Ignition Curves and How to Create Optimal Performance

Ignition timing is easily the single most important tuning adjustment on an internal-combustion engine, yet the concept of ignition curves continues to be elusive for many enthusiasts. All it takes to improve torque, horsepower, and driveability is a simple timing light and an informed tuning process. Think of this as “free” horsepower, because it costs very little to optimize timing as long as you know the tricks.

The plan behind optimized ignition timing hasn’t changed since Nikolaus Otto began fooling around with four-stroke, internal combustion engines in the 1870s. The idea is to light the charge in the cylinder with enough lead time (advance) to create maximum cylinder pressure at the ideal point after top dead center (ATDC) to push the piston down, exerting leverage on the crank. It’s generally acknowledged that peak cylinder pressure needs to occur at roughly 15 to 18 degrees ATDC to maximize advantage on the crankshaft. If the spark timing is too early, the cylinder may experience detonation and potentially cause damage.

01] This is a typical mechanical-advance mechanism on an HEI distributor with a pair of weights that move outward as engine speed increases. You can create a custom curve by mixing springs from an aftermarket spring kit. One of the two slots is indicated by the arrow. The only way to reduce the total mechanical advance is to shorten the length of the slot. This will require disassembly and some brazing or welding.

02] MSD distributors use a single slot and pin with a bushing that is retained by a nut. Changing the bushing diameter allows the tuner to increase or decrease the amount of mechanical advance. MSD distributors come with the largest (black) bushing that minimizes the mechanical advance, but smaller bushings are supplied with the distributor. Make sure to put a spot of Loctite on the threads when changing the bushing. We’ve seen these nuts fall off.

03] Vacuum-advance canisters move the plate in the distributor when vacuum is applied to the internal diaphragm. Vacuum applied to the diaphragm advances the pickup position, altering the timing. Adjustable vacuum canisters are available for most popular distributors and are usually identified by their hexagonal shape. This one uses a 3⁄32-inch Allen wrench to adjust the rate at which advance is applied.

04–05] This Innova digital, dial-back timing light displays the total advance (32 degrees) and engine rpm (2,580). To use this dial-back light, press the advance (up arrow) or retard (down arrow) buttons until the TDC mark lines up with the zero mark on the engine’s timing tab. This display tells us we have 32 degrees of advance at 2,580 rpm.

If the spark occurs too late, the engine runs flat, makes less power, and may overheat. For a typical distributor-equipped street engine on pump gas, this means we should take the Goldilocks approach to ignition timing. This discussion will focus on street-driven engines running pump gas, although the generic components are all the same for any engine.
An engine’s ignition-timing requirements will vary, depending on dozens of variables like compression ratio, fuel octane, combustion chamber shape, and inlet air temperature to name a few biggies. However, condensing this down to its simplest aspects, timing is dependent on engine speed and load. Load is determined by the throttle and is easily monitored with a vacuum gauge. When the throttle is barely open, the engine demands more air than the throttle allows, creating manifold vacuum (low pressure). A typical streetcar with a mild cam might idle at 12 to 16 inches of mercury (in-Hg) on a vacuum gauge. As the throttle is opened, manifold vacuum begins to drop. At wide-open throttle (WOT), manifold vacuum drops to near zero. Most engines will pull roughly 0.5 in-Hg of manifold vacuum at WOT.

The next step is to separate ignition timing into three basic components: initial timing, mechanical advance, and vacuum advance. Our approach is to optimize the spark timing over the engine’s entire operating range while minimizing the chance of detonation.

All this starts with initial timing. This is the amount of advance at idle with the spark triggered before top dead center (BTDC). Most stock street engines call for 6 to 8 degrees of initial advance, but this is not set in stone. Engines with longer-duration camshafts and other modifications often demand more initial timing. It’s common to input 14 to 18 degrees of initial timing for engines with big cams. Timing is checked with a timing light that compares the position of the No. 1 cylinder TDC mark on the harmonic balancer with a timing-reference tab located most often on the timing-chain cover. Initial timing is set by loosening the distributor hold-down bolt and rotating the distributor body. This changes the relationship between the distributor body and the spinning rotor. Twisting the distributor opposite to the direction of rotation advances the initial timing.

