Hi-flow cat [Archive] - SaabCentral Forums

: Hi-flow cat


M4Gunner
19-04-04, 01:23 PM
so i just got back from my buddy's speed shop. he can make me a hi flow cat/exhaust in any size i choose. my question is this, will the T8 ECU play any role in taming this mod? my thinking is that its out of the engine by the time it gets to the cat, so it wouldnt matter? yes, no, maybe?

joshd2012
19-04-04, 02:01 PM
Make sure he can work with the sensors. There are two cats on the stock system. Both have sensors in them that will go off if something is out of the ordinary. You need to get a cat that will support that. But besides that, a custom exhaust is a great idea. The T8 will recognize an increased airflow and adjust the engine to make more horse and torque. A free flowing system can give you some pretty good gains.

M4Gunner
19-04-04, 02:06 PM
no, i understand the "gains" part, i just want to make sure the ECU wont go the other way with it, for lack of better words. and restrict airflow, to keep the same flow as stock.

he can do the O2 sensors, and id probably just have him eliminate the second resonator. we'll see i guess. the nice thing is that w/ a universal hi-flo cat, a magnaflow muffler and custom exhaust piping, its only going to run me about 350! SAAWWEEEEETT!

ctrlz
19-04-04, 02:53 PM
Guys,

Can you describe the dual cat/dual sensor system a little more? I haven't had my car on a lift.

What I'm imagining is the exhaust manifold goes to a Y-shaped head pipe. Somewhere in each branch of the "Y" two oxygen sensors are inserted (upstream of the cat convertors). The "Y" dumps into two cats, then followed by some intermediate pipes which combine either before or at the muffler. This makes the most sense, there being two identical oxygen sensors in front of two cats.

Tell me what it really looks like. Also what are the stock pipe diameters and what are you planning to go to? Something like exhaust manifold to ONE 2 1/2" pipe, then big cat, then 2 1/2" to big muffler, etc.? Is that the plan?

M4Gunner
19-04-04, 03:14 PM
thats pretty much the idea. im not even 100% on the layout, but ive done this type of stuff before, and its really all pretty cut & dry.

the plan is, as you said, bigger better badder! factory is 2.25, im going to go 2.5 just to let it breath a bit more. should do just fine.

ctrlz
19-04-04, 04:31 PM
Well, just guessing, I'd say they used dual cats to get increased flow without sacrificing underbody clearance.

The two sensors are probably both oxygen sensors. The cats need 2% oxygen in the entering exhaust to do their job properly. Using two sensors may be a way to insure BOTH cats are EACH getting 2%. The other possibility is that one sensor can serve as a backup. If a sensor goes bad, the computer will throw a code, and you get an engine light. Since you still have one GOOD sensor, the ECM can still run in "closed loop" mode. On a car with just one sensor, the ECM would have to go into "open loop" mode, also know as "limp home."

If I'm correct in my assumptions, I would say you should just install BOTH O2 sensors in your new head pipe. That saves you the trouble of trying to fool the ECM into thinking one sensor is really two, etc. A well-equipped shop can quickly cut the holes with a plasma cutter.

You are probably correct in your overall assumption that the factory exhaust is the major restriction in airflow. Changing the exhaust system and air filter are easy, but it's nice to see what someone else gets trying this approach before you spend $$$.

joshd2012
19-04-04, 04:47 PM
To the best of my knowledge, there is nothing in the ECU which will restrict airflow after you open it up.

M4Gunner
19-04-04, 04:55 PM
Well, just guessing, I'd say they used dual cats to get increased flow without sacrificing underbody clearance.

The two sensors are probably both oxygen sensors. The cats need 2% oxygen in the entering exhaust to do their job properly. Using two sensors may be a way to insure BOTH cats are EACH getting 2%. The other possibility is that one sensor can serve as a backup. If a sensor goes bad, the computer will throw a code, and you get an engine light. Since you still have one GOOD sensor, the ECM can still run in "closed loop" mode. On a car with just one sensor, the ECM would have to go into "open loop" mode, also know as "limp home."

If I'm correct in my assumptions, I would say you should just install BOTH O2 sensors in your new head pipe. That saves you the trouble of trying to fool the ECM into thinking one sensor is really two, etc. A well-equipped shop can quickly cut the holes with a plasma cutter.

You are probably correct in your overall assumption that the factory exhaust is the major restriction in airflow. Changing the exhaust system and air filter are easy, but it's nice to see what someone else gets trying this approach before you spend $$$.

im definatly going to run both sensors. itll just be a custom setup! me likey!

RED
19-04-04, 07:50 PM
A well designed custom exhaust systems can really "open up" a turbocharged engine. I have such a system on my SVO. However... Note that I say "well designed". Unfortunately, I have seen several systems on various cars (and know of many more, turbo and otherwise) that were NOT better than stock, as shown to their rather chagrined owners by dyno sessions. They were not amused that their louder exhaust systems were only that - louder - than stock systems running on the same dyno on the same day.

