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.