of wack,but it can happen.
So when they begin the dialing process yes they might end up with 15 base and 34 total for sweet spot to run well with no ping because maybe like you say the ballencer /timing tab are not that acurrate..
But i stick by the base 18 & total 36-38 because unless someones timing marks are way off like 8-10 deg the stock perf 18/36 perf timing curve is a good place to start for most street sbc/bbc mild to faily hot normally asperated perf motors and the dialng in process after that will get you right where your setup need to be IMHO.
I have been using that method for over 37yrs and had mayby 1-2x over all thoses yrs where i ran info issues with the ballencer slipped on the ring or timing tab issues being way out of wack but its apparent when you run into that situation.
And when the timing tab or ballencer are way out of wack when you set the timing anywhere close to where it should be and it runs like crap you know something is up with the ballancer/timing tab/dial back timing light in some cases /cam wasn't installed properly/or the motor has a stock gm cam in it that requires 4-8 deg base timing when someone thinks its an aftermarket perf cam requiring 16-18 deg base when it doesnt .
But checking TDC like you stated can help pinpoint a problem if you find the motor is not responding correctly to a normal perf timing curve when running an aftermarket perf cam.
Just my 2 cents on that.
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