So I’m new to building my engine and upgrades. So here’s my question. I’m installing MSD street fire distributor. It comes with a plate to limit vacuum advance. What do the degree settings mean? Is that above initial timing?
Scenario: Sbc 355 initial Timing set at 10* then put limit plate on C setting 8-11* on vacuum advance. What’s does that setting mean? Is that that many more degrees added to initial timing?
I’m not looking for history on vacuum advance. I get it. I just can’t find much of anything on this type of setup.

 

When you have timing it will typically be base+vac advance+mechanical advance. With a 355 I'd probably want a bit more base advance honestly. That said you should have roughly 30-36 all in with base+mech and then I'd start at 10 with vacuum if I was going to set and forget. If you had a vacuum gauge I'd end up setting it by going and doing steady state part throttle cruise (say 55mph on the highway) and figuring out which timing setting gave best vacuum.

Regardless if you went conservative and ran 8 degrees I'd run vac no matter what. It's very important in terms of mileage and manners.

ED:I take the timing recommendations back as an absolute. That's if you're running standard wedge heads. You may need considerably less timing overall if you're running modern heads (swirl port, Vortec, aftermarket depending.)

 

Thanks that’s what I wanted to know is the vacuum is added to base and with limit plate will only go that much more. Got it. Just wanted it in layman’s terms.

 

That's not quite exactly right.

"Base" or "static" timing is what you set the dist body to. Centrifugal advance is added to that, to give "total" advance. This is BEFORE figuring in vacuum advance.

It's better to think about it like you would if there was only ONE timing setting; like a lawn mower or something. That setting would then be the "total". You'd set it for the motor to run "The Best"; which might be some combination of max HP at whatever RPM that occurs at, best torque at whatever RPM that occurs at, best gas mileage, cool running, etc. You wouldn't really care about idling (as long as it could successfully), "cruise" (high RPM low load) conditions, etc; you just want the damn thing to cut as much grass as possible, right?. It's dependent almost exclusively on the fuel you're burning, with a slight input from the combustion efficiency of the chambers. For gasoline as we know it, the optimum "total" setting is about 34 - 36°; a head design with very rapid flame propagation (Vortec for example) might want only 32°, and a slow one (BBC) might want 38°. Most engines aren't too closely sensitive to it, in terms of making max power; a degree or 2 might make only a 2% difference in max HP. Too much however, can lead to detonation, which is premature explosion of the mixture instead of a controlled burn, which can be highly destructive. As in, break parts. Not enough will make the motor run hotter than it should when running for extended periods of time at full power; like, how trucks sometimes overheat while towing.

From there, imagine that as the RPMs decrease, starting at some point, you'd want to also decrease the amount of advance; after all, burning takes place over a period of TIME if the circumstances around it aren't changing rapidly, not DEGREES of crank motion, therefore you need fewer degrees to equal the necessary time. A typical hot street motor on pump gas will run best with about 16° or so of advance at low (idle) RPMs. So let's say your motor wants 34° at max power (always ASK the motor what IT wants, not try to force what YOU THINK IT SHOULD HAVE on it... it will tell you in no uncertain terms when you give it something it likes better) and 16° at idle; that's 18° of difference. The more timing you can run at low RPMs, the better the throttle response will be; but if you run too much, you get pinging, or spark knock. Sounds like marbles rattling around in a can. That's the sign you've gone to far.

In reality though, you don't physically set the distributor up in a car that way, unfortunately. You have to start with the idle advance as its fixed setting, then as RPMs increase past some point and up to some other point, you want to gradually increase the advance. So, you set the "static" to whatever works best at idle (say, 16°), then allow the centrifugal to start adding advance just off idle (say, 1200) and have it "all in", 34°, at some higher RPM near max torque (say, 2800).

So in that case you'd "set your timing" to 16°, and set the limiter to 18°, to give your target of 34° total.

Vac adv is something altogether else. As the engine reaches high RPMs and very low load (cruising), the mixture becomes rarified because the manifold vacuum is high. If for example you have 15" of manifold vacuum at cruise, then the density of the air going into the cylinders is about half of atmospheric pressure, and therefore half as dense. This burns slower, therefore needs to be started sooner, i.e. needs more advance. Then on top of that, the mixture usually needs to be MUCH leaner at cruise than at max power; 14.7:1 air:fuel by mass is "stoichiometric", which is where every atom making up the fuel completely combines with atoms of oxygen to form water and carbon dioxide. At max power, it will ALWAYS take more fuel than that to guarantee that every oxygen atom is consumed; a ratio usually somewhere around 12.7:1 is required. (this is why "tuning" is so important... every engine is a little different, and some are EXQUISITELY sensitive to the A/F ratio at max power, such that only acoupla tenths off of whatever the engine wants, can result in 10% less HP) At cruise, you might be able to get away with even less fuel than that. But the leaner the mixture, the slower the burn, just like the lower density. So for best possible gas mileage and running and all that, your cruise mixture might end up as lean as 15.5:1 or maybe even leaner. Most engines will appreciate 12 - 15° of vacuum advance ON TOP OF the "total" timing, and one can often end up with over 50° of timing at cruise. Most distributors in which it is adjustable, put the adjustment on the vacuum can. Usually you'd want to set it up so that there's full vac adv at whatever your cruise vac is, and it should drop out starting at maybe 3" or so below that, and be fully gone by 5" of vacuum. There's a port on the carb that prevents vac adv from being available at idle, by being drawn from a port that's above the throttle blades; the throttle then has to be open some minimum amount before the port is exposed to vacuum at all. Most engines will run better with hard manifold vacuum fed to the vac adv, but some will have unstable idle, pinging at throttle tip-in, or other adverse effects, and will run better from ported vacuum. Try both and see which works better for YOU.

So your first "advance limiter" probably wants to be set at 18°, and tune it from there for best results. Which is some combination, determined by YOU - NOT "spec" from "book" - of max power, max torque, best gas mileage, smoothest idle, crispest throttle response, freedom from pinging, coolest running, no trouble starting, and so forth. Then the vac adv will want to be tuned for best gas mileage, coolest running for long stretches on the highway, no pinging when you just barely ease into the throttle on the road or go up a slight hill, etc.