Next is mechanical advance. Mechanical advance is tied strictly to engine speed (rpm). It is determined by a centrifugal advance mechanism, first used on James Watt’s steam engines in the 1780s. However, even Watt admits he borrowed the idea from an earlier design that appeared on a 1600s gristmill.

01] Here’s a quick tip for determining rotation on any distributor with a vacuum-advance can. Position your hand parallel with the vacuum-advance can as shown. Your fingers will point in the direction of distributor rotation. This Chevrolet HEI distributor rotates in the clockwise direction. Ford distributors locate the vacuum can on the opposite side of the housing, which means they rotate counter-clockwise. Simple, no?

02] You can buy a timing tape from MSD that will display the timing marks so you don’t need a dial-back light. Alternatively, you can make your own tape as we’ve done here. Multiply the balancer diameter by 3.1416 (pi) and divide that value by 180 to get a distance per 2 degrees. For an 8-inch-diameter balancer, we rounded that 2-degree value to 0.140 inch. That positions the 30-degree mark at 2.1 inches from the zero mark on the tape.

03] All this tuning assumes the ignition system is already in peak condition. Always use a high-quality distributor cap with brass connections like this MSD piece instead of the cheap aluminum ones and spend the money on quality spark-plug wires.

04] Even the little things can make a difference. Projected nose spark plugs (left) move the spark a little closer to the middle of the chamber and offer a small advantage over standard plugs (right).

The typical centrifugal advance uses a pair of weights that pivot on pins. The weights are attached to a plate that locates a pin moving within a fixed slot. The distance the pin travels is the amount of mechanical advance, which advances the position of the rotor. On a typical Chevrolet distributor that spins clockwise, as the mechanical advance weights open, this moves the rotor in the same direction, advancing the timing. The rpm at which the weights begin to move and the point of their maximum travel is determined mainly by the strength of the springs that hold the weights in place. Lighter springs allow the advance to begin at a lower rpm. Heavier springs delay the onset and slow the rate of advance.

A typical mechanical-advance curve might start at 1,500 rpm and achieve full advance by 2,600 rpm. If that full advance moves the rotor by 25 crankshaft degrees and our initial timing is set at 10 degrees BTDC, then our total mechanical-advance reading at the harmonic balancer at 2,600 rpm or higher would be 35 degrees (10 initial + 25 mechanical = 35 degrees total). We can adjust this total by either adding or subtracting initial or mechanical advance. Changing the quantity of mechanical advance requires modifications to the slot or by changing the bushing diameter that fits over the pin in the slot. Both of these methods change the physical amount of space the rotor can move.

Checking the mechanical advance with a timing light should always be done with the vacuum-advance canister disconnected. If not disconnected, the readings will be a combination of initial, mechanical, and vacuum advance.

Now we can introduce vacuum advance into this system. There’s a popular but misguided view among enthusiasts that vacuum advance is only for bone-stock engines and/or emissions-controlled engines. The more enlightened way to look at vacuum advance is as load-based timing. It’s worth a peek down the rabbit hole of the combustion process to understand why load-based timing is important.

Let’s start with a typical carbureted small-block cruising at 70 mph at 2,800 rpm on level ground. The engine could be pulling between 14 to 18 inches of vacuum. As mentioned before, high vacuum means low load and a nearly closed throttle. A little-known fact is that most mild street engines cruise down the freeway pulling fuel from the carburetor’s idle circuit. That’s not a misprint. Engines with long-duration cams or cars with tall gears in overdrive might transition into the main circuit, but most mild street engines with high vacuum at cruise will actually be running on the idle circuit.

With a minimum of air and fuel entering each cylinder, the mixture is not tightly packed. Here’s where things get tricky: You may think of the combustion process as an explosion—the spark goes off and, boom, combustion occurs like a bomb. That’s not what happens. The reality is the spark plug fires and it takes a generous period for the combustion gases to burn completely across the top of the piston—more like a prairie fire across a large valley. The more densely packed the grass is, the faster it burns, while sparse areas burn more slowly.

