Catalytic Converter Face-Off


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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

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. 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.

Next we installed the test pipe to get a read on the upper bounds of performance via flow improvement. 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.

Next up was the Jackson Racing piece, which showed gains similar to those achieved with the test pipe but at lower revs. We gained one to two horsepower between 5300 and 6300 rpm.

Off came the Jackson Racing and on went its apparent twin, the direct-fit MagnaFlow. 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.

Once we finished the new baseline run, we could install the custom-fabbed, lightweight MagnaFlow universal catalytic converter. 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.

The final catalytic converter in our test was the high-gloss Flyin’ Miata model. 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, (facing page) 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.

Catalytic Converter Results
























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Comments

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alfadriver
alfadriver UltimaDork
8/13/15 2:53 p.m.

I'm glad this article has finally been posted.

flatlander937
flatlander937 Reader
8/13/15 6:41 p.m.

So the Magnaflow cat is 0.5lbs lighter than the test pipe?

Hungary Bill
Hungary Bill SuperDork
8/13/15 7:06 p.m.

Impressive results and great article.

I wonder what the test would be like with something older. Say mid-80's ish?

KyAllroad
KyAllroad Dork
8/13/15 7:15 p.m.

Makes me feel better about keeping the cat on the race car. It'd be interesting to repeat the test (circa 2008) on something turbo and something bigger.

irish44j
irish44j PowerDork
8/13/15 7:37 p.m.
Hungary Bill wrote: Impressive results and great article. I wonder what the test would be like with something older. Say mid-80's ish?

ditto.

I just keep the cat in the e30 because without it, the M42 exhaust sounds like complete raspy garbage.

alfadriver
alfadriver UltimaDork
8/13/15 7:39 p.m.
irish44j wrote:
Hungary Bill wrote: Impressive results and great article. I wonder what the test would be like with something older. Say mid-80's ish?

ditto.

I just keep the cat in the e30 because without it, the M42 exhaust sounds like complete raspy garbage.

Something of that era, you'd be better off with a modern HP cat. Flow better, lighter, and even though I'm not a fan of aftermarket cats (not good info), it will still be better than the original hardware.

Advan046
Advan046 SuperDork
8/13/15 8:24 p.m.

This was a refreshing article. I learned that my impressions were some what accurate about cat delete and high flow cat impact on modern-ish cars.

novaderrik
novaderrik UltimaDork
8/14/15 2:11 a.m.

now do the same test on a late 70's/early 80's GM car with the stock pellet style converter... those things were restrictions on engines that only made just over 100hp..

Tyler H
Tyler H SuperDork
8/14/15 7:13 a.m.

Great article. We don't have emissions tests here and I find myself huffing Hondabro (or new Silverado with dual straight pipes)exhaust every time I get stuck at a light. WHY? A Magnaflow cat is $150 and does nothing to hurt performance.

Roll by and they inevitably have that glowing check engine light. That would drive me crazy.

foxtrapper
foxtrapper UltimaDork
8/14/15 7:22 a.m.

The results don't surprise me.

BUT... this isn't the whole picture.

1, these were all matrix or honeycomb type converters. Those flow great! Throw in the a pellet type, and the results will be different, very much to the worse. Yes, I know pellet types are old, but they are still around. Especially on older GM vehicles.

2, none of this dealt with a damaged cat, particularly an overheated and melted. This isn't always as obvious. Get into home-tuning and get the mixture overly rich, and it's not to hard to get the cat partially melted, killing flow through it.

alfadriver
alfadriver UltimaDork
8/14/15 7:40 a.m.
foxtrapper wrote: The results don't surprise me. BUT... this isn't the whole picture. 1, these were all matrix or honeycomb type converters. Those flow great! Throw in the a pellet type, and the results will be different, very much to the worse. Yes, I know pellet types are old, but they are still around. Especially on older GM vehicles. 2, none of this dealt with a damaged cat, particularly an overheated and melted. This isn't always as obvious. Get into home-tuning and get the mixture overly rich, and it's not to hard to get the cat partially melted, killing flow through it.

Why would they want to test technology that's over 30 years old? What's the point of that? This article was to illustrate how things have progressed. Nobody in the aftermarket does pellet cats, and I'm not even sure one can get them from an OEM. That would be pointless.

