I've been speaking to a tuner in Brisbane who has done a custom headers and single high flow cat setup which then runs back into a "normal" twin system for the 350Z.

He tells me that one of the downsides to the twin high flow cats is the potential loss of bottom end power & flexibilty and that the single unit effectively achieves what a crossover pipe would and overcomes the risk of loss of flexibility.

He is a good guy (did beautiful work on my Sti) and very honest so I'm inclined to believe what he's telling me.

You guys who have twin high flow cats - what are your thoughts and experiences?

Cheers,

 

I think it's all depend on the design of the hi-flow cats.

I find i got a Good power all range. yet Some people said the Hi-flow cats only gain a Good Torque than a Power.

 

Well, twin high flow cats will flow twice as much as a single high flow cat. Also the people who install test pipes, which flow more then twin high flow cats gain torque through the whole rpm range.

 

You would loose flexibility if you got HUGE duration camshafts which only come on power at 5000rpm, not from hi-flow cats. If they did loose any power it wouldnt be measureable.

 

How can something gain good torque without gaining good power?

Power = Torque x RPM x constant

If you're making more torque at a certain RPM, then you make more power at that RPM. Unless these "high flow" cats are strangling it in the top-end (which seems like an oxymoron to me), where peak power is being generated, then this shouldn't be the case.

 

I think the point that Zdownunder's exhaust man is making is not that twin high flow cats drop power or flexibility, but that they don't make as much power as a single cat that acts as a crossover pipe.

Crossover pipes have been known to make good gains, but in my mind unless this single cat is large, it will only act as a bottleneck.

The problem is that some people (me included, half the time) hear something, only half understand it, and then expound on it like its the truth. In fact, they've been told a concept and missed the point.

The crossover exhaust setups I've seen have not shrunk two pipes into a single pipe of the same diameter, but linked two pipes together with another pipe (or merged two pipes together and cut a hole in between them. That way the exhaust pipe diameter is not reduced, but on a V engine the exhaust gas can exit out of the "other" pipe, and by process of scavenging improve exhaust flow compared to two, completely independent, pipes.

The only bolt-on 350Z exhaust I've seen with anything resembling a cross-pipe is stillen's. All the rest either merge pipes, or are completely independent.

What's Stillen's power gains like compared to the Injen / Borla (which are independent dual), or Hi Tech / Fujitsubo (which merge into a single pipe)?

 

I think the most cheapest and highest flow cat is run a strange pipe through your original cat. No one can really tells unless Full EPA test which they have to book you in. Even the exhaust noise test can't tell!! In any case you can say the cat was damaged.

 

Thanks everyone for your thoughts on this.

I'll be seeing the tuner next week so should be able to advise more at that stage.

 

Actually they can tell from the temperature using a non contact IR probe. At least that's what they use in NSW.