mine is a 305 TBI I have heard that running dual cats in place of a single will increase HP is this true. Will a dual cat system improve my chances of passing a smog test lessen them?? Is installing a dual cat system on a 305 TBI a waste of time? My car is a 91' Camaro is it legal to swap a single for a dual?

Mark

 

Technically, to be legal, it must have been offered on the car with the engine/equipment you have, or be a CARB-certified replacement. Since dual cats were never offered on LO3's, and nobody offers a "certified" dual cat replacement for the LO3, technically, it isn't legal. But, I doubt you'd get pinged for having them.

As to being worth it, the factory system is a good place to look for improvement, but a single 3" cat should be more than adequate for all but the most wildly modified TBI engines. Same legal technicality as above.

 

The price should be illegal.
GM charges over $800 for a dual cat setup for 3rdgens.

 

my dad had dual cc's on his 85 Z, it sounded sweet...

 

I'm just wondering but... how is having dual cats better than having a single cat? I'd think that it would be more restrictive...

 

For the same reason why dual exhaust is better (more or less) than single exhaust. Four cylinders flowing thru 2.25" pipe is less restrictive than eight cylinders flowing thru 2.75" pipe. Plus each cat only has to clean up half as many cylinders too. But as was said above, you can only switch on TPI engines, as carb/tbi was never offered with dual cats. I remember Tom Keliher getting hassled by a clueless smog tech, when the tech's book said dual cats didnt come on Tom's 89 Formula 350. The book was wrong.

 

Ok, I'm not sure if this is how it is but, isn't it one cat right after another? Or does the exhaust do some kind of y into 2 cats, then y back down to one pipe again? Can u post a pic or sumthing? I've never really understood this dual cat thing, and I'm ready to learn so...

 

I don't know where U R from, but in CT it is illegal to remove ANY working cat from a vehicle. I guess the only way you can get away with it legaly in CT is if you were to fail emissions with the present one.

 

Ok, people always take 'remove' so literally. To me it means 'Remove and never put one back on'. Example: Say the cat goes bad, you have to remove the bad cat to put a new one on right??? So is it illegal to remove the cat in that case? More than likely not.

 

Stock exhaust head pipe has one catalytic converter per bank of cylinders. Immediately after the catalytic converter, it merges into one 2.5" pipe back to muffler.

 

I just found a pic of a dualcat setup, so can I put the dual cat setup on a '87 Z28 305 TPI??? This has sparked my interest. It looks like it makes the 3rd gens that much closer to dual exhaust Would I need a new Y pipe or nething? Problem is, if I got the dual cat setup, I wasted 300$ on the flowmaster catback I have waiting to go on... What do you guys think I should do, and would it fit on my car?

 

My website isnt working anymore so I cant post my pics, but its not one cat after another. Its the left side of four cylinders going into one cat, and the right side of four cylinders going into one cat. Then after the two cats, they join back together into one pipe going to the muffler. The size of the pipes depends on the engine. Its either 2.25" y-pipe and 2.5" i-pipe for regular engines, and 2.5" y-pipe and 2.75" i-pipe for the higher performance engines. If you want me to send you my pics, email me.

 

I'm thinking about doing this too, only because I may get a good deal on a set of dual cat headers. My question is how do you route the A.I.R. tubes? Right now there's just one going from the cat. into the engine bay, but how do you incorporate that second tube from the second cat. into the whole system? I also thought I heard once before that you need a 1LE heat shield for the second cat or something. Does anyone know about this?

Has anyone actually performed this change before?

 

Hmmm....I have dual cat 305, and it has a STOCK 3" intermediate pipe...

 

Just to comment on the remarks about price and AIR tubes, if i were to do dual cats, i would put on dual cat headers/y-pipe, then use random technology cats(which flow as well as modern day cats found on brand new cars, that have no restriction and don't require AIR), then through the catbaack of my choosing. I would think that just switching to a random tech single cat would be more beneficial than putting all of the money into dual crap cats. Random techs may cost more, but the $230-250 for one 3" is cheaper than converting to dual cats.

 

There is a junction block that splits into two pipes that feed the dual cats. I dont have pics of it tho...

It might look like 3", but its not. Its 2.75" diameter.

Why spend $230 on a new RT cat when you can spend $100 on a new Car Sound cat, and get one that is PROVEN to flow the most, as tested on a flow bench? Modern day stock cats are not all that great, in fact, most suck on purpose. The thing about having dual cats is as I said in my first post on this thread. Also the dual cat y-pipe is a better shape and design than the single cat y-pipe. The joint is horrible where they come together, its almost a right angle.

 

GMs conversion kit for the carbed 3rd gens includes dual cats and carries smog exemtion numbers for all fifty states. so they should be legal as long as you have the automatic trans. Same with 350 and carb

 

And what that would have to do with a TBI LO3 would be...

 

When I did my exhaust I went to true dual 2.5" exhaust. I had to take my inspection sticker/city sticker/base sticker off (carefully) and had AAA tow it to the exhaust shop. I told them it was a wierd SCCA class that demanded 2.5" max exhaust with cats. They did it for me. Some more details and pics are in this link towards the bottom.

https://www.thirdgen.org/techbb2/sho...threadid=73181