How to Design a Custom Exhaust System

[Flowmaster Exhaust]

How to Design a Custom Exhaust System

For customers who are considering designing a custom exhaust system for their vehicle, we have listed some of the basic things to keep in mind when designing a true dual system. Most of these points can also be applied to a single system.



Pipe Size:
The size of pipe used in an exhaust system is a critical item to consider. Pipe diameter will affect the sound level and performance characteristics of the muffler, but keep in mind that bigger is not always better. Too large of a pipe can actually hinder exhaust scavenging by allowing atmospheric pressure up the pipe.

Balance Pipe(H-pipes and X-pipes):
Flowmaster strongly recommends using a crossover tube, otherwise known as an “H” pipe or balance pipe on all true dual exhaust systems. The crossover pipe equalizes the exhaust pulses and allows the sounds waves to communicate between both banks of the engine. Not only does this usually improve torque in the low to mid rpm range, it also creates a deeper mellower tone both inside and outside of the vehicle and helps eliminate “back-rap” on deceleration.

Flowmaster has performed extensive testing and determined that the “H” and “X” pipes deliver no significant difference in performance, but do affect tone, the “X” pipe giving off a higher pitched tone. Flowmaster Scavenger “X” pipes however, do increase torque through use of their patented D-Port technology and retain the deep Flowmaster muffler tone desired by most performance enthusiasts.

Muffler Selection:
The sound of your exhaust is much like music; we all have different preferences so it’s important that the system be “tuned” to what you like. With the Flowmaster family of products we can help accomplish this by answering the questions below. This approach will allow you to choose the best product to suit your needs and expectations.

1. What is the vehicle year, make, and model?
2. Which engine do you have or how much horsepower does it produce?
3. What is the vehicle used for? (i.e.: daily driver, race, towing etc.)
4. Preferred sound levels (interior and exterior?)
5. How are the space requirements?
6. Would you like a complete exhaust system or universal pipe kit if available? Single or dual outlet?

Header Collector Ball Flange Kits:
The Flowmaster Ball Flange Kit takes the place of messy three bolt flange type connections and allows for a leak free and gasket free ball flange type connection. All kits are 16ga aluminized tubing and are sold in pairs.

System Hangers:
Adequate hangers should be used to properly support the system. Flowmaster uses OE style hangers in all systems.

Resonators:
Resonators are designed to be used in conjunction with your mufflers for improved sound reduction. Resonators will help to eliminate interior resonance.

Stainless Tips/ Exit Location:
Flowmaster highly recommends running a minimum of 12” tailpipe after the muffler. This will not only greatly help in sound control, it will also provide an increase in exhaust efficiency (more power). Running tailpipes out from under the car in street applications will also direct the potentially harmful exhaust fumes away from the vehicle, as well as minimize the sound transmitted into the cab area of the vehicle.
Flowmaster's line of Stainless Steel Exhaust Tips are a great way to let people know that you have Flowmasters under your ride! They come in a variety of styles from brushed to polished and will greatly enhance the look of your vehicle!

If you have further questions regarding any of our products, please call (707) 544-4761 or visit our website, www.flowmastermufflers.com.

[Josh@Ridetech]

Nice write-up!

[Ron@Flowmaster]

Originally Posted by Josh@Ridetech

Nice write-up!

Thanks, Josh!

[Jafa]

Agreed, nice write up with some good information.

Question if I may, on the website it gives pipe diameter for approximate horsepower, but that's for v8's. What pipe diameter would you recommend for a 4 cylinder, race-only ( road course ) engine putting out 180hp at the flywheel. 12:1 compression ratio, running straight avgas ( piston aeroplane engine fuel ). Car weighs 2300lbs.

Thanks in advance.

[Ron@Flowmaster]

Originally Posted by Jafa

Agreed, nice write up with some good information.

Question if I may, on the website it gives pipe diameter for approximate horsepower, but that's for v8's. What pipe diameter would you recommend for a 4 cylinder, race-only ( road course ) engine putting out 180hp at the flywheel. 12:1 compression ratio, running straight avgas ( piston aeroplane engine fuel ). Car weighs 2300lbs.

Thanks in advance.

