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Thread: Can exhaust be to free flowing for these cars?

  1. #1
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    Can exhaust be to free flowing for these cars?

    Ok, On my mission to figure out why my car isint running well I remembered last year when all I had was a IC fan, single 2.5 exhaust, cat delete, and CAI and 10% sc pulley. I bought the car with those mods and one day on the quest for more power for free I unbolted the rear section of the exhaust, thinking that less restriction has to equal more power. Well I was wrong, the car was very loud and much slower everywhere in the rpm range than when It had full exhaust and mufflers.

    SO my question now is that with my ported late model, mp inlet, 75mm tb, 3.5 intake, lmaf, 42# injectors,255lph walbro, FMIC, dual 2.5 exhaust off the manifolds to single 3 behind tranny and dump, could that exhaust be a issue? Should I run it to the back of the car and put mufflers on it?

  2. #2
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    Quote Originally Posted by 90tbirdsc View Post
    Ok, On my mission to figure out why my car isint running well I remembered last year when all I had was a IC fan, single 2.5 exhaust, cat delete, and CAI and 10% sc pulley. I bought the car with those mods and one day on the quest for more power for free I unbolted the rear section of the exhaust, thinking that less restriction has to equal more power. Well I was wrong, the car was very loud and much slower everywhere in the rpm range than when It had full exhaust and mufflers.

    SO my question now is that with my ported late model, mp inlet, 75mm tb, 3.5 intake, lmaf, 42# injectors,255lph walbro, FMIC, dual 2.5 exhaust off the manifolds to single 3 behind tranny and dump, could that exhaust be a issue? Should I run it to the back of the car and put mufflers on it?
    No a 2.5-3-2.5 set up will handle just about anything u throw at it

    Stock catz are a restriction
    Stock resonator is biggest restriction
    Stock mufflers not so much but yes

    Port ur stock manifolds that will also help

    The motor needs a bit of backpressure from the exhaust
    But not as much as it had stock
    However something like dual 3" exhaust would be complete Overkill

    Now aftermarket resonators X pipes H pipes true duals and different mufflers will be mainly sound difference

    How ever I felt a better response on mine after adding dual borlas and its so much more quiet matter of preferences
    Also to help with drone if u use mufflers keep tips the way stock were aiming down sort off helps alot with drone...

  3. #3
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    It's never a good idea to have any exhaust back pressure. Think about it. Back pressure leads to exhaust being pushed back into the cylinder through the exhaust port under operation. When is that ever a good idea?

    My setup is kooks mid length to two 2.5 inch into a magnaflow resonator with twin 2.5 inlet out to a single 3 inch then back to a Y fitting that splits to two 2.5 inch into two magnaflow mufflers.

    As a special note you can see straight throu my mufflers and resonator so there is no restriction in my exhaust at all and I have to to say my exhaust is quiet and has no drone. So you can have a free flowing exhaust that isn't obnoxiously loud. You would never know my car has headers with the exhaust as opened up as it is.

    Very stealthy............until wot. LOL
    Last edited by 90sc35thann; 06-13-2013 at 05:28 AM.

  4. #4
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    Wink

    Quote Originally Posted by 90sc35thann View Post
    It's never a good idea to have any bexhaust back pressure. Think about it. Back pressure leads to exhaust being pushed back into the cylinder through the exhaust port under operation. When is that ever a good idea?
    It does tho the engine does need some backpressure , not the 17psi of back pressure the stock system offers lol but run open headers on dyno will be loud
    Then Run dual 3" no back pressure
    Run an exhaust thats about what the car needs and there will be differences open headers or oversized free flowing piping will hurt performance
    I saw a 15 muffler comparison sound and power levels on ls1 forum let me search it , where no mufflers had a loss in hp compared to having having mufflers

    I Had shorty headers and ram the car a couple miles to muffler shop it was very unresposive and loud

    AFter that I got 2.5 downpipes no catz to a magnaflow resonator single 3" out all the way back to differential oumpkin it made a big difference in response and quickness .... couole months after I got a Y pipe that split that 3" to dual 2.5s with dual borlas and the car feels More responsive now
    From my experience the little back pressure helped
    compared to 17psi

  5. #5
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    Quote Originally Posted by 90sc35thann View Post
    It's never a good idea to have any exhaust back pressure. Think about it. Back pressure leads to exhaust being pushed back into the cylinder through the exhaust port under operation. When is that ever a good idea?

    My setup is kooks mid length to two 2.5 inch into a magnaflow resonator with twin 2.5 inlet out to a single 3 inch then back to a Y fitting that splits to two 2.5 inch into two magnaflow mufflers.

    As a special note you can see straight throu my mufflers and resonator so there is no restriction in my exhaust at all and I have to to say my exhaust is quiet and has no drone. So you can have a free flowing exhaust that isn't obnoxiously loud. You would never know my car has headers with the exhaust as opened up as it is.

