I know theres been a lot of talk about stock or High Flow cats lately but no one seemed to anwser it directly.

I have a stock manifold but a dynomax cat back and i need a new Catalitic converter.


THE BIG QUESTION: Do I get a high flow Cat or a stock one. WHich will be better performance wise??????????

 

None - put a pipe in its place.

 

BUT for the emissions-concerned..... a high-flow cat outflows a stock cat, even brand new. Hence its name..high flow. And the stock cat probably isn't much cheaper than the high flow..and if it is, it's probably a piece of crap. High flow cats aren't priced up too much I think cause they know people with truly high performance cars don't use them... I recommend a Catco high flow..it's what I have, not priced too bad, works great.

 

that's what I did.

 

Regular...the flow numbers are VERY close between high flow and normal cats, but everyone goes for High Flow just cuz it says High Flow.

 

I just ask because there was a discussion on whether a stock replacement one might be better for a v-6 becauuse our cars "need backpressure" for power.

So is the verdict a high flow one even though Our cars "need backpressure"?????

 

straight pipe- lose bottom end, gain top end




cat- keep bottom end, loose top end, better for the street- seat of the pants feeling

its just physics related to the laws of air flow

more air you can flow, the more effecient your engine will be
efficient engines have their power placed higher up the RPM power-range

and without doing ALOT of work to these V6 motors, they arent too efficient

i would keep the cat

 

No car "Needs" backpressure. Low end will run slightly better with a little backpressure for like a 4x4 in low going up steep hills but will hurt topend.Its only helps real low end torque.

A high flow cat will flow better above aprox 4000 rpm's on a 2.8 V6 than will the stck unit. They will both flow about the same under those rpm's.

 

Then tell me why I have seen many a vavle burned out because there was nothing holding the pressure on the seal?
I have also seen cars WITH converters run 11.297 EXACT SAME CAR W/o Converter 13.114. Do not tell me that modern cars do not NEED backpressure.

 

Heavier valve springs will solve burnt valves, not backpressure.

Your car you claim runs 11's w/ cat and 13's w/out cat is said to be seen? If this is so then the only explaination for this is the guy has too large of primary header tube and the installation of a cat will trap or slow up exhaust flow from the primaries to the collectors and cat and keep the exhaust gases hotter so they will flow out the tailend better. I still don't buy your scenerio though- its hearsay. If he goes down to the proper size primaries and remove the cat he will get the same or better results.

 

i have the same issue here as 3.1...i just put the dynomax catback on and need a new cat...i can get a stock for about 65...and then a magnaflow stainless steel for 120...wouldn't it be better to go with the stainless steel one for longer lasting...or is it me...just wondering...

 

running 420 CI W/ 1 5/8 primaries. 2 runs back to back. This is also a street driven car. He lost both MPH and overall time with cat removed.

 

I think some of you may be thinking the right things, but attributing it to the wrong things.

(1) No car needs backpressure in the exhaust. All backpressure can do is rob the engine of power because it requires more pumping work to get rid of the exhaust gases. "Well, why not just get the biggest pipe that will possibly fit, if that is the case?" you ask? Well, it is because backpressure, or lack thereof, is not the only phenomenon happening. What gives you a peak in your powerband is when you have the best and most efficient flow velocity into and out of your engine, so that the engine can get a good scavenging effect, evacuate the cylinders properly, and give them the fullest fresh charge of air and fuel with the smallest pumping and friction losses. If you have a super huge exhaust, then you are going to have so little flow speed at the lower RPMs, that you won't be clearing out the cylinders well, or getting any scavenging effects. This means that you will loose a bunch of power down low. The converse is also true that a smaller exhaust system will give you higher exhaust velocity at lower RPMs so that you have good torque. At the higher RPM's, you will build up BACKPRESSURE which will increase the pumping losses in your engine and cost you horsepower. That is why the exhaust must be treated as a system with your goals in mind, not as just something that is stuck on there. Compromises will almost always have to be made to get a system that is an all around good performer.

A recent magazine article I read said that at WOT at high RPM, a good flowing exhaust should be between 3 PSI and 0 PSI in exhaust backpressure. Anything above 6 PSI is considered to be restrictive, and HP can be gained by a different exhaust.

(2) A high flow cat is kind of a misnomer. It will flow better than a stock one, but how much better is the question? 10%? 25%? I don't think that having a high flow cat will hurt you any unless other parts of your exhaust system are way oversized. I have a high flow cat and a 3 inch catback on my 1992 3.1 liter, which most people say is way too large. I would tend to agree, if I wasn't using Nitrous. I did notice some loss of low end torque because of them, but it wasn't that dramatic. It helps me most when I use the nitrous because I don't build up backpressure.

