Exhaust Explanations
Exhaust system design has been a “hot” topic of academic discussion and bench racing since before the 1950’s. In reality, the exhaust system begins at the exhaust valve and ends somewhere behind the rear bumper. Exhaust gasses need to find their way out of the cylinder after the combustion event to evacuate the cylinder, making room for a fresh intake charge to make power once again.
Aftermarket exhaust “upgrades” may offer improved fuel economy, but usually aren’t cost effective... unless you need a replacement anyways. At that point, the “return on investment” is no longer factoring the entire cost of the exhaust system, but merely the difference between a cheap exhaust and a “performance” system.
There are several scientific laws applicable to the exhaust system design: Helmholtz Effect, fluid dynamics, thermodynamics, and electrostatics.
Helmholtz Effect
Back in the 1950’s many racers designed exhaust systems they believed exemplified superior engineering principles, only to find their engine fell flat. Somehow Mother Nature didn’t subscribe to the same science journals and racing magazines as these racers. By the 1960’s, most of the basic engineering math had been worked out through trial, error, and evaluation.
When it comes to induction and exhaust systems, the “Helmholtz Effect” capitalizes on the “bouncing ball” effect. The Helmholtz Effect is why a long hollowed out log with a slit at the top can produce a middle “C” in a 17th century cathedral organ. Back to engines; the exhaust valve opens, the charge begins moving down the exhaust pipe picking up speed and momentum — it becomes a mass in motion. When the valve closes, a vacuum forms just behind the valve. The mass in motion no longer has more “stuff” to fill the void created behind it. That vacuum bounces (or “sucks”) the charge back, creating a pressure pocket behind the valve. It then bounces back the other way... again. The trick is to get the exhaust valve to open while the mass in motion is moving away from the valve. The momentum is already there, helping to better extract the spent exhaust.
Astute observers realized there were harmonics and frequencies at play inside an engine. Somewhere, someone put 2 and 2 together and came up with 4. These frequencies and harmonics interplayed perfectly with already-established Helmholtz observations in music. The trouble was that a 1st harmonic could only be achieved with intake runners and header tubes longer than the vehicle. Luckily, someone realized that 2nd, 3rd, 4th, and even 6th harmonics could provide practical power increases. In practice, intake manifolds are tuned to 5th or 6th harmonic resonance, while long exhaust header tubes are tuned for 4th (or so) harmonics.
Headers
Long-ish Tube Header, with Paired Pipes; 4-2-1
Exhaust headers serve the function of the exhaust manifold. Headers usually have longer tubes to replace short cast iron pieces. For fuel economy, you want efficiency in the low-throttle low-RPM range. Since velocity is your friend, smaller diameter primary tubes will give you better fuel economy than large tubes. Taking Mr. Helmholtz’s lead, longer tubes will give you higher level harmonics at lower frequencies/RPM. Therefore, what you want probably isn’t what is commercially available. If you have an old small block Chevy (or some other really popular application), there just might be exactly what you’re looking for. If you have a Nissan Juke, probably not. A good source of information is Headers By Ed.
Again, just swapping in a header (or two) is not going to be cost effective (reasonable ROI). However, many engines are known for cracked exhaust manifolds. This is bad for several reasons. First off, this leaks exhaust gasses into the engine compartment (pre-cat) that may find their way into the passenger compartment through the HVAC system. This is directly bad for YOU. Next, between pulses where it kicks exhaust gas out, it sucks fresh air in. This will show your oxygen sensor a lean mixture. The ECU will dump (pronounced “waste”) needless fuel trying to bring the AFR back in line. Lastly, there’s the extra noise factor.
If you have a cracked exhaust manifold, at least replace it. If you can find a header(s) with small diameter long primary tubes, this just may give you better throttle response and fuel economy.