As for 2- this does somewhat illustrate that- as it's a cat swap. So if you damage the catalyst- instead of just putting a straight pipe in, here's the performance impact of various choices VS. a pipe. The damaging of catalysts could be fit into a sidebar.

Basically, this is the picture a couple of years ago. And it's not changed much since then.

pushrod36
pushrod36 Reader
8/14/15 8:12 a.m.

I would be interested to see the same test on a turbo engine making over 500hp. Even if the results were teh same (test pipe isn't gaining you anything) I think it would add resolution to the test results (5-10HP changes instead of 1-2HP changes).

Power curves and backpressure measurements would be good information, too.

dean1484
dean1484 MegaDork
8/14/15 8:24 a.m.

The insides of the cat on my 924s came apart and I ended up removing them as they clogged up my exaust to the point that it would not run over 4,000 rpm. I immediately found that the down side was that it stunk. Stand behind it and your eyes wattered. It also made the car much louder and it had a horrible drone at highway speeds. I actually got a magnaflow cat based on that article and put it in. Huge improvement in day to day drivability of the car. No more eye wattering exhaust and you coil have a conversation on the highway with your passenger. I don't know if performances changed but it made the car much better to live with.

Driven5
Driven5 Dork
8/14/15 9:32 a.m.
foxtrapper wrote: BUT... this isn't the whole picture.

I beg to differ. If you have an old technology or damaged cat on your car and are worried about performance, get a new modern replacement instead of a test pipe. If you already have a test pipe on your car, installing a cat will greatly lower your "d-bag coefficient". It is my longstanding belief that there are no good reasons to run totally catless on most any EFI car that is registered for street use, and have yet to see a decent argument to the contrary.

foxtrapper
foxtrapper UltimaDork
8/14/15 10:13 a.m.
alfadriver wrote: Why would they want to test technology that's over 30 years old?

Because the pellet type cats are still around and were on the cars favored by American muscle hot rodders, GM.

I'm not anti-cat, and the matrix type has always been around. It's not a new technology by any means.

bmwbav
bmwbav Reader
8/14/15 10:30 a.m.

This is relevant to my interests. I'd love to add cats to my BMW project car to reduce drone and not choke on it's exhaust all the time. My worries are in three areas. How close do they need to be to the engine? Is my backyard tuned megasquirt going to ruin them in two weeks? How much extra heat is this going to generate?

alfadriver
alfadriver UltimaDork
8/14/15 11:31 a.m.
foxtrapper wrote:
alfadriver wrote: Why would they want to test technology that's over 30 years old?
Because the pellet type cats are still around and were on the cars favored by American muscle hot rodders, GM. I'm not anti-cat, and the matrix type has always been around. It's not a new technology by any means.

Where? As in the last 20 years, who sold a pellet style catalyst?

If hot rodders use them, it's because they are either lazy or just don't know things have changed a lot.

alfadriver
alfadriver UltimaDork
8/14/15 1:35 p.m.
bmwbav wrote: This is relevant to my interests. I'd love to add cats to my BMW project car to reduce drone and not choke on it's exhaust all the time. My worries are in three areas. How close do they need to be to the engine? Is my backyard tuned megasquirt going to ruin them in two weeks? How much extra heat is this going to generate?

Now that I'm back at a real keyboard....

Start with ruining a cat- so what you are trying to make sure is that the calibration does not over temp the cat- and the way it will, so badly that you damage anything, is misfires. Lean or rich misfires will hurt- the rich ones being really painful- with plenty of fuel to go with the air. Ignition problems that cause misfires are equally as bad.

So if you can prevent all of those now, you should be fine. And if you notice a misfire, stop and fix the problem quickly, and you should be ok,too.

Can you tune it in a weekend to work well? Sure. If you are using a narrow band O2 sensor, just keep that going back and forth around stoich, it should work. Keep that oscillation small enough that you don't feel it, even better. For a WB system, I'm not that familiar with the MS fuel control, but stick it at stoich all the time- it will work fine. There will be enough noise in the system to keep it from being perfectly constant- and that helps.