I personally would always go with the aftermarket 'high flow' using the stock size inlet and outlet. The main reason being economic. Yes, you could see some minimal performance gains, but the main thing I have found is that a stock converter will cost you about 2 times as much as an aftermarket 'high flow' type. That's what I would recommend. Sorry for writing a book.

 

no problem with the book, it was full of info, not just crap. I knew that too big an exhaust was to much and wouldn't help the car and that too little would deffinitly hurt performance, but not exactly why the exhaust system changed the torque. Good stuff.

 

Thanks 2_point8_boy. I'm just trying to pass on some of the stuff that I have had the great fortune to have either been shown or have experienced.

I forgot to say that a valve will not burn because it doesn't have backpressure. It burns because it overheats when you have a bad exhaust leak. The high exhaust velocity also carries most of the heat away from the engine with it. When you have a leak right at the manifold, it decreases your velocity and allows those hot gases to be exposed to more area of the head, and thus more heat is absorbed by the head. This will cause localized overheating and the valve's edge will begin to melt. This is what is usually referred to as a 'burned' valve. It is the leak that makes the valve burn, not the lack of backpressure on the engine.

As far as the car that ran slower with the catalytic converter removed... Exhaust changes can affect other things. Maybe a better flowing exhaust caused his car to run lean. Maybe it threw the O2 sensor off, or something else got accidentally changed. It may be that in this particular case, the converter actually did give better flow velocity at the powerband he was running. There are lots of reasons why that could have happened. I don't try to flame, just discuss. I hope I haven't pissed you off.

 

A high flow cat will flow much better than a NEW factory cat from an old stockpile of parts from the car's birth year. (EX, finding a cat from 1986 that's in a warehouse.) Back in the 82-92 years, cats were restrictive.

A high flow cat will flow much better than an old factory cat that's been on the car since the car was made. Chances are that the old cat has broken up inside, and is blocking the exhaust flow. Even if the factory original cat isn't broken up, it's still restrictive because it's an old design.

A high flow cat will NOT flow much better than a brand new 2003 GM factory cat!! You'll also pay about $300 for the brand new factory cat. The new factory cat will also reduce emissions more than a high-flow cat, which might be a selling point if you live somewhere strict like CA or NJ. As GM got smarter, they realized a fundamental hot rod fact- the easier an engine works (breathing IN (good intakes) and OUT (headers, catalytic convertors), the lower the emissions.

A high flow cat is usually cheaper than a parts store no-name cat. (Major reason why I'm running a high-flow convertor!!!)

A high flow cat usually bolts right in, unlike a parts store no-name cat, which either has to be welded, or the $20 "bolt-in kit" has to be bought.

 

I just busted mine out gotta love non-emissions countys here in missouri. it did seem to improve milage but at the same time non-braked burnouts seemed to become exceedingly more difficult

100 posts cool took me long enough

 

A high flow cat will flow slightly better than a regular one, but not nearly enough to even notice or measure.

A straight pipe would be best performance wise.


I went with a high flow cat because it was only $20 more and a 1/4" larger than stock. I like to keep my car emmissions friendly. I care more about my car burning cleanly and efficiently than having an extra bhp.

I might be moving to a large city and I would like to be able to pass an emmissions test so I don't have to spend $600 to put something back on my car that I took off in the first place.

 

I went around and around with my muffler guys about this. In the end I got a cat with a 2.5 in and out. Our cars are suppose to have 2 & 1/4 if I remember correctly. So its not really high flow, but if is...Anyway they said this would be the best set up...they said for a cat to be functional it has to filter said elements. To filter them all cats must have the same grids to flow thru, so how can one be high flow? They all flow the same based on the grids they go thru. So a bigger cat gives the air a larger cavity to flow thru, ergo giving you some flow increase. But you can go to large. They rec only one step up in size.

For what its worth the car sounds better then any other 2.8 or 3.1 I have ever heard and people tell me the same at shows all the time. Now I did notice that I had some of the Gurggling (sp) in the exhaust when I would be driving say 45 and then I get off the gas and coast to a red light. The car would gurggle when I got off the gas. Like the exhaust was just a touch too big. Then I got the new cam and the gurggle is gone. The exhaust is now perfectly matched.

So take all that, for what it is worth, I hope it helps!
Shannon