High Flow Cat - Looking for advice

E28 technical advice asked and given! Troubleshooting, modifications and more.
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ahab
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High Flow Cat - Looking for advice

Post by ahab »

I finally pulled the trigger and bought a Supersprint exhaust. I've been looking for a dual in dual out high flow catalytic converter to sandwich between the header and the resonator. I'd like to stay dual pipe all the way back and so far the Magnaflow 94008 looks to be a good choice. The spec says 2" inlets/outlets but I can't find any info on the distance between the centerlines. The resonator inlet CLs are 3" apart and I think that's going to be close enough. I plan on having a muffler shop fab some short pipes to connect it up to the header and resonator so I have some slop room. Anyone here have any experience with this type of setup? Is there a different cat I should consider?

TIA,
Aaron
LA
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cat

Post by LA »

I have a high flow cat! It was damaged in an accident and the ceramic was cracked & rattled. A few stabs with a large screwdriver and shake it around...hey,hey,hey - a high flow cat. Sounds better too. 8)

BTW - your factory cat will bring $200 or so on fleabay if it't the 'three buscuit' (square holes in the ceramic in both ends). It takes some creative writing to prevent it being pulled by the legal eagles.
rlomba8204
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Post by rlomba8204 »

Aaron-

I understand Magnaflow is the best of the aftermarket cats, so it sounds like a reasonable plan to me. The only better high flow option, as I understand it, are the factory units and given how expensive they are, they should flow pretty well.

Good luck, and let us know how it goes. This question comes up a lot, so photos of how it is done would be helpful for the archives too if you have a chance for those who are looking for ideas on how to set up a system that doesn't cost silly money.

LA -- just so you know, in general gutting a cat doesn't add that much, if any, power, by itself. The cars, especially the newer engines, are designed with a certain amount of backpressure in mind so, while I know you're joking around, that's fine and all, but it's not accurate, in general, to think that deleting a cat will give you a power advantage vis a vis a functioning factory cat, which is pretty high flow. It might give you a power advantage verus an overly restrictive, too tiny aftermarket unit, sure. I won't even go into the impracticalities / illegality associated with trying to get the car to pass emissions testing in some of the more restrictive jurisdictions . . . .

Ray
ismellfish2
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Post by ismellfish2 »

rlomba8204 wrote:The cars, especially the newer engines, are designed with a certain amount of backpressure in mind
I'm pretty sure this is a common misconception. I don't see how resistive force applied in the form of backpressure could possibly help the engine's performance. I'm sure running sans cat doesn't get you 50 horses, but it should get you at least a little more power.
LA
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BOTH

Post by LA »

ismellfish2 wrote:
rlomba8204 wrote:The cars, especially the newer engines, are designed with a certain amount of backpressure in mind
I'm pretty sure this is a common misconception. I don't see how resistive force applied in the form of backpressure could possibly help the engine's performance. I'm sure running sans cat doesn't get you 50 horses, but it should get you at least a little more power.
And the answer is...You are both right! For any reasonably balanced baseline setup, an increase in backpressure will improve low end performance a bit at the the expense of high RPM HP. Conversely, for the same baseline, a decrease in backpressure will bump, slightly, HP at high RPM but low end (dare I say torque?) will be compromised. This will probably be another long thread similar to the flywheel weight debate...oh well, bring it on. :blah:
rlomba8204
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Post by rlomba8204 »

I know what you mean, ismellfish, it doesn't seem to make sense. But here's why it does. I had this explained to me by my old BMW tech, so if I garble it, it's on me. Also bear in mind it was in the context of a discussion about my "newer" car, so to the extent that an M30 is a less electronically-refined motor, in terms of engine management, it might be less of an issue.

Basically, the cat creates some back-pressure due to what it does -- the exhaust has to flow throw a platinum infused ceramic honeycomb. This has the effect of "straightening" or removing turbulence from the exhaust, and reduces noise. The center resonator and rear muffler create some backpressure too.