We can apply this prairie-fire analogy to the combustion space. At WOT, the air and fuel are tightly packed and burn quickly, so we don’t need as much timing. At 2,800 rpm at WOT, 32 to 34 degrees of timing could be just about perfect for a typical pump-gas street engine. However, at very light throttle (14 to 16 inches of manifold vacuum), the air, and fuel are far less densely packed in the cylinder. To make the most power possible at part throttle, we need to start the combustion process much sooner—perhaps as much as 40 to 44 degrees BTDC, depending on the engine’s individual demands.

This timing is only needed when the engine is under very light load. Since manifold vacuum is a great indicator of load, early engine designers used a small vacuum canister attached to the distributor to advance the timing under high manifold vacuum to create a load-based timing curve independent of mechanical advance.

The two graphs illustrate very simple mechanical and vacuum advance curves. Mechanical advance is very dependent on engine speed, while vacuum advance is controlled only by engine load. We need both because on the street we can have low load at very high engine speeds, say 6,000 with the throttle barely open or very high load (WOT) at very low engine speeds like 1,500 rpm. These two situations have very different ignition-timing requirements.

Now let’s introduce the critical variable of cam timing. Let’s use an extreme example with a small-displacement engine like a carbureted Ford 5.0L with a big hydraulic roller cam with 230 degrees of duration at 0.050-inch and 0.565-inch lift. Even with 16 degrees of initial timing, let’s say our engine barely idles at 8 inches of manifold vacuum, and it’s backed by a tight torque converter because it also has nitrous.

With 9.5 or 10.0:1 compression, the application of a big camshaft means the cylinder pressure at low speeds will be greatly reduced compared to a milder cam. This engine would respond to more vacuum advance at cruise speeds at part throttle to improve its driveability and throttle response. Connecting the vacuum advance to a manifold vacuum source will add timing and help the engine idle better in gear with an automatic transmission. Milder applications can also benefit from this idea, but will require some experimentation. Several companies like Crane, Moroso, Pertronix, and Summit Racing offer adjustable vacuum-advance canisters that allow you to customize the advance curve to fit your engine’s requirements.

Let’s put these ideas into action with a specific example. We dropped a very mild 383ci small-block into an early El Camino pushing through a TH350 Trans and a very tight 11-inch converter. With 16 degrees of initial timing and a properly adjusted idle circuit in the Holley carburetor, the engine struggled to idle with the in-gear vacuum dropping to below 8 in-Hg. Adding more initial timing meant making major changes to the HEI distributor to limit the mechanical advance that was ideal at 20 degrees advance (16 initial + 20 mech. = 36 degrees total).

The distributor was fitted with an adjustable vacuum-advance canister, so we connected the can to manifold vacuum, which added 14 degrees of advance and created 30 degrees of advance at idle. The idle vacuum instantly improved to 12 inches in gear and allowed us to lower the idle speed to minimize that annoying in-gear clunk. The added vacuum advance also allowed us to lean the idle mixture slightly. This engine only had 8.5:1 compression, so it prefers more timing. After additional driving and tuning, we finalized this combination with 14 degrees initial, 20 degrees of mechanical advance, and 14 degrees of vacuum advance for 48 degrees at highway cruise speeds, yet it runs fine even with 87-octane fuel.

Every engine will have different timing requirements based on its combination of combustion-chamber design, compression, octane, cam timing, and ignition-curve variables. The best way to determine your ideal curve is to make small changes and evaluate them for a few days of driving before attempting further changes. Pay attention to what your engine is telling you and record all your changes in a notebook.

This is just one example, but it serves to illustrate how you can juggle ignition timing to improve part-throttle engine performance. Very few magazine stories talk about part-throttle performance, but it’s critical for street-driven engines. If you think about it, a street engine easily spends 95 percent of its life at part throttle and idle. Why wouldn’t you take the time to ensure your engine runs its best where it will be spending nearly all of its operating life? Spend a little quality time with your engine and a timing light, and we guarantee you will be glad you did.

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