[quote name='cowvette' post='166300' date='10/11/2008, 00:00']34 gradi di anticipo a 3000 giri con vacum disinserito ti trpvi meglio del classico 10 di anticipo al minimo[/quote]
la mano di tom suggerisce di cambiare il tasso della molla di primavera
(Editors Note: Rocky Rotella is an avid Pontiac enthusiast and a noted collector of Pontiac information. He is a member of the Omaha chapter of POCI and has graciously shared this information with us. A special thanks to Rocky from the Arkansas Pontiac Association.)
By Rocky Rotella
Most people would consider the ignition distributor as the source of spark distribution to the plugs. In basic operation that is it's true function but a distributor must offer spark in more than one way to provide optimum performance at all RPM.
Whether Pontiac or another manufacturer produced your vehicle, it will have points or an electronic firing source like that found in the HEI. Either distributor relies on the same basic function of increasing spark timing to provide more spark when needed. We'll look at the three basic stages of spark timing, initial, mechanical and vacuum advances and how they affect your car's economy and performance.
For any type of fire to exist it requires three key elements; fuel, air and spark. The internal combustion engine is no exception but it relies on compression of the fuel/air mixture to create a more volatile explosion producing greater amounts of power. As the piston exerts force upward, compression of the fuel/air mixture is ignited from the spark plug just before the piston reaches the top of its travel. This is known as "spark lead" and is referred to in crankshaft degrees Before Top Dead Center (BTDC). As overall engine speed increases so does individual piston speed requiring spark lead to be even greater or when the piston is further from TDC, to achieve optimum performance.
Initial timing is the base figure of spark lead at and slightly above idle speed. Initial timing is a set amount, however it can be adjusted by rotating the distributor clockwise (advance) or counterclockwise (retard). Most street driven cars require initial timing in the 10 to 14 degrees BTDC range but specific combinations will vary. Initial timing can and should be checked with the use of a timing light. Increasing initial timing will raise idle RPM which will require lowering of the idle set-screw. Too much initial timing may result in too high of idle even with idle set-screw completely backed out. Additionally, excessive amounts of initial timing can lead to starter drag when hot.
Because the fuel/air mixture requires a specific amount of burn time, when engine speed increases, spark lead must occur with the piston deeper in the hole or more degrees BTDC for optimum burn to result. A pair of springs and specially shaped weights work in conjunction with a floating plate on the distributor shaft to advance the rotor's electrode in relation to the firing time of the distributor. Through centrifugal forces, the weights begin to spread apart and advance timing until they reach maximum travel which is limited by the weight's shape. Spring tension works against the forces to control the rate at which the weights spread apart thus producing the advance curve. This action is known as mechanical advance. Too much mechanical advance before the opportune time will induce pre-detonation from the entire fuel/air mixture being completely burnt before the piston reaches TDC. Noticeable power loss or audible detonation are signs that may indicate too much advance or too quick of advance curve.
The third stage in the distributor's role is for improved economy. Conditions where manifold vacuum is at its highest like at part throttle cruise are where engines are most efficient. A special "can" mounted on the distributor housing and operated by vacuum advances the floating "advance plate" further providing additional timing for increased fuel economy and better overall efficiency.
Vacuum advance has no bearing on overall performance because the vacuum signal drops under heavy load or WOT conditions. Certain aftermarket advance cans from manufacturers like Crane are adjustable allowing the tuner or owner to tailor the amount of vacuum required to advance but also the amount of advance available. These are very beneficial as generic vacuum advance cans are not calibrated for any specific application and could over-advance or advance too quickly and induce detonation.
The factory-set mechanical advance curves are somewhat lazy and not optimum for maximum performance. Mechanical advance curves can be modified to provide better performance in both the amount of advance by replacing the weights and the rate of advance by replacing the springs. Ideally, the advance should begin slightly above idle and stop advancing or "all in" between 2500 and 3000 rpm.
By marking the harmonic balancer, use of a timing light and much trial and error one could modify the advance curve by changing weights/springs. There are kits available from aftermarket manufacturers through your local parts store but those should be avoided. They are rarely accurate and are more generic in calibration. The most productive way to adjust the advance curve would be to remove the distributor and have it "recurved" using a distributor tester.
Both the Allen Electric and Sun Electric companies produced machines that would test operation of the advance allowing the operator to change and retest the unit. Certain models included a vacuum pump to check operation of the vacuum advance unit. It is almost necessary to use a machine to adjust the aftermarket adjustable advance cans. Today, finding a shop with an operational machine is tough and trying to purchase a one would prove costly.
The most confusing aspect of ignition timing is the math! Most do not realize that a 4-stroke motor's camshaft will spin half that of the crankshaft. Since the crankshaft must intake-and-compress on one revolution and power-and-exhaust on the next, the valves only need to be open on the respective intake stroke or exhaust stroke. If you have ever noticed, the cam gear of the timing chain set is twice the size of the crank gear. If spun in a 1:1 ratio, the cam would open the valves every crankshaft revolution allowing no compression stroke or power stroke to happen.
Since the cam operates the distributor, the distributor runs at half speed making its output multiplied twice for crank output. Therefore specifications of distributor RPM are half of crank RPM.
For instance, a mechanical advance curve may begin at 1200 crank rpm and be in by 2600 crank rpm and provide 24 degrees advance at the crankshaft. At distributor RPM, the advance will begin at 600 rpm, end at 1300 rpm and only advance 12 degrees. It's very confusing but re-read the paragraph and you may grasp more another time through.
Total timing is the sum of initial timing and mechanical advance. Vacuum advance pshould never be factored into total timing. In fact, when checking timing the vacuum advance can should be disconnected and its hose plugged to prevent a vacuum leak.
A mathematical example of total timing would be having 24 degrees mechanical advance from the distributor. Setting the initial timing to 10 degrees would produce 34 degrees total timing (10 + 24 = 34). Moving the initial timing to 14 degrees would produce 38 total timing (14 + 24 = 38). If the hypothetical vehicle required 14 degrees initial advance but only 34 degrees total advance, recurving the distributor to provide 20 mechanical advance would provide the desired results (14 + 20 = 34). Due to combustion chamber design and spark plug placement, most Pontiacs run best with total timing in the 34 to 38 degree range.
As overwhelming as it may seem, all these elements must be addressed when trying to achieve optimum performance from your car. Advance curves are not a "one-size-fits-all" concept, each aspect of a combination must be considered and sometimes it may require more than one attempt to find what suits your vehicle's needs.
Distributors may indeed distribute spark but the distributor itself needs to function properly and provide spark at the right time. Timing is a key element in a car's operation and without proper timing, performance and economy will suffer. Next time you pop your hood and see the distributor, think of all that goes on under that cap!
