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Another timing question.

845 views 2 replies 3 participants last post by  BogiesAnnex1 
#1 ·
On another thread I started a while back some guys said that my 355 should be 24-26 initial and 34-36 all in. Im at 17 initial right now. If I bump the initial to 24-26 how do I control the upper end to keep it at 34 -36. Isnt it going to advance right along with the initial. I searched but really didnt find a laymen explanation for this. Thanks in advance.
 
#3 ·
First thought is that initial advance is determined by the cam. As the cam timeing becomes more radical the more initial ignition timing is required. This can go as far as having all the advance in the initial for extreme cams.

In general the total amount of ignition advance isn't quite as sensitve to the cam. This is largely controlled by the effectiveness of the compbustion chamber and the Dynamic Compression (DCR) Ratio which is tied the where in crankshaft degrees the intake valve is closed so yes this is where it links to the cam. Where heads and their combustion chamber are concerned a good example of this difference can be found between SMOG type open chambers and the L31 Vortec. The former open chamber head will require 45 to 55 degrees advance for best power, the latter L31 type chamber will develop best power between 32 and 36 degrees of spark advance.

The total advance is the addition of the base and the variable advances. So where the total can't be exceeded then you need to know how the distributor sees this. A typical non smog with a moderate street cam distributor is set up for 10 to 14 degrees of base and 22 to 25 degrees in the centrifugal. If your in a cam situation where more base is needed than that added amount has to be removed from the centrifugal so the total is not affected otherwise the engine is likely to go into detonation.. This is done by restraining the centrifugal using methods that range from welding guide slots to fill some of their lenght to using various types stops its are available for this.

Above a certain point that being about 3000 RPM the flame speed naturally accelerates in concert with increasing RPM which is why advance systems stop at about that point. Flame speed is dependant upon compression ratio and mixture ratio. Higher compression ratios generate faster flame speeds. Mixture ratios that get too far from chemically (stoichiometric) correct slow down. That's happens for lean because there is too much distance between fuel molecules and it happens to rich mixtures as well where the excess fuel burns slowly due to a lack of oxygen for the avalible fuel. There's other causes at work as well but just to keep it simple.

Vacuum advance is used with milder cams to support cruise RPM power where the RPM is fairly low as is the load on the engine, yet high speeds are expected. This condition generates high manifold vaccum with low mixture density in the cylinders, this burns slowly so more advance is needed. This brings up the power without having to add more throttle which improves cruise mileage.

Bogie
 
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