This initial timing guideline...
This initial timing guideline was used from Barry Grant to determine what the initial timing should be set at based on the camshaft that is in the engine.
It has been said that the exhaust emissions from a "typical" classic carburetor-equipped street rod can be as much as 100 times that of a new '10 vehicle. That may be possible if the owner of this "typical" street rod has an engine that has not been properly maintained or tuned for today's reformulated gasoline. But in the real world, the exhaust emissions from a properly tuned street rod engine with a mild camshaft exhaust are maybe five to seven times that of a new computer-controlled, fuel-injected engine with catalytic converters. Take a look at how nice the "typical" street rod looks and you will know that it has been taken care of by a person who truly cares for both their car and the world they live in.
The proper tuning of a carburetor-equipped street rod engine that was designed and tuned for leaded gasoline has become a little more complicated in recent years since the gasoline of today is quite different than the leaded gasoline that was sold at a corner gas station in the '60s and '70s. The main differences between today's gasoline and the leaded gasoline of days past that effect how an engine performs are the density, the volatility, and the burn time of the gasoline. The leaded gasoline that most carbureted engines were designed and tuned to use allowed the engine to perform quite well even if the ignition spark timing or the air/fuel mixture was not properly tuned. The same cannot be said for the reformulated unleaded gasoline of today, unless the ignition spark timing and the air/fuel mixtures are properly tuned to match the needs of the engine with the blend of gasoline it is burning, the engine's performance and driveability will suffer, plus the exhaust emissions from the engine will also be higher than they should be.
A cylinder leak-down test...
A cylinder leak-down test is a good way to check the condition of the engine.
We have had long-term owners of vintage carbureted street rod and other classic cars who have expressed to us that their engine does not perform today as well as it did in years past. They were asking if there was any way we could restore the lost engine performance. We also have heard from hot rodders who have a high-end, carburetor-equipped crate engine in their car that does not perform as well as they assumed it would. It seems that they thought because their high-performance engine was run on a dyno at the factory therefore it would perform flawlessly once it was in their car but they were often left wondering where all the power was. More often than not, the reason these engines lack power and driveability is that the engine's ignition spark advance system(s) and carburetor's air/fuel mixture curves are not properly tuned for the blend of gasoline they are using. Unless your engine package is properly tuned for the blend of gasoline you are using, a lot of the energy that is in the gasoline is just going out your tailpipe as exhaust pollution and wasted energy!
An ignition scope allows a...
An ignition scope allows a tuner to check the condition of the ignition system.
Tuning the Ignition Advance Systems for Today's Gasoline
The conventional, reformulated, and oxygenated unleaded gasoline of today has both a different burn rate during the combustion process and a different distillation profile than the leaded gasoline of the '50s, '60s, and '70s. These changes in the gasoline's formulation cause an engine that was tuned for leaded gasoline to need to have the initial timing, mechanical advance, and the vacuum advance systems tuned for the gasoline of today if the engine is expected to perform its best. The computer of a new modern, fuel-injected engine is continually adjusting the ignition spark timing for maximum engine efficiency and performance but a vintage carburetor-equipped street rod engine does not have a computer to make the necessary ignition spark adjustments that are needed for it to properly burn the gasoline of today. This means that if you expect your carburetor-equipped engine to perform its best it will need to have the mechanical and vacuum ignition spark advance curves tuned for the gasoline of today.
The mechanical advance system...
The mechanical advance system on the right is from an MSD Pro-Billet HEI distributor. The MSD Pro-Billet and Ready-to-Run distributor lines have a very tuner-friendly, mechanical-advance system. The mechanical advance system on the left, from a GM HEI distributor, can be a challenge to get the correct amount of mechanical advance.
The amount of mechanical advance...
The amount of mechanical advance on a distributor, such as this aftermarket HEI distributor, is not as easy to modify as an MSD Pro-Billet distributor.
The engine that this distributor...
The engine that this distributor came out of was lacking power because the advance system was too slow for the engine's needs because the installer did not read the instructions that came with the distributor.
The MSD Pro-Billet and Ready-to-Run...
