I'm looking at the N10 down pipes from the manifolds to the CAT's. The outer pipe is a lousy design. The pipe is folded in, so the inside pipe sits right next to it. One is concave, the other is convex. Definitely a restricted flow on the passenger side down pipe.

 

This is part of why we always tell people who have that system, that when it needs major work (new cats for instance), it's time to ditch it, in favor of a good quality aftermarket single exhaust. While that system may have been better than a factory single-cat setup, it doesn't hold a candle to GOOD AFTERMARKET single-cat ones. OTOH if you're in a place where that's not allowed, CA for example, some creativity might be called for.

 

Well…it did add 10hp back in the day.

 

Who is WE ??
My 2 cats, that I will gut out, using a sewer machine will outflow any single pipe.

 

So an open pipe flows more than an open pipe? A high flow cat does not create any restrictions. What sofa is saying is that the stock exhaust, even if "upgraded" optioned from the factory is hot garbage. The entire system needs to be replaced, tweaking it is not worth the effort.

 

Indeed it did: COMPARED TO A STOCK SINGLE-CAT SYSTEM.

Maybe they will. BUTT THE REST OF IT WILL NOT. As you yourself just pointed out.

Overall, in general, if you want better flow, you're best off, economically especially, leaving that behind and going with a good-quality ATERMARKET (not STOCK) single-cat setup. OTOH if you're in a place where that's not allowed, CA for example, some creativity might be called for. Such as, finding 2 AFTERMARKET REPLACEMENT cats that will fit in their places, and re-making the Y-pipe, all with as little as possible to lead the "visual" inspector to suspect that it's not factory. Whether that's practical, let alone possible, I don't know, because I've never done it myself. Only pointing in the direction that one would have to go in.

 

If I did anything aftermarket, it would be the Magnaflow 23479

 

If you live in a place with strict emissions like Cali, it might be better just to keep the original dual cat Y-pipe if you have the N10 option. These cars are slow anyway compared to stuff today so why not roll with the original stock parts and be proud you have an N10 dual cat car.

However, if you have money burning a hole in your pocket, you can configure the stock N10 Y-pipe to have two mandrel bent pipes leading to twin high flow cats and then a big single exit to the mid pipe after that. Make it look as stock as possible, but get rid of that stupid concave outer pipe. That alone should free up some HP, but you won’t be racing stock new Mustangs or 6th gen Camaros. Unless the cats are burned out and you want to spend the money, I say just leave it alone.

 

Why not just go with the magnaflow? They have CA CAT's and 50 state CAT's. All the work is already done, and mandrel bent. and it's a DIYer job, because of the slip fittings.

and... if you call magnaflow, there is no such thing as high flow CAT's. I called them twice.

 

Whoever that is at Magnaflow is dead wrong. 400 cpi ceramic brick vs 100 cpi metallic brick. It is plain to see which one flows better.

 

The 100 will not remove the stench. ... or not all of it.

 

The Magnaflow dual cat Y-pipe is a nice piece, but unless your original N10 is totally trashed I could think of better things to spend almost a $1K bucks on for an old 3rd gen than a slightly better flowing Y-pipe.

 

If you're halfway decent with a welder and mandrel bent pipe, you can build your own 2.5" pipe from manifolds to the cats. The inlet is 2.5 on the cats but GM went tiny pipe infront and a terrible Y merge behind the cats.
f-body can't be faster than the vet so lots of intake and exhaust restrictions.

 

I don’t think even a fully optimized Y-pipe would have made the F-body faster than the Vette. The Vette got aluminum heads in ‘86 and then the ZF 6sp manual in ‘89. It was also a few hundred pounds lighter, on average, than the typical TPI F-body.

I’m really surprised Pontiac was able to get away with using the turbo 3.8L from Buick for the 20th anniv. cars. That car was definitely faster than the Vette. In my mind, I figured that was a response to having to go corporate SBC back in ‘82. If Pontiac was able to keep their unique 4.9L turbo for the 3rd gen Firebird in ‘82, it would have been faster than the Corvette back then.

 

I'm looking at my N10 under the car. It's not rattling at the moment, but I digress.... The outer pipe is very close to the lower fender mount. Maybe 1" tops. I was considering cutting off the concave pipe. and welding in a full round pipe. But then all the offsets would change.
There seems to be plenty of room if both pipes were moved inwards. But that would change the CAT angles., and the HOG clamp would never line up correctly.

