Batteries and Stereos That Pump Up the Volume

The term thumper might once have referred to an engine with a bad piston rod. Now it designates a new generation of high-capacity sound systems that car audio enthusiasts install to achieve volume that can thump in another driver’s ears four lanes over.

These systems, with hardwood frames and motorized music changers, can cost up to $50,000--far more than the cars that carry them. And they can consume more electrical power than a residence, raising questions about the impact of stereos on car generating systems.

This concern led one Encino man to write me, saying he is worried that a Kenwood cassette system he installed in his 1993 Mazda 2600 pickup may cause problems. “Is there a way of having a secondary battery installed solely for the stereo?” he asked.

He doesn’t need an auxiliary battery. His Kenwood stereo is a standard system that draws a maximum of perhaps 60 watts or about five amps. His truck is more than capable of generating the power required for these kinds of sound systems.

Any standard electrical device on a car gets its power from the generating system, not from the battery. The battery provides power to start the car.

The battery also temporarily provides power when electrical demands exceed the car’s generating capacity, which would not occur with most stereo equipment. An overload typically occurs when a car is stopped in traffic with a lot of electrical devices turned on.

Even the highest-powered original equipment stereo--the Mach 460 system on the 1994 Ford Mustang--would not tax a car’s electrical system. The Mach draws about 260 watts of power or about 20 amps. Steve Mazer, an engineer at the Automobile Club of Southern California, says the average automobile generating system produces about 60 amps of power. Some systems on campers can generate as much as 110 amps.

The heavy-duty batteries found on many new cars and trucks can provide power to a cassette player for a long time even when the engine is not running. Mazer estimates that in these vehicles, a typical cassette player could operate for 24 hours before running a battery down completely.

The thumper is another matter, but the average driver won’t need or want one. When owners have volume contests, no one can sit in the cars because the decibel level hits the ear-damage zone. Mazur says these systems can draw 10,000 watts of power at their peak and to run one on a car electrical system, you’d need a heavy duty alternator and as many as five batteries to provide temporary power.

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Question: I’m confused about the concept of back pressure in an exhaust system. What does it do and what good is it? If I were to free up the exhaust, would I get better efficiency?--V.Q.

Answer: The exhaust pipe and muffler restrict to some extent the venting of exhaust gases, creating a pressure in the system that extends from the end of the exhaust pipe back to the engine manifold.

You can reduce back pressure by changing the muffler or putting in a header, a pressure-reducing type of manifold.

You may increase fuel economy and engine performance by reducing back pressure, but a certain amount of back pressure is necessary to meet emissions standards.

Your Wheels is published every other Friday. Ralph Vartabedian cannot answer mail personally but will attempt to respond in this column to automotive questions of general interest. Please do not telephone. Write to Your Wheels, 1875 I St. N.W., No. 1100, Washington, D.C. 20006.