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Understanding Flow Rate And Calculating The Best Exhaust System For Your Vehicle

Updated: Aug 19, 2023

In this article we will touch mostly on the flow rate of exhaust piping, but also on different techniques to an exhaust system in which knowing what it takes in this department can easily enhance your engine with a 20-40HP advantage over your competition.

Grand National showing off his horsepower created from his new exhaust system.
Grand National

Exhaust Valve Timing and Manifold Porting

In some advanced exhaust systems, variable valve timing technology adjusts the timing of the exhaust valves to optimize flow rates at different engine speeds. This improves overall performance by enhancing scavenging and reducing backpressure. When properly tuning the exhaust system and using an overlapping cam you can achieve a suction from exhaust to pull exhaust gases from the cylinder more efficiently and pull fuel into the cylinder from the intake port before the piston has left TDC.

Example 1: When traveling down the bore, a piston of a 350ci displaces 727cc. If the engine has a compression ratio of 12:1 The combustion chamber volume above the 727cc is 63cc. Now if a negative pressure wave draws out the exhaust gasses remaining in the combustion chamber at TDC, then the cylinder can draw in 790cc and run as if it was a 385ci engine.

Example 2: In an unrestricted NASCAR Cup Car engine, the finely tuned exhaust will exert a suction of 6 to 7 PSI on the cylinder. This occurs during the overlap period, much of this suction is applied to the intake port.

Now on to the porting of an exhaust manifold. A man by the name of Randy Thomas Brzezinski came up with the anti-reversion dam. In a nutshell when port matching a stock exhaust manifold to the cylinder head, you don’t match the bottom of the port. You leave a lip to cut the reverse flow of exhaust gasses. When this reversal happens it brings about a (20-30 ft-lbs) reduction in low speed torque. When the dam is applied the results on average were an increase of at least 10HP but on average the number was 15HP.

These are just 2 types of old school ingenuity I speak of in the HiddenHP welcome mission statement. The lost minds of a generation who probably started build things at a young age and asked questions and came up with solutions.

Flow Rate and Its Importance The Significance of Flow Rate in an Exhaust System: The flow rate in an exhaust system refers to the volume of exhaust gases expelled per unit of time. It directly affects engine performance by influencing several key factors, including horsepower, torque, fuel efficiency, and even the sound produced by the vehicle. A well-designed exhaust system aims to maximize flow rate to achieve optimal engine performance. Measuring and Assessing Exhaust System Flow Rate Directly measuring the flow rate of an exhaust system is a complex task and often requires specialized equipment. However, there are indirect ways to assess and optimize flow rates:

a. Exhaust Gas Temperature: Monitoring exhaust gas temperatures can provide insights into flow rates. Higher temperatures may indicate increased backpressure and restricted flow, while lower temperatures suggest improved flow rates. b. Dyno Testing: Dynamometer testing can assess engine performance and measure horsepower and torque outputs. By comparing results with different exhaust system configurations, the impact on flow rates can be evaluated. c. Sound and Performance Evaluation: The sound produced by the exhaust system can serve as an indicator of flow rate. A deep and aggressive exhaust note often indicates better flow rates due to reduced restrictions.

Exhaust Manifold Design The exhaust manifold's design and construction significantly affect flow rate. A properly engineered manifold ensures efficient exhaust gas collection from each cylinder, minimizing restrictions and promoting smooth flow into the rest of the system. From test done by some of the world’s best engine builders and tuners we learn that primary exhaust pipe length makes a difference, but the secondary length (Collector Length) is critical. We have learned from our research you need to know RPM range as well as converter tightness. The collector extension pipe can range from 8 inches - 40 inches in some cases. This is where trial and error on a dyno for a finely tuned collector length comes into play. For a 7,500-rpm race-cammed small-block Chevy, a collector length of 8 to 12 inches proves to be the most effective, while the optimal primary pipe lengths are 24 to 42 inches. If we use David Vizard’s primary length finder we see starting at 6,500RPMs a 29in long tube works best but at 7,500RPMs a 25in primary does an even better job. Knowing the RPM range of the engine being built is a must to find the optimal primary tube length.

Pipe Diameter and Length The diameter and length of the exhaust pipes play a crucial role in flow rate. Larger diameter pipes allow for increased gas flow, reducing backpressure and improving flow rate. However, it is important to maintain an optimal balance as excessively large pipes can lead to decreased exhaust gas velocity, affecting low-end torque. Mandrel Bends The use of mandrel bends in the exhaust piping allows for smoother airflow compared to standard press bends. Mandrel bends maintain a constant internal diameter, minimizing restrictions and turbulence, thereby promoting better flow rates.

How do we take this information and find hidden horsepower?

By using pipe size to determine requirements when adequate system flow is the ultimate goal. All the information I am about to provide now is for an exhaust muffler system for a vehicle to has an optimal exhaust output. First, we will touch on finding the optimal primary tube diameter of the exhaust header.

