wat do silencers do as far as back pressure??

The type of silencers that cap off the exhaust tip muffle the sound somewhat by virtue of restricting exhaust flow, which is to say increasing backpressure. For a good example of that very concept as a tuning tool look up Supertrapp mufflers. They are mostly used on two-strokes and dune buggies, most for their simplicity and low weight. Gotta hand it to our new friend Ricky, at least he is thinking and asking rather than jumping in and making false statements out of ignorance. May we kick around the idea as to why it is that 99% of the time those of us who swap to a 3" catless exhaust experience impressive gains even though our engines are said to have no intake/exhaust overlap?

The silencers restrict the tips to dampen noise. Which creates some back pressure. :)

yeh lets . I have no idea - do you have any thoughts ?

I have three concurrent curiosities going right now; the first involves MM's assertion that the Renesis' VE drops above 7,800 rpms. The second is the lack of intake/exhaust overlap that the Renesis apparently has. If they are both related in the sense that Mazda designed the exhaust to flow enough for a linear torque curve until 7,800 that's probably why 3" catless exhausts only show noticeable gains above 8K. That's what my G-Tech showed me on repeated ocassions, anyway. The third involves whether or not it is true that the Renny was also designed to have a built-in EGR effect with the lack of overlap. I am also curious as to what the vacuum readings would be through the rpm band in 3rd and 4th gear so as to get an idea on the design of the intake side. I still have to read through Dave's porting project thread. As it stands, if all this stuff is true then the gains we see are strictly caused by reducing backpressure so that the gains are actually from reduced exhaust pumping inefficiencies and not from any overlap-induced scavenging. That's why porting on the exhaust side was/is such a interesting topic. If we induce a bit of overlap and tune accordingly, how much more beneficial will our 3" systems be?

wow so much info. hahaha im trying to process all this in. hahah

hey guys,
juss read an article in a automobile magazine and i thought it might help with some of us disputing both sides. My problem or doubt WAS that i thought i was having a loss of power due to too big of an exhaust 3". But as i read in the article, it said that when a tuner was trying to make an exhaust for a 911 turbo, he was making a bigger one than stock to DECREASE any backpressure. I'm not sure whether the article had stated that backpressure increases or decreases power, but the tone that i derived from it is that backpressure does not matter at all. so yea... juss letting you guys know. hahah :fingersx:

Charles - what were you measuring with gtech ? HP , 0-60 , 1\4 mile . Can you definately rule out technique as a factor ?

When using the G all that data is gathered at the same time. There is also a c.d. included which allows the users to upload their run data in a spreadsheet-like fashion. When spring comes I'll load that software back on my computer and maybe that'll be another thread I can host: "Getting the Most from Your G-Tech", or something like that. When you ask about ruling out technique, I am not sure that technique couldn't be ruled out in any scientific endeavor. Since each person has their own peculiar ways in which they do things I am sure mine might differ in the details as much as anyone's might. I can say with confidence that I use the G-Tech in a manner indicated in the operations manual and with a firm grasp of the concepts therein. With a strong drag racing background I understand things like "roll-out", significance of the 60' times, and etc.

Will adding a full Agency power catback including the midpipe that takes away the cat make my 04 AT rx8 a lil faster? if not should i go with a 2.5" exhaust instead? or 3" all the way?

yes it'll make your car faster.

2.5" exhausts will be more quiet.

for me, 3" all the way.

would it be better if i do an agency power midpipe with the greddy sp2? how many inches is the greddy sp2?

From what Ive read less backpressure is desireable but in a NA car a properly sized/tuned system will induce a scavenging effect by tuning exhaust pulses to line up casuing a suction effect, the pulse in front pulling the one behind it out the pipe. Quality headers have tuned length runners to line up exhasut pulses. I dont know if this is practiced or possible on the 8 bc of the shared mani pipe in the middle. Im new w/ the 8.

Ive seen plenty of 350-400 hp turbo DSMs on a 2.5" system so Im feelin a 3" system on a 250-300hp 8 is too big unless you go FI.

Who makes a 2.5" cat back and a 2.5" race pipe or race cat/pipe?

Also ive read losing this scavenging effect will result in a loss of tourqe at lower RPM and all HP gains are on the top end. I think Im a few lb/ft better than the S2000 stock and dont want to lose this bragging right:) For around town regular driving more tourqe is better imo.

http://www.nsxprime.com/FAQ/Miscella...austtheory.htm

this has some info about exhaust gasses cooling too much becoming denser than hot exhaust as well but not sure if that counts with the high ex temps of the 8

the greddy sp2 is 60mm or about 2.4 inches so you would want a 2.5 test pipe or a different 3" catback




https://www.wesellcarparts.com/store...idProduct=3768

I'm so torn between the Agency Power vs JIC Magic Exhaust. such a HARD decision :(