Some back-pressure is needed to aid exhaust gas recirculation, which helps clean up the emissions. To accomplish this goal, the engine's valves are timed so that the engine "inhales" some of the exhaust gases to mix with the fresh fuel air mixture during the intake stroke of the motor. The DME is set-up to expect this slightly lower intake pressure and is programmed to calculate the mixture to compensate for it. At idle, because of the lower rate of exhaust flow, there is very little exhaust gas recirculation. But, as engine speed increases, and we know that BMW engines like to rev, so does exhaust back-pressure. Given these pre-set parameters of backpressure that the engineers figure into the equation, removing that back-pressure usually provides no engine performance advantage because the computer is not optimized for it. All of this only becomes more true when you add in things like VANOS, Valvetronic, etc.

This goes back to, at the end of the day, a conversation that has come up here before: which is that a single, stand-alone engine (I am using the term "engine" to refer to everything from the intake to the tailpipes) modification is not likely to produce all that much power, if any, by itself. It has to be done as part of a well-engineered plan. Without a comprehensive approach, it is more likely than not that you are simply moving the peak power band around, but not increasing it in the aggregate. Sometimes, you might even be hurting it. And alot of the time, the unscientific, butt-dyno "it feels faster" argument is more often than not the result of someone removing a clogged cat and gutting it, in my experience. I've rarely see anyone verify these claimed power increases, fwiw of a gutted / no-cat car versus a car with an OE cat, which is pretty large.

Hope this helps. I've read about alot this area because I've often been tempted to undertake modifications, but I've come to the conclusion that without going the whole way, which is very expensive and time consuming, it is usually not worth the trouble.
rlomba8204
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Re: BOTH

Post by rlomba8204 »

LA wrote:
ismellfish2 wrote:
rlomba8204 wrote:The cars, especially the newer engines, are designed with a certain amount of backpressure in mind
I'm pretty sure this is a common misconception. I don't see how resistive force applied in the form of backpressure could possibly help the engine's performance. I'm sure running sans cat doesn't get you 50 horses, but it should get you at least a little more power.
And the answer is...You are both right! For any reasonably balanced baseline setup, an increase in backpressure will improve low end performance a bit at the the expense of high RPM HP. Conversely, for the same baseline, a decrease in backpressure will bump, slightly, HP at high RPM but low end (dare I say torque?) will be compromised. This will probably be another long thread similar to the flywheel weight debate...oh well, bring it on. :blah:
LA-
This is exactly what I meant, in the sense that you are moving the powerband around but not increasing power in the aggregate. I am not saying that's bad or good, but rather just trying to correct a common misconception that gutting a cat is a performance enhancement usually. It isn't if your car has a well functioning, properly flowing cat. That's all.

And of course, again, this puts to the side questions of social responsibility, legality, practicality, and the like. But an interesting thread nonetheless.
Take care.
LA
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cat

Post by LA »

rlomba8204 wrote:'...more likely than not that you are simply moving the peak power band around....'

Bingo! :up: Actually I like the way you presented that better than the way I stated it! :rockon: Great explaination; your BMW tech mentor should be proud.

Help bottom + hurt top -or- hurt bottom + help top = moving the peak power band around. ;)

:beer:
LA

BTW - I gutted my cat to stop a rattle, not in expectation of a noticeable performance difference.
LA
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single vs. dual

Post by LA »

Hey Aaron,

I hesitate to start this debate on the board...but I encourage you to talk w/ Jim at Metric Mechanic regarding his sys 300 (which I use) that has only one slightly larger pipe from the cat back. It has to do with the timing of exhaust pulses and the scavenging effect at the exhaust ports for a continuous downstream flow - vs. – pulsed downstream flow. I wish I had dyno results and don't know if he does but I honestly think it makes sense. You can always go back to twin outlets coming out of the muffler in the very back just for the 'look' if that is important to you.

Think about the highly tuned 'bag of snakes' exhaust systems on formula racers. They usually reduce down to a single outlet! The idea is to not waste energy slowing down then speeding back up the exhaust gasses on the way through the exhaust system.