How do we determine the primary tube size?

We know from standard research that primary header tubes range from 1.25in all the way to 2.75in. Again, David Vizard has a chart to search for, if we use his primary exhaust piping sizing chart, we need to know the flow of the exhaust valve at full lift. To determine this, you should have a spec card/sheet for the heads you have purchased. If you do not have this, you can simply google search the product numbers and find the specs online. If you have ported and polished the heads or done any modification to the heads you should have them flow tested so you know the new numbers. In the chart there is a blue, purple, and green line. The primary tube size is to the left of the chart and the flow numbers are at the bottom of the chart. The green is for all around street use, the purple is for street and strip and the blue is for high RPM drag use and some low torque may suffer from using the blue line.

Example 1: We have a cylinder head that flows 175CFM at the exhaust valve if we follow the chart. Green = 1.575in Purple = 1.625in Blue = 1.675in

Example 2: Brodix WP BD 2300 flow 267CFM at .800 Green = 1.95in Purple = 2.0in Blue = 2.065in

Having the optimal primary tube length and size on the exhaust header will maximize our exhaust scavenging during the overlapping of the cam. When this is done properly it creates what master builders call the 5th cycle.

The 5th Cycle 1: Piston Driven Induction Stroke 2: Compression Stroke 3: Power Stroke 4: Exhaust Stroke 5th: Exhaust Driven Induction Event a.Cycle 5: has cleared the combustion chamber and put a large amount of kinetic energy into the incoming charge before the piston even starts its way down the bore. The result of the 5th cycle is an engine that can achieve a volumetric efficiency well over 100%. The bottom line, extra Horsepower, Torque, and Fuel Mileage. All 3 of these combined are what wins races.

Understanding Flow Rate And Calculating The Best Exhaust

Now we understand the process in finding the proper header for our build, let's look at the CFM flow rating of exhaust pipes. From my past experience people typically lean on the people before them to tell them what exhaust pipe size their vehicle needs. When in reality with some simple math formulas you can come to the best possible conclusion. 2.2CFM per HP or 2.2 x HP This works obviously best if you already know from an engine dyno what an engine produced with the optimal RPM primary header tube with a 8-12 inch collector tip. As for the rest of us we have to make an educated guess on the horsepower we believe we will produce, or we can simply enter our numbers into the following formulas. Horsepower = Torque x RPM / 5,252

How do you come up with the torque value?

Where does this 5,252 keep coming from?

Torque = Horsepower x 5,252 / RPM I’ll quickly explain where the 5,252 comes into play. If you were to graph the engines horsepower and torque as on a dyno screen at 5,252RPM the horsepower and torque cross; this is how we determine the two. Unfortunately, without one or the other you will just simply have to guess at a resemble horsepower calculation of the engine being used or go into deeper calculation which can be done. If you bought the engine from a manufacturer, they should have some numbers from when the engine was tested or numbers from the builds they do.

Example 1: 350ci Crate Engine 341hp/371tq 2.2CFM x 341hp = 750.2CFM of piping needed to produce an optimal exhaust output. The 750.2CFM is for both banks of the engine. This would be for a single pipe flowing the entire engine horsepower. For an engine with dual exhaust, you simply divide it by 2 which equals 375.1CFM per dual pipe.

Example 2: Pro Series 454ci SBC Crate Engine 563hp/545tq 2.2CFM x 563 = 1,238.6CFM of single piping needed for exhaust output or 619.3CFM per dual pipe.

A chart showing the flow rating of exhaust pipe from 1.5 inches all the way to 5.25 inches in diameter.
CFM Flow Rating of Exhaust Pipe

In this chart I have listed the estimated CFM flow of exhaust piping from 1.5 inches to 5.25 inches. I took into count the piping being 1/16th thick, and and average 115 CFM per straight pipe area inch squared.

Quick explanation of math used.

1.5 / 2 = .75radius , .75 - .0625 = .6875Radius - Thickness,

3.14 * .6875 * .6875 = 1.4841Area in², 1.4841 * 115 = 170.6715CFM

170.6715 / 2.2 = 77.5779HP, 77.5779 * 2 = 155.1601HP w/ Duals

With that said, let's look back at the engine examples I gave to determine the piping needed.

Example 1:

350ci Crate Engine 341hp/371tq 750.2CFM single 375.1CFM dual

A 3.25in pipe for a single exhaust or 2.25in for dual exhaust.

Example 2:

Pro S 454ci SBC Crate Engine 563hp/545tq 1,238.6CFM single 619.3CFM dual

In this example a 4in pipe for a single or 2.75in for duals.

Exhaust System Components

The inclusion of high-flow catalytic converters, resonators, and mufflers can impact flow rate. Performance-oriented components with less restrictive designs promote better flow and higher flow rates compared to stock or restrictive aftermarket components.

