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

Tuning a carburetor-equipped engine to perform its best with the reformulated unleaded gasoline of today can prove to be a challenge for many kit car owners and builders. Back in the days of leaded gasoline a tuner would tune the ignition spark timing and the carburetor's jetting by "reading" the spark plugs, but quite often the reformulated unleaded gasoline of today does not leave any "color" on the spark plug unless the air/mixture is very rich. The unleaded reformulated gasoline of today is blended for a modern computer-controlled, electronic fuel-injected engine with the goal of the lowest possible exhaust emissions.

Modern gasoline is composed of hundreds of different hydrocarbons blended together to provide the engine with a fuel that hopefully will allow the engine to perform its best from the moment you start your engine. There are complicated differences between the reformulated unleaded gasoline of today and the leaded gasoline of years past. The following changes are the ones most important to tuners. Removal of the lead that was used as an octane enhancer from the gasoline is the clearest difference, but the distillation profile of the gasoline has also been changed to help reduce evaporative emissions from the fuel. The fuels also have density differences, as well as flow changes. This means the fuel burns at a different rate during the combustion process and the fuel even flows differently through the jets of a carburetor when compared to the leaded gasoline of days past.

The map on the previous page shows different blends of gasoline mandated in the United States. Many areas of the country are voluntarily adding oxygenates such as ethanol to their gasoline.

There are at the least 14 different government-mandated blends of conventional, reformulated, and oxygenated gasoline that are available at your local gas station depending on the part of the country where you live. The formulation of gasoline available at your local petrol station will also vary by the season of the year and the predicted temperatures in that part of the country. Gasoline in many parts of the country may also contain oxygenates such as ethanol. This is true even in areas of the country where there is not yet a government mandate for its use.

The new fuel-injected cars of today have an onboard computer that is continually adjusting the air/fuel mixture and ignition spark advance to match the formulation of the gasoline and the engine's needs. When you use this modern unleaded reformulated and/or oxygenated gasoline in an engine that was designed and tuned for leaded gasoline, it changes the air/fuel mixture the engine is getting from its carburetor. The burn rate of modern gasoline is also different from the leaded gasoline of days past, so the ignition spark-timing advance needs of the engine are also different than they were. This means the air/fuel mixture the engine is getting from its carburetor and the ignition spark advance curves from its ignition distributor must be tuned to match the needs of the engine and the blend of gasoline you are using.

In this tuning article we are going to show you how we tuned the ignition and carburetor systems on two Cobra replica kit cars to solve the driveability problems their owners were experiencing. Hopefully, you should also be able to do this for yourself. The first Cobra has a high-end 427ci Ford crate engine that was rated at 550 hp, while the second Cobra has a 347ci Ford engine that has since been dyno tested to show that it has over 350 hp at the rear wheels. The first Cobra with the 427 crate engine had a very bad stumble off idle. When you drove it at 2,200 rpm on the highway, the engine would almost shut off even though the engine was "hot tested" and "dyno tested" at the factory before they shipped it to the customer. The second Cobra with the 347 engine had a hesitation on acceleration, a surge at part throttle, and it did not have the power the owner expected from the engine he built.

Ignition Spark Timing
The first step in the process of tuning an engine to use today's reformulated unleaded gasoline is to tune the ignition spark advance curve(s). The proper tuning of the initial and the ignition spark timing advance curve(s) may be the easiest and lowest-cost way to unlock the power that was built into an engine.

The distributor ignition advance curve that is in most new aftermarket distributors is a very conservative advance curve for a generic engine. It is up to the person installing this new distributor to set the amount and rate of ignition spark advance curve of the distributor and how much initial spark timing the engine needs. The variables that need to be taken into account when you pick an ignition advance curve are: the vehicle's weight, how the driver will operate the engine (the rpm range of engine operation, or if the driver tends to lug the engine), the fuel octane, the engine compression, altitude of the vehicle's main usage, and the heat of the air. The distributor's mechanical and vacuum advance curves must be correct for the engine and the fuel being used or the engine's performance will suffer-and there's also the possibility of engine damage from detonation.

Ignition Spark Advance Curves
The ideal ignition timing for power and fuel economy is just short of the point where detonation or pinging occurs. The correct ignition timing (the initial timing plus the advance from both mechanical and vacuum advance mechanisms) will cause the pressure created by the fuel being burned in the combustion chamber that is pushing the piston down to be at its peak/maximum when the piston is at about 12 and 15 degrees ATDC (after top dead center). If the peak cylinder pressure is reached too soon, the engine will lose power as the piston fights to compress the burning air/fuel mixture. Detonation may also be experienced, which can lead to engine failure. If peak cylinder pressure is reached too late, the engine will just be wasting the energy the combustion process creates as wasted heat out through the exhaust system.

The amount of ignition advance needed changes with engine speed, engine load, engine temperature, air temperature, compression ratio, the fuel octane being used, and the air/fuel mixture. When an engine is operating with a rich (power) air/fuel mixture for maximum power, it needs less ignition spark advance than it does when it is operating with a lean (cruise) air/fuel mixture. A distributor with a vacuum advance adds a little more ignition spark advance during a low-load highway cruise condition, so the engine can more completely burn the slower-burning, leaner air/fuel mixture that is seen at part-throttle conditions. A 12.5-13.5:1 air/fuel mixture burns the fuel faster and will make more power, but a leaner air/fuel mixture of 14-15:1 is better for fuel economy.

If an engine does not have enough ignition spark advance, the engine may lack power, plus the engine may tend to run hot or overheat. If the engine has too much ignition spark advance, the engine will also want to run hot and lack power. If excessive pinging or detonation is present, you are also risking engine damage. Any distributor, whether it is original or performance replacement, should have the mechanical and vacuum advance curves checked to confirm they are correct for use with your driving style, engine package, and today's reformulated unleaded gasoline.

The best way to check both the vacuum and mechanical advance curves of a distributor is in a distributor test stand; this is because you can check the spark advance curve at any rpm without fear of over-revving the engine. Back in the 1960s and 1970s almost every automotive repair shop had a distributor test stand so they could check the advance systems of a distributor. In current times, it's getting hard to find a shop that has one.