Ignition curves are the key to creating optimal performance.

Ignition timing is easily the single most important tuning adjustment on an internal combustion engine and yet the concept of ignition curves continues to be elusive for many enthusiasts. Yet all it takes to put the tweak to improve torque, horsepower and drivability is a simple timing light and an informed tuning process. Think of this as "free" horsepower. Too much timing can cause serious engine damage, so it's best to be an informed tuner.

The plan behind optimized ignition timing hasn't changed since Nikolaus Otto began fooling around with the four-stroke internal combustion engine 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 15-18 degrees After Top Dead Center in order to maximize leverage on the crankshaft.

If the spark timing is initiated too early, the cylinder may experience detonation and potentially cause damage. If the spark occurs too late, the engine runs flat, makes less power, and may overheat. This discussion will focus on a typical distributor-equipped street engine running pump gas.

An engine's ignition timing requirements will vary depending upon dozens of variables like compression ratio, fuel octane, air-fuel ratio, combustion chamber shape, mixture motion, and inlet air temperature to name a few biggies. But 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 street car with a mild cam might idle at 12 to 16 inches of mercury (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 inches of 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 with this engine is to optimize the spark timing over the engine's entire operating range while minimizing the chance of detonation.

All discussion of ignition timing starts with the 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 not unusual to input 14 to perhaps 18 degrees of initial timing for engines with big cams. This timing is checked with a timing light that compares the position of the Number One 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.

This initial timing is used as a starting point for our next step, which is mechanical advance. Mechanical advance is tied directly to engine rpm. Mechanical advance is determined by use of a centrifugal advance mechanism that was first used on James Watt's steam engines in the 1780s. But even Watt admits that he borrowed the idea from an earlier design that appeared on a 1600s gristmill.

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, accomplished by advancing 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 mainly determined by the strength of the springs that hold the weights in place. Lighter springs allow the advance to begin and achieve maximum advance at a lower rpm. Heavier springs delay the onset and slow the rate of advance.

So a typical mechanical advance curve might start advancing 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 was 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 mechanical advance requires modifications to the slot or by changing the bushing diameter that fits over the pin in the slot. This is how MSD distributors allow easy changes to mechanical advance in its distributors.

It's important to mention that checking the mechanical advance with a timing light should always be done with the vacuum advance canister disconnected. If the canister is 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 yet misguided view among many enthusiasts that vacuum advance is only for bone stock and/or emissions-controlled engines. The more enlightened way to look at vacuum advance is to view it 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 use the example of a typical carbureted small-block cruising down the freeway at 70 mph at 2,800 rpm on level ground. The engine could be pulling anywhere from 12 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 overdrive 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, this means the mixture is not tightly packed. Here's where things get tricky. The common perception of the combustion process is 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 of time for the combustion gases to burn completely across the top of the piston, much 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 right for a typical pump gas street engine. However, at nearly closed throttle (14-16 inches of manifold vacuum), the air and fuel are far less densely packed in the cylinder. In order to make the most power possible at part throttle, we need to start the combustion process much sooner—perhaps as much as 40 degrees BTDC or more depending upon the engine's individual demands.

But we only need this much timing 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 that would be in addition to the mechanical advance.

We've created two graphs that illustrate very simple mechanical and vacuum advance curves. Mechanical advance is totally dependent on engine speed while vacuum advance is solely controlled 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 inches of valve lift. Even with 16 degrees of initial timing, let's say our engine barely idles at 8 inches of manifold vacuum and it is backed by a tight torque converter because it also has nitrous.

Even with 9.5 or 10.0:1 compression, the application of a long-duration 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 drivability and throttle response. Our experience shows that connecting the vacuum advance to a manifold vacuum source will add timing at idle and improve idle quality 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 inches 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 merely connected the can to manifold vacuum, which added 14 degrees of advance, creating 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 clunk of the engine dropping into gear. The added vacuum advance also allowed us to further 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 on 87 octane fuel.

