There really is no specific initial timing specs for any performance engine. You certainly can't go by the factory specs when your engine isn't all bone stock and isn't running all factory components, especially when running an aftermarket distributor, or even a stock replacement distributor. This is because the factory KNOWS how much advance your distributor has in it (because they built it) in conjunction with the amount of initial timing they tell you to set it at. Replacement distributors, whether they are performance or stock, are all different than your original distributor so you obviously can't use the factory setting, besides, stock engines, stock distributors and stock timing specs weren't designed for optimum performance, even in factory muscle cars they were kind of mediocre. Also, (and pretty obvious), performance engines aren't stock, they run differently so they need different timing settings. You really have to time an engine of this sort solely by the "total timing". Total timing is a combination of your initial timing (on the crank) and the amount of mechanical advance you get from your distributor.
Most naturally aspirated engines like a total timing of 34 to 36 degrees BTDC, (Before Top Dead Center) AKA "Advance". Nitrous and supercharged engines usually run less than that, unless you plan on blowing the heads off the engine or blowing holes through your pistons. usually a typical supercharged or nitrous engine with a moderate amount of boost ora moderate shot of nitrous will have a total timing of about 28-32 degrees. But that is variable on the type of fuel you are using and the amount of boost or nitrous you are running.
To check and set the total triming, you need a timing light with a timing offset built-in. You can tell this kind of light from a standard light because it will have a dial on the back side. These lights are usually a few bucks more than the regular lights but they do a lot more.
Most street engines use either a vacuum advance and/or a mechanical advance (the weights inside the distributor). More serious performance engines "usually" only use the mechanical advance, and most true race engines are usually "locked-out" and have no advance in the distributor at all. They also have no vacuum advances mounted on them in most cases.
There are several types of timing that you need to know about. First, there's the initial timing, which is also known as "idle timing", which is where the timing is at an idle without the help of the mechanical weights inside the distributor, and without the vacuum advance hooked-up. Then there's the mechanical advance, which is the advance you get from the weights inside the distributor. As the engine RPM comes-up, it's the initial timing AND the mechanical advance that gives you your TOTAL timing. For instance; say your distributor has 26 degrees of mechanical advance built into the weights inside and you want a total of 36 degrees of total timing; you would need to set the initial timing (idle timing) at 10 degrees (10 degrees on the crank and 26 degrees from the mechanical weights = 36 degrees of total timing).
The trick is; for performance use you want as much advance as you can get on the crank and the least amount from the mechanical weights in the distributor, hence why race distributors have no mechanical advance at all. To really make a trick street set-up you should only have about 10 degrees or so of mechanical advance in the distributor and about 24 - 26 degrees of initial timing on the crank to obtain your total of 34 - 36 degrees of total timing. This will launch you out of the hole much harder, and give you great jack rabbit starts from street light to street light. You NEED advance at low RPM to make that engine pull hard. It'll have MUCH better throttle response, a better idle vacuum signal, and it'll run cooler & cleaner. Unfortunately stock engines & distributors are set-up completely the opposite so they need to be re-curved to fix this problem. This is why some guys can tune an engine and make it run killer (like we do), and other's just can't seem to get their car to get out of its own way.
Now; to find and set the total timing all you need to do is set the dial on your timing light to 36. Now rev your engine up to about 3,500 RPM (to insure that the mechanical weights are fully activated) and watch your timing mark on the harmonic balancer. Now rotate your distributor until the marks line-up at "0" on the crank. When it reads "0", (yet the light is set at 36), you have a total timing of 36 degrees. Make sure you do this with your vacuum advance NOT hooked-up. If you don't run a vacuum advance, then don't do anything, just leave everything as it is and tighten it down and you're good to go. Your engine isn't really at "0" or Top Dead Center. The timing light is offsetting the light beam by 36 degrees, so you should be reading "0" on the crank.
