PERFORMANCE!!! High flow cats/mufflers? Looking for testimonials/advice.

Hello all, I have a 98 GMC Z-71 5.7L, K&N intake, MSD Ignition + 8.5 wires, Hypertech low temp thermo, Superchips programmer, dual remote oil filters, and full synthetic fluids. Recently the crankshaft broke, so the engine is being rebuilt with a Comp Cam custom grind for the elevation here in colorado springs, and flat top pistons. In addtion they are also installing the Doug Thorley's Tri-Y headers that I bought right before it blew. I've tried researching everywhere but I cannot decide how to finish the exhaust system. My plan is the headers down to dual cats, to a Magnaflow x-pipe (I already bought) to single?/dual? muffler(s), all to running 2.5 inch pipses until the exhaust tips. To me sound is a byproduct of performance, and I have to keep it emmisions legal. It is hard to find info on which companies put out the best hp gains and reliability, especially for cats. I'm in the army and I want to do this right the first time, it's hard to buy experimental parts on a government paycheck. Any help is greatly appreciated.

We'll i too am looking for that same exact answer, i have HIGHFLOW mufflers on my 94 GMC SIERRA with the 5.7L and they sound great but im not sure im getting all the peformance out of them that i could. I have posted this same question at the GMC forums and i will keep you updated as too what i have heard from people atleast hopfully to give you a better perspective of what is the best thing out there. From what i know i think i might have to either go with a good set of Magnaflow's or CherryBomb Mufflers. But as to the performance out put of either muffler im still in search of that answer.

Cool thanks for the heads up, I to will keep you posted on what I hear. So far, I've heard good about Magnaflow cats, but also have heard that they don't last. The one shop I talked to said they had no luck with Magnaflow and they prefer Catco, but I can't find any info to support this. As for the mufflers, a guy told me they they all basically flow the same, but each manufacturer does minor changes for their unique sound. I personally think that the performance/torgue gains from every manufacturer has to differ somehow. Anyways goodluck on the hunt.

That is exactly what i thought, i mean if they didn't differ in some way why is it that the prices on some mufflers are so much higher then the other. And, if it was all because of the name why wouldn't everyone just buy the cheaper ones if they were all pretty identical. Im sure there has to be a noticable chance in sound and performance throughout each muffler that is out there , it really all depends on the quality that is put into the product. And, i have found out that FlowMaster are supposed to be one of the better sounding mufflers, and the Magnaflows are supposed to flow better. Also i do know that the Magnaflow mufflers have a more deep rouble where is the FlowMaster has a more crack type sound. I havn't heard anything from CherryBomb other then there alot like Magnaflow but that really doesn't help much. So today im goin to call a few of the shop within the area and see what they say about all this . I also will give Summit/Jegs a call to see what they have to say about this.

-Will keep you updated.

Just got off the phone with both Jegs/Summit and they both told me that
Flowmaster is a better muffler, it flows better because the baffals inside of the muffler actually speed up the air flow which should also increase hp/trq , but that also depends on the size of pipe u were to use with it, I also asked them about magnaflow/cherry bomb and both were said to be cheaply made mufflers designed to muffle the sound but also give you sume what of a deep rumble. So what i think im getting out of this is FlowMasters are the best mufflers that you can get? So really i guess im going to have to go with the flowmaster 40series, or the 50series depending on how much interior sound i would want but the fact is i guess flowmasters over all is the best muffler money could buy. Hopfully this could help you out, and if you find any additional information please inform me so i dont go out and spend money on sumthing that really doesn't do what it's said to do.