Cat-Back Exhaust Systems (Fluidynamics)
Bad Compression Bend
Two decades ago I was the Assistant Manager at a Meineke Muffler shop. I dealt directly with customers, made recommendations, sourced out parts for repairs, and sold the repairs. When a customer would request a Cat-Back exhaust system, I would always ask, “Why?”. If they replied they wanted more performance or better fuel economy, I would do my best to talk them out of it (aw shucks, I was honest). I would suggest a shotgun list of alternatives. However, if their response was, “I want the throaty exhaust sound” or “I want 2 pipes sticking out from the rear bumper”, I would work with them. Honestly, I sold a fair number of Cat- Back systems! Regardless of YouTube videos you may have seen, or the sales pitch from exhaust system manufacturers, you will never recoup your investment in better fuel economy. The laws of physics are at play between the combustion chamber and the one thing that doesn’t change with a Cat-Back system — the catalytic converter. If you want to improve the exhaust system for fuel economy, you are better concentrating on the Cat-Forward part of the system.
Smooth Mandrel Bends
If you are replacing your exhaust system anyways (rusted out), make sure the pipes are mandrel bent as opposed to compression bent. Compression bends reduce the cross- sectional surface area of the pipe, causing restriction and undesirable turbulence. Mandrel bends are smooth and allow the exhaust to flow with less inhibition, maintaining better velocity.
Materials
The cheapest exhaust pipe you can probably buy is aluminized steel. This is what the chain muffler shops sell, and lasts a mere 2-3 years. Most vehicles made over the past 3 decades were equipped from the factory with 409 stainless steel exhaust. This is an economical upgrade from aluminized steel, and lasts 10 to 20 years. Many exhaust shops can install 304 stainless steel, the next step up. This stuff will outlast your grandchildren. At the top of the scale (not at all needed for fuel economy) is 316 stainless steel. When you see polished exhaust, it’s probably 316 SS.
Thermodynamics
Gasses expand when heated and contract when cooled. The exhaust gasses will be the hottest as they pass the exhaust valve going into the cylinder head. From that point on, they get cooler and cooler. Exhaust pipes act somewhat as a heat exchanger. Many factory exhaust systems will run a 2.25” pipe to the catalytic converter, then a smaller 2” pipe the rest of the way out the back. Cooler exhaust gasses take up less space. The smaller pipe downstream maintains velocity. It may be tempting to “upgrade” to a larger exhaust post-cat. Unless you have a turbocharged engine, it will probably do more harm than good — at least for fuel economy.
To preserve velocity, header wraps (and even some space-age ceramic thermal coatings) hold the heat in the pipe. Since the exhaust cannot cool as easily, it doesn’t contract as readily. This helps maintain the higher velocity through the system. A second benefit is that it reduces under-hood temperatures. The intake may breathe cooler denser air, belts and hoses last longer. It may even reduce stress on the cooling system.
Aftermarket headers/factory exhaust manifolds and the pipe that connects to the cat inlet should be wrapped; basically from the cylinder head to the cat. An additional benefit is that the cat sees hotter exhaust. One of the strategies employed by car companies is to monitor the cat temperature. If it cools below a programmed threshold, extra fuel is dumped to “stoke up the fire”. Keeping the cat hotter reduces wasted fuel for keeping the fires burning.
Anything Else? (Electrostatics)
Since you asked, there is, actually. Exhaust gasses are fresh from the furnace, loaded with active ions and charged radicals. These ions build a static electrical charge on the exhaust pipe. This charge attracts the exhaust gasses like a magnet, and act as an invisible restriction. Most exhaust systems are suspended with rubber (translated, electrically non-conducting) exhaust hangers. Adding a ground strap or 3 can short out that static charge and actually improve flow. Start at the rear bumper and work forwards. You want a minimum of 1 grounding point at the rear. More is better. Use at least 18 AWG wire, or braided straps grounded to the body.
FE2 - FE3 Depending
MPGenie Basics 051 Training - Elaboration on Exhaust