Ahab, beats rehab!
LA
ahab
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Post by ahab »

LA

I considered the MM system but truthfully was put off by the price. I'm not at all against the single pipe design either, I always take function over form, and I even like a black can better (!). I'm hip to the pulsed exhaust and the scavaging benefits too, the long tube Stahl header does exactly that. The flow work and Schrick should help get the air though the head and I'm figuring the header adds to the improvement but I have an obvious concern about keeping it moving all the way down the line. I was also considering an individual cat setup like Rich did in this post (looks really nice, is she on yet Rich?) to help maintain the exhaust flow timing. While the Supersprint resonator isn't exactly 100% isolated due to the perforations in the internal tubing, it is straight through piping in there. I think the rear can is essentially the same construction so most of the flow integrity should be intact if I went that route. I don't quite understand Jim's theory of maintaining the pulsed stream by condensing the dual inlet cat into a single outlet. If that's true then I suppose the Magnaflow 2in/2out should be as effective, if not more?? Part of my beef with buying a new direct fit cat (like the S300), is having to cut 2+ feet off the inlet pipes of a brand new pipe to fit up to the header. This is also mostly why I didn't take you up on your old one at 5er Fest. (Thanks again for the feeler gauge btw). Anyway, I'm still sussing this one out (sans before/after dyno charts :oops:) but I think it's going to be a Magnaflow and I will take some photos of the installation for the archives. Maybe when all is said and done I'll actually get it up on the dyno and find out what I've accomplished, ...beyond hemorrage money.
Bingo! :up: Actually I like the way you presented that better than the way I stated it! :rockon: Great explaination; your BMW tech mentor should be proud.
X2, Spinedocab and I have debated this a time or two and I think it finally makes some sense.

Thanks for the comments,
Aaron
Jeremy
Beamter
Beamter
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Post by Jeremy »

I'm not going to argue with LA (this time!), but will say this. Exhaust sizing is a tricky business. Too small and you get hurt on top without gaining enough down low to really justify. The reverse happens if you go too big. You lose a lot down low but gain very little up top.

The MM 300 is very good and very expensive. It can be replicated for MUCH less than Metric Mechanic sells theirs for. I've detailed it before, but I'd be willing to go through it again if anyone is interested.

My 535i with MM300 exhaust made 162.4 rwhp (191 bph) with 233k miles on the original engine. My only complaint was that it was a bit raspy for my tastes.

Jeremy
LA
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J

Post by LA »

Jeremy wrote:I'm not going to argue with LA (this time!)...
.

Quick! Could someone in the Southington CT area run over & check on Jeremy? Something is very, very wrong here!
Jeremy wrote:The MM 300 is very good and very expensive. It can be replicated for MUCH less than Metric Mechanic sells theirs for. I've detailed it before, but I'd be willing to go through it again if anyone is interested.
Aaron - that's what I was suggesting; it uses a 524td rear muffler I believe and no resonator at all so replicating at any exhaust shop should be straight forward.
Jeremy wrote:My 535i with MM300 exhaust made 162.4 rwhp (191 bph) with 233k miles on the original engine.
Jeremy
.

Jeremy - What, if any, other mods did that car have? I'm not sure how literal to interpret 'original engine.' Those are really good numbers if the sys 300 was the only mod!

Oh yes...any real 'car guy' will love the 'raspy' sound of the MM system 300 design. I'm sure it's even better behind Stahl headers.
ahab
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Re: High Flow Cat - Looking for advice

Post by ahab »

If anyone is interested, I just purchased a replacement Magnaflow 94008 to the one I ended up buying after discussing in this thread. The Supersprint is still going strong but certainly starting to show its age. I turned 300k in January of 2007 and I'm at 498k now so I guess they exhaust has done a good 180-ish k in the last 16 years. That seems low but I've had so many other cars between now and then that I guess it works.

Anyway, $90 in 2007 and $119 in 2023. I hate exhaust work.
Mike W.
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Re: High Flow Cat - Looking for advice

Post by Mike W. »

180 on a aftermarket cat is very impressive, do you have to pass smog there? And 500K just on the chassis back in the rust belt is nothing short of amazing. Original engine? Tranny?
ahab
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Re: High Flow Cat - Looking for advice

Post by ahab »

PA has an exemption for cars over 25 years old, or driven less than 5k per year. I only need to pass a visual at this point. I never had any problems with emissions whenever it was tested but it's likely no longer doing much beyond the visual. Which it's barely doing. Pretty certain the only things original to the car are the rear wheel bearings and the steering box.
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