First of all, check with your local laws about changing the cat setup. It's illegal in some place to do anything but change your cat out for the same setup (single cat.) I've been warned many times (by inspectors and by a referee) about mine but since the truck changed before I moved to an area with inspections, they overlook it with a warning. Flowmasters are better for performance but not for sound. Borla, Magnaflow, and the like all sound better. Having cats to act as resonators helps tone down the droning sound from the Flowmasters. Without cats, it's very annoying and gets old for a daily driver. My 40 series Flowmasters are 12 years old and still going. The cats are Catco but I've replaced both of them in the 10 years or so they've been on it. One because of an ignition problem dumping raw gas into the exhuast then detonating one the cats. The other started rattling after about five years (before the detonation incident.) After I took it off I saw that the catalyst had separated from the housing and was able to shift around inside.

I live in Northern Michigan, and we dont have any type of inspections or anything just have to have your exhaust at a low db, even though no one around here really has it low , and really all you see and hear around here is annoying sounding exhausts , because people just buy glasspacks for like $20 and think it sounds "Cool" i can't stand that bs. :banghead: :banghead: but anyways enough with the venting, doesn't the cat restrict airflow ? because that's one reason why i took my cat off and just have true duals on my truck. And, i mean it sounds good and all but i want to actaully get some performance out of it not only sound , and i was told flowmasters were the best out there, but i always was told there sorta "RATTY" sounding so then i though about magnaflows and i was told that exactly what i had said above your post so i dont know what to think right now, i guess i just have to get more testimonials from other users of all or either muffler and figureout what's best for me and my pickup?

First of all, check with your local laws about changing the cat setup. It's illegal in some place to do anything but change your cat out for the same setup (single cat.)

I know some manufacturers sell higher flow cats still emissions legal for the vehicle rating and my truck came stock with two cats and four:shakehead O2 sensors.

Marsal420-Cats do create back pressure, which some is needed. If your exhaust is too free flowing, your powerband will move too high in the RPM range, which will result in loosing a substantial amount of power in the low RPM range.

Today I am going with my bud to a dyno shop to get his "Z06" tuned to elevation.(A-Hole found an '01 with performance/visual upgrades for 28 grand, what a steal!!) Anyways, I will ask them about what performance differences they have noticed with different cat/muffler setups and brand names. I'll repost as soon as I can. I've heard Magnaflow and Flowmaster are "the same company/made by the same people" it's just that Magnaflow is a "high class" Flowmaster, anyone hear anything like that before?

Also, I know people usually put the big dual inlet/outlet mufflers on these trucks, but I was thinking about installing two seperate mufflers offset like the cats are, which setup would you guys think have the better performance gains? Can mufflers be mounted verticle or at like a foretyfive degree angle if space is an issue? Just a thought.

Finally, the only company I've seen with O2 bosses are pre-drilled into the cats is Magnaflow. My headers have an O2 boss per side, which will leave me with the two rear O2 sensors just hangin out. My other buddy has a '99 Camaro, when he took his in for tuning, they were able to turn off the rear two O2 sensors because they really didn't do anything at all. I'm pretty sure my truck is setup the same way, but due to my different computer setup, I don't think they can tap into my computer like that(The shop uses the HP Tuners software which doesn't offer tuning for my truck). So, besides buying drill and welding O2 bosses into the aftermarket exhaust, the other option is 02 simulaters I found on Summit Racing for about eighty eight dollars. Opinions/Experiences about O2 sims?

Maybe I'm putting way too much thought into this whole thing:screwy:

Since today's engines are tuned for back pressure, removing most of it without retuning (possibly to a point of expense and time you won't put in) will loose a little performance so you can go too big on exhaust. I can't comment on the O2 sensors as mine is an '88 with a single O2 sensor in the header. An '88 is OBD I so loosing the back pressure was definitely beneficial but you're talking OBD II and the computer contols alot more and doesnt take to drastic changes like that as well. But as far as the muffler, if you have a crossover, it doesn't matter if you use one or two mufflers. I sure there is a little difference but nothing you'd notice in the seat-of-the-pants-ometer. As far as placement, closer to the headers is better for sound but not possible for most trucks. You can mount them however you have to under there, it doesn't matter on orientation. As for cats being restrictive, most definitely. For an '88 (got 12/16 MPG stock), it went to 16/21 MPG after going to headers, dual 2 1/4" with dual Flowmasters (no cats.) With high flow Catcos in place, it went to 14/18 MPG. Also, with only very mild mods (nothing internal), the truck was chirping the tires at the 1-2 shift. With the cats back, it can't anymore.

