355 Bypass valve and Y pipe...good or bad??

From reading a few threads over the years I can remember many saying the Y pipe and valve are not the best design, why is this?

Also, I can remember reading comments regarding removing the bypass valve and the reduction in torque in the lower rpm's, is that a correct analysis of what happens when the valve is removed? Are there any other negative/positive points from removing the valve?

Or is it that a new system (like the gothexhaust?) runs in a different way (no Y pipe) and therefore will not suffer the negatives from simply removing the bypass valve in a OEM set up?

 

I've have also read alot about bypass valve in this forum. My car has been running the bypass open for a long time, with no apparent problems. I fitted a capristo bypass valve not too long ago. I can tell you that the car sounds better with valve open full time, but is a bit noisey when cruising. With the valve open, the car 'pops' when lifting off the throttle which sounds great. I will be fitting the capristo remote control kit so that i can switch between 'open' and 'closed/oem setting' at a flick of a switch. I can't really feel any performance difference with the valve 'open' or in the 'oem setting'. If i had to do a drag race, i would definitely prefer the valve open, simply because you have to rev the car pretty hard in neutral to keep the valve open for a full throttle launch.

 

Lets see if I can make sense ......

The factory by-pass was a good idea, in that it creates an exhaust path with less back pressure. This was needed because the mighty 355 engine was flowing exhaust gasses beyond the factory cats capacity.

IMO the functional compromise in the stock design, are when the exhaust columns are split up, then re-merged after the y-pipe inside the muffler. Not having all the exhaust pressure/volume at the y-pipe, limits maximum cross-bank scavenge/velocity.

..... for power at WOT .... lower back pressure is good ........ lower back pressure with 'full cross-bank scavenge' is better .....

The stock 355 exhaust is heavy as hell and does not 'cross scavenge' all the exhaust ........

My first 355 exhaust 'cross-scavenges' all the exhaust, but runs primary cats full time .... so top RPM WOT power suffers a tad ............

...... so .... I am currently working on 'Version II' of my 355 exhaust ....... which will 'cat' at lower RPMS .... then be a 'full monty' cross-scavenging monster at WOT ............


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Personally, I couldn't care less about squeezing every once of power out of the motor. The undisputed fact is that the Y-pipe makes the 355 the best sounding car on the planet. Ever.

Why mess with that?

 

These exhaust mods are not only for power ....... it is just under 40lbs lighter than stock ..... generates noticeably less heat ...... and actually makes the best sounding car on the planet ........ sound even more F1 ......

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With all due respect (and adding the caveat that I have not heard your stuff in person...) so far I have not heard an exhaust system that I would swap my cat-free Cappy 1 for

 

.......... I had a cappy 1 w/hyperflows ..... and it is indeed a fantastic sounding exhaust .............. I sold it because I made myself a new system (version 1) ............. but in the never ending quest for perfection ............. Version II is now in the works ......... we will see what that sounds like ............

 

Oz, let's at least spell "Monte" correctly

 

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I can't find the post right now, but an aftermarket manufacturer has posted here that they tested a 355 with the bypass open, using (I think) their own design Y-pipe that eliminates the bypass valve and found significant low rpm torque loss on the dyno. I forget the exact numbers. While he said he loved the sound of driving his own 355 with the valve removed, he realized there was definitely a performance penalty to do so.

 

I finally came up with a fix for the buzzing bypass valve and have gone back to the factory configuration. While I love the sound of an open bypass, my "seat of the pants' dyno tells me the car has more snap with the bypass valve so I would concur that there is a loss of low end torque without it. Plus my car now passes emissions

I think Goth has some dyno sheets that could help dispel this though.

 

As an exercise only I removed my buzzing bypass valve last winter. Completely disassembled and had the butterfly valve shaft plasma sprayed to repair the wear. Allowed 0.10mm clearance to allow for thermal expansion. Works like new. The valve is not designed or manufactured to be rebuilt. Not really a cost effective process but interesting none the less.

 

I like the idea. Well done!

 

If anyone needs one, I have a good factory one. Mine rattled, but when I got it out the small bolts that held the flap in place were both loose! Some locktite and an allen key, and it works like new.