How close? At the end of headers will work well. If you do any active work to get the cats hot, that will help, but unless you are being tested, you probably are not so worried about the starting emissions. On my Alfa, I was planning where the headers end below my feet- which ends up being about 2 1/2 feet. OEM's have them super close to get lit off quickly. But at the longer distance, it will light off reasonably well, and it also reduces the risk of overheating just from aggressive driving.

As for extra heat- your headers will be hotter. Catalyst construction really makes the outer skin of those cooler than the metal exhaust part. Under normal conditions- you'll see about 100F increase across it. When running rich for power, that will go down to below 50F. As a matter of fact, we go rich to keep the exhaust components cool. So shoot for 12:1 at peak power.

rcutclif
rcutclif Dork
8/14/15 1:52 p.m.

how can you tell if a cat is clogged or ruined?

Trackmouse
Trackmouse HalfDork
8/14/15 1:56 p.m.

You guys must be retired to have this much time to argue about crap that doesn't matter.

bmwbav
bmwbav Reader
8/14/15 1:56 p.m.

In reply to alfadriver:

Wow, Thank you for such a detailed response!

I'm using MS3 with speed density fuel control on a modern BMW V8 engine, WB O2 sensor. It's targeted to Stoich. Not concerned with start-up emissions, the car is almost 45 years old. (1971 BMW Bavaria)

It's tuned well enough, but A/F ratios do move around quite a bit. Am I reading that right that being rich lowers exhaust temp, but may overstress the cat?

Honestly I haven't data logged it for a while, It's been running this way for over a year. I just haven't made time to get on a dyno since it works well enough for street use and occasional track days.

That said, it does have a fairly consistent misfire under load at higher RPM's, probably tuned too rich or conservatively. It's never lean in that range of operation, It could also be ignition related.(Megasquirt sucks for diagnosing that type of stuff) It doesn't really cause issues on the street, as I would be doing something illegal to hit that part of the power band.

I am really concerned with additional heat. It's a big engine squeezed into a pretty small car with some components close to the headers/manifolds. So, I'm going to see heat increases on anything upstream of the cats? I was planning on putting them under my feet and passengers feet, probably 2-3 feet from the manifolds. If it's more like 4 feet, is that substantial?

alfadriver
alfadriver UltimaDork
8/14/15 1:56 p.m.

Clogged is pretty easy- engine performance will drop considerably- since there's a nice blockage in it.

Over temped so the metal is sintered, but the substrate is still ok? Outside of gas measurement, you need to have a second O2 sensor to compare to the first one.

If it rattles- it's shot (not including the heat shield).

alfadriver
alfadriver UltimaDork
8/14/15 2:02 p.m.
bmwbav wrote: In reply to alfadriver: Wow, Thank you for such a detailed response! I'm using MS3 with speed density fuel control on a modern BMW V8 engine, WB O2 sensor. It's targeted to Stoich. Not concerned with start-up emissions, the car is almost 45 years old. (1971 BMW Bavaria) It's tuned well enough, but A/F ratios do move around quite a bit. Am I reading that right that being rich lowers exhaust temp, but may overstress the cat?

No, it won't stress the catalyst. Unless it misfires. Trucks with trailers run very rich going up passes for as long as the pass is, and it won't damage anything.

Honestly I haven't data logged it for a while, It's been running this way for over a year. I just haven't made time to get on a dyno since it works well enough for street use and occasional track days. That said, it does have a fairly consistent misfire under load at higher RPM's, probably tuned too rich or conservatively. It's never lean in that range of operation, It could also be timing related.(Megasquirt sucks for diagnosing that type of stuff) It doesn't really cause issues on the street, as I would be doing something illegal to hit that part of the power band.

You need to deal with the misfire at some point- at high speed and loads- that will burn up the cat really quickly. As in a few seconds- picture the lost power you notice from the misfire, and ALL of that lost energy is being released in the catalyst. Not good.

I am really concerned with additional heat. It's a big engine squeezed into a pretty small car with some components close to the headers/manifolds. So, I'm going to see heat increases on anything upstream of the cats? I was planning on putting them under my feet and passengers feet, probably 2-3 feet from the manifolds. If it's more like 4 feet, is that substantial?

