Ignition Timing and How to Modify
Engine timing and it’s effects on performance.
Ignition timing is measured in several different ways for distributor type engines. Distributor less engines do not have mechanical components to manipulate timing and are measured differently. In this article I am explaining distributor timing control . I will discuss distributor less ignition systems (DIS) in a later article. Here are the terms used when asked about an engine with a distributor . Initial timing, total timing, mechanical advance, and vacuum advance. Initial timing is what amount the distributor is advanced or retarded from top dead center before any mechanical or vacuum advance is introduced. Total timing is just what it suggest. The total amount of timing of the combined initial setting, mechanical advance mechanism inside the distributor and the vacuum canister if used or hooked up. As the engine changes rpm it requires a different timing curve to make the most power efficiently. Mechanical advance is usually located under the distributors rotor button. Some aftermarket distributors and OEM’s will put it under the plate located inside the distributor that holds the points or trigger wheels pick up. The mechanical advance is controlled by weights and springs. They are carefully matched to bring in more timing as the engine starts to increase in rpm’s. There is a slot that a pin rides in to limit how far the mechanical advance can move and is sized according to the amount of timing desired when fully advanced. Vacuum advance is controlled by a dash pot or round canister looking thing mounted on the side of the distributor housing. If used it can be hooked up to ported (vacuum at part throttle) or direct vacuum (vacuum at idle) depending on the desired timing required by the engine.
Now that you know what all the terms are and what their intended purpose is. You can begin to modify and change your distributors settings to improve horsepower and torque. Keep in mind that engine damage will result if not properly executed and tailored exactly to what the engine wants. More total advance is usually not good or desired. Quicker advance is good for an engine with a larger camshaft with more than stock overlap. Vacuum advance is mostly used for fuel economy at cruising speeds. Initial advance cannot be to high or it will make the engine hard to start when it’s hot by kicking against the starter.
A few examples of how a distributor can be altered to compliment an engine. For stock or near stock engines the mechanical advance springs can be changed to a lighter tension spring for quicker advancing. This if not over done, can improve off idle performance and seat of the pants feel. Improving fuel economy possibly as well. It speeds up the flame front at lower engine speeds and increases cylinder pressure down low. Fuel octane can only tolerate so much cylinder pressure especially at lower engine rpm’s.So go in small increments and listen very carefully for detonation or misfires. For a mildly cammed engine with moderate compression up to 9.5-1 with iron heads and 10.5-1 with aluminum heads. Changing the springs and weights can offer an improvement in performance. Advance curve kits usually come with weights and springs. The weights are heavier than the factory weights which helps the mechanical advance to come in quicker. A side note: Most all HEI distributors have way to much mechanical advance. This isn’t a problem with stock springs and weights, but becomes a big problem when using lighter springs and heavier weights. This is when it’s time to modify the slot I told you about that limits the mechanical advance mechanism. That can be done in several different ways. One way is to dis assemble the distributor and weld the slot to reduce its length which I prefer. Or in some advance kits you get a bushing that goes over the pin that rides in the slot which increases it’s diameter and limiting its travel. Some aftermarket distributors will give you the extra advance curve parts. In these parts kit are bushings of different diameters just for this. Heavily modified engines with big camshafts will require the distributor to be locked not allowing any mechanical advance at all. These types of engines have so much camshaft overlap that at lower rpm’s they need as much help as possible building cylinder pressure. So whatever the engines required total advance is, is where you set the initial.
All engines are different and require different curves. So the only true and correct way to re-curve a distributor for your engine is through trial and error. Before doing any timing curve changes. Make sure the engine is in good mechanical condition and good tune. Always use the same grade of fuel and test in several different scenario’s. Such as hot and cold temperatures, loads up and down hills, steady cruising, and full throttle. Never ASSUME !!!!
Pontiac V8 535 E head 750 HP pump gas engine
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