So -- get a before and after dyno reading from an independent shop, or you may only be making noise. :cheesy: Not that there's anything wrong with that :cheesy:

M4Gunner
19-04-04, 09:07 PM
yeah, i hear ya Red. the fellow i use is quite respected and does a hell of a job. however, im sure midas would do i for you as well... and im also sure it would $uck! unfortunatly dyno's are scarce around here, so im not sure how likely it will be that i get on one first. but, if i can, ill deffinatly try. if not, ill just have to use the *****-dyno... "yup, feels faster!"

ctrlz
19-04-04, 09:16 PM
RED's point about LOUDER vs FASTER is well taken.

This car's been out for over a year now, though. Someone somewhere has tweaked the exhaust to be sure! I'll bet if you troll around, you'll find somebody who dynoed before and after.

M4Gunner
19-04-04, 09:19 PM
ya, thats tru too. hmmm... I GOT IT!! ill post a new thread!!!

oslowley
20-04-04, 01:31 PM
If you plan on doing the downpipe also let me know and i'll probably have your guy make me one at the same time (maybe that will bring the cost down). right now I'm away at work but as soon as I get back home, hopefully my cat-back from genuinesaab will be there, I am going to have my muffler guy install a free flow cat as well plus take a good look at the downpipe and see if he can work one up.

pjku79
20-04-04, 02:35 PM
thats pretty much the idea. im not even 100% on the layout, but ive done this type of stuff before, and its really all pretty cut & dry.

the plan is, as you said, bigger better badder! factory is 2.25, im going to go 2.5 just to let it breath a bit more. should do just fine.

just a heads up bigger isnt always better. you want the optimal size not just bigger.

pjku79
20-04-04, 02:45 PM
another question, everyone keeps talking about cats. If a straight pipe is added would the sensor trip or would they just measure the more air and adjust. no emissions in michigan.

M4Gunner
20-04-04, 02:51 PM
i think the main reason for a hi flo is just for legal purposes. "it has to keep the same ehaust layout as factory." at least thats what ive been told here in NH.

and youre right, bigger isnt ALWAYS better, but it is 95% of the time, especially w/ turbos! :) i think im only going to go up to a 2.5 anyway. the rule of thumb is no more than a .5 inch step up from stock.

Cayman1
20-04-04, 03:37 PM
another question, everyone keeps talking about cats. If a straight pipe is added would the sensor trip or would they just measure the more air and adjust. no emissions in michigan.

Cat's are used to regulate the amount of toxins that is released into the atmosphere by the engine.

They are pretty restrictive and that is why people get a high flow one or just remove them and put a straight pipe if you live in a state that has no emissions like you stated.

if you remove the cat and put a straight pipe, all the muffler shop needs to do is make a hole and re-mount the O2 sensor somewhere on the exhaust pipe and all will be fine.
the 02 sensor has nothing to do with the amount of air flow passing through the exhaust.

On my last tubo car, i lived where there were no emissions so i gutted my cat out and them put it back on. therefore there was no need to have a shop mount the 02 sensor. You would be amazed to see how much stuff is in a cat and will understand how it restricts air flow.

pjku79
20-04-04, 04:16 PM
On my last tubo car, i lived where there were no emissions so i gutted my cat out and them put it back on. therefore there was no need to have a shop mount the 02 sensor. You would be amazed to see how much stuff is in a cat and will understand how it restricts air flow.

how did you gut the cats? i have never done that. on my mustang i just bought an offroad pipe (no cats).

Cayman1
20-04-04, 04:20 PM
On my last tubo car, i lived where there were no emissions so i gutted my cat out and them put it back on. therefore there was no need to have a shop mount the 02 sensor. You would be amazed to see how much stuff is in a cat and will understand how it restricts air flow.

how did you gut the cats? i have never done that. on my mustang i just bought an offroad pipe (no cats).

I lived out of the country and I had a nissan skyline and i did not want to pay to have such a part shipped from japan so i just unbolted the cat, put it on a block and pounded out the interior with a steel rod and hammer. it was a pain but it was free and spool up was a little faster.

ctrlz
20-04-04, 04:29 PM
another question, everyone keeps talking about cats. If a straight pipe is added would the sensor trip or would they just measure the more air and adjust. no emissions in michigan.

O2 sensor is inserted upstream of the cat. It reports to the ECM, and the fuel/air mixture is adjusted to maintain 2% oxygen AHEAD of the catalytic converter. The O2 sensor won't care whether the cat converter is functional or not.