With your horsepower at 180hp, we’d recommend 2.5” mandrel bent tubing. In terms of sound, a Flowmaster Super HP-2 or DBX Series muffler would also be a nice addition to your system. Mandrel bent is key because compression bending will make the 2.5” more like a 2.25” at the bends and disturb exhaust flow. For any other questions or concerns, give us a call at 707-544-4761.

[Ben@SpeedTech]

Hey Ron, thanks for the input so far.

I have been trying to get some research done on exhaust flow in effort to maximize my exhaust in a tight package but actual data backed information has been hard to find. I have 3 main questions that seem to mostly only invoke uneducated opinions like "race cars run open headers so bigger is better" when I ask on muscle car based forums. I'd really like to know a professional's data based opinion.

About the car- It's a G body with single exhaust platform. I stuck with the single because less weight, components and cost. I currently run no cats, with 2 1/2" dual head pipes with the left bank crossing over behind the trans and into a Flowmaster Y pipe with 3" tube through the high flow muffler an continuing 3" over the axle and out the back under the bumper. My old .480 lift 406 SBC motor had roughly 325 hp and 400+ ft lbs net at the flywheel and it ran great, good midrange torque and decent power from about 2000-4500, but hp noticeably dropped off after that. Higher rpms were likely limited by the factory 400 smog heads and small 600 cfm carb. With this 406 the 3700lb w/me car on a stock convertered TH350 and 3.42 gears ran sea level 13.40s at 98 through the full exhaust. I now have a new 12.5:1 SBC 406 that should be roughly 500+ hp/500 + ft lbs net at the flywheel. The motor will see mostly street driving and autocross, and occasional drag racing. Realistically the engine will likely spend most of it's racing time between 3500 and 5500, despite the tq peak is around 5000 and hp peak 6500. Likely only in drag racing will it ever see above 6000. I'm hoping for around mid-low 12's with this new engine and a new 3000 stall converter combination.

1. In my own research reading internet posts, mostly supported by first hand info including time slips or dyno #s, there's some information indicating that smaller tube, (opposite of most people's "opinions"), has an advantage in full exhaust cars because it will create more velocity which improves lower to mid range power. Along with that, a single exhaust of an appropriate size will help scavenge as well if not better than an X pipe, the combination of which will create decent exhaust flow through the entire rpm range. Backing that up I've read of 650 rwhp small displacement turbo cars on 3" single exhaust.

Since Flowmaster markets big power gains through use of your Y pipe in a single exhaust system, I'm curious what your experience with single exhaust has been with an average 450-550 hp N/A V8? Will my current 2 1/2" dual to 3" system continue to work well or will it restrict the new engine's potential significantly enough (upwards of 20+ hp/tq loss) to warrant the expense of an entire new larger system?

2. I've read that a dual 2.5" system can support upwards of 500+ hp and 3" duals is better for 500+ up to ___ hp. At what horsepower level would I want to go to 3" dual into a 3.5" (or 4") single Y pipe and then follow through as single 3.5" (or 4") out the back? Since most of my racing will be revolving around 3200-5500 mid range autocross power range, would the larger pipe help across the whole rpm range or like swapping to a single plane intake mainly just shift the rpm range up, decreasing low to mid tq and increasing high rpm hp potential?

3. What about tail pipes? How much of a power increase or loss difference if any do they make? I have been considering running my current system with the 3" exiting out the pass. side in front of the rear tire to avoid the general internet assumed opinion of power robbing bends of the tailpipe. However I'm not excited about the increased noise level, the car is already pretty loud as it is. I've also read on occasion that as the exhaust cools it has less and less affect on hp, and by the time it hits the tail pipes they don't make a significant difference to hp figures whether on the car or eliminated with dumps in front of the axle.

Your/ Flowmaster's Thoughts?

Thanks in advance, trying to find quality info on single exhaust systems is a bit tough.

Pics of current exhaust...

[Jafa]

Originally Posted by Ron@Flowmaster

With your horsepower at 180hp, we’d recommend 2.5” mandrel bent tubing. In terms of sound, a Flowmaster Super HP-2 or DBX Series muffler would also be a nice addition to your system. Mandrel bent is key because compression bending will make the 2.5” more like a 2.25” at the bends and disturb exhaust flow. For any other questions or concerns, give us a call at 707-544-4761.

Thank you very much Ron.