We eventually added a looser converter, which allowed us to remove the manifold vacuum advance at idle technique. This looser converter allowed us to reduce the total advance at idle in gear to a more conservative 18 degrees initial, which improved idle quality in gear because of the reduced load.

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 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. Recently, HOT ROD did a To The Rescue column in which a poor-running stroker Ford small-block radically improved its throttle response just by the simple application of timing and jetting. Very few magazine stories address 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 a timing light and we guarantee your engine will be glad you did.

See all 12 photos

See all 12 photos 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.

See all 12 photos 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 are factory equipped with the largest (black) bushing that minimizes the mechanical advance. 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.

See all 12 photos 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 octagonal shape. This one uses a 3/32-inch Allen wrench to adjust the rate at which advance is applied.

See all 12 photos

See all 12 photos This is an Innova digital, dial-back timing light from Summit Racing. The display reveals both the total advance (32 degrees) along with engine rpm (2,580). To use this dial-back light, merely 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. The display then tells us we have 32 degrees of advance at 2,580 rpm.

See all 12 photos 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.

See all 12 photos You can buy a timing tape from MSD that will display the timing marks just like a degreed balancer so you don't need a dial-back light. Or you can make your own tape as we've done here. Multiply the balancer diameter by 3.1417 () 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.

See all 12 photos 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 like those from MSD, Moroso, and others.

See all 12 photos 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).

See all 12 photos This Graph illustrates a typical mechanical advance curve that includes the initial timing of 10 degrees with a total of 32 degrees. This equates to a mechanical advance of 22 degrees.

See all 12 photos This graph reveals a vacuum advance curve adding up to 14 degrees of additional timing at 18 inches Hg. Combining these two curves, it's possible to have as much as 46 degrees of advance at a 3,000 rpm cruising speed if the manifold vacuum is at or above 18 inches Hg (32 + 14 = 46).

5.3L LS Timing vs. Load Map

Load (Throttle Percentage) 1,000 2,000 3,000 4,000 5,000 6,000 10% 40 50 53 52 49 44 20% 32 34 38 40 36 32 30% 24 28 31 33 32 30 40% 18 25 28 32 31 29 50% 10 16 21 26 29 29 60% 4 12 17 26 28 28 70% -11 8 14 26 28 28 80% -11 6 14 26 28 28 90% -11 6 14 26 28 28 100% -11 4 14 26 28 28 Show All

If you refer to the graphs, you will notice they are both linear (straight line) curves. Electronically controlled engines offer the advantage of non-linear ignition curves. This chart is a simplified example of a load-based timing map that originated from an 87-octane GM 5.3L LS truck engine. Essentially this map is a combination of initial, mechanical, and vacuum advance. The vertical scale is the percentage of throttle opening (load) while rpm is presented on the horizontal scale. As you would expect, as load increases, timing decreases. As an extreme example, you would never be at WOT (100 percent) at 1,000 rpm, but if this did occur, you can see that the map minimizes the timing to -11 degrees, which is 11 degrees After TDC which is drastically retarded to prevent detonation. Conversely, at 10 percent throttle opening at 3,000 rpm the timing is at 53 degrees BTDC. This is load-based timing.

Parts List

Description Part No: Source: Price: Innova electronic dial-back timing light 3568 Summit Racing $99.97 Crane HEI adj. vac. can and springs kit 99600-1 Summit Racing $35.40 ACCEL HEI adjustable vacuum canister 31035 Summit Racing $24.32 Pertronix HEI adjustable vacuum canister D9006 Summit Racing $18.97 Summit HEI adjustable vacuum canister 850314 Summit Racing $12.97 Standard Motor SB Ford adj. vac. canister VC192 Summit Racing $36.97 Summit LA Mopar adj. vacuum canister 850426 Summit Racing $19.97 Crane GM points dist. vacuum adv. kit 99601-1 Summit Racing $35.43 MSD timing tape 8985 Summit Racing $4.25 Show All

Sources:

Crane Cams

866-388-5120

cranecams.com

Performance Distributors

901-396-5782

performancedistributors.com

Summit Racing

800-230-3030

summitracing.com