Modern catalytic converters are two-stage devices, reduction followed by oxidation. The 2% oxygen is there to feed the oxidation stage (along with some oxygen formed in the reduction stage). Here's a nice explanation:
http://auto.howstuffworks.com/catalytic-converter2.htm

Even if you gut the converter, you'll still need the O2 sensor for the car to run right. But it won't behave differently if there's a straight pipe downstream.

Many states which don't monitor emissions still have laws requiring all factory-installed emissions components to be maintained in unmodified form. I know Pennsylvania is like that, though they still always passed my musclecars at annual inspection. Some states may actually be strict about it.

pjku79
20-04-04, 05:26 PM
michigan doesnt even have inspections. I know i have to keep the o2 sensors in there.

RED
20-04-04, 08:18 PM
Some states may actually be strict about it.

... and there is a reason for that!

Point one - you do NOT need to break the law to produce impressive power gains from a custom exhaust system. It can be done quite effectively and keep you legal - even here in California. I know, I have done it.

Point two - see my earlier post about "well designed". Gutting a cat is not "design". Replace it with a straight pipe and take your chances, or do the job right and run a bigger cat and downpipe.

Point three and an OFFICIAL WARNING - "SaabCentral cannot condone the promotion of any illegal activities through these forums, any such posts will be locked or removed." This thread is getting to be a very dark shade of grey. Not over the line, not yet. So stay on topic and lets not see any advice on how to evade smog laws, shall we?

mbodo
21-04-04, 12:43 AM
So, assuming we're all talking about offroad use only:

Wouldn't the sensor that lives behind the cat get abnormal readings, thereby sending some fairly screwy signals to the ECU, probably running the engine outside of it's normal operating range?

If so, I wonder (it's anyone's guess) if the PPC can be updated to accomodate this.

M4Gunner
21-04-04, 03:50 AM
at that point, i dont think itll matter. but, im really not sure. i dont think youll need to have the PPC recognize this. it would be the same as adding an exhaust to a PPC, (stage II) where it actually uses the same software.

pjku79
21-04-04, 08:32 AM
[quote]Some states may actually be strict about it.
Point three and an OFFICIAL WARNING - "SaabCentral cannot condone the promotion of any illegal activities through these forums, any such posts will be locked or removed." This thread is getting to be a very dark shade of grey. Not over the line, not yet. So stay on topic and lets not see any advice on how to evade smog laws, shall we?

I just want to point out that I was speaking strictly for my case MICHIGAN, where there aren't any emissions tests. Therefore, I am not promoting anything illegal.

ctrlz
21-04-04, 09:20 AM
So, assuming we're all talking about offroad use only:

Wouldn't the sensor that lives behind the cat get abnormal readings, thereby sending some fairly screwy signals to the ECU, probably running the engine outside of it's normal operating range?

If so, I wonder (it's anyone's guess) if the PPC can be updated to accomodate this.

If there's a sensor BEHIND the cat, then the emision controls are more sophisticated than the typical system. The O2 sensor sits IN FRONT OF the cat to make sure there is enough oxygen for the cat to do its job. It also ensures that a bit of excess oxygen is still present in the exhaust manifold, as things are still hot enough in there to burn off hydrocarbons that weren't consumed in the combustion chamber.

It was stated the stock setup uses two sensors. I assumed it is a redundant system which uses two oxygen sensors in front of two cats. If there are sensors BEHIND the cats, I'm not sure what they are.

mbodo would then be right about some ECM reprogramming being necessary, and probably illegal.

Frankly, I hope there are redundant O2 sensors. Having been caught in expressway traffic in "limp" mode several times, it's just something I'd rather not do again. You guys with Sentronic automatic transmissions might be in for an even bigger surprise. In "limp" mode some electronically controlled automatics won't shift above 2nd or 3rd gear.

ctrlz
21-04-04, 10:16 AM
Looks like my guess was wrong:
In addition to two manifold sensors, 1994 and later OBDII equipped vehicles may also utilize a pre-catalytic and post-catalytic converter oxygen sensor to indicate converter efficiency. Most 1990 and later oxygen sensors are heated with battery voltage to accelerate sensor activity and decrease cold-start emissions.
This is from:
http://www.forparts.com/emission.htm

So the ECM is also monitoring oxygen DOWNSTREAM of the cat. The second O2 sensor is specifically there to report how well the cat is performing. Saab probably has it tweaked so there's a teeny bit of oxygen left in the exhaust exiting the cat. The parameters will change as the cat heats up. Pretty efficient way of doing things from an emissions standpoint.

From a performance standpoint, you may be behind the 8-ball. If you go to a high-flow cat, it may not be as efficient as stock, and the ECM will compensate by leaning out the fuel/air mixture. So, the performance gained by reducing exhaust restriction is hampered by the ECM leaning out the mixture.

To optimize performance, you either need to reprogram the ECM to NOT CARE how well the cat is functioning, or install "dummy" devices which report the optimum voltages to the ECM in place of one or both O2 sensors. I guarantee THAT'S NOT LEGAL!