You should not seem much of a temp increase upstream of the cat. Some, but not enough to really worry about.

2-3 feet will be ok- 4 feet would be stretching it if you do a lot of stop an go driving- as the idling will cool things down. But that should be ok. On turbo cars- the turbo is acts just like a 2 foot stretch of pipe, and we can make that work well.

alfadriver
alfadriver UltimaDork
8/14/15 2:03 p.m.
Trackmouse wrote: You guys must be retired to have this much time to argue about crap that doesn't matter.

I have to retire to STOP worrying about catalysts.

bmwbav
bmwbav Reader
8/14/15 2:11 p.m.

In reply to Trackmouse:

Of course it matters, it's on the Internet!

bmwbav
bmwbav Reader
8/14/15 2:21 p.m.
alfadriver wrote:
bmwbav wrote: In reply to alfadriver: Wow, Thank you for such a detailed response! I'm using MS3 with speed density fuel control on a modern BMW V8 engine, WB O2 sensor. It's targeted to Stoich. Not concerned with start-up emissions, the car is almost 45 years old. (1971 BMW Bavaria) It's tuned well enough, but A/F ratios do move around quite a bit. Am I reading that right that being rich lowers exhaust temp, but may overstress the cat?

No, it won't stress the catalyst. Unless it misfires. Trucks with trailers run very rich going up passes for as long as the pass is, and it won't damage anything.

Honestly I haven't data logged it for a while, It's been running this way for over a year. I just haven't made time to get on a dyno since it works well enough for street use and occasional track days. That said, it does have a fairly consistent misfire under load at higher RPM's, probably tuned too rich or conservatively. It's never lean in that range of operation, It could also be timing related.(Megasquirt sucks for diagnosing that type of stuff) It doesn't really cause issues on the street, as I would be doing something illegal to hit that part of the power band.

You need to deal with the misfire at some point- at high speed and loads- that will burn up the cat really quickly. As in a few seconds- picture the lost power you notice from the misfire, and ALL of that lost energy is being released in the catalyst. Not good.

I am really concerned with additional heat. It's a big engine squeezed into a pretty small car with some components close to the headers/manifolds. So, I'm going to see heat increases on anything upstream of the cats? I was planning on putting them under my feet and passengers feet, probably 2-3 feet from the manifolds. If it's more like 4 feet, is that substantial?

You should not seem much of a temp increase upstream of the cat. Some, but not enough to really worry about.

2-3 feet will be ok- 4 feet would be stretching it if you do a lot of stop an go driving- as the idling will cool things down. But that should be ok. On turbo cars- the turbo is acts just like a 2 foot stretch of pipe, and we can make that work well.

Again, thanks so much for the knowledge!

So, here's what I've absorbed.

  1. Fix my damn misfire (Need to anyway)
  2. As long as A/F is in a semi-sane operating range, the cat will handle it
  3. Location - closer will keep it at operating temp better

So, final clarifying question about location..

I live in the bay area, so, yes, there is a bit of stop and go :) Having it further away will cool it down while idling, is that a terrible thing?

alfadriver
alfadriver UltimaDork
8/14/15 2:24 p.m.

In reply to bmwbav:

Not terrible- just that cooling it down reduces the effectiveness. For a car that has no catalyst- it's still better than nothing by a very wide margin. It will warm back up quickly enough. You might smell it.

(one has to put my view into some perspective- my job is to try to meet low emissions, for 150k miles. Some of the suggestions come from that work- and you really need 80's-90's goodness to be good.)

bmwbav
bmwbav Reader
8/14/15 2:43 p.m.

In reply to alfadriver:

Fair enough, thank you!

Toebra
Toebra Reader
8/14/15 6:25 p.m.

High flow cat, excellent, great idea, unless you live in Kalifornia, where it is OEM or it is not street legal.

Desy
Desy New Reader
8/15/15 1:47 a.m.

What about cars with 2 or more? Sure the single CAT test shows results, but lets add another variable.

