Excessive back pressure can be a real drag

Back pressure can be nice. When my chiropractor applies pressure to my lower back, it makes me feel good. I cry — or is it laugh? — for more.

But there are other times when back pressure can be a pain — for instance, if too much of it is applied to your car’s exhaust system.

All engines, and the exhaust systems attached to them, are subject to varying amounts of back pressure, the resistance exerted by all exhaust-system components on the flow of exhaust gases to the outside air.

Mufflers, catalytic converters, heat-riser valves, the diameter of the exhaust pipe, the number of turns in the pipe … all of these influence how much exhaust gas is expelled from an engine, and how fast that expulsion is. No two engines will expel gases in exactly the same way.

Nonetheless, all engines great and small must get rid of waste gases pronto, or they will be in big trouble. All engines are therefore engineered to discharge these byproduct gases efficiently, minimizing as much as possible the amount of resistance — a.k.a. back pressure — produced by their exhaust systems.

If too much back pressure develops in the exhaust system, the waste gases can’t escape and both heat and pressure build up in the system. This directly affects the performance of the car, most noticeably through a loss of power and a drop in fuel economy.

What causes excessive back pressure? A number of things, some of which are so simple that we may ignore them, only to pay a tremendous price later on for having done so.

Let’s start with the tail pipe. When was the last time you checked it closely? It should be nice and round, with no obstructions or dents. Even a minor crumpling can cause a bottleneck for escaping gases and substantially increase the entire system’s back pressure.

I once received a letter from a man who had backed his car into a dirt bank and rammed his tail pipe full of mud. Not noticing this problem, he drove the car that way for several days, all the while wondering why it seemed to be losing power. He even had the engine checked, but no one thought to look at the tail pipe. A pity, because it cost him a rebuilt engine.

There’s no time like the present to check your tail pipe for obstructions. While you’re at it, check the entire exhaust system. Look for dents or crimps in the muffler, the pipes and the catalytic converter.

If you spot a dent that looks deep enough to impede the free movement of gases, have the part in question removed and the dent straightened or, if necessary, replace the entire pipe — it’s a small price to pay for saving your entire engine from damage.

There are other, more insidious ways in which back pressure can play its engine-defeating game. If you use dirty gasoline, for example, or use leaded gas in a car meant to use unleaded, you will slowly but surely clog the catalytic converter. Remember, the converter is a part of the exhaust system. All waste gases must pass through it on their way to freedom. If you notice a gradual loss of power, the converter is one of the things that you should have checked.

Have the exhaust manifold’s heat-riser valve checked as well, to make sure that it opens and closes freely. It’s the first restriction that exhaust gases meet on their trip out, and problems there can jinx the whole system. This valve closes when the engine is cold, to allow the exhaust gases to help heat the engine. As the engine warms, the valve opens to allow the free flow of gases. If the valve is sluggish in opening or even is stuck closed, it will limit the gases’ escape route and build up, yes, excessive back pressure on the engine.

Then there’s your muffler. If cars didn’t have them, we’d all be wearing hearing aids, the same way that, without catalytic converters, we’d all be choking and wheezing on polluted air.

But engines pay a price for both items. They are major restrictions to the exhaust system, and they force the engine to run at less than maximal efficiency. Not that I have any quarrel with that — in return for that loss of efficiency, we get peace, quiet and cleaner air.

All the same, don’t go putting any old muffler or catalytic converter on your car. These stock components are designed with a predetermined amount of back pressure built in, and the entire exhaust system and engine are tuned to operate at those pressures. If you install a “universal” muffler or a bargain “any brand” muffler in place of the factory-recommended one, you’re asking for trouble.

Keeping your exhaust system clean is an important part of getting the most from your car. These days neither you nor your car can afford anything less.