You got yourself an interesting conundrum :-?

ctrlz
21-04-04, 11:12 AM
Some GREAT stuff here:
http://home.flash.net/~lorint/lorin/fuel/lambda.htm

Don't think you can bypass the front O2 sensor in any way without really screwing up performance. The output voltage fluctuates dramatically at hard acceleration.

RED
21-04-04, 11:17 AM
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Excellent post! Should be required reading for all those interested in how exhaust gasses are used to tune engine performance.

ctrlz
21-04-04, 11:23 AM
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Excellent post! Should be required reading for all those interested in how exhaust gasses are used to tune engine performance.

Yeah, that's EASILY the best page I've ever seen on O2 sensors (and I've looked around before). Really set ME straight!

Just shows you really gotta use your brain these days when trying for performance gains.

ctrlz
21-04-04, 12:58 PM
Think I got your solution.

You need the front O2 sensor to be sure, otherwise the fuel/air mixture can't be tuned to driving conditions. The rear can be replaced with an "oxygen sensor simulator." The simulator will prevent a "check engine light" from coming on.

Do an ebay search for "oxygen sensor simulator" to learn more.

Some simulators are for dual exhaust cars. Also, note that some ECM's check for continuity in the sensor heating circuit as a way to confirm the sensor is actually installed. (I'll bet the 9-3 does just that.) So the old sensor may need to be retained, just so the heating element can be wired. The actual "signal" generating job is performed by the simulator.

ctrlz
21-04-04, 01:09 PM
Now you guys got me interested!

I'm betting just replacing the stock diameter cat with a less restrictive one might give a nice low-budget horsepower gain, and a better exhaust growl. The cat has got to be the main bottleneck in the exhaust.

Opinions, please!

And please post any sites which sell appropriate hi-flow replacements for the 9-3. Even if it's for 2.5" pipe, I could always use adapters. I take it the stock is 2.25"? Is the exhaust all stainless? I really should know this, I know.

moz
21-04-04, 02:05 PM
Ok, I'll try to give it a shot - though a lot of information is scattered in this forum.

As far as I know there are two options in a cat-back system for our cars.

1) BSR 2.5"
2) Ferrita 2.5"

Both are stainless steel, single oval outlet varieties (though Ferrita does have a double pipe outlet which I have personally never seen on a SS). The Ferrita is hand-made and costs around $7-800 with shipping through Nordic Tuning (WIGWAM Saab is the distributor in the US). You can refer to my thread (http://www.saabcentral.com/forums/viewtopic.php?t=26169&highlight=ferrita) on it for more details. The BSR system can be found at GenuineSaab, and runs around $500 with shipping (or thereabouts). Both are bolt on solutions afaik.

The stock piping in our car is indeed 2.25".

HTH!

Cayman1
21-04-04, 02:07 PM
I would agree with the cat being the bottleneck of the exhaust system.

I was once told that the cat only works to reduce emissions when the engine is cold and that if you car is well up to running temp you can pass an emission test without one even in.

does that sound right? cause it doesn't to me.

ctrlz
21-04-04, 02:41 PM
I would agree with the cat being the bottleneck of the exhaust system.

I was once told that the cat only works to reduce emissions when the engine is cold and that if you car is well up to running temp you can pass an emission test without one even in.

does that sound right? cause it doesn't to me.
Uhh, no.

All the emissions components need to be heated up to work properly. When the car is cold, the ECM works in an open loop mode, running a rich mixture just to keep things running smoothly until the O2 sensor is pre-heated. There is a heating element in the sensor, so the ECM has a built-in time delay until it believes the reading from the O2 sensor.

The catalytic converter works whether cold or hot. It's just most efficient when hot.

I think you're pretty certain to fail an emissions test without a cat, unless your state has very generous allowances for emission levels. I wouldn't try.

I'm starting to wonder about the hi-flow cat idea. I've been reading that modern cats aren't nearly as restrictive as before, and that even with a hi-flow cat PLUS bigger exhaust one might only get a 5-7 HP increase.

Cayman1
21-04-04, 02:43 PM
yeah, i thought it was bull.

M4Gunner
21-04-04, 11:54 PM
I think you're pretty certain to fail an emissions test without a cat, unless your state has very generous allowances for emission levels. I wouldn't try.

I'm starting to wonder about the hi-flow cat idea. I've been reading that modern cats aren't nearly as restrictive as before, and that even with a hi-flow cat PLUS bigger exhaust one might only get a 5-7 HP increase.

i agree 100%, however, in NH we dont have emissions tests! :wink: (gotta love that!) also, the fact that the header back would add (im guessing) 7-15 hp, (in all reality, we dont know what itll make yet) AND make it sound groovy.. well sir.. THATS what ill do! i guess well find out sooner or later tho, right!? :wink: