Proof of larger DP/ Fluid Dynamics

[rst95eclipse]

Okay, I've done some research, and I'm sure that it can be debated until nights end. I don't want people giving their opinions or what they've heard. I just want proof. Proof as in dyno charts/numbers or the word from someone who has done this.

I want to know if you have a pretty sizeable setup that's pushing 500+whp that a 4" downpipe with a 3" testpipe/catback would yield a gain of more power than just a full 3" exhaust.

I'm asking this because I'm questioning if the bottleneck created downstream from the turbo would cause a loss in power. Or if the extra 4 feet of larger diameter piping allowed the turbo to spool faster. Something.

[niterydr]

A 4" downpipe is cool, but if you have ever been under your car and looked at your oil filter, you would realize that there is no way in hell to get a 4" pipe through there without seriously sacrificing ground clearance.
Also, exhaust gases expand, so should your piping diameter.

[Shotgun!]

With out a cat, your gases don't expand. Other way, they are cooling.

[niterydr]

With out a cat, your gases don't expand. Other way, they are cooling.
Thats what I meant. They are cooling, and you are gaining density because of it. In order to "get it all out" you need to open up the pipe. You loose even more velocity, but that is counter-acted against because you have a turbo. In the case of a turbocharged setup, the less (or sometimes the larger diameter) exhaust you have, the better. This is of course within reason.
Good catch on my wording!

Back to his question:
I remember reading a long time ago, on a "unmentionable" board a "certain well know member" switched his "vehicle" from a 3" catback to a 4" catback and saw something like "50hp/xxx 1/4 ET gains".
If someone looks up the "archives" on that certain "board" I bet the specifics can be found.

[Goat Blower]

If you're talking about Shep, he switched to the same 3.5" Buschur system I have a fewy years ago. Then he went to a shorty pipe out the front bumper last year. I've only heard of three actual 4" systems on DSM's. Trust me, the 3.5" is plenty huge.

I've never seen actual back to back comparisons above 3" systems. But rough guesstimation by some NABR big hitters says a true 3" system isn't a restriction until at least 550 whp.

[slowbubblecar]

I don't think it would even be possible for a 4" downpipe unless you were able to move the turbo over towards the tranny a ways or start with a smaller 02 housing.

[rst95eclipse]

A 4" downpipe is cool, but if you have ever been under your car and looked at your oil filter, you would realize that there is no way in hell to get a 4" pipe through there without seriously sacrificing ground clearance.
Also, exhaust gases expand, so should your piping diameter.
Oh there are ways my friend.

I don't think it would even be possible for a 4" downpipe unless you were able to move the turbo over towards the tranny a ways or start with a smaller 02 housing.
There are ways of doing this.


Back to the question. It's not about a FULL 4" exhaust. It's questioning if the 4" to 3" reduction under the car is going to cause a bottle neck that would cause a loss in effientcy or if it actually has positive results. Because obviously the best exhaust on a turboed car is no exhaust.

[niterydr]

If you're talking about Shep, he switched to the same 3.5" Buschur system I have a fewy years ago. Then he went to a shorty pipe out the front bumper last year. I've only heard of three actual 4" systems on DSM's. Trust me, the 3.5" is plenty huge.

I've never seen actual back to back comparisons above 3" systems. But rough guesstimation by some NABR big hitters says a true 3" system isn't a restriction until at least 550 whp.
I wasn't going to drop names ;)
I forgot if it was a 3.5" or a 4", I couldn't remember.
I know I'll need a 4" catback.

[Goat Blower]

To make a long story short, going to a smaller diameter somewhere downstream in the system is going to be a restriction. If you're doing a 4" downpipe, why the hell would you go down to a 3.5" catback? Run 4" all the way, the best exhaust is no exhaust.

[Super Bleeder!!]

i say go 4" all the way back....until the muffler. then find a muffler with a 4" inlet and a 0.5" outlet

surely you will see the benefits of this across the board