Tuning A Carbureted Kit Car Engine- Fine-Tuning The Ignition System

If you don't have access to a distributor test stand, an optional method to check the vacuum and mechanical advance curves is a dial-back timing light. A dial-back timing light allows you to read the advance curve of an engine at different engine speeds even if it doesn't have a degreed vibration dampener. Just take care not to over-rev the unloaded engine. The vacuum advance curve can also be checked with a hand vacuum pump to vary the vacuum supplied to the vacuum advance. Use the timing light to read the amount of advance given by the vacuum advance at different amounts of vacuum from 1 to 23 inches.

Ignition Spark Advance Guidelines
The "hot-rod" advance curve used most on a typical 9.5:1 compression engine with a mild camshaft (duration less than 220 degrees at 0.050 inch) is 10-12 degrees initial timing plus 22-24 engine degrees of additional advance from the mechanical advance mechanism. In most cases, full advance (32-36 degrees) is in by 3,500 rpm. An engine with camshaft duration above 220 degrees at 0.050 inch will like more initial timing; however, the total timing will stay the same (32-36 degrees).

Any original or high-performance aftermarket replacement distributor must have the ignition spark advance curve(s) tuned for your engine and the gasoline you will be using. Most of the high-performance replacement ignition systems, such as an MSD distributor, come with a very conservative ignition advance curve installed in the distributor. A new MSD distributor comes with a selection of advance springs and bushings so you can set the advance curve you want. However, most vacuum-advance-equipped distributors have too much advance for the reformulated gasoline of today.

The mechanical advance should not start advancing until just above the base idle speed. Too much advance with the engine speed too low may cause a ping or detonation problem, which can lead to engine damage. The guideline we have found for determining what initial timing is best for a gasoline engine is in the Barry Grant Inc. catalog and/or Web site in the Demon carburetor selection guide. The booklet recommends 10 to 12 degrees of initial timing when the camshaft duration is less than 220 degrees @ 0.050 valve lift, 14 to 16 degrees of initial timing with less than 240 degrees @ 0.050, and 18 to 20 degrees of initial timing with a cam with less than 260 @ 0.050 valve lift. The total ignition mechanical advance from the distributor must be reduced when you increase the initial timing because engine damage will result if the total advance is excessive for the engine compression and fuel being used. A modern 9.5:1 compression Chevrolet, Ford, or Chrysler engine will in most cases like 34-36 degrees of total mechanical ignition spark advance, including the initial timing with the reformulated gasoline of today.

An engine with a high-performance camshaft designed to create power above 3,000 rpm will respond well to 18 degrees of initial timing because the air/fuel mixture is not uniformly mixed together at lower engine speeds. The additional initial timing allows more time for this air/fuel mixture to burn in the cylinder (the same theory also applies to an engine with a race-designed intake manifold or an air-gap intake manifold). A high-performance cam and/or intake manifold will decrease the velocity of the air/fuel mixture at low rpm. This low velocity causes the fuel to separate from the air at lower engine speeds.

Tuning The Ignition Spark Advance For Fuel Economy
The ignition spark advance curve an engine needs for maximum power with the rich air/fuel mixture the engine has during wide open throttle acceleration is different than the ignition spark advance it needs for the leaner air/fuel mixture the engine has when you are cruising at 65-75 mph on the highway. The vacuum advance curve we use most will provide the engine with an additional 10 to 12 degrees of spark advance when the engine vacuum is above 10 inches. This way the engine has the extra spark advance it needs to burn the leaner cruise air/fuel mixtures it sees at part throttle.

Tuning The Cobras Advance
When we referenced the Barry Grant initial timing guide, we decided that 18 degrees of initial timing would work quite well with the camshafts that are in both of the high-performance engines in the Cobras we are working on. We then recurved the distributor so it would give us 16 degrees of mechanical advance that starts advancing at 800 rpm and is all in by 3,000 rpm to give us 34 degrees of total mechanical advance that the 427 engine manufacturer recommended. We set the total mechanical advance on the 347 Ford engine at 36 degrees since this engine did not have the same fast burn cylinder heads as the 427 engine.

The changes improved the engines' performance. The idle quality was much improved on both engines, and both engines had more power at low- and mid-range rpm driving conditions. Now that the ignition system is tuned for the reformulated unleaded gasoline it's using, it's time to look at the fuel system.

Fuel Supply Systems
The fuel supply system in every gasoline-powered vehicle must supply the carburetor with an adequate volume of fuel delivered at the correct fuel pressure during every operating condition the engine will be driven at. The job of keeping fuel pressure constant on a high-performance engine may not be as simple as it seems. Everything from the fuel tank vent system to the point where the fuel is delivered to the carburetor's needle and seats must be designed to flow enough fuel for the needs of the engine.

If the fuel pressure supplied to the carburetor by the fuel pump drops below the ideal fuel pressure as measured at the carburetor's needle and seat, even within a second, the engine may run out of fuel. If the air/fuel mixture shifts too lean, engine performance will suffer, and/or if you run too lean for too long engine damage may result.

Fuel Tank Venting
Fuel tank venting is an often ignored area that can cause quite a few fuel volume and pressure-related driveability problems. As the fuel pump pulls fuel from the fuel tank, outside air must be allowed in to replace the fuel. The fuel tank gets this outside air through the vent in a vented gas cap, a fuel tank vent hose, or the evaporative canister system, depending on what fuel tank vent system that was installed on your kit car. A vented gas cap has a vent restriction that is in the 0.080-inch range. This small of a fuel tank vent may be OK for normal driving, but if you drive at high speeds for an extended length of time, it could lead to low fuel volume and pressure-related problems. If your kit car has a sealed gas tank with an evaporative canister system, the fuel tank vents through the canister. The evaporative canister has a filter system built into it. As this filter gets dirty, it will restrict the flow of outside air into the gas tank.

The next critical component in the tuning process is tuning the air/fuel mixture the engine is getting from its carburetor for both power and fuel economy. Stay tuned for part two, tuning the air/fuel mixture, in the next issue of KIT CAR magazine.