I can see what your saying about the cats actually dwelling the performance, but my biggest question is, on a 1994 Pickup like mine or how 67malibu just stated it does restrict the airflow and everywhere i look around here people drive with no cats what so ever and there doesn't seem to be a problem, so what im getting at is, Why bother getting aftermarket cats if there going to restrict your airflow, and give you less performance and less mpg? I just got off the phone with Midas and they told me that taking the cats off will give you the benifit of less restrictive airflow but also like you said does contribute to a minimal if any loss in your low rpm range. And, see i though the same thing that i would prob loose low RPM because of a straight threw flowing of exhaust rather then some type of pressure , but since iv gotten the exhaust put on i havn't really seen any loss in my low rpm range , so mayb it just varies from truck to truck? Or possibly if it's a newer truck that is more bassed on computer controlls rather then our truck that is morely based on our pickups where there based on more of a less restrictive less computerized setup, but which does have it's restrictions also .. I know this might not make much sence i just woke up so im trying to make the best of sence of all of it. But i guess we have to wait and see what you find out at that shop your going to, and hopfully we can get this thing straightened out .

Thanks for the help

Well YIPPIDEE-DOO!!!! All three guys at the shop said that basically all cats/mufflers are the same in performance/quality, along with a bunch of shrugs and "i don't know" expressions. Doesn't make any sense, the one guy who tunes on the dyno and is good at it, you would think would pay attention to different setups of mods to see the differrence in performance. I guess that's what I get for looking for information at a "mustang" shop. Too bad it's the best dyno shop in town. Don't get me wrong, I do have an appreciation for anything/everything fast and anyone with knowledge, but in my experience "ford" people generally aren't that bright, but there are exceptions to everything. They all kinda hinted around about not running cats, which leads me to my next point,

"WE WERE ALL RIGHT"

I was thinking way way way too much in depth about all this, and running no/gutted cats seems to be the best bet, and I was thinking to far in depth about all this. So my remaining info to hunt is how/if running no/gutted cats will effect my pre and post cat O2 sensors. I'll just have to rig something back up when I move back to Illinois since they still have emissions testing and Colorado just got rid of theirs.

As for the mufflers they all hinted towards flowmaster after they found out it was for a truck. All together I was not happy with the lack of firm answeres I got, and I will continue to hunt for more info. Wish I had more for you guys, I'm doing the best I can, and thank you for the info you are posting. I think by the end of this thread it will be a great source for alot of good information.

removal of your cats will do nothing to your O2 sensors before the cat. if you have 2 before and 2 after, the ones after the cat dont affect A/F mixture, they simply throw a code and tell you if your cat went bad. If you have one infront and one behind the cat, the one before isnt affected by no cat but the 2nd one is affected. it also contributes to the A/F ratio. So either weld it into the exhaust pre cat or get an )2 simulator for it. The muffler i was very happy with was the Warlock muffler made by Flowtech. I put a pair of them where the cats were (moved the mufflers forward). They have built in exhaust dumps for when you get moody and wanna take her out to the strip. Flowmaster's i have had decent results with. The one i put on a 6 cylinder was a 50 series 3 chamber, sounded awesome but didnt notice much of a performance increase.

flair666369- sounds like the day i had today, callled a few more shops and actually called a few places in *CHICAGO* since im originally from there and asked them down there and yet they gave me the same answer that u got... Wow i dont see how there can't be anyone that can give any of us a straight answer as too which one is the best overall for the money, and even so why is it so difficult for the die hard car enthusiasts *Sp to answer that type of a question. What it seems to me is that everyone is alittle un sure as too what performance you would be getting out of the mufflers as too more or less how they sound, and so then what im thinking is " this is stupid " lets go out each one of us buy a different muffler and really see what we get from each one ? Although knowing my luck i would go buy a muffler put it on my truck not like it and be stuck with a muffler that dont do anything for me so i guess we gotta go to plan *B* which ..... when i figure out i will let u know.

check my signature... love the sound and gained roughly 35 horses from it with nothing on the intake side.

