Justin Fivella
August 14, 2014

It's an age-old question that's nearly impossible to answer without the help of a dyno or the track. At what point should you upgrade from a 2.5-inch exhaust system to a 3-inch system and when should you move from 15⁄8-inch headers to bigger 13⁄4-inch tubes?

Sure, there are mathematical formulas and engineering dictations that can predict optimal exhaust size in relation to power. There are also indicators like power curves, exhaust gas temperatures, and boost curves that can also help decipher at what point your exhaust system is a restriction—but not all engine combinations are created equal.

Naturally aspirated engines and those with power adders, respond differently to exhaust-size changes, and other variables such as cylinder-head flow, rev ceiling, cam design, and more. In other words, one can't say that at "X" horsepower you need "Y-sized" exhaust.

The Flowmaster 3-inch Delta Flow header-back race exhaust system (PN 17389) is a great system that is specifically designed for the Fox-body chassis with shorty headers. It features massive 3-inch aluminized piping and the infamous Super 40 series mufflers for that famous Flowmaster sound.

Emissions-Friendly Recap

Despite Project Smog-Legal Killer's combo that breathes through a small-catted exhaust, inhales through an emissions-friendly intake manifold, and still spins the factory smog pump, it has continued to impress us on the dyno.

So much in fact that we wondered if the exhaust was a restriction. Last time we strapped the ol' coupe on the dyno it made 610 hp and 595 lb-ft at the wheels, gains of nearly 90 hp over the previous runs thanks to the Pro-M Racing standalone computer system and 92mm MAF along with the Anderson Motorsports Power Pipe. The new combo relieved inlet restriction, sending boost to roughly 15.5 psi and raising some concern along with it. Not only was 600-plus hp at the wheels on a stock-block 347 a bit of a gamble, it was worsened by the 91-octane pump gas.

Since we have a lot of cool track stories still in the works, we thought it wise to back the power levels down to preserve our AFR headed, CHP 347E engine for the long haul. We decided the best way to kill power was with less boost, so we swapped the supplied Vortech 3.33-inch pulley on our V3si for a 3.70-inch "sissy" version that dropped boost to a safer 10 psi. Upon removal we noticed the bypass valve connector was loosely screwed into place. A look at the air/fuel ratio and boost logs with our AEM Failsafe gauges proved that this was a significant boost leak that only showed its face when pressurized for more than about a second. This meant it was undetectable on the dyno, but at the track we were leaking upwards of 6 pounds of boost and the air/fuel ratio was pig rich at 9.8:1. We could never understand why the car only trapped 124 mph on its 10.80 pass, but discovering the boost leak and subsequent rich condition answered that question. With the boost leaks buttoned up, our next order of business was the search for efficient horsepower.

Relieving Restriction

Even at 10 psi, we knew stepping up to larger headers and a bigger exhaust system would help relieve restriction and increase horsepower. The previous system consisted of 15⁄8-inch shorty headers and a 2.5-inch catted X-style midpipe along with 2.5-inch mufflers and matching exhaust tips—it's a great system, but with this much power it was a tad small. But here's where it gets tricky—how could we fit a big-power exhaust system and still stay emissions friendly? Simple, we had to get creative. Thanks to the help of Flowmaster, Kooks Headers, and AED, we checked all the right boxes.

Let's start with the Flowmaster 3-inch Delta Flow header-back race exhaust system (PN 17389) that comes pre-welded and already designed for the Fox-body chassis. This high-performance system is made in the USA, comes with a three-year warranty, and uses massive 3-inch aluminized piping that mates to the factory headers with a 2.5-inch ball-socket. That's right, it mounts to the stock header location so upgrading to larger 13⁄4-inch shorty wouldn't be a problem.

The Flowmaster 3-inch system utilizes the infamous Super 40 series mufflers for the patented deep growl that only a Flowmaster can produce. The chambered design of the Flowmaster muffler uses a sequence of chambers instead of a glasspack design to control airflow and sound. The included Scavenger Series X-style midpipe also adds a raspy snap that sounds wicked.


01. This is the first time anyone has dyno tested the new Kooks Green cats that are EPA certified and are as green as they are mean. We also added Kooks V-band clamps so that swapping the cats out for test-pipes at the track only requires 10 minutes and a single wrench.