My goal is to keep the HOG clamp, because I have an SLP TOTL cat back from back in the day. I do need the CATs for visual inspection. No punk inspection kid will know the AIR is gone, but the CATs are obvious on a visual.

 

I have dual 3" in/out 200s going into a single 4" exhaust on one of my small blocks, no stench at all and it is nearly 500 hp engine.

 

Man you are all tore up about the cats and high flow vs normal and this N10 system and blah, blah, blah. I'm pretty sure I've even told you before which replacement Magnaflow cats to use to cut out the OEM GM "AC" N10 cats and weld in the proper Magnaflow replacement cats for a stock N10 Y-pipe. I don't remember the part number right off hand but I'm pretty sure it ends in 15. What Magnaflow has listed on their site now for their OEM N10 replacement cats is wrong.


Magnaflow replacement cats welded in place of the factory OEM GM "AC" cats on a 27k mile 1990 Formula 350.




27k mile factory muffler starting to rust at the seam. See the pretty new Magnaflow cats installed just like GM put them there. Measure twice and cut once.

1989 Trans Am GTA ACS Convertible with a replacement "Hog Clamp" style N10 intermediate pipe, AP/Eastern muffler and tailpipes. Squint closely toward the front of the car and you will see the OEM GM "AC" cats have been replaced with Magnaflow cats welded in their place.


The "Hog Clamp" for a factory 2.75 inch N10 dual cat exhaust system. Bet everyone wished you could still buy these new.

Last I checked you could still get the "Hog Clamp" style N10 intermediate pipe new from Davico.

Walker/Dynomax factory replacement N10 intermediate pipe for 89 to early 91.5 "Hog Clamp" style N10 exhaust system.

Magnaflow individual replacement cats for factory N10 exhaust system on 89 GTA ASC convertible.

 

And rinse and repeat, and rinse and repeat about 6 more times on other Formulas and GTAs.

The biggest restriction on a factory GM N10 dual cat Y-pipe is the after cat 2 into 1 merge. It kills 8 Net HP on a 5.7L engine that makes 308 Net HP with the much less restrictive Flowmaster after cat 2 into 1 merge pipe.


The 2 into 1 after cat merge on the stock GM N10 exhaust system kills 8 Net HP.

Flowmaster American Thunder Exhaust System 17228 - N10 Catback Exhaust System after cat Y-pipe restores the lost 8 Net HP.

 

The aluminum vet heads, especially the 128s, weren't all that great. Maybe save some weight and a touch of compression but not a huge gain over the centbolt iron heads.
the ZF, while very strong, is pretty heavy. Weight of the cars probably wasn't far apart. (Haven't looked tho).
the tta was detuned and weight was added in the hatch during testing to help convince gm brass it was ok. (Slower than the vet). According to the gentleman that spoke at the TA nats a couple yrs ago and was in on the program.
the ZR1 was the ace in Chevys sleeve.

 

That hog clamp pictured is the service replacement clamp for N10 Y-pipe to mid pipe junction. The original factory clamp was a slightly different design and the bolt that secured the clamp was totally different . The threaded part that the bolt went into was part of the clamp on the factory part, not a separate nut.

Of course, I was kind of bummed when I got the replacement clamp from the dealer back then, but at least I saved the original clamp even though it was somewhat corroded. Maybe I dip it in some strong vinegar and paint it some day to get it somewhat decent looking just for nostalgia's sake.

 

That's my thing. I need to save the hog clamp rearwards. I paid alot of money back in the day for my CAT back. I too have the OEM hog clamp, that I saved by sitting in oil for a week.

 

A "T" merge like that is terrible. I bet it kills even more low-speed torque than high rpm HP. A good merge Y definitely helps things out especially with a straight through muffler design. I recently went from a Dual 3" in/Single 4" out Walker Dynomax 6.0/8.1L replacement muffler that had somewhat of an OEM design (Horribly Restrictive) to a Howe Racing H4022 merge Y-pipe and straight through 4" muffler. My 383 gained 20-30 ft/lbs across the whole power band and sounds so much cleaner at WOT. The old muffler made it sound like a vacuum cleaner with a full bag above ~3,000 rpm. Now it has an actual V8 exhaust note all the way through the RPM band.

Howe Racing makes some really nicely designed Merge Ys. Just do not order one from SummitRacing unless it is in stock. They will slow boat it on their stock order and it will take 3-4 weeks to actually arrive.