Are catalytic converters just a cork in the system? Many enthusiasts think so, and we can’t say we’re surprised. After all, these components were born out of federally mandated emissions regulations, an origin story that doesn’t exactly scream “performance boost.” In fact, issues with cats have been fueling performance woes since their introduction more than 30 years ago.
Those first catalytic converters put a substantial stranglehold on horsepower. For a while, easy tuner hop-ups combated this problem: Enthusiasts simply bored out the guts of the cat or installed a “test pipe.” The pipe was just a straight piece of exhaust tube marketed as a means to “test” whether or not a cat was plugged up. And since there were no after-sale emissions tests at the time, folks could leave the test pipe in place and enjoy the extra performance with no worries.
However, this fix didn’t travel under the official radar long. Fast-forward to today and we have even tighter federal emissions requirements. Additionally, many metropolitan areas are required to regularly monitor the emissions equipment of their citizens in order to receive federal highway funds.
These government-designated nonattainment zones have resulted in a variety of city-based and statewide programs, each of which implements one or more federally approved test procedures. Some of these directly measure the tailpipe emissions with a probe, while others rely on a properly functioning On-Board Diagnostics II system. The latter monitors the exhaust’s oxygen content both before and after the cat in an effort to verify that the emissions equipment is working properly. An OBD-II system is required on newer cars to maintain a constant vigil over the cat’s functioning and record any occurring faults.
Too bad there were no systems in place to monitor the actions of car owners when cats were introduced. Accidentally filling the tank with leaded fuel would ruin the cat and plug up the works—in fact, at one time the EPA estimated that one in six owners pulled this maneuver.
Considering all this, we’d still argue that catalytic converters have gotten a bad rap. Once again technology has triumphed, and today’s cats are much better than their early counterparts, both in terms of emissions cleanup and performance efficiency. In fact, many unmodified modern vehicles don’t see any performance gains when fitted with an aftermarket high-flow catalytic converter. When you allow for extra breathing due to typical tuner mods like a cold-air intake, header and cat-back exhaust, though, the O.E. cat can still be a bottleneck, especially at higher engine speeds.
Dose of Thunder
The Street Touring category in SCCA autocross includes a basic tenet: Common modifications are allowed so long as the car remains street-legal. Emissions legality is key, as participants won’t have to undo their mods for yearly smog tests on their cars.
Since much of the issue centers around the catalytic converter, the STS/STS2 class rules originally took the easy way out. They simply required that the cat be original equipment—or equivalent—and fitted in the original location. The downside of this approach was that the inexpensive, off-the-shelf replacement cats that many of the older cars in the class needed were not always legal.
As a result, the rules were modified for 2008. Participants can now run any EPA-approved catalytic converter, including high-flow units.
The Environmental Protection Agency regulates aftermarket cats through a program that approves manufacturers and their engineering processes. Violators can face huge penalties.
An approved manufacturer certifies that each of its direct-fit cats are specifically made for a particular year and model application. Universal weld-in replacements must be specified for a certain engine displacement and vehicle weight. Approved cats receive an EPA code on the case and must be installed in the original location.
Any time the rules grant a new allowance, smart racers figure out how to best take advantage of the situation while still meeting the restrictions. In our case, we ordered up a variety of SCCA-legal catalytic converters for our Street Touring 1992 Miata to conduct a little testing session.
While all-out horsepower numbers are always enticing, we also wanted to look at weight as well as emissions output. As with any project like this, logic dictates many of the results but surprises lurk around every corner.
We're the Replacements
Miatas have a number of marque-specific equipment suppliers, and two of our players in this experiment come from those sources. Sometimes it’s nice working with a car that is so well served by the industry.
From left to right: Test pipe, Jackson Racing cat, MagnaFlow Direct-Fit cat, Flyin’ Miata cat and MagnaFlow Spun-Cat universal cats.
The first is a handsome, polished high-flow unit from Flyin’ Miata, a company well known for its pioneering work with forced-induction Miatas. This piece has a 2-inch inlet that matches most aftermarket Miata headers and a whopping 2.5-inch outlet to mate to the most common turbo exhaust systems.
Our next two players appear to be identical parts—different EPA-stampings are the only visible variation. One is from Jackson Racing, sold under license by Moss Motors, and the other is a direct-fit unit from MagnaFlow. Both have 2.25-inch inlet and outlet pipes. (Note that the MagnaFlow catalog shows this unit as having a 2-inch pipe, but we measured it as having a 2.25-inch outside diameter.)
Our final aftermarket cat is a universal piece from MagnaFlow, and it’s sized to accept either 2- or 2.25-inch tubing. (This is done by putting the pipe inside or outside the opening.) This is one in the company’s line of Spun-Cats, which come in a variety of shapes and sizes for close-fit applications. We also had a choice between a typical (and less expensive) ceramic core and a higher-flow metallic substrate. Of course, we chose the premium version.
To prepare our universal cats for service, we broke out the chop saw, the welder and, yes, the grinder—also known as the unwelder. We then paused to think about where to mount the cats.
The new rules require the “same location” for the replacement cat, but what exactly does this mean? Is it flange-to-flange? How does this apply to universal units or applications where the cat is originally welded in? We decided to mount our universals much like the direct-fit models and keep the brick centered.
