Thread: Question on 3" setups

Good question from the OP.
I was wondering why 3" is necessary if I was running 2.5 SSAC
I am gonna go 3" anyway with my SSAC,into a 3" in/3"out cat or resonator to 3" cat back,mandrel of course....but I was just pondering the question.
I definately would never use a 2.5 with a 3" header like fuji,but what about SSAC?
Anyway,if the collector on the header is 2.5", it seems like there would not be enough exhaust flowing through it to max out ,or need to be 3" from there.
Can some enlighten me here?
How could the 3" make a difference if it is 2.5 up till that point?

Not trying to rob the thread,just"expand on it"

I had a 3" when I dyno'ed my SE-R with its JDM DE(also intake +2.5 header w/ Calum RT). It made more tq than other SE-Rs I've seen with similar set-up and 2.5/2.25 pipes.

http://www.sr20-forum.com/members-rides/8870-swappage-weekend.html

Dyno chart is at the bottom of the page. We only set the fuel. Had timing all the way advanced mechanically. Rough tune.

You could see tq everywhere except below 2000+- rpm. Below that level I could not confidently assert there was improvement, but it was not distinguishable to me.
I'd be pretty confident to rn a 3" on a VE. Remember when everyone down there told you 60mm was too much for a DE, and told me 2.25 was too much for my B12?

This should make it pretty clear:

Headers

Still not clear enough, at least for me. Any one else?

Thanks

Sure, I'll explain it some more.

Backpressure is the enemy. Everything done with the exhaust is with the goal of decreasing back pressure. If you understand that an exhaust pulse has a high pressure head and a low pressure tail then you can easily understand the scavenging effect that is utilized inside a header. As the low pressure tail passes by the primary of the next cylinder it will draw out the exhaust gas from the current exhaust event. Exhaust header primary or secondary diameter is important here as it determines how fast the exhaust pulses travel. A faster exhaust pulse will create a lower pressure tail which is desired. This is why exhaust header primaries and secondaries should be smaller in diameter. Once the exhaust has traveled through the primaries to the secondaries (if any) and to the final collector they've maintained their velocity and low pressure tail. After the collector there is no reason to keep the exhaust diameter small because you've already taken full advantage of the scavenging effect of the exhaust pulse tail. At this point it's best to have the largest exhaust diameter you can possibly have to continue to keep backpressure low.

Now there is some turbulence in the exhaust after the collector so if you're going to run an open downpipe it's best to have some smaller diameter pipe (2-4 inches) after the collector to lessen turbulence but this isn't usually necessary with a full exhaust.

I've dumbed it down a bit, but those are the basics. Anything unclear I can try and explain better...

@ Benfenner,

So in essence you are saying that a header with 2.5" collector with a 3" catback exhaust is ideal since it (header) keeps the back pressure for the scavenging effect?

What about a header with 60mm (2.36") collector into a 3"?

Peace

Now that's weird, you would usually hear such a thing from American shops, because they are used to working with big block or american muscles, thus anything with less cylinders then a V6 is useless and is not a manly engine.

I say, after you get the exhaust done to be 3", take that guy for a ride in the VVL monster, and he'll understand

In essence, I am saying that yes, except I'd like to make one thing very clear. It is not that the header keeps the back pressure for the scavenging effect. Back pressure (high pressure in the exhaust) is bad always. The header with it's small diameter pipes actually creates low pressure (the opposite of exhaust back pressure) behind the high pressure exhaust waves. So get the notion of back pressure = good (ever) out of your head.

About your question of 60mm collector to 3" exhaust. Let me say this. The best thing you could do for power (everywhere in the rev range) is to run an open horn after the collector. Next best thing would be a 5" (or bigger) exhaust. Next best, 4". Next best is 3". Get the idea?

Edit: This all assumes a proper header design.

What if All I have is 20ve motor with sr16 cams (not N1) tuned with a Greddy emanage netting 170-175WHP with stock header and 60mm catback. Without doing any more mods or bolt on, will a 3" be an overkill? Why I asked is That quite a few peeps running 3" are using N1 cams or plan to. Will be staying at the current set up so just want to know.

Not really be-laboring the point, if you know what I mean.

Peace

You should see gains with a 3" on that setup, but I'm not sure of how much, or if it's worth it to you. Also I'm just guessing. Someone else might know for sure.