One of the most commonly asked performance questions is, "what's first?" The answer can vary somewhat from vehicle to vehicle, but most often it's the exhaust system. We aim to give you the 101 course on exhausts and tell you how and why they can give you an increase in performance and mileage at the same time. Cooler running under load and increased engine life are side benefits. The principles you will read below apply to all engines, gas or diesel.
The Savage Scavenger
We need to start the discussion on exhaust system on the clean air side. Power and economy are all about volumetric efficiency (VE). That's the amount of air the engine really takes in versus the calculated volume of the cylinder. The ideal is to fill that cylinder with as much air and proportionally correct amount of fuel as possible over as much of the engine's rpm range as possible. Most engines are 80-85 percent efficient, meaning a 350 ci V8 engine at 85 percent VE is delivering the power of a 297 ci V8. VE will vary at different engine speeds, usually topping out somewhere in the engine's peak torque range. Whether that torque range is at a high rpm or a low one doesn't matter. Anything you can do to get a larger charge of air and fuel into the cylinder at any given moment results in improved VE. At wide open throttle (WOT) and part throttle too,you'll be using more of your engine's displacement the more efficient your intake and exhaust of air is. Better fuel economy results because you don't need to use as much throttle to achieve the same work.
A naturally aspirated (NA) engine has limits on how much air it can take in. The normal 14.7 psi air pressure (at sea level, less at altitude) is the natural limitation. The camshaft profile is the mechanical "software" that controls how and when the engine breathes. The downward intake stroke of the piston combines with the air pressure and a little bit of velocity charging (ram air) to help fill the cylinder. The intake restriction from the air filter, manifolding, etc., as well as the primary restriction in a gas engine ... the throttle butterfly... also limits VE. You don't reach peak VE at part throttle, but anything you do to aid full throttle VE will also help at part throttle.
Exhaust restriction is yet another impediment to wide open, and part throttle, efficiency. Part of that is due to the remaining pressure in the cylinder as the piston starts upward on the exhaust stroke. It takes some power to push against that pressure, so the easier the exhaust system flows, the more easily the piston can push that exhaust out the valve and out the pipe. How much harder is it to breathe out through a straw than through an open mouth? Also, the faster the engine can get rid of its hot air, the cooler it runs.
The outflow of exhaust is aided by an effect called scavenging. It's a lot like a siphon effect, where the "weight" of water flowing through the hose "sucks" the water out of a bucket in a continuous flow. Though exhaust gasses are considerably lighter than water and they are a series of pluses rather than a continuous flow, the "weight" and velocity of the pulse "sucks" the pulse behind it.
Scavenging helps clear the cylinder of inert exhaust gases sooner, leaving more room for a fresh charge of air and fuel. Exhaust scavenging also affects the intake side of the cycle during the camshaft overlap period, when both the intake and exhaust valves are open. The scavenging effect helps draw a larger charge of air into the cylinder by adding to the "suck" of the downward traveling piston.
Scavenging is present to a degree in all NA engines and exhaust systems, but a tuned exhaust can add greatly to it. Typically the effect peaks at some point in the engine's rpm range and, again, that's usually inside the engine's torque peak and often where the engine's fuel economy is best. The exceptions are with forced induction (read below).
Another negative effect is called reversion. It's when exhaust actually backs up into the intake system. You can imagine the effect of that. If the cylinder pressure is still too high (because the exhaust flow is backed up by a restrictive exhaust), exhaust pressure is higher than atmospheric and as soon as the intake opens during overlap, a slug of exhaust pushed up into the intake.
The scavenging effect is partially dependent on heat. As the exhaust gas temperature decreases, so does the volume of the gas. The balancing point for good scavenging is pipe size. If it's too large for the volume of exhaust, the flow slows and scavenging is reduced. The same thing happens when the exhaust cools as it travels down the pipe. That's why you may see pipe diameter changes in some parts of an exhaust system. A common spot is after the muffler. The exhaust is often very hot right to the muffler but is considerably cooler when it exits, so reducing the size of the pipe after the muffler may maintain more velocity. The temperature effect is somewhat variable according to the workload of the engine (hotter, more expanded exhaust gas at heaver loads), so pipe sizes are a compromise designed to be most efficient at the expected "normal" operation of the engine.
Savage Scavenging: Forced induction
The intake part of the scavenging effect in the cylinder is effectively non-existent with forced-induction engines (turbos, superchargers) because the compressor is blowing air into the cylinder under pressure. The velocity of the exhaust flow will still have a scavenging effect to aid flow in the pipe.