As luck would have it, an old catalytic converter sacrificed itself for the cause. With a nice perpendicular slice of the chop saw, we now had a perfect tubing/flange front section ready to weld onto our universal cat. The 2-inch inlet matched our header, so we carefully butt-welded it onto the front of the cat.
We could have fabricated the outlet section the same way as the front, but instead we decided to attempt some weight savings. Our slip-fit SuperTrapp muffler typically mates up directly to the back of the cat via an adapter flange and short tube. In a moment of brilliance, we saw that we could eliminate the adapter with a direct slip-fit arrangement just after the cat. This saved the weight of two flanges and some tube—a whopping pound or two. It also allowed for a true 2.25-inch exit from the cat.
The only difficulty in our setup was getting the proper 30-degree outlet bend, which took some time with the grinder. A better solution would be to buy a Spun-Cat already fitted with an angled outlet, but we aren’t that smart.
Back to Back
With our load of cats in hand, we headed off to see Steve Hudson at Power Curve Dyno. This shop is located in the confines of Applied Racing Technology’s Spec Miata shop and sees plenty of Miata action. Our car is no stranger to these rollers, and we started the test with the O.E. cat to get a solid baseline.
Figure 1; The test pipe showed solid gains at about 5500 rpm.
Next we installed the test pipe to get a read on the upper bounds of performance via flow improvement (Figure 1). The results followed logic fairly well, with gains of one to two horsepower starting at 5500 rpm and running up to 6500. There were minor positive and negative variations between 3000 and 5000 rpm. This can be attributed to wave-tuning of the exhaust signal and the associated changes in ideal air-fuel ratio. A cat serves as an endpoint to the exhaust wave and affects the tuned length of the header collector somewhat. The test pipe changes that wavelength.
Figure 2; The Jackson Racing cat provided slightly lower gains.
Next up was the Jackson Racing piece, which showed gains similar to those achieved with the test pipe but at lower revs (Figure 2). We gained one to two horsepower between 5300 and 6300 rpm.
Figure 3; The MagnaFlow Direct-Fit cat produced a curve similar to the test pipe.
Off came the Jackson Racing and on went its apparent twin, the direct-fit MagnaFlow (Figure 3). Surprisingly, the results went up dramatically, mirroring the test pipe on high-end power—we gained a horsepower or two above 5500 rpm—with none of the wave-oriented changes down low. The Magnaflow had good capacity for exhaust flow and had the correct tuning to eliminate the exhaust pulse wave issues we encountered with the test pipe.
At this point, our testing conditions changed for the warmer, a variable that’s hard to avoid in anything but a totally climate-controlled facility. Up to this stage, we had enjoyed very consistent intake air temperatures at both the air filter and the manifold. Despite the dyno’s SAE correction factors, the power at the wheels showed a slight decrease. We did a second baseline pull with the stock cat to realign our figures.
Figure 4; We saw the best high-end gains with the MagnaFlow Spun-Cat.
Once we finished the new baseline run, we could install the custom-fabbed, lightweight MagnaFlow universal catalytic converter (Figure 4). The custom cat beat the stock piece by one to two horsepower from 5000 rpm all the way to redline, which was 7000 for this test. Like the test pipe, the MagnaFlow cat also brought an additional one to two horsepower from 3000 to 4000 rpm and lost only one pony from 4300 to 5000 rpm.
Figure 5; The Flyin’ Miata piece gave 1-2 horsepower gains everywhere. At right, armed with his trusty array of sensors, Dan Pedroza of JCAutoSpec tested the effectiveness of each catalytic converter.
The final catalytic converter in our test was the high-gloss Flyin’ Miata model (Figure 5). The prettiest girl at the dance did not disappoint, posting the best gains of the day by beating the O.E. piece by a solid one to two horsepower across the entire board. Compared to the custom MagnaFlow cat, the Flyin’ Miata piece fell off a little on the high and low ends but made it up in the mid-range. Not that these are huge differences, mind you.
Color Me Impressed
While the high-flow cats can indeed produce the same numbers as a test pipe, is there a downside? Our search for an answer brought us to John & Corky’s Automotive Specialist, a full-range repair facility. Of note, JCAutoSpec is an official Texas Emissions Test facility and Repair Center, allowing it to administer yearly smog checks and make any necessary repairs.
Since our car is pre-OBD, we decided to perform a real tailpipe sniffer test to validate the effectiveness of our cats. Texas only monitors carbon monoxide (CO) and hydrocarbons (HC) for non-OBD cars; some states also check nitrogen oxides. We opted to use the shop’s Snap-on five-gas analyzer to get a read on everything coming out the back.
We tested the O.E. cat both at the beginning and end of the session, and right off the bat we noticed a bit of a difference. Based on experience, we'd guess that the cause was probably due to the ambient temperature changes that occurred during the test, which put the ECU into different areas of its mixture tables. (As the day went on, the car continued to lean out.)
The full results from this test are shown in the table, and the most obvious finding is how much a catalytic converter cleans up emissions. On a related note, however, even though the test pipe is a gross polluter in our eyes, our little Mazda still met the rather lax Texas limits on emissions: 220 ppm of hydrocarbons and 1.2 percent of carbon monoxide.
However, we’d rather keep Texas beautiful—and be SCCA-legal—so we’ll keep a cat on our Miata.
Photography by Andy Hollis
