The cool thing about ignition timing is you can usually get a considerable performance increase for little or no money. But, some time and effort will be required.
Let’s start with a simple explanation of why we need spark advance and how much of it we want.
First – When you hook up the timing light to the #1 spark plugwire and it flashes a light beam onto the timing tab, it’s showing youwhen the spark occurs, NOT when the ignition occurs and the explosion that starts to push the piston down the cylinder.
We want this to happen just as the pistons reach the top of thecompression stroke, but it takes time for the spark plug to ignite thefuel mixture. This “time” is measured in crankshaft degrees and is thedifference between when the plug fires and the explosion occurs to pushthe piston down the cylinder – producing the power stroke.
Next – How much “time” or how many degrees of advance is correct.Well, that changes with each application but in most cases it will beas much spark advance as we can get as soon as we can get it withouthaving detonation or “ping”.
Now that we know why we want spark advance and that we want all we can get without detonation – How do we get it?
If the engine is already in the car and running, we will have tohope the timing mark is correct. If you are building an engine orhaving one built at a shop, make sure the timing mark is correct; use adial indicator on the #1 piston head to find “top dead center.” Also,if the damper is easy to get to, put a 38° mark on it to use as areference on the stock timing tab. The formula we use is Dia.×3.1416÷360×38 (Diameter of thedampener/balancer × PI ÷ 360°×38°, or with numbers:8×3.1416(25.1328)÷360(0.0698" per degree)×38°(2.6529" which you mark onyour balancer away from your TDC mark), this distance will be 38° on whatever diameter damper youare using.
Now we know the timing tabis correct, and we have a 38° mark on the damper, we can now start towork on our timing “curve.”
Most racingand street performance ignitions do not have vacuum advance and thestock type distributors that are sent to a shop to be recurved shouldhave the vacuum advance removed and locked out. For this reason we willdeal only with mechanical advance. If you send yourdistributor to a shop to be recurved they can get you pretty close tothe correct curve on a distributor machine. We usually install a 26°mechanical curve that starts about 100 rpm higher than the engines idlerpm and have all 26° in by 2,800 rpm, this is a good general purposetiming curve when used with 12° initial timing set at engine idle.
This, however, is not perfect or optimum for any one combination.
A street/strip car that runs on pump gas may “ping” with this muchspark advance and will require less initial advance or heavier springsto slow down the “curve.” Where as a low compression, low stall speedconverter car may respond better with more initial spark advance orlighter springs for a quicker “curve” – but watch it if you begin yourmechanical advance curve at or below the idle rpm, the car will be areal pain in the ass to drive and tune.
For circle track applications, these motors almost always operate overthe rpm where total advance is needed so the timing “curve” is notnearly as important as having the correct total spark advance. This iswhere the 38° mark is very handy to have. We have had much success withinstant advance curves in these engines. This is where we start engineson about 10° of spark advance and the instant it fires, the timing goesto the total advance – as long as you use high enough octane fuel thisgives a nice clean idle and very quick response.
As you can see, there’s a lot you can do with ignition timing, andevery engine will require a specific curve and total spark advance, butif you take the time to sort out what’s best for your engine, you willgain performance without spending a lot of money, and isn’t that whatwe are all after.
– Tech Tip courtesty of Jensens Engine Tech
Engine Builder Staff
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