The MSD Pro-Billet and Ready-to-Run distributors come with a set of bushings and springs that are used to match the advance system to the needs of an engine.
An engine that has too much or too little ignition spark advance may lack power, tend to run hot, or overheat. An engine that has too much mechanical ignition spark advance for the octane of the gasoline may also suffer from a ping or detonation problem, which can lead to engine failure. The vacuum advance system can also supply an engine with too much or too little spark advance for the needs of the engine. The distributor's mechanical and vacuum advance curves must be correct for both the needs of the engine and the octane of the fuel being used or the engine's performance will suffer as well as the possibility of engine damage from detonation.
Initial Timing Guideline
The initial timing guideline we use to determine what initial timing an engine should use with unleaded gasoline of today is from the Barry Grant catalog and/or website in the Demon carburetor selection guide. The guideline recommends 10 to 12 degrees of initial timing for an engine with a stock/mild camshaft (duration is less than 220 degress at 0.050 valve lift), 14 to 16 degrees of initial timing with a high-performance cam (duration less than 240 degrees at 0.050), and 18 to 20 degrees of initial timing with a radical/race cam (duration less than 260 degrees at 0.050 valve lift). When you increase the initial timing, you should re-curve or modify the mechanical advance system of the distributor so it does not supply the engine with too much ignition spark advance for the needs of the engine
An MSD Pro-Billet HEI Chevy...
An MSD Pro-Billet HEI Chevy distributor in the distributor test stands.
Tuning the Vacuum-Based Ignition Spark Advance for Fuel Economy
The ignition spark advance curve an engine needs for maximum power with the rich power air/fuel mixture the engine has during wide-open throttle acceleration is different than the ignition spark advance it needs for the leaner cruise air/fuel mixture the engine has when it is cruising at 65-75 mph on the highway. A vacuum advance-equipped distributor adds in the extra ignition spark timing advance the engine needs to fully burn the slower burner leaner air/fuel mixtures the engine sees during low load/high vacuum cruise driving conditions.
This vacuum advance unit from...
This vacuum advance unit from an MSD distributor has a bushing to limit its advance to 10 degrees.
Back in the days of leaded gasoline, most tuners limited the total advance (initial, mechanical, and vacuum) of the typical overhead valve engine to 52 to 56 degrees but today's gasoline has a different burn rate than leaded gasoline, so we limit the total advance (including the vacuum advance) to the 46 to 50 degree range. The vacuum spark 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. We have found whenever the advance from the vacuum advance exceeds 15 degrees the engine will often suffer from ignition misfire and/or ping problems at cruise speeds with the blend of reformulated gasoline we have here in California.
This shows the advance system...
This shows the advance system of an ACCEL distributor.
Tools for Checking the Ignition Timing Spark Advance Curves
The best way to check both the vacuum and mechanical advance curves of a distributor is with the use of a distributor test stand because you can check the spark advance curve at any rpm without fear of over-revving the engine. If you do not have access to a distributor test stand, an optional method that you can use to check the vacuum and mechanical advance curves is the use of a dial-back timing light. A dial-back timing can allow you to read the advance curve of an engine at different engine speeds but take care not to over-rev an unloaded engine. The vacuum advance curve can also be checked with the use of a hand vacuum pump to vary the vacuum supplied to the vacuum advance, just 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 of vacuum.
The arrow points to the vacuum...
The arrow points to the vacuum advance stop that we installed to limit the vacuum advance system to 10 degrees of spark advance of the Mopar distributor.
Once you have the ignition spark timing properly tuned for the blend of gasoline you are using, the next step is the tuning of the carburetor's air/fuel mixture curves. In the final part of this tuning article we will explain to you how and why we tune a carburetor's idle, off-idle, cruise mixture, power mixture, and accelerator pump systems for the best power, driveability, and the lowest possible exhaust emissions with the blend of gasoline you are using.
A distributor test stand is...
A distributor test stand is the best way to check the advance systems of a distributor.
If you do not have access...
If you do not have access to a distributor test, a Dial-Back timing light is the next best thing.