For instance, the OEM exhaust system on my car (2014 tC) has lots of variations of exhaust diameter. This provided great low end torque and feel. But when I removed the midpipe for a aftermarket unit it upgraded the size to a 2.25" stainless system removed 1 CAT and had 1 20" resonator in line. The results? Less low end torque, but after about 3500RPM's much better flow and pull going through the mid-upper power band. So I went from 2 CATS and a resonator to 1 Cat, 1 resonator, and a larger consistent pipe.

novaderrik
novaderrik UltimaDork
8/15/15 7:07 a.m.

if you need a cat converter to keep your exhaust from smelling like gas, then you have tuning issues.

jsquared
jsquared Reader
8/21/15 8:46 p.m.
alfadriver said:
bmwbav said: In reply to alfadriver: Wow, Thank you for such a detailed response! I'm using MS3 with speed density fuel control on a modern BMW V8 engine, WB O2 sensor. It's targeted to Stoich. Not concerned with start-up emissions, the car is almost 45 years old. (1971 BMW Bavaria) It's tuned well enough, but A/F ratios do move around quite a bit. Am I reading that right that being rich lowers exhaust temp, but may overstress the cat?

No, it won't stress the catalyst. Unless it misfires. Trucks with trailers run very rich going up passes for as long as the pass is, and it won't damage anything.

This sort of skirts around my primary issue: high-power turbo engines. Obviously with the current proliferation of OEM turbo engines, running a cat on a moderate-power turbo car is possible without any major drawbacks. My concern is with a much elevated power level over stock. Under boost and power and with proper tuning I wouldn't be too concerned, since the richness is basically unburned HC which the cat is designed to deal with, but the "misfires" and such are what concern me. I have to run a slightly rich tune to protect the engine on pump gas, and when running track events the exhaust heat/richness/etc is enough to get pops/firespit between shifts after a few laps even with a recirculated BPV. It is my suspicion that this would fry a high-flow cat in short order. Any thoughts?

Also, medium-term plans (i.e. spare time during house-move prep and job-search) include plans to install my Aquamist setup; theoretically this should help by dropping temps a bit, especially on track, and leaning out the tailpipe mixture a bit. Is this an accurate assumption?

alfadriver
alfadriver UltimaDork
8/22/15 8:02 a.m.

In reply to jsquared:

A lot of your concern is covered, to me, by who calibrated it. And OEM application that is intended to pull heavy trailers up a long hill on a hot day is calibrated to not misfire and still be able to go up the hill. One thing is may do is reduce power to keep the system from breaking, but exhaust wise, the rich limit will run and not misfire. Besides, OEM systems have full range misfire monitors. Lots of testing, quite a few cars, many studies to do that.

Some aftermarket calibrators are good, but I don't have a very high opinion of all of them. Can't really comment how much work they do to make sure not to go too rich to prevent misfires.

I assume you mean a mist system for water injection in the engine. For that, the primary benefit is knock reduction, which greatly improves exhaust temps, which, as you think, leans out the need to keep the fuel mixture at best richness for power.

Again, how well that is figured out on an aftermarket side is not known- one has to do a lot of work to figure out what the knock limit is, and understand the amount of richness that is used to keep temps down, etc- not sure what kind of tools and experiments they do to figure that out. It can be done- OEM's spend a lot of time and effort on that.

Spoolpigeon
Spoolpigeon UberDork
8/22/15 9:50 a.m.

Great article, especially since I'm looking at replacing the current one on my S2000. Were all of these cats similar cell count? The rules for ST* have a minimum 100 cell count, and that is what I'm leaning towards, just curious what count these were.

Shaun
Shaun HalfDork
8/27/15 11:44 a.m.

Thanks Alfadriver!

jsquared
jsquared Reader
8/27/15 8:51 p.m.
alfadriver wrote: In reply to jsquared: A lot of your concern is covered, to me, by who calibrated it. [...] Some aftermarket calibrators are good, but I don't have a very high opinion of all of them. Can't really comment how much work they do to make sure not to go too rich to prevent misfires.

