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View Full Version : Zen and the search for more power... Phase 2: Headers & Catalytic Converters



JoeT
01-18-2004, 12:30 AM
In the quest for power, acceleration, speed, after the cat back exhaust, the next step usually is to look for a set of headers to help augment the flow of gasses and make it easier for the engine to breathe. Besides, what's an engine anyway but a large air pump. The easier for it to inhale and exhale, the freeier it is to rev until it's not allowed to.

4 Stroke-Cycle engines are all basically the same, Intake->Compression->Combustion->Exhaust are the 4 strokes per cycle that occurs at every 720 degree revolution.

Getting back to the Quest for Power. Once again my goal was to increast the available torque and power available for my hobby / sport of Autocross. Which means I need power under the curve to stretch as far as possible, and start as low as possible. I've already installed my 2.25" cat back exhaust, now I need to get rid of my restrictive stock exhaust manifold and stock catalytic converters.

Again, I did research, lots of it to look for the absolutely the best headers for my application. Keep in mind, the 2.5L Boxer engine from our scoobs have a redline of 6250 or 6500 depending on SOHC or DOHC. My car was a SOHC variation with a slightly lower redline, which means at this low rpm range and a volume of 625cc / exhaust pulse (add temperature), I needed a header with fairly long primaries to do "scavengine" within the primaries, relying primarily on the length of the primaries for increased torque, rather than scavenging at the collectors.

This meant a Loooooooong set of primaries to do the job. In the ideal world, the primaries should be in the range of 30 - 34 inches with an ID of no smaller than 1.5 inches but no larger than 2", and equal length would mean equal scavenging for every cylinder.

So the search went on, and on, and on ..... On the road to utopia, we discovered some interesting designes out there. Borla is a popular one, but here are the facts. Borla primaries are nowhere close to 30" and their unequal length secondaries resulted in untimed exhaust pulses. The only benefit from using short runner unequal length secondary headers are from the reduced backpressure from eliminating the restrictive stock catalytic converters and replacing them with a freer flowing type. No biggie.

Where to find good headers, Maxim Works makes the crown Jewel of them all, Brullen makes a functional but ugly header, and TWE makes nice equal length long primary headers but they are made of mild steel. Also, from Australia MRT makes headers that work quite well.

I bought my self a set of Maxim works headers, cause they fit the criteria I was looking for, but they came with no Catalytic converter, so on to the next topic.

Catalytic converters:

Our cars use 3 way catalytic converters, and no matter which converters you use, you'll get the P0420 CEL code unless you add a 1 Meg Ohm resistor with a 1 microfarad capacitor to the rear O2 sensor. (another story).

3 way catalytic converters use 3 reactive catalysts to control the exhaust gasses and to catalyze the hydrocarbons to harmless CO2 and O2 and H2O. They do this by burning the remaining hydrocarbons through chemical reaction with the Platinum, Rhodium, and Iridium embedded in the catalytic converter. The exhaust out of the catalytic converter is hotter than the input temperature, that's how mechanics check if your catalytic converter is actually working. They use an IR pyrometer and point it to the inlet and exhaust of the converter and see the difference in temp.

Anyways, what's a high flow cat?

Before I get into that, let me tell you about the stock cats, the stock cats are ceramic substrate with extremely fine honeycombe pipettes running in parallel throughout the length of the element. Each cell or pipette is about the diameter if a pin head, and there are hundreds or even thousand of these little pipettes straining the exhaust gasses, our scoobs have 2 of these in the NA version. The pre-cat is a full cat which pre-burns the exhausts and leads them immediately to the second cat where the burning continues. The OEM Catalytic substrate is high quality and will usually last the lifetime of the vehicle if good quality unleaded fuel is used. Otherwise they get coated and clogged. Run them too rich and the cats can melt themselves down.

What about those 100 dollar aftermarket cats? Well put it this way, the aftermarket can make 10 cats out of 1 of our stock cats, which means that 2 of OEM cats can make up to 20 aftermarket quality cats. (Generalization, but it's true)

What is a high flow cat?
By definition, a high flow cat is any cat which flows higher than the stock cat. Which means if you change the inlet and outlet size of the "cheap" cats, you get a high flow cat. I've seen quite a few of those, Random tech sells one for big bucks, Brullen can supply you with a variety of cheap cats. What makes them cheap is they use the similar ceramic type of substrate with tiny pipettes just like the OEM, the only difference is the size of the inlet pipe and outlet pipe.

What's a real high flow cat?
In my books, the only real high flow cats out there are the ones that are called "Metal Core" high flow cats. They are sold in 3 cell sizes, 300c, 150c, 80c. I don't really know what the c stands for, but if you take a look at a typical metal core cat, it looks like a spiral wound cardboard flap, but it's made of metal.

Advantages of metal core: Can't be burned out, will accept anti-lag without a hint of damage, lasts forever, and filters exhaust gasses 140% better than the OEM cats, without the restriction.

Disadvantage: $$$

Net result after spending mega bucks on Maxim Works headers and high flow cat...

Flatter torque and power curve and an increase in the fabled HP of 15 hp, but the most significant gain was the flatter torque curve.