It's late errr...early, but I've thought of a way to easily bolt in different mufflers and cats in different setups to dyno after each one. The only problem is the price issue. Still in debate, I'll get back to you after some price research.

After looking at prices, I have decided that it was too expensive to test out all or even most of the major name brands for performance gains. So the question still remains, WHO PUTS OUT THE MOST HP GAINS???

Lol, Were right back in the same boat, hmmm I know there has to be a way to get this answer out .......... Let me make a few calls and hopfully i can get some answers. Will post the information i get later on today.

So.. After spending most of my day in the house i got a few calls out and between today and yesterdsays here is exactly what i found out.

Every entry is based on each companys information. *HOPFULLY IT HELPS*

Jegs- All mufflers ( Flowmaster,Magnaflow,Dynamax,CherryBomb ) will give you the same performance but the sound that you would get from each muffler varies. They also told me that by getting Mandert bent exhaust pipes it will increse airflow because there is no change in the size of the pipeing.

Summit- They pretty much stated the exact same thing as jegs did.

Midas- Since they clearly sell Magnaflow muffles there all for it but the guy told me that if i want to actually see an increase get a exhaust kit. * manderat bent piping flows the best *

Cliff's Muffler Shop- Told me to go straight pipe , lmao! figure that was funny to add on here

That is all the places i called and got a answer that was actually worth putting on here .

Hopfully some of the information helps, but i think im gonna get myself a nice set of either Magnaflow or CherryBomb there both have a nice *RUMBLE* to them not a crackle type muffler like Flowmaster is. But i gotta wait and see if anything else developes from any of the informaiton. But im pretty much set on One of the 2.

Here's a wrench in the gears; check out www.borla.com they seem to be the only ones driven by performance just as much if not more than sound. Read the FAQ, there is a ton of great info. Only one problem, the price!!! But if it's as good as they claim, and from what I hear after asking around, you get what you pay for. I'm going to be calling them on monday to figure out the difference between the different universals since how they don't make a kit for our trucks. But i think i found the mufflers I'm going to get.

As I said before, Borla and Magnaflow will definitely sound better than Flowmaster. I can't say I've heard a set of Borlas on a truck before but I have heard the before (stock) and after on a Camaro SS and a Z06 vette. At least on high output engines, the Borlas sounded all out MEAN. They'll last longer than the vehicle but they are one of the most expensive options.

Yeah Borla is a very expensive option, considering just for a kit for my 2002 Impala it runs around $1,250. And, really im not sure if i wnat to go that route just yet, i might have to settle for Magnaflow's or even a nice set of CherryBombs, since they are more around my price range.

Yes, this thread is a bit old but I've seen little to nothing on Dynomax exhausts for the C/K. I've just installed a split dual cat-back on my '95 2wd with 350 and 3.73:1 rear and OEM headers and downpipes to the cat.

Compared to what it had before (it had the original exhaust but with a Quietflow3 muffler welded in), idle sound is deeper but not much louder in-cabin with a nice, low burble. Light acceleration doesn't increase loudness much but go more than 1/3 to 1/2 throttle and it emits a low-pitch rumble that is defintely louder. Highway speeds is a bit louder than before, but as before if you keep it cruising and stay out of the throttle, the noise level is good. Not bwappy at all. Stays pretty quiet at 65 mph & 2000 rpms. Again, the low note is still there but doesn't overpower all other noises such as tires, radio, wind, etc...real good for those who go long trips.

Can't tell if there's much performance improvement...I don't drive the truck enough to really tell, but it feels a little more responsive from a stop. As I work it, I'll try to update this.