It's going to be a guy who builds and tunes race engines, somewhat known in the Subaru world, wins Time Trials and has huge-HP Time Attack cars, so relatively speaking it should be a pretty good calibration once it's done (and given Subaru's issues with the OEM tune, probably better than a stock tune on a stock engine). I'm wondering about the difference, though, between the temps from a between-shift mini-fireball out the tailpipe from the momentary over-richness and the temps from a combustion-chamber misfire under load. Is the unburnt fuel igniting in the exhaust pipe on it's way out going to be at a heat that could hurt the cat, or is the EGT spike from a misfire in the combustion chamber going to be way higher? I don't have an EGT gauge on the car or else I probably wouldn't have to ask that question

alfadriver wrote: I assume you mean a mist system for water injection in the engine. For that, the primary benefit is knock reduction, which greatly improves exhaust temps, which, as you think, leans out the need to keep the fuel mixture at best richness for power.

Yep, the Aquamist is a brand name for a water-injection system (uses a pulse-width-modulated valve rather than pump speed to control injection as well, and my particular model can be operated off of an additional map in my standalone, so in effect I have a 3D map control over a single injector in the intake manifold... if I ever get around to installing it, that is! hahaha)

Big thanks, BTW, for dropping the knowledge on us Always nice to have a pro's viewpoint.

alfadriver
alfadriver UltimaDork
8/28/15 8:43 a.m.

In reply to jsquared:

Quick misfires from shifts should be ok (but I'll suggest doing fuel off shifts- engine speed will drop faster). Decel misfires- even slowing from 60mph- can destroy a cat. Again- lots of time is spent making sure that misfires don't happen on tip outs, to the point of being really aggressive turning the fuel off.

As for the aftermarket tuner- spark is still hard to calibrate. Takes a lot of time if one wants to do it really well- harder if the engine is knock limited, as the knock on effects (see what I did there....) are numerous. Mostly in exhaust temp effects and keeping that in a safe range not to break something.

Coupefan
Coupefan Reader
9/1/15 12:19 p.m.

Having added a modern cat to a 34 year old car, I can say it made an extraordinary difference. Now likely the old cat was slowly dying and plugging up, but the addition of the new cat allowed the car to breath at higher RPMs. Additionally, from a numbers perspective during emissions testing, the car ran numbers comparable to a late 90s/early 2000s car. Win-win, as they say.

DAVEG
DAVEG New Reader
9/18/15 1:04 p.m.

Good article...I have both CAT which I reinstall when not racing and bypass pipe. I have not had the car checked with the CAT to see HP difference. I know that a revised chip made a big difference in HP and according to the chip maker a big difference in emissions with a leaner air fuel ratio.

Ian F
Ian F MegaDork
2/27/18 8:23 a.m.

While it tends to be an unpopular opinion, I've wondered about adding one of these high-flow cats to an older car that never had one to reduce exhaust smelliness. 

alfadriver
alfadriver MegaDork
2/27/18 8:29 a.m.
Ian F said:

While it tends to be an unpopular opinion, I've wondered about adding one of these high-flow cats to an older car that never had one to reduce exhaust smelliness. 

While a catalyst will be reasonably effective with a carb, you would be far better off with a programmable EFI system for effectiveness and durability.  The dawn of EFI and exhaust sensors really was a turning point in vehicle emissions.

Ian F
Ian F MegaDork
2/27/18 8:35 a.m.

In reply to alfadriver :

That is sort of my plan - add a high flow cat to the exhaust of my 1800ES which will get a closed loop EFI system.  Still on the fence on the two Triumphs, but I suspect they'll get EFI eventually, given my general dislike for carburetors. 

rslifkin
rslifkin SuperDork
2/27/18 8:43 a.m.

Being that decel misfires have been mentioned, does that mean anything that gives off some crackles and pops on decel is probably beating up the cat?  

Any chance of getting a quick run-down of what's safe for a cat and what will damage it from a tuning perspective?  (for those of us who are tuning our own stuff)

alfadriver
alfadriver MegaDork
2/27/18 8:51 a.m.
Ian F said:

In reply to alfadriver :

That is sort of my plan - add a high flow cat to the exhaust of my 1800ES which will get a closed loop EFI system.  Still on the fence on the two Triumphs, but I suspect they'll get EFI eventually, given my general dislike for carburetors. 

I, too, have plans to convert my GTV to EFI and add a catalyst.  Even have the catalyst in hand.  Not sure if I will ever do it, though.

rslifkin
rslifkin SuperDork
2/27/18 8:53 a.m.