02. The Flowmaster full system comes with 3-inch chrome tips that are show-car cool and look great under the bumper of any Fox-body.

03. How does Flowmaster snake a 3-inch system up and over the rear axle without clearance issues? Easy, by taking the time to design a two-piece rear section that literally fits like factory and offers more tire clearance inside the fenders than most 2.5-inch systems.

04. The Flowmaster Scavenger Series X-style midpipe promotes a smooth merging of exhaust flow, but it also sounds nasty with plenty of rasp. Trust us, it sounds wicked.

05. According to Kooks its Green cats are the only high-performance catalytic converters on the market that are EPA certified to pass emissions, won’t throw CELs, and are tested up to 950 hp and 23 psi of boost. Maybe that’s because their cores are platinum dipped for the utmost in bling … er, we mean performance.

06. The Kooks V-band clamps bring race car performance to your street car by making cat removal a one-wrench process. Just spin the nut down, unhook the clamps and off comes the cat.

07. Have a look at the 3-inch Flowmaster Scavenger “X” compared to the 2.5-inch catted “X” we’re removing. The smaller unit has been a great midpipe, but we’re at a power level where a full 3-inch system is beneficial.

08. Here’s the Flowmaster 3-inch piping against the outgoing 2.5-inch example: How’s that for a big increase?

09. We found these used 13/4 unequal-length ceramic coated shorty headers on the forums for a great price and decided to move up from the outgoing 15/8-inch shorties we’d been previously running.

10. Note the larger 13/4-inch primaries of the headers pictured at the top of both photos—the difference is especially noticeable where the primaries meet the flanges.

11. The merge collectors on the used 13/4-inch headers were supposed to measure a full 2.5 inches to match the downpipes, but they were a few millimeters small so Greg Wallace of AED opened them up.

12. Red RTV is said to help the exhaust gaskets better seal between the head and the headers. Be sure to let it set a little before installing everything.

13. Take your time during the installation since bigger headers means even less clearance for spark plugs (and hands). It’s a tight squeeze, but with patience all of the bolts can be inserted.


More than just killer sound and performance, the system fits like a glove. Flowmaster's unique over-the-axle rear portions create more tire clearance near the inner fenders than most 2.5-inch systems on the market. Unlike cumbrous one-piece tips, the Flowmaster units are easily installed thanks to a multi-piece design that clears the axle on a stock Fox by a mile and hangs the beautiful 3-inch chrome tips in the perfect location. Flowmaster really did its homework with fitment because it's hard to believe how well the system lines up despite housing a set of massive 3-inch pipes. The unit also comes with all necessary hangers, connectors, and clamps, so it's literally a bolt-in affair.

But remember the part where we said it was a race system? Well, this means while it has the appropriate air-injection-tube provision and the necessary O2 bungs, it doesn't have cats. That's OK because we're used to improvising for emission's sake by now and thankfully Kooks Headers had the mean and green solution.

"The Kooks Green catalytic converters are the only 49-state, EPA-certified, high-performance cats on the market, and we spent a lot of time and R&D ensuring they preserve power and still burn clean," Chris Clark of Kooks Headers said.

That's right, we had AED in Shingle Springs, California, help us modify the Flowmaster "X" with Kooks Green 3-inch cats and matching 3-inch V-band clamps for an ultraclean install that's emissions friendly on the street and can be race ready at the track in minutes.

"V-band clamps allow enthusiast to swap the cats for test (straight) pipes with only a wrench and a few minutes of their time; they're handy on a street car that's also going to see closed-course racing," Greg Wallace of AED said.

According to Clark, Kooks searched high and low before partnering with Global Emissions System Incorporated to create the Green cats to ensure performance and reliability—Kooks wanted to ensure the cats were good enough not to trigger a check-engine light (CEL), while still preserving performance.

"They pass the 49-state EPA emissions standards and don't trigger CELs on the rear O2 sensors of the newer cars because the cores are platinum-coated unlike others on the market, which allows our cats to burn cleaner without costing power," Clark explained.

"We've tested the cats up to 950 hp and 23 psi of boost without any problems since the 300-cell cores are of the highest quality—we've done a lot of testing to ensure our customers get the best product possible," Clark added.

