building the best exhaust for your ride
Nov 6, 2003, 12:39 PM
#1
building the best exhaust for your ride
today i was late for english class. the reason? i spent about 20 minutes chatting with a campus physics instructor about how the velocity of exiting gases relate to engine power. he showed me mathematically how gases will exit more quickly when going big to small, not more slowly with sputter as you may expect. honestly, when i purchased my kamikaze header and downpipe a few weeks ago, i was expecting to be building a full 2.5" exhaust system. today i turned 180 on this idea...
the reason can actually be explained in very simple terms. take a garden hose... turn the water up to max and let it spray out. now, take your finger and place it over part of the hose. because of the back pressure, the same amout still exits the hose except now more quickly and further. this same principal can be applied to our cars.
sticking to the assumption that our cars will be street legal (or close), we can assume that a high-flow cat will be used. it would actually make more sense to go from the 2.5" piping to 2.25" and basically force the exhaust through the cat more quickly. on the other side, keep the 2.25" piping the rest of the way out (i.e. greddy catback) providing optimal performance.
any comments? i'm personally planning on a circular 2.25" high-flow along with a greddy catback in the future... but i'll have to wait for my tax return to do that much
the reason can actually be explained in very simple terms. take a garden hose... turn the water up to max and let it spray out. now, take your finger and place it over part of the hose. because of the back pressure, the same amout still exits the hose except now more quickly and further. this same principal can be applied to our cars.
sticking to the assumption that our cars will be street legal (or close), we can assume that a high-flow cat will be used. it would actually make more sense to go from the 2.5" piping to 2.25" and basically force the exhaust through the cat more quickly. on the other side, keep the 2.25" piping the rest of the way out (i.e. greddy catback) providing optimal performance.
any comments? i'm personally planning on a circular 2.25" high-flow along with a greddy catback in the future... but i'll have to wait for my tax return to do that much
Nov 6, 2003, 12:53 PM
#3
just about the only thing i can think of that would better performance would be a test pipe... but after hearing horror stories about people getting slammed with tickets (especially here on the west coast), it doesn't seem worth it. $140 for an extra 2% increase over a high-flow? i think not. besides, the test pipe sound seems whiny to me... our cars will never be FAST fast, so i figure i at least want some show with an extra dash of go. (you kno what i'm sayin.. the non-rice approach)
Nov 6, 2003, 12:56 PM
#4
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it all depends on what you PLAN your setup, mine is much higher in HP than most for obvious reasons, but from larger to bigger wouldnt work in a high HP case, you want your exhaust to flow out the best it can to keep temperatures down, optimal flow, and personally im going with: 0 cats, 0 resonators 
Nov 6, 2003, 01:01 PM
#5
Originally posted by TURBOit
it all depends on what you PLAN your setup, mine is much higher in HP than most for obvious reasons, but from larger to bigger wouldnt work in a high HP case, you want your exhaust to flow out the best it can to keep temperatures down, optimal flow, and personally im going with: 0 cats, 0 resonators
it all depends on what you PLAN your setup, mine is much higher in HP than most for obvious reasons, but from larger to bigger wouldnt work in a high HP case, you want your exhaust to flow out the best it can to keep temperatures down, optimal flow, and personally im going with: 0 cats, 0 resonators
with a turbocharged setup, of course you would want straight through with no restriction... you have far less of a back pressure issue to consider
. besides, if you can afford to go turbo, you probably aren't worried about a (most-likely one time) ticket from a cop that thinks your exhaust is too loud and actually checks
Nov 6, 2003, 01:08 PM
#6
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yup, but the exhaust actually quiets down a bit, but as far as NA setups are concerned, i wouldn't go with the downpipe, when i had my car NA i wanted to increase power across the whole band, not just highend, those huge downpipes kill the low end
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Nov 6, 2003, 07:05 PM
#8
My father watched a video on exiting gases and tuned exhausts. What you posted is the basics of that. Keep in mind the volume of exiting gases is not increased with such a setup, but the velocity is increased. Somehow, this combined with tuning an exhaust (sound waves can actually improve or hinder the flow of the exhaust somehow) can increase both torque and horsepower. I wish I understood it more, but I bet you're on the right track, and would love to hear final results if you go that far.
