Old muffler tricks on ECM controlled engines? (archive)

The complete automotive resource for buyers, sellers, and owners like you.

In the carb days, an exhaust system played a huge part in the air-fuel delivery to the engine. Opening the exhaust system up not only freed breathing, it changed the fuel curve at the intake side. For staters, headers and big exhaust (or open) tended to create a richer lower-end fuel curve, which leaned out as the RPM's increased. This produces increased low-end torque, and increased top-end power. The trick was to jet the carb(s) so the top end wasn't too lean, and the bottom end wasn't too rich. Most carb systems had two metering sections, where you did this tuning. But one it was done, it was fixed, mechanically, and would only react to the conditions of vacuum and airflow through the venturies. In this, the exhaust system dynamics were very important.

The theory in all this is, leaner mixtures at higher RPM's make more power, while lean mixtures at low RPM's trim torque. Fixed exhaust systems can only be tuned to perform optimally at one narrow RPM range, so in the old days we sacrificed torque for peak HP (race engines), or peak power for some torque (street or tow vehicles).

Now, with the ECM controlling fuel curves, in responce to oxegen sensors, airflow, rpm, throttle position, engine temp, etc... etc... we're not dealing with a fixed carb system anymore.

Mix this with variable valve timing, and the computers ability to control valve, iginition and fuel system curves all at once. Then add in the fact that on cars like the M coupe, the exhaust system is already quite large and free-flowing.

The ECM is calcualting input, making adjustments thousands of times a second, to optimize the air/fuel mixture. Carburetors, on the other hand, were described once, by Smoky Unich, as the perfect device for delivering exactly the wrong air/fuel mixture to the engine at all times.

You really have to wonder, how much impact does a new muffler really have today, beyond just changing what thew car sounds like?

Certainly the idea that is behind the Supertrapp design is possibly moot. Since the ECM controls the fuel curve, then adding or removing discs to increase or decrease torque or power (which worked great on crab cars, by the way), will not be anywhere near as effective, if at all, on ECM controlled systems.

Looks to me like you need to make modifications on modern engine systems as a system. This would require the exhaust system mods to include throttle body/airflow revisions, and ECM programming changes at the same time, to avoid system compensation erasing any single component potential improvement.

I do seriously doubt that, in modern engines, that the old 15%-20% horsepower gains once claimed (seldom backed up) by header and muffler makers, is valid anymore. Older engines needed a lot of help (like the old small block Chev) making 20% was pretty easy. An M-motor is an entirely different animal, making 15% is going to take a lot more than a simple muffler change out!

Questions, comments, or problems, please visit the Roadfly help desk.