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.