Nov 7, 2003, 12:11 AM
#9
so...make my exhaust like a funnel???
like...3" down pipe, and than a hi flow cat than 2 1/4" piping than my muffler and tip?
but..wut if say i get 2.5" than switch to 2.25" from my catback to muffler...like cata. 2.5" piping 2.25" piping, muffler/tip.
or would that defeat the purpose? just curious
like...3" down pipe, and than a hi flow cat than 2 1/4" piping than my muffler and tip?
but..wut if say i get 2.5" than switch to 2.25" from my catback to muffler...like cata. 2.5" piping 2.25" piping, muffler/tip.
or would that defeat the purpose? just curious
Last edited by Drive02Lancer; Nov 7, 2003 at 12:14 AM.
Nov 7, 2003, 09:15 AM
#11
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Why make a small cat-back exhaust when the catalytic converter is the major restrictor in the exhaust? You'll still have the pressure build in the downpipe because of the cat. Honestly though I don't think you should have a problem running a 2.5" full exhaust (such as a Kamikaze or RPW header and dp w/ GReddy or Buschur cat-back).
Ah...I wrote that, had to do some work and now I don't remember what I was talking about so I'm leaving it as is.
Ah...I wrote that, had to do some work and now I don't remember what I was talking about so I'm leaving it as is.
Nov 7, 2003, 11:34 AM
#12
Originally posted by HobieKopek
Why make a small cat-back exhaust when the catalytic converter is the major restrictor in the exhaust? You'll still have the pressure build in the downpipe because of the cat. Honestly though I don't think you should have a problem running a 2.5" full exhaust (such as a Kamikaze or RPW header and dp w/ GReddy or Buschur cat-back).
Ah...I wrote that, had to do some work and now I don't remember what I was talking about so I'm leaving it as is.
Why make a small cat-back exhaust when the catalytic converter is the major restrictor in the exhaust? You'll still have the pressure build in the downpipe because of the cat. Honestly though I don't think you should have a problem running a 2.5" full exhaust (such as a Kamikaze or RPW header and dp w/ GReddy or Buschur cat-back).
Ah...I wrote that, had to do some work and now I don't remember what I was talking about so I'm leaving it as is.
not sure about the buschur tho... tired looking for it a while back and couldn't find a place to check it out. i did like the looks of it better... not much on 4" exhaust tips
it is true that moving to a smaller pipe does not increase volume, but only velocity. this makes sense... take the hose example back into consideration. putting your finger over the hose doesn't make the faucet spew out more water
true also that the cat is the most restrictive portion, but forcing exhaust through at a higher velocity all the way out would in fact increase performance. odd to think about, but going from a 3" header/dp to a 2.5" cat to a 2.25" catback would in theory be your best bet.
am i crazy... maybe. will i try this setup? no
i will, however, give the 2.5 -> 2.25 cat + catback a shot tho. i'm way to po' to ever afford a turbo, so making the best of na is my best bet
Nov 7, 2003, 04:28 PM
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Larger to small will increase velocity, but not overall volume. The trick set-up would be to incorporate some sort of expansion chamber (think V8, ala NASCAR) , to move the gases to a "holding area", then expel them. I do not know the physics or gas expansion rules, but I do know that many sanctioning bodies outlaw these chambers to keep costs down. You can tune a torque output vs. high power production through the exhaust without changing any internal/external engine parts. That is the end of my knowledge.
Paging any engineers in the forum?
Jason
Paging any engineers in the forum?
Jason
Nov 7, 2003, 10:49 PM
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Originally posted by kensuke
... the greddy is 2.25"
... the greddy is 2.25"