The keenest of eyes will catch that while the Green cats would likely pass the strict California tailpipes emissions standards, they're only 49-state EPA improved, not CARB EO exempt as required by California. This means that like other pieces on the coupe, the exhaust system falls within what some might call, "the gray area." Truthfully, with the right person they might not pass the visual test because they lack a CARB EO number, but to the majority of people, they'll easily pass without a second look. As for the most important aspect of the smog test, the tailpipe emissions, they passed with flying colors. Sure we're playing to some gray areas as far as visual inspections, but the car makes over 600 hp at the wheels and still passes the tailpipe emissions—sometimes you just have to play to the gray.

Like the rest of the exhaust system, finding 13⁄4-inch shorty headers took some creativity. Unlike long-tubes that come in all different sizes, shapes and forms, if you want big-tube shorty headers, the options are limited. Anderson Ford Motorsports used to make wicked examples, but has since stopped production.

We hear they might be making a comeback—if you're reading this AFM, please bring them back! Mac also makes a pair, as does Ford Racing Performance Parts (FRPP), but they're designed for the Z-heads with raised exhaust ports. The FRPP headers will bolt to any other SBF head, but the ball-socket flange hangs lower to account for the raised exhaust ports since the headers were designed for a Fox-body with a stock H-pipe and the raised Z-port heads. We searched high and low for a used set of AFM headers without luck and eventually ended up with a used set of 13⁄4 ceramic coated shorty headers we found online.

As with most of the build, we knew our emissions compliance was hurting horsepower, especially since long-tubes would have promoted better exhaust scavenging, but we had to work within the constraints and get creative. In this case, we combined the killer header-back Flowmaster 3-inch system with Kooks 3-inch Green cats and some used 13⁄4-inch headers to create our own high-horsepower (sort of) smog-legal exhaust system. As you'll discover from the dyno results, we uncorked some serious restriction and netted solid gains.

Installation Process

As some of you might have seen from the photos, there are suspension additions we were hoping to keep a secret, but due to some interference issues, we must come clean. The Flowmaster full 3-inch system fits like a glove on a factory Fox-body, but the Smog-Legal Killer isn't exactly stock. As a result, we ran into some clearance issues both with the Tremec T-56 Magnum and the X-pipe as well as the tailpipes and the Maximum Motorsports Panhard bar.

In all honesty, these are easy fixes that a local exhaust shop can remedy since we're talking a dimple here and a tweak there. Like we mentioned earlier, this is by no fault of any of the companies since all of the parts were designed for a stock Fox-body—in other words, welcome to the world of hot rodding. For comparison's sake we also weighed the car before and after the installation process and despite increasing the tube diameter to a full 3 inches, the entire system only added 4 pounds.


14. Our larger aftermarket steering shaft (more on that in the coming issues) required us to lightly clearance the rear primary on the new headers. Don’t worry, we didn’t dink it much.

15. It was already mean, but we needed to keep it green: Here Greg Wallace measures the exact area where he’ll weld in the V-band clamps and the Kooks Green cats.

16. Remember folks, measure twice, cut once! It’s time to remove the piping and replace it with the cats.

17. Don’t forget to clean up the exhaust surfaces before welding in the V-band clamps.

18. Greg must have hands of steel because even tack-welding without gloves can burn your mitts. The tack welds are done for the mock-up phase to ensure everything will install as needed.

19. After the V-band clamp has been tacked to the cat, it’s time to tack the other end to the exhaust piping. Be sure to note there are male and female ends to the clamp collars.

20. Here’s the Kooks Green cat and V-band clamps are tack welded into place so AED can check for any alignment problems or fitment issues.

21. It’s time to finish welding the V-band clamp collars to the cats and exhaust system. Here Greg can be seen laying a bead down.

22. After the clamps are cool, it’s time to throw the cats back into the exhaust system and fire it up.

23. The Flowmaster exhaust system even comes with the air-injection inlet barb for a factory-like fitment.

24. You’re not supposed to see the Maximum Motorsports suspension pieces yet, so ignore them. However, we couldn’t because the original one-piece exhaust tips couldn’t be removed with the Panhard bar in place. In the end we snipped them for removal. Thankfully the Flowmaster units are a two-piece design so clearing the axle isn’t an issue.

25. We folded the seam weld on the gas tank and barely got the massive Flowmaster exhaust system to clear the Maximum Motorsports Panhard bar. This isn’t the fault of either company since both pieces fit perfectly on a stock Fox-body, but our coupe is anything but stock and some dimpling to the rear piping and a few tweaks to the over-the-axle piping will get everything to settle in place.

