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.
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!
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.