I called MF technical support. Magnaflow does NOT offer high flow cats. They are a standard CAT replacement company.
I was surprised....

 

Meow

 

Magnaflow does offer a high flow replacement N10 dual cat Y-pipe for our cars. It is 409 SS with cat bodies made of 304 SS and the pipes are 2.5" mandrel bent with 2.5" inlet and outlet cats and a beautiful Y merge after the dual cats into a 3.0" outlet. To go with this N10 system they offer a 409 SS cat back exhaust system with a 3.0" intermediate pipe into a 3.0" inlet 304 SS straight through flow high muffler with dual 2.5" outlets into 2.5" 409 SS tailpipes with 3.0" polished 304 SS tips that are ugly as sin in my opinion.

The factory OEM GM "AC" N10 dual cat system is 2.25" pipes with 2.25" cats and a horrible Y merge into a 2.75" outlet. The factory cat back on an N10 car is 2.75" necked down to 2.5" into the factory baffled muffler with dual 2.5" tailpipes.

The factory OEM GM "AC" dual cat system killed 10 HP on a 355 HP 350 H.O. ZZ4 create engine and the Y merge after the cats killed 7 HP on that same engine. The factory H.O. / TPI cast iron manifolds killed 18 HP on that same 355 HP 350 ZZ4 create engine. The cat back exhaust used in this test was a Flow Master 3.0" system for N10 cars with a wonderful Flow Master Y merge after the cats for N10 cars. It gained 7 HP just from this much better Y merge pipe.

I don't have a link to this information. It was from an article about the ZZ4 350 H.O. Camaro engine conversion kit offered by the Chevrolet Race Shop back in the day and the numbers came from a GM engineer doing the dyno testing.

 

These are 20% off retail price with the promo code and have FREE shipping.

Magnaflow 23479 | CHEVROLET CAMARO, PONTIAC FIREBIRD | 5.0L / 5.7L | Dual Catalytic Converter-Direct Fit | Standard Grade EPA

Magnaflow 16829 | Chevrolet Camaro(Z-28)-Pontiac Firebird(Trans Am) | Stainless Cat-Back Performance Exhaust System

 

I remember those. I think it was Kevin McClelland that headed that project.
He worked for Flowmaster and did work for GM at the time if I'm not mistaken. May have written for CHP magazine as well if my memory is good.

 

I think they are still technically 'high flow' but were told they could no longer advertise emissions equipment/cats as high flow.
I might be wrong though....

 

You are absolutely correct, most catalytic converters have been monolithic core since around 1984.
They quit using pellet cats around that time, so they are all "high" flow now.
They didn't want manufacturers putting High Flow on their boxes because people
would cut off the good cats and replace them with aftermarket lower quality models
thinking they were getting something better.

 

I called MF again. They do not offer flow ratings. and again claim: "standard CATs only". Nothing high flow.

 

We replaced the factory GM assembly line installed rattling "AC" cats on the N10 dual cat systems on a 1989 L98 GTA ASC convertible, 1990 Formula 350, and 1991 Formula LB9 5 speed with 2 Magnaflow 94015 2.25" inlet and outlet 3 way converters with oxidation (A.I.R. tubes).

If you need a replacement 2.75" aluminized mild steel mandrel bent intermediate pipe that necks down to 2.5" at the muffler inlet for a 1989 to 1991.5 N10 dual cat exhaust system it's a DAVICO 526744.

DAVICO 526744 - Prebent Intermediate Exhaust Pipe - Center Rear; w/ Dual Catalytic Converters

Pictured below is the Walker/DYNOMAX Performance Exhaust Clamp 35430. I ain't seen one of these in years. It connects the N10 dual cat Y pipe to the intermediate pipe. Spray the nut and bolt with rust penetrant good a few times and let it soak in well. If it doesn't want to budge put some heat to it or cut the bolt in two and replace the nut and bolt assembly. Make sure to not mess that clamp up. This is the clamp for the N10 dual cat exhaust that everyone calls the "Hog Clamp". It looks like a V-Band clamp but I don't think it exactly is one. It's also for 2.75 inch pipe. The wider cutout at the top left of the "Hog Clamp" indexes the two mating pipes to each other.



If you carefully measure, mark, and cut everything just right and do a nice job welding everything together the new Magnaflow cats will look like they came factory installed on the OEM GM N10 dual cat exhaust system.

You can carefully remove the flange on the inside convertor that attaches to the N10 specific transmission bracket/hanger assembly and weld it on where it should go.

Don't screw up the N10 transmission bracket/hanger assembly. They are specific to automatic or 5 speed N10 systems. So there are 2 of them for N10 systems and the transmission brackets/hanger assemblies from any of the single cats systems are totally different.

 

Following

 

I corrected my rattling problem... by taking out the AIR components. For whatever reason, once I pulled the diverter valve, and exhaust manifold tubing. The rattling stopped. Then I capped the AIR tube to the cats, with a 1/2" galvanized plumbing cap. It wasn't my intentions; I was just cleaning up the engine compartment. But no more N10 CAT problems.