what is the best cat setup for an engine producing over 450hp? is it single 3" hi-flow or dual 2 1/2"? i currently have a dual cat setup and 3" catback but am considering going single cat. anybody ever tried a single 3 1/2" cat? does anybody have access to flow test numbers on various aftermarket cats? i've heard good things about carsound from magnaflow. tell me what you know.

 

3" of total flow or 5" of total flow. Which do you think would perform better?

 

well if you want to answer your own question, look up the flow bench test numbers. there are several 3" cats that easily outflow dual stock cats. im looking for the most practical way to get a great flowing cat setup. im sure somebody out there has researched this one...

 

I don't see how one brands cat at 3" will outflow 2x 2 1/2" cats of that same brand. It comes down to how well the catalyst block will flow.

 

You can't go with a single cat. You have to use what your car came with from the factory. Plain and simple. Why? Because Uncle Sam says so.

Seriously though, I would imagine it has something to do with the way the ECM is set up compared to the number of catalysts in order for the catalysts to do their job right. They have to operate at a certain temperature in order to work correctly.

Even if that's not true, the government thinks it is, and that's why you have to keep the factory set up.

 

in the event the cat system is damaged or fails, uncle sam allows an equivalent replacement part. now i doubt an inspection station is likely to research whether your particular 89 f-body came with one cat or two as long as it has a functioning ststem with all the emmissions equipment intact.

 

Doubt what you want, but it'd be terrible if you were wrong.

 

Your bliss is possible because you're about as far away from Kalifornia as you can get and still be called "US".

I was pretty nervous when I took mine in the first time after the extensive mods. The previous two inspections, the VIN said "2.8l" and under the hood was an LG4. They didn't seem to care, just checked off the equipment list and pulled it onto the rollers. This last time, though, with headers, a pretty healthy growl, etc., they spent as much time reading the underhood stickers (which went with the LG4) as they did checking off the equipment list (which still said "2.8l"). After all, LG4's never came with 3" cats. They didn't say anything, though, finally pulled it onto the rollers and after their computer gave it a "fast pass" because of the low readings, and said, "Good job," as they handed me the emissions certificate.

However...

Herr Inspektor in the Peoples Republik of Kalifornia has been known to check block casting numbers at inspection time. You pull up to the check point with a 450 HP engine, you're: 1) Probably already in inviolation of at least one of Uncle Sam's rules, let alone Kalifornia's, and 2) probably in for an intensive interrogation by said Inspektor.

 

yeah i suppose im spoiled for not being from cali. we dont even have emmissions testing here. so i guess the answer totally depends what state your in.

 

Yeah, it's illegal to replace a 2 cat sys with a 1 cat sys. But I did it anyways. I even got lower emissions numbers with the 1 cat over the 2 cats. That was with just my exhaust and the catco cat. Then I did the rest of my mods and switched to a Random cat and passed with flying colors again.

 

im thinking about fabricating a custom y pipe and running dual 3" cats. that would eliminate any restriction from the cats.

 

Somebody needs to go back to math class.... 1 3" cat has a flow area of 7.065 sqin whereas 2 2.5" cats has a flow area of 9.813 sqin. All other things equal, the duals would flow better. But why have dual cats if you are just running the pipes together into a single exhaust after them... you are just moving the bottleneck to another part of the system? A true dual system will give you the best flow results. Just remember that the exhaust is only part of a system in which all of the components need to work together to produce the results that you want.
Somebody would have to pay me a whole lot to even think about living in the Republik of Kalifornia.

 

wake up. that il-logic is flawed. you're trying to say that cat flow capability is based on pipe diameter alone. tell me that a stock cat will flow the same as an FLP or Carsound cat of the same diameter. wrong, not even close. and why use a dual cat setup for a single exhaust? well if that single exhaust pipe is 4", i'd say it would flow pretty well in comparison. it all depends on your hp goals and every component should be balanced to those goals. in some instances, a 3" single hi-flow cat will be fine and quite possibly better than stock duals, but obviously a dual hi-flow setup is optimal.

 

Note that I did say, ' all other things equal.'
And that part of the post was more aimed at the person that was comparing 3" to 5" (as in 2x2.5).
I do know you can have a single, or dual into single, that will outperform a dual system, especially if everything is matched correctly.
Your original post did say dual 2.5 into single 3" catback

 

sorry, i guess i misunderstood your post. anyway, after doing some calculating it appears that dual 3" hi flow cats may flow 1200cfm or better. thats pretty damn good. wondering if anybody has done this and passed inspection. btw, i also plan on ditching the 3" catback for a 3.5" in the very near futire.