355 Exhaust Manifold vs 348

The manifolds? They're both 4-2-1, no? Or do you mean the whole exhaust on the 355, with the dual paths?

 

Noise control--if anything, it makes emissions a bit harder rather than easier.

Why so convoluted, F355 was in a time where European noise emission levels were tightening (also why the valve does not open until 3,000 RPMs in 3rd gear::Swiss noise limits) but Ferrari wanted a noise more like their older cars so, some king of switch/valve was necessary.

 

The split is to enable a bypass of the primary (ceramic) cats and muffler restriction for a path with a single metallic cat and reduced muffler restriction at higher rpm. This allowed them to hit idle and drive by emissions and noise standards while getting higher top end power. I don't know if anyone's measured how much that is worth power wise, but it does provide better sound. At the time it was the highest specific output NA production motor built, and Ferrari definitely had to throw a bunch of stuff at it to get there.

So to answer your original question - it was for both emissions and power purposes. It enabled more top end power with less emissions where typically operated and measured (idle and cruise).

 

Same reason Ferrari did it - it allows sane sound and a less stinky exhaust at idle or when driving around, and then rowdy, unobstructed exhaust when you wring it's neck. I added cats back onto my 355 as the exhaust would make your eyes water at cold idle without them, but still get the sound and top end charge it (mostly) had without them. That complicated system is also why the 355 is one of the best sounding cars ever made, which is probably the main reason nobody wants to get rid of it.

 

On the Capristo website there is a claim that the 3 path layout of the F355 when using a single chamber silencer/muffler triggers a "flow loop" when the valve opens.
Fluid dynamics says that a fluid will always follow the path of less restriction, so following the Capristo claim this suggests that exhaust gases flow more easily, when the valve is open, through the Y-pipe despite its featuring secondary cats, than through primary cats, all of these 3 paths leading to the single chamber in the (stock) silencer/muffler.

Assuming main and secondary cats are made of the same webbing I do not understand how exhaust gases could flow more easily through the Y-pipe when the valve is open as the Y-pipe diameter divides basically per two, right before the open valve, compared with the main piping, if we consider things in terms of exhaust gas mass. This also means that the speed of exhaust gases is theoretically 33% higher through the valve than through the main cats.

Does anyone have picture of an opened-up stock silencer? It would be interesting to see how it is baffled inside. This might give a clue or two if there is any vacuum at both bottom inlets when there is pressure at the top one.

 

The '95 (2.7) has a less restrictive Y-section than the '96+ (pipes are larger in diameter) .....

In both cases, the non-y-section gasses flow down to the muffler and through at least 2 sections of the muffler before joining a central section of the muffler where the Y-section gasses enter. Thus, not only is the Y-section path shorter, it has less restriction in the muffler and flows a lot more gas.

The standard exhaust section CAT has your standard ceramic matrix.
The Y-section exhaust CAT was a metallic converter that could heat up and cool down faster.

 

@Mitch Alsup
Thanks. Do you know how 2.7 and 5.2 mufflers differ inside? Pictures show that the 5.2 unit features an extra lower bulge (there is even a cut out in the 5.2 bumper to make room for it), which suggests a slightly different internal routing from the 2.7 muffler.
I have a hard time believing that the "flow loop" issue was unknown to Ferrari engineers, to the point that I suspect all stock mufflers are indeed dual rather than single chamber. Digging through the archives on here has not provided a definitive answer.

 

I’d be interested to hear a 355 with non split manifolds with a an X pipe and no cats or mufflers. That would have to be the most efficient/powerful right?


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The mufflers have 7 chambers in a 3-1-3 configuration (looking horizontally).
The standard exhaust goes through 2 chambers (3 and 3) before reaching the common chamber (1)
The y-section exhaust arrives directly at the common chamber (1)
After the common chamber, L&R exit through the final (3) chamber to the exhaust tips.

 

Sure but it'd blast your eardrums out. I have just high flow cats on my car and challenge (ie straight) exhaust and it's extremely loud on bypass. I had just test pipes on the primary before and it was too loud, the cats calmed it down at idle and low rpm but when the bypass opens it's full on race car. Fun, but if you removed the cats completely and just had straight pipes with an x pipe it'd be full race car the whole time.

 

I’m wondering if it would create the high pitch we’re all after. No bypass, y pipe etc. just primaries into an X pipe… obviously loud but I’m wondering if there would be some symphonic harmonies achieved with such a configuration where the challenge exhaust is primarily just loud


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The challenge exhaust Y's on bypass, so there's something else going on with eg capristo and kreisseg exhausts. Not sure what, but basically on bypass the challenge merges and then splits with nothing in between. My thought is it must have something to do with the length of of the merged tube before it's split and the high pitched exhausts have them merged for a certain extra length to achieve that pitch where the stock and challenge exhausts split right after the bypass valve. So I don't know that an x-pipe achieves that, there's something else going on.