Advance the timing before TDC. Why?

Nobody ever really talks about the real reason why. It's actually very simple from a physics standpoint. Everything with the engine running happens very quickly. Even at idle, when you talk about a valve opening and closing, for example, by the time you've talked about it, it's opened and closed probably a hundred times......or more.

With that mindset, you can see that if you set timing to say 0 degrees, or top dead center (TDC), that's very late in regards to engine real time running. And all of this timing talk is relative to when the spark is lit. Light the spark at 0 degrees, for example and you're "missing" a lot of time that could have been used for making power. At 0 degrees, you're lighting the mixture with the piston at TDC. That makes the power stroke very short and decreases engine performance.

However, lighting the mixture several crankshaft degrees BEFORE TDC (BTDC) lengthens the power stroke. It allows combustion to be created much earlier in the power stroke, so it lasts a longer time, creating a harder "push" against the piston going down. Once the piston is on the way down, it's too late. That's why it's important to create the combustion before top dead center (BTDC).

This also has some negative effects (according to some). It burns more hydrocarbons, since the power stroke becomes longer, burning more fuel. This is why through the beginning of the emissions era, you saw timing specifications down around and even some AFTER TDC. I've always said the EPA is stupid there. I say that, because more advanced timing burns more fuel, but it creates FAR more power than the extra fuel burned, so its efficiency goes way up. In other words, more power and more work being done per gallon of gas. So in the long run around the barn, advanced timing is far more efficient. This is why if you still have inspections in your area, you need to retard the timing before the inspection, so the vehicle will pass. Less timing advance = fewer hydrocarbons out the tailpipe.

I've always chosen to give one as much timing as it can stand without dragging the starter, or having spark knock.....whichever happens first. I run just under 15* initial on my stock 170 slant six and it runs fantastic. I tried right at 15* and it ran a little rough so I backed it down. How much timing one can stand is directly related to how much cylinder it produces. The more cylinder pressure, the less timing it can stand......unless of course, you run race gas.....but we're not talking about "that".