Bypass valve/Y pipe

When the bypass valve opens up and exhaust gases flow through the muffler, does anyone have an idea how much of the exhaust gas flows through the y pipe compared with the primary pipes? Is it 1/3 or do the primary pipes still flow more than the Y pipe?

I was considering about having a muffler built. Combining a challenge exhaust with a more traditional exhaust. Keeping the primary exhaust pipes separate from the Y Pipe. Each set of pipes having their own muffler.

 

Probably about 60% though the Y-Pipe.

 

Fabspeed makes a secondary catbypass Y-pipe (for the 5.2 and 2.7) that will help improve the sound in those upper RPM's when that valve opens. It is necessary to keep the Valve in any event to help preserve low RPM performance.
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Thanks Mitch for the info. Wasn't there a thread a while back where Capristo described how the factory exhaust/bypass system worked and how they developed their mufflers for the 355? I thought he stated that the Y-Pipe and the primary exhaust should be separate. I read this quite a while ago so I could be wrong.

Evan I have an aftermarket Y-Pipe but decided to keep my OEM one on the car for now. I recently purchased another OEM Y-Pipe and gutted it. I may eventually put that one on.

 

There are many things which can affect this 'percentage' ....... many things .....

° which headers are used, the divergent rate/angles/bends at which the collector exit splits off into the separate paths, is each path equal in that respect?

° which cats are being used, ceramic or metallic, how many cells per inch, what is the flow area and length of the catalyst matrix?

° the shape and merge rate/angle of the y-pipe and how much flow area is allocated per bank before the bypass? what size is the tube post merge? is there a divider in there? how long is it? .... Example; stock 2.7 y-pipe has a pretty long divider till just before the bypass valve

° number of bends in each path, flow area of each path along the way and how long is each path overall?

Then;

° what happens after the bypass and the cats, has its own set of similar factors ..... the factors above and some I missed, will affect that ultimate 'percentage'

° finally ..... sound preferences and goals ...... what sound attenuation methods are being used and how are they applied?


But, if you are just looking to put a muffler together to quiet the engine and not too concerned about a well planned, fully optimized system ...... then disregard most of this post ......

 

Oz, I was hoping that you would reply given the fact that you have experimented with a few different exhaust systems for the 355. I am not as concerned about sound as much as a well developed system. Are there any advantages of keeping the bypass and the main exhaust separate?

 

I found the article from Capristo on their website that talks about the 355 exhaust system.

http://www.capristoexhaust.com/shop-online/ferrari-products/ferrari-355?page=shop.product_details&flypage=vmj_color_plus.tpl&product_id=95&category_id=37

 

Jim, without going into exhaustive details and speaking strictly from a cat-back perspective. One of the primary goals in proper exhaust design, is to make sure the ECU is getting accurate and timely information from the o2 sensors. So essentially maintaining a representative exhaust flow over the o2 sensors at all times, (example; during closed and open bypass positions). This maintains proper A/F ratio as intended by the ECU program, for given operating conditions. When proper A/F ratio is the case, the life of the engine and exhaust components is less likely to be hastened ...... at least not by improper A/F ratio .....

 

Oz, have you ever taken a factory muffler apart? Is the Y-pipe kept separate from the main exhaust? Are the main pipes kept separate as well or do they merge inside the chamber?
On Capristo's exhaust system ,the main pipes are kept separate from the y-pipe.

 

I have seen a thread on here that showed an OEM muffler diagram of the internals. It showed the bypass and primary flow ultimately share all four tips, with the bypass having a more direct path.
But getting to the point, one can create a 'proper' 355 exhaust using either the 'shared' or the 'separate' method. Keeping the exhaust paths 'separate' after the bypass and cats, in and of itself, is not a 'golden bullet', it is only one design variation.

That said .......... I have designed 355 exhaust systems using a few 'methods' ..........

.
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Thanks Oz. That was what I was wondering about. It was after reading that article from Capristo that made it sound imperative that the y-pipe and main system be kept separate in order to prevent excess heat build up and ruined headers. It seems that this may not be the case. Thanks again.