Just built a 383 TPI. 10:1 compression, comp 268 xfi cam, ARF 195 heads, comp pro magnum 1.6 rockers, edelbrock base, Arizona spped and marine runners, ported plenum.
I plan to use Dyno Don headers, but can't decide between 1 or 2 cat converters. Common sense says 2 will flow better and when GM adds 2 on the stock l98 I think it picks up 10 hp.
However, after the converters it goes into one 2.5" pipe, into a flowmaster muffler, and to stock dual tailpipes. Can dual converters really help given the rest of the exhaust system? Anybody ever checked/verified?
Ive read through some threads but cannt seem to find a good answer.
My thought is that one converter will flow as much as the rest of the exhaust system if capable of; but I'm just not sure. "

 

Cat converters are bottlenecks. Having two of them will allow a more free flowing exhaust, even when they go back into a single pipe.

That being said, I'm converting my dual cat setup to a true dual exhaust by having my muffler guy run 2x pipes from the cats to 2x independent mufflers in the back.

 

"However, after the converters it goes into one 2.5" pipe"

It should go into a minimum 3" pipe to the rear.

 

is there any downside (besides the extra cost) to dual cats? also, any idea if I need a different size heat shield? (I have looked but cannot find any info on the heat shield)

 

I never changed the heat shield, the one there seems to be fine.

Remember a 383 flows more air so a dual system is a must. IMO

 

makes sense. That's what I'll do.

 

the stock setup goes into a single 2.75" pipe that necks down to 2.5" on the inlet of the muffler (loosing a 1/4" that far back in the exhaust won't make that big a difference). Nothing came stock with a 3" pipe...

 

ok. Then am I ok to leave the exhaust alone from the intermediate pipe back? I heard the flowmaster robs some power, but...it's already there and in good shape....

 

Have a look at the Firehawk exhaust setup. This should help find a starting point in exhaust design.

 

that is a great idea!
I just looked and can't find any pics on here; any suggestions on where else to look? (I googled too and all I get are tail pipe pics)

 

If I run into info, I'll get back. Personally, I would avoid a dual exhaust on these cars due to the way the under body is designed. Seems like I've read 3" pipe is plenty big to handle up to X horsepower, which is a bunch. The Firehawk used headers, had N10 option, 3" cat back and probably other goodies. If you want to get really fancy, you could have a computer open up your muffler when pedal is detected to be on floor so that the exhaust just spills out the 3" pipe. If you're racing the 1/4 mile, it only needs to be open about 13 seconds or less

http://www.hotrod.com/cars/featured/...tiac-firehawk/

 

Oh, here's a good one, the girl if nothing else!

http://thethrottle.com/2014/02/05/on...awk-31-photos/

 

damn. that's nice! (oh, and the cars were ok too....)

 

fer sur!

 

The chart I saw said 3" is good up to about 360HP. I don't really know if that's WHP or FWHP though. That's way more than any near stock motor will put out either way.

 

I'm thinking of adding a factory style dual cat to my 91 Z28.
The unknown for me is if the dual cat setup uses a different heat shield on the car body side. My single cat shield looks kinda small and wouldn't cover the 2nd cat.
Does anyone have a pic or info about the shield on a factory N10 car?

 

Don't waste your effort.

A FACTORY dual cat setup is better than a FACTORY single cat one. Doesn't say a damn thing about AFTERMARKET ones.

If your car has the single cat now, just get the best single cat you conveniently can, and the best cat-back. It'll be VASTLY superior to the FACTORY dual cat stuff.

 

The dual cat cars have a heat shield roughly the size of Texas. I'm not sure it'd matter with aftermarket cats, they should have built on heat shields. The single heat shield doesn't do much good when the stock single cat fails and glows red hot, even with the shield the carpet melts to the floor.

Generally speaking, I'd personally run the dual cat setup if you have it available to you. The OE cats won't be much good after 25 years. The stock setup also has a less than ideal Y-pipe after the cats. But I still think the benefit is worth working around those problems. To most people it probably wont matter much either way.