Next Article... in the Zen series... Camshafts!

Note: There will be a surprize later in the series when talking about ignition and fuel curves, and how important it is to extracting the most out of your engine.

DonS
01-18-2004, 12:51 PM
Are one of the tradeoffs of equal length some loss of the distinctive boxer sound? The Brullens I've seen had equal length primaries and unequal secondaries.

I'm starting the longer term plan as time, money, warranty allow. Headers and intake work are on the shorter end of the list.

Speaking of sound, cruising back roads in the middle of the night with 15+ cms of fresh snow underneath makes for a very nice sound in a wagon. This is with stock exhaust.

JoeT
01-18-2004, 01:43 PM
The boxer sound will be there in one form or another, because of the firing order 13 24 (Right band of cylinders then left bank of cylinders). But it smoothes out the sound slightly, and reduces the Misfire sound.

The Brullens were designed to scavenge each cylinder individually through the looooong primaries, the secondary or collector really does not make that much of a difference in terms of scavenging, it does if you have "shorties" as headers. Note: The Brullen NA headers don't really have secondaries, it goes directly into the collector.

The Brullen's give a noticeable improvement in low end + midrange torque. It's not just "I thing there's a difference" it's more line "Why did I wait so long to do this, wow".

Hope this helps.

DonS
01-18-2004, 03:09 PM
Ah yes, there is the boxer sound. Trying to apply some old theories from my olden cycle days. I had always preferred twins (in-line, V, and opposed) with some single "thumpers" over the too common 4's. Two strokers were a blast since with a sealed lower end crankcase you could cut the tranny off and make a lovely go cart engine. A whole different exhaust theory on two stroke motors. I had a 100cc twin engine with the tranny sliced off. The cutest thing, you could hold it in your hands. There it was timing the pulses in the expansion chamber with the intended load and range.

Twin 4 strokers were great for tuning. The apparent roughness made them sound more boxer like than a buzzy 4 hole. The Norton 750 had a really sweet sound and pull. I can only dream of a Ducati V.

So for my NA I'm after more grunt and mid range. Or lots of mid range and no loss of low end. The Brullen's were looking good. I'm just browsing now since money will be a ways off. Good to plan it out.

cams, balancing, porting.... oh my. Further down the cycle. I think my best hope is a spare 2.5 that can be built over a long period as money, ideas, etc occur. Keep one stock and pristine. My old 2.8 Datsun was built slowly with lots of care. Everything was balanced and polished with lots of elbow grease. Sweet.

Navigator
01-18-2004, 11:19 PM
I can't wait for the rest of the series, I have found the first to parts informative, clear and easy to understand. I would suggest a section on the boards, where after a select period of time important and/or technical articles can be placed for easy reference. It makes learning easier when all the good information is in one spot.

Navigator

01-23-2004, 04:17 PM
A good idea to make this kind of information easy to find and universaly available.

The search works very fast and will continue to be fast into the future but we should also cover the folks that won't use it or are just browsing around. Most of the site content gets indexed in the global search engines. Useful information brings good people and hopefuly even more information.

Right now this thread and a few others are sticky. Perhaps a (maybe read-only) "Award Winning" forum? Make it stand out so it gets reviewed and people find the information quickly and comfortably. It might cut down on redundant questions. And we become ever more authoritive in the industry, err "scene", err I don't like that term either, we'll think of something.

Good comments, it helps me massage the site functionality..

Yipes, I just hijacked the thread. Sorry, it will be edited if this gets moved to the Pulitzer Prize Forum.

02-14-2004, 11:17 AM
Please disregard my last reply posting. I am a as they everyone calls it a newbie and am just getting to know the regular people of this forum and how it works. This is the best form I've found so far with information that I've been looking for.

evang
07-15-2005, 05:36 PM
Very interesting discussion - thanks for posting it.

Concerning Brullen headers + their catback on a DOHC Legacy GT, some info and a question.

- Since fitting them, the engine feels as if it has much better torque in the 3-5K rpm range
- I have not analyzed the exhaust, but the tailpipe and decreased gas mileage suggest the car is running rich, and CEL is more-or-less permanently on (O2 sensor code)
- At the first try, the car would not pass an e-test (HC & NOx too high)
- After noticing that the cat was cold enough to touch at idle, applying header wrap to the individual runners resulted in a huge drop in both types of emissions. It easily passed the next e-test.

However, the engine still appears to be running rich. From your experience any suggestions as to how to correct this?

JoeT
07-15-2005, 09:28 PM
Actually the symptoms you mentioned sounds like your front O2 sensor is on it's way out.

With a high flow cat, you will get cat inefficiency codes, like the P0420 basically you replaced 2 cats with 1, so there will be more HC going out your tailpipe and possibly some sooting on the exhaust tips.

Your milage will stabilize, on cruise you'll notice more fuel economy with the headers and 2.25 exhaust.

Since your car has a MAF, please do not use a short ram CAI with adapter. The 98 - 99's have been known to run slightly lean at 3500 - 4000 rpm which is can be compounded by the use of CAI... So please do not put a CAI on your car, keep the stock aribox in place an use a good quality panel filter.

Hope this helps.