26. We found it easier to install the Flowmaster mufflers onto the X-pipe before mounting it as a single unit in the car. Look at the size of that piping!


On the Dyno

As mentioned earlier, this was the first time we dyno'd the car with the bigger pulley at 10 psi, down from the last 15.5-psi pull. With the fluids up to temp our baseline figures with the smaller exhaust checked in at 571.4 hp and 552 lb-ft at the wheels on AED's in-house dyno. With the full Flowmaster system, the bigger shorty headers and the Kooks Green 3-inch cats in place we again hit the dyno to the tune of 583 hp and 560.5 lb-ft at the tires for peak gains of 12 hp and 8 lb-ft at the tires. But there's more to the story than just peak power, as we saw gains from 4,000 to redline with the greatest increases in the middle measuring over 15 hp and 15 lb-ft. The bigger exhaust also pushed the torque peak up several hundred rpm without losses anywhere on the curve.

But here's where it gets interesting: With the big exhaust system in place and only huffing 10 psi, the coupe is within 28 hp of the 15.5 psi run with the small exhaust. This means that while we saw gains of roughly 15 hp at 10 psi, at 15.5 psi we would have seen seriously stout gains. In other words, the original 2.5-inch exhaust system was less of a restriction at 10 psi than it would have been at higher boost levels.

But wait, there's even more perplexing information from our dyno sessions. We swapped the Kooks Green cats for test pipes and again hit the dyno. Without cats in place we saw the very same midrange gains of 15 hp and 15 lb-ft, but as the car approached redline the graph fell back down to the original baseline so that peak gains were insignificant. Even though the dyno corrects for atmospheric conditions, we attempted the dyno runs at various times throughout the day to see if the lack of gains was caused by dyno error, but each run netted the same results. Some calls to industry pros at Kooks and other companies all netted the same results; that gaining horsepower with high-flow cats isn't uncommon and that it's likely the result of a slight increase in back pressure. Whatever you believe, we call it proof that the Kooks Green cats are great for power and the environment. As for the full system, we consider it a success even at 10 psi—just imagine the gains if we had an aftermarket block and 17 psi.

What's Next?

Now that our Smog-Legal Killer is making efficient horsepower with new lungs, it's time to tie up some loose ends and introduce you to the big surprises to come, which include some dragstrip and autocross action with results you're not going to believe. Hang tight, there's plenty more to come.

Pro-M plug-and-play standalone ECU

Normally we'd be sweating bullets about swapping supercharger pulleys because each swap would likely call for a costly re-tune, but thanks to the Pro-M standalone, we've been able to swap pulleys on the dyno without consequence. The Pro-M's unique load-based tuning means that making changes, no matter how extreme is easy since the computer automatically adjusts for the changes. Seriously, it saved us from a day of tuning fees as we let the ECU sort it out.

A quick call to the company revealed that Pro-M has just released plenty of updates for the system as well, like coil-on-plug technology that does away with the factory distributor as well as integrated nitrous controls and there's even a methanol controller in the works—how's that for versatile? For the full story on the Pro-M system, visit Mustang360.net.

Decibel Test

Since we'd heard Internet rumors that the Flowmaster 3-inch race system was obnoxiously loud, we decided to test the different systems with an high-quality decibel meter at idle and under WOT from 10-feet behind the car while it was on the dyno.

We'll admit that dyno noise added a lot to the outright values, but what's more important is the comparison between the different systems since the original 2.5-inch system sounded healthy, but wasn't loud at all. For the record, the Flowmaster system wasn't any louder than the original system. Here's how they compare.

db at idle
2.5-inch catted exhaust: 82
3-inch catted exhaust: 80
3-inch exhaust without cats: 82

db under WOT on the dyno
2.5-inch catted exhaust: 114
3-inch catted exhaust: 108
3-inch exhaust without cats: 116

As for exhaust tone, the Flowmaster system has a more aggressive snap at idle that makes the car sound more cammed than before. Under WOT it's also got a raspy crack that sounds wicked and is several octaves lower than before. But it's also civilized at part-throttle cruising, as it measured slightly lower in decibels with the cats in place than the original 2.5-inch catted system—how's that for having your cake and eating it too?