As an additional thought / question on cats: I know a standard 3 way catalyst won't do much to reduce NOx when running lean burn.  But what's out there for catalysts that would?  Maybe running a diesel catalyst behind the standard one?  

RossD
RossD MegaDork
2/27/18 8:58 a.m.

I plan on adding a cat to the Renault 8 while I add MS2 and a turbo. I hate smelly exhaust. It smells.

alfadriver
alfadriver MegaDork
2/27/18 8:59 a.m.
rslifkin said:

Being that decel misfires have been mentioned, does that mean anything that gives off some crackles and pops on decel is probably beating up the cat?  

Any chance of getting a quick run-down of what's safe for a cat and what will damage it from a tuning perspective?  (for those of us who are tuning our own stuff)

Depends on how bad they are.  Seems to me that many of the current OEM's that are doing it are likely to be doing tricks to make it sound like you are getting pops and crackles in the exhaust without hurting the cat- and that is possible.

But, generally, *some* should be ok, a lot will burn the catalyst up in short order.  Basically- all the energy that is used to move the car forward would end up in the catalyst to burn.  The hard part for tuners is that the only real way to know what is "safe" is to measure the cat temp.  Which isn't exactly an easy thing to do without some decent resources (which then makes it really easy).  In that regard, I would just suggest that instead of leaving the fuel on during decelerations longer than 2 seconds or so- just turn it off.  That actually saves fuel, too.

For everyone here, if you are adding a cat, the issue that OEM's face about recovering the cat for performance isn't an issue- the big issue for you would be to make sure there's enough fuel to re-start the engine.

Now if you can measure gas temps (which is easier for most fabricators than cat temp- just due to drilling the catalyst material)- I would suggest keeping the cat inlet temps below 1600F, and that should keep the cat temps below 1700-1750F- which is where most cats get damaged.  If you get close to that temp, run a little richer.  But since it's not common for people here to have that kind of instrumentation- a "safe" cal would be to run about 11 to 11.5:1 (relative to gas of 14.6:1) which should keep the cat temps down, and not result in a big loss in power from best fuel of 12.5:1 or so.

And below WOT, just run closed loop around stoich.  It will work great.

The0retical
The0retical UltraDork
2/27/18 9:20 a.m.

I feel like this article needs to be posted once a year. It's such a great showing of how far the tech has come and the comments have so much info from people who do this for a living.

It gets tiring to hear about "ripping out all that emissions crap" from other people when you can build quite a bit of power with catalysts and not smell like a gas station when you get to work.

I worked in aviation long enough to get tired of smelling like gas all the time and now that I work at a desk I don't want to reek when I get there.

Stanger2000
Stanger2000 New Reader
2/27/18 10:03 a.m.

Not sure if this has been mentioned but most aftermarket cats last nowhere near as long as OEM, especially most high flow cats.  

mad_machine
mad_machine MegaDork
2/27/18 10:33 a.m.

good to see that the Magnaflow universal is so good. I need a cat for my Saab, so I think that is the way to go over the original

snowrx
snowrx
2/27/18 11:54 a.m.

In reply to mad_machine :

Just bear in mind that the spun universal is single wall construction, so there's no heat shield to protect the vicinity from heat, possibly red hot with a cat failure. The shell is heavy enough gauge to weld on some studs to mount a shield to protect whatever's too close. As a previous poster noted, you can get the housing with an angled outlet that helps a lot with system layout.

carguy123
carguy123 UltimaDork
2/27/18 3:22 p.m.

In reply to Spoolpigeon :

I sent my stock S2000 cat to Random Technologies for them to test to see which cat they had that would flow better.  The stock S2000 cat flowed better than Random's 3" cat.  Because of this they began offering the 4" cat.  After seeing that I elected to keep my stock cat.

 

Also I've seen about 125 dyno pulls on various S2000 configurations and a few other Hondas and every time the power bands were much smoother with the cat than without.  I'm going to run a cat on the race car I'm building.

maschinenbau
maschinenbau Dork
2/27/18 3:40 p.m.