Think of it this way, if we know in our example of the 350-crate engine, we needed 750CFM for a single pipe or 375CFM per dual pipe. We know that in the single pipe scenario a 3.25in pipe will flow and estimated 881.59CFM. So straight piped no bends to the back theoretically ideal flow plus some. With 105dbs at 3000RPMs right out the back. Well, this is why cities have made noise ordnances or else ever car enthusiast would be out there rattling the pictures off your walls. So now back to the muffler situation, we have a major problem here. Most people don’t even know of this problem. Mufflers are not rated in CFM Flow; most are rated in sound.

I’m going to guess and say you are thinking to yourself; If I have a 3-inch exhaust system and I add a 3 inch in and out muffler what is the problem? It will flow 746CFM right? Not necessarily, you have to determine the internal make-up of the muffler, you have so many different types and no not all mufflers flow a 100% efficiently. One of the more popular mufflers is a chambered muffler.

Chambered Style muffler

The chambered muffler consists of a series of internal chambers or compartments through which the exhaust gases flow. These chambers are typically shaped like cylinders or tubes and are connected in a specific configuration. Each chamber is separated from the others by perforated plates or baffles, which allow the exhaust gases to pass through while disrupting and dissipating the sound waves.

As the exhaust gases travel through the chambers, they encounter the perforated plates and baffles. These obstacles cause the sound waves to bounce around and change direction, leading to sound wave cancellation and reduction. The exhaust noise is muffled as the sound waves collide with each other and lose energy.

These chambered mufflers can actually be used as a pressure wave termination box, Which I plan on writing an article on to piggybacking off this article’s information. In other words, the chambered muffler as the engine sees it is an opening in the exhaust system.

We often believe what a manufacturer tells us to be true since; How will we know? We don’t have all the fancy equipment they have. Well in some cases these mufflers flow a significantly lower amount of air through them than the piping entering them. In other words, take a straw and blow through it. Now pinch the straw and do the same. That is obviously an elaborate example of the situation here, but it gets the point across. What we so desire from a muffler is either 100% flow efficient equaled to the in and out piping used or greater.

You can and I have gone online and looked for CFM flow ratings of mufflers. The one that seems to be floating around is quite old and some of the part numbers don’t even exist anymore. I would love for a company that sells mufflers, to get a flow machine and just test the mufflers. So, us as customers looking for a certain flow can see the CFM of the muffler in the description box of any muffler. In just my personal opinion, if you are buying a muffler for a build go .25 - .5 bigger on piping of the in and out of the muffler. This will insure at least the minimum flow or close to it. Look some of the charts up in your spare time and you will see in some cases these mufflers are only 50% efficient to their piping diameter.

In conclusion, understanding and optimizing the flow rate of an exhaust system can significantly enhance engine performance, providing a competitive advantage over others. Factors such as exhaust valve timing, manifold porting, pipe diameter and length, and the use of mandrel bends all contribute to maximizing flow rates. The design and construction of the exhaust manifold, as well as the inclusion of high-flow catalytic converters, resonators, and mufflers, also play crucial roles in achieving optimal flow. By measuring and assessing flow rates through techniques like exhaust gas temperature monitoring, dyno testing, and sound evaluation, engine builders and tuners can make informed decisions to improve performance. Furthermore, determining the proper header tube size and length, as well as selecting the appropriate pipe diameter and muffler type, are essential steps in finding hidden horsepower. By considering these factors and making educated choices, engine builders can unleash the full potential of their builds.

Thank you for a moment of your time. Stay safe and save the classics. The Hobbyist

Here are ten performance mufflers that are often praised for their performance and sound characteristics.

1. Flowmaster Super 44: Known for its aggressive and deep tone, the Super 44 is a popular choice among car enthusiasts.

2. MagnaFlow Performance Muffler: MagnaFlow mufflers are well-regarded for their excellent sound quality and increased exhaust flow.

3. Borla S-Type: Borla is known for producing high-quality mufflers, and the S-Type offers a refined, sporty sound.

4. Gibson Performance Exhaust: Gibson mufflers provide a powerful and deep tone, delivering increased performance and flow.

5. Dynomax Ultra Flo: The Ultra Flo mufflers are designed to reduce backpressure and provide a deep, aggressive sound.

6. Corsa Performance Exhaust: Corsa mufflers are engineered to deliver a sporty sound without excessive drone, and they offer various sound levels.

7. Vibrant Performance Streetpower: Vibrant Performance mufflers are known for their straight-through design, providing excellent flow and a deep tone.

8. Hooker Aero Chamber: The Aero Chamber mufflers offer a unique design that maximizes exhaust flow and provides a moderate, aggressive sound.

9. Walker Quiet-Flow: These mufflers focus on reducing noise while maintaining optimal exhaust flow, making them ideal for daily drivers.

10. Thrush Turbo: Thrush Turbo mufflers are affordable and provide a deep tone and improved exhaust flow.


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