questions on timing an engine

You need to do some reading on the subject of engine timing. As it has been said previously there are three components that dictate the timing curve an engine has. In this example I am talking about a street driven engine, not a track only race engine.

First component is initial or base timing. This is the degrees of advance an engine sees at idle rpm generally 900 rpm or less. It is constant, does not change with rpm or vacuum.

Second component is mechanical or centrifugal advance. This advance is added based on an increase of engine rpm over idle to the base setting up to a predetermined value. Most street driven engines like around 30 degrees of advance give or take of base plus mechanical. Base timing added to mechanical advance timing = the sweet spot where engine can develop maximum power without any pinging or spark knock.

Mechanical advance is controlled by the length of two slots located inside the distributor that allow, or limit a pair of weights to travel based on engine rpm and keeper spring tension out from distributor’s center shaft not unlike a governor would function. Previously someone listed the length of these slots needed to produce a given amount of advance. The shorter the slot is the less mechanical timing that can be added to the mix. To read mechanical advance with timing light, vacuum advance vacuum hose must be disconnected from carburetor, and its tap caped off to prevent a vacuum leak condition.

Third component of advance is rather ingenious, the vacuum advance. Previously I said an engine can only be advance to around 30 degrees to keep it from pinging under hard acceleration, or load. However that same engine under light load, such as steady cruise on level ground, can tolerate an additional 20 degrees or so of timing to return better fuel economy. The ingenious part of this device is based on how or when the engine develops or loses vacuum under different load conditions.

You know that when engine is under wide open throttle (WOT) vacuum drops to near zero, and when coasting down a hill with little load, it will develop maximum vacuum perhaps into the low 20’s inches Hg. The vacuum advance diaphragm reads these different vacuum conditions and adjusts timing automatically. As vacuum increases timing is added in up to around 20 additional degrees over the base and mechanical advance. As engine comes under load, that additional 20 degrees of advance can be reduced to zero leaving just the base and mechanical component.

Vacuum advance will not activate at idle if connected to ported vacuum tap on carburetor. This tap is located on atmospheric pressure side of throttle plates, or in other words, above the throttle plates, and won’t see any engine vacuum at idle. As throttle plates are opened this tap is exposed to engine vacuum, at which time the vacuum advance can is activated.

To sum up; base timing + mechanical timing + vacuum advance = total timing.
Depending on rpm and load conditions, timing can vary between base setting at idle to somewhere near 50 degrees of advance.

The tricky part to this timing biz is to determine what your engine likes for timing throughout its rpm range under varying load conditions. Not all engines have the same timing requirements, and each vehicle has different weight and aerodynamics that contribute to the load an engine sees. In other words a light car will have different timing requirements than a heavy truck.

Generally 10 to 16 degrees of base advance a good starting point.
Full load condition, most engines like 28 to 35 degrees of timing, just how fast the timing needs to be dialed in will depend on a particular engine’s build and converter stall speed. Perhaps someone with a 360 build similar to yours could chime in with their settings for a rough starting point.

Vacuum advance will also have to be dialed in. The diaphragm can be adjusted by an Allen wrench when to react to a given vacuum level and propensity of a given engine to ping under light load.

Curving, or developing an advance curve for a particular engine without a machine designed for such projects, is a trial and error endeavor. Here is an article on re-curving a distributor with nice photos showing slots, and weights.
http://www.slantsix.org/forum/viewtopic.php?t=46266

Here is a list of vacuum cans, not all of them are V8 cans, but one can see that there is a selection based on vacuum level and amount of advance available.
http://www.slantsix.org/forum/viewtopic.php?t=13198

Here is more reading on re-curving showing how I worked through my first attempt on a high performance slant six. A straight six has different timing requirements than a V8, but the curving process is the same. Recommended reading if you are having problems sleeping… LOL
http://www.slantsix.org/forum/viewtopic.php?t=38471