The critical part to remember on an engine with forced induction is that the exhaust system needs to be larger and of higher flow capacity according to the amount of boost. With forced induction, your engine is using all of it's cubic inches (100 percent VE or more), so the airflow in and out of the engine is much greater. An engine at around 7 psi boost has effectively gone to a VE of 150 percent. In other words, it's taking in approximately 50 percent more air and fuel than the volume of the cylinder. That yields a "bang" in the cylinder 50 percent more powerful than stock. A 350 ci engine at 7 psi is blowing as much exhaust as a 525ci engine. That means you need an exhaust system big enough for a 525ci engine. It's not quite that simple, but you get the idea.
Great Expectations
The need for exhaust improvements are almost a given in the performance world, even for a vehicle that will see no other mods. In the thrilling days of yesteryear, with those dinky pipes and mufflers a Briggs & Stratton one-lunger would have trouble breathing through, it was a virtual certainty that when you installed a performance exhaust, you'd get a noticeable uptick on the old "butt-dyno."... and a good bump on the real dyno as well.
Today, we find that OE installed exhaust systems are generally better than we saw previously. Rule one in the performance realm is that what you gain will be relative to how much better the replacement part is over the original. As a result, an exhaust system may yield only modest results by itself. The three to eight percent gains (my averages) may be subtle but they are there! You may feel it in your wallet with an extra 25 miles to your next tank of fuel. You may discover for the first time your truck pulls that notorious hill in overdrive where previously it always dropped down a gear. You may find that, on those blistering summer days, your truck is staying a few more needle widths below the red zone than previously.
Where exhaust system improvement become absolutely vital is when other mods are made. If you increase the VE and the power output, a factory exhaust isn't big enough, even at low speeds. Most performance improvements improve the efficiency at all speeds, even down low. Too small an exhaust system will hit you most at higher speeds, but if the stock system began to have a serious detrimental effect at stock power outputs at 4000 rpm, if you boost power 20 percent by other means, you lower that detrimental rpm threshold by at least that the same amount..
Myths, Pitfalls and Testing
You've heard the old myth that engines need backpressure. That myth is largely based on incorrectly applied modifications and not realizing changes beget the need for more changes. For example, in the old days, adding a free flow exhaust tended to lean out the carburetor. If the carb was not rejetted to compensate, power gains could be limited or non-existent and sometimes burned valves resulted from not correcting the fuel mixture.
Today's EFI (Electronic Fuel Injection) engines can adjust to exhaust changes but there are a few caveats. Exhaust flow increases can effect oxygen sensor performance by not letting them heat up sufficiently. If this happens, it's often a result of the owner removing the catalytic converter, which we all know is a no-no anyway.
One way to minimize the gains is going too big on pipe size for the application. We'll cover this more in part two. Too large a system is not the impediment that too small a system can be, but pipes that are too large tend to cost low end torque. Until there is enough flow to get the scavenging effect going well, the engine is actually "pushing" the exhaust out. That's why, to a large degree, a stock exhaust works well in the very low rpm ranges. The small pipes encourage scavenging. The stock system is big enough... until it's not!
One objective test you can make to determine if, or how badly, you need exhaust improvements, is to measure backpressure. It first requires a sensitive gauge with a large scale. Zero to ten psi is ideal but higher will work if the scale is large enough. Next, you need to tap the exhaust for a fitting. The gauge will read the pressure at the point you install the tap. If you want an overall reading, tap in up front, ahead of the cat. If you place it father back, such as in front of the muffler, you'll get a reading of the system from there back and will not include the restriction of the cat.
You'll have to hook up the gauge to a long hose and mount it inside through an open window or hung securely from a side mirror where it can be read by a helper. Taking readings in the driveway will tell you little. You need to make full load tests. That means a few full throttle acceleration tests on a safe bit of pavement. Even better is a grade long enough to let you rev the engine up and put a good load on it without excessive speed. Note the pressure at 500 to 1000 rpm increments from 1500 rpm on.
Where the restriction comes is as important as the actual number. If it comes at low "everyday" speeds, you have a serious problem. If it comes only at 4800 rpm, a speed you have never seen before that day, you have less to worry about. What are good numbers? A system that delivers less than about 2.5 psi under a full load is one you have nothing to worry about. From there to about 5 psi, there's room for improvement but you're doing pretty good. Above five psi is problematic but over 10 psi, especially if it comes at low rpm, you need help.
Obviously, high backpressure is bad but zero backpressure isn't always good. It could mean you have precisely the right size exhaust system or one that's too big. No easy way to tell which without a dyno test. You could look at a little backpressure (around 1 psi) as indicating your exhaust is sized about right.
Sources
Magnaflow
www.magnaflow.com
Gibson
www.gibsonperformance.com