I'm putting a cat on my Ford Model A hot rod and you can't stop me!

tolyarutunoff
tolyarutunoff
2/27/18 4:17 p.m.

i had a mazda rx/7 turbo2 that i ordered a cat-back for from a major supplier, about 20 years ago.  instead, i received a long pipe with a phony "cat box" where the cat should've been.  it released so much power that the fuel pump couldn't keep up:  i couldn't get within about 700rpm of the redline, let alone set off the buzzer!  mileage went up too.  heckuva deal!

jdoc90
jdoc90 New Reader
2/27/18 10:48 p.m.

In reply to irish44j :

raspy garbage can be easily corrected by a turbo install ....lol

te72
te72 New Reader
2/28/18 12:47 a.m.

I wouldn't mind my car smelling a bit better. It's tuned fairly well so that in most cases it isn't offensive, even to open top cars behind me, but an improvement would still be nice.

 

However, I think anti-lag and two step shenanigans would be blowing substrate components out the back at a fairly regular intervals...

StuntmanMike
StuntmanMike New Reader
2/28/18 7:08 a.m.

Great article, I've been trying to tell everyone not to run test pipes and how its not worth it. Now I have proof! Without cats, exhaust stinks (cause its full of poison) and is raspy. I've been using the Magnaflow universal cats for a long time and they are cheap, easy to fit anywhere and don't take hardly any power away.. 

rslifkin
rslifkin SuperDork
2/28/18 8:53 a.m.
te72 said:

However, I think anti-lag and two step shenanigans would be blowing substrate components out the back at a fairly regular intervals...

Yeah, those would be kinda bad for a cat.  There might be some metal substrate (instead of ceramic) cats that would hold up better to the abuse if you shop around.  Or rig up a setup with cutouts to bypass (and block off the inlet of) the cats for when you're using those features.  

te72
te72 New Reader
2/28/18 8:37 p.m.
rslifkin said:
te72 said:

However, I think anti-lag and two step shenanigans would be blowing substrate components out the back at a fairly regular intervals...

Yeah, those would be kinda bad for a cat.  There might be some metal substrate (instead of ceramic) cats that would hold up better to the abuse if you shop around.  Or rig up a setup with cutouts to bypass (and block off the inlet of) the cats for when you're using those features.  

Interesting you say cutouts, plural. At first, I thought you meant only one, which, on my setup, would likely have me being politely asked to leave due to noise levels at some tracks... However, with two cutouts... That's an interesting idea.

 

Alternatively, I could run one exhaust setup strictly for street and road trip use, and another catless setup for track use. Thanks for the suggestion!

rslifkin
rslifkin SuperDork
3/1/18 7:48 a.m.

In reply to te72 :

Yeah, the thought was 2 cutouts.  1 controlling flow through the cat bypass pipe, the other controlling flow through the cat itself with the 2 pipes merging back together after the cat.  To switch the cat out of the system, you open the bypass cutout and close the cat cutout.  

te72
te72 New Reader
3/1/18 11:29 p.m.

In reply to rslifkin :

Would basically be the same idea, plumbing wise, as recirculating an external wastegate. Not a big deal if you can find room for 3" pipe plus the cutouts... combine that with my desire to build a flat floor for the car, and well... probably not gonna happen. Do appreciate the suggestion though!

Toebra
Toebra HalfDork
3/8/18 7:00 p.m.

Flyin Miata makes a downpipe with a cat for my car now that is street legal.  Costs less than half what the one I would otherwise have had to use does too, so double bonus.

Floating Doc
Floating Doc Reader
3/17/18 1:31 p.m.

My 88 TBI 350 Silverado still has the original exhaust. Tail pipe is rusted through at the end, so I've been thinking about a replacement. 

Seems to make sense to look at changing it from the manifolds back (the Y-pipe is said to be very restricting). 

docwyte
docwyte SuperDork
3/17/18 4:51 p.m.

Depends on the car and on the cat.  On my old VW Mk2 16v GTI, there was no power to be had by going with the Euro downpipe, although you did gain some torque.  On my 944 Turbo, definite power gains by losing the cat.  On my E36 M3, 10-12rwhp by dropping the cats.

Even on the latter two cars tho, the power gains aren't so much that I'd remove the cats on a street car, only on a race prepped car...

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