The total advance will change if you replace your distributor or install an advance curve kit, so always check it and KNOW where your timing is at! It's REAL important on more radical engines, especially on high compression / supercharged and/or nitrous engines! One degree off on the timing can mean anywhere fron 20, 30 or even as much as 40 horsepower loss in some engines!
Now, if you use a vacuum advance, (which is pretty much only there for part throttle economy), hook it up AFTER you have set the total timing. Of course when you are driving down the highway at part throttle, the vacuum advance will pull anywhere from 10 to 15 degrees MORE advance (beyond your total timing), BUT under hard acceleration, the vacuum advance doesn't work anyway, so you're back on your total timing when you're on the throttle. Personally? I almost never use a vacuum advance on performance engines, although a vacuum advance can also be used to help engines get rid of "run-on" or "Dieseling" problems when you shut the engine off. You can do this by hooking it up to a manifold vacuum source to pull more advance when the engine is idling. For those of you that can follow along with that concept, the reason for doing this is to make that extra timing being pulled at an idle to increase the idle RPM, which in turn allows you to back-off on the idle speed screw, which in turn closes-off the throttle plates so when you shut the engine off, it can not "mechanically" continue to pull fuel and air into the engine causing it to Diesel.
Remember this; a correctly timed engine produces the most horsepower. An engine with timing too late (retarded) will have a low idle vacuum, have slow throttle response, feel like a turd at low RPMs and will run hotter than normal. An engine with the timing too soon (advanced) will have a high and erratic vacuum signal. It might have a snappy throttle response but it won't pull very well under a load, and it will have pre-ignition (detonation) problems, sometimes called "pinging", which will certainly lead to either a blown head gasket and/or serious piston damage. It will also idle rough, like it has a bigger cam than it actually does.
Keep this in mind too; once you set your timing and you don't physically move the distributor, the timing will pretty much never go out. The first thing people do (who have no clue what the hell they're doing) when their engine starts running funny, is to start twisting the distributor around and screwing with the timing. The timing will NOT affect any ONE particular cylinder. It can ONLY affect ALL of them at once, so if you have a back fire or a missfire or a dead cylinder, there is no need to start messing around with the timing. Again, if you don't move it, it'll stay set pretty much forever.
I had someone ask me; "If I set my total timing at 36 degrees at 3,500 RPM, will it still be 36 degrees at 7,000 RPM? If not, how much will it change?"
Below is the answer I gave him:
Well that depends. If you've set your distributor up to get full mechanical advance at or below 3,500 RPM, or have at least checked it to see that it’s all in by 3,500 RPM, then your answer is yes, once you hit full advance it won’t keep advancing as more RPM comes up. Once the mechanical advance weights sling-out at their given RPM, which is set by the type of weights it has, and more-so by the stiffness of the springs to give you your “mechanical advance”, they don’t (and can’t) come-out any farther. Most stock distributors are set to get full mechanical advance at about 4,000 RPM. Brand new MSD distributors come with ridiculously stiff springs in them that don’t see full advance until upwards of 4,500 – 5,000 RPM. You want all of your advance in at about 2,400 RPM. 3,000 is too high, 2,000 “might be” a bit low, so somewhere in there is where you want to be. This is why re-curving your distributor is essential. Not having full advance at or below 3,000 RPM is pretty useless considering an engine needs its advance to run at its best and to get that car moving. Stock, or un-curved, distributors that don’t see full advance until 4,000 – 4,500 RPM are a bit late in most cars and WILL cause your car to not move off the line or accelerate very well below it’s full advance RPM. But like I said, once the weights have slung-out… they’re out and won’t give it any more advance. The trick is to know how much advance you have, and WHEN it comes in. If it comes-in too late, then you need to change the springs and maybe even the weights to make it come-in sooner. If it has too much advance, then you need to limit the amount of advance inside the distributor and give it more “initial” timing on the crank to achieve the 34-36 degrees of desired total. I usually like to see about 10 – 14 degrees in the distributor and the rest on the crank in most performance engines.