    Very stealthy............until wot. LOL
    ^ mine is like that lol but on idle it sounds choppy like cammed and it isnt lol


    What I mean is as long as he ditched the stock horrible exhaust components he will be ok with a 2.5-3-2.5 or dual 2.5 or 2.25 dual or whats the other set up some use dual 2.5 to a single 3.5 to 2.5 again any of those set ups will be good for anything most SCs will see unless its a crazy 10s SC

  6. #6
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    Heres what I was sort of talking about

    Gray chart is off tccoa others of the Ls1 forums
    Last edited by Miguel5671; 06-27-2013 at 11:37 AM.

  7. #7
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    Let me say this. When you drove to the shop with open headers the reason it hurt performance is because your ECM had no idea how much fuel to add. Your O2 sensors were too close to fresh air. For example when I was running logs with open headers my KAMFR's were all over the place because they kept getting signals from the O2's that were all over the place.

    The two charts you provided were comparing decibals and mufflers. None of which has anything to do with power. As for the dyno charts. Different mufflers can certainly affect performance based on restriction.

    Still I have never had an engine where a little back pressure was better than none.

    The ls1 article doesn't tell me much either as the delta in HP numbers is negligible. Ditto to the tccoa link.

    If anything a good exhaust system will help to suck out the exhaust, which is why valve overlap on a cam helps improve performance. The exhaust system actually helps pull the intake charge into the cylinder. A system with back pressure would defeat this purpose.

    Read the second paragraph of the article at the section called "overlap: how much is too much?"

    http://www.popularhotrodding.com/tec...s/viewall.html
    Last edited by 90sc35thann; 06-13-2013 at 06:52 AM.

  8. #8
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    Quote Originally Posted by 90sc35thann View Post
    Let me say this. When you drove to the shop with open headers the reason it hurt performance is because your ECM had no idea how much fuel to add. Your O2 sensors were too close to fresh air. For example when I was running logs with open headers my KAMFR's were all over the place because they kept getting signals from the O2's that were all over the place.

    The two charts you provided were comparing decibals and mufflers. None of which has anything to do with power. As for the dyno charts. Different mufflers can certainly affect performance based on restriction.

    Still I have never had an engine where a little back pressure was better than none.

    The ls1 article doesn't tell me much either as the delta in HP numbers is negligible. Ditto to the tccoa link.

    If anything a good exhaust system will help to suck out the exhaust, which is why valve overlap on a cam helps improve performance. The exhaust system actually helps pull the intake charge into the cylinder. A system with back pressure would defeat this purpose.
    Ok I understand completely about the o2s reading ambient air,
    The LS1 charts dynoed the same car swapping mufflers
    Even tho the differences were minor having no mufflers hurt performance and torque lets say if all those mufflers were no restriction (just example) why did running no mufflers made less power ? I have the borla pro xs and they are not a straight through design I do believe they make .5psi back pressure each, now my next question is why then would a free flowing 3" dual exhaust hurt performance it is no restriction isnt it?

    ANd yes I know the exhaust scavanges the gases out that is why a big pipe will make the car loose torque
    Also then a true free flowing exhaust would be a mandrel bend straight through headers
    Ok so say back pressure isnt needed but velocity if gas is at low rpms u need the velocity for gasses to come out quicker but at high rpm a small pipe diameter creates back pressure doesnt it ? Because the engine is trying to oush the gas through a smaller volume path so in a true free flowing free back pressure exhaust would be an overkill pipe with no back pressure headers mufflers resonators yet it would kill ur low end torque
    That is why an exhaust set up must be chosen right for each application? As little back pressure from components as possible yet a diameter pipe that can be at the optimal point to maintain power and velocity

    Or im I way off and should just go to sleep ?
    Last edited by Miguel5671; 06-13-2013 at 07:23 AM.

  9. #9
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    I read it and I DO understand that back pressure isnt needed
    But, there will be back pressure in an exhaust system to make it optimum for both low and high rpm
    So the lowest back pressure possible while having a pipe diameter that helps velocity is optimum


    http://www.sccoa.com/articles/cwexhaust.php

    TAke a read on this even having 2.5 piping and free flowing mufflers and catz and resonator u will still have back pressure then very little and that's the good part the least amount there the better but u will have it if u have the right piping for the motors power range
    ONLY if u go to an oversized diameter pipe then u almost completely eliminate backpressure but that would kill ur low end torque like using dual 3" or 3.5"
    That is why the pipe must be sized correctly to have good gas velocity at low rpms even if it means it will have a little bit of back pressure at high rpms

    I Think only this wouldnt apply to turbo vehicles because the exhaust is driven by the turbo
    Both pulling and pushing the gas

  10. #10
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    Exhaust Scavenging

    I was talking to a co-worker the other day about back pressure in exhaust.

    We were discussing this article which explains it well, IMO.

    http://www.veryuseful.com/mustang/te...Scavenging.pdf

    I'd love to put a variable exhaust valve like Turbo Subaru STIs have.

  11. #11
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    There are many reasons no mufflers hurt the engine performance but none of which had anything to do with the loss of back pressure in the exhaust system. IMO


    It probably had to do more with the afr and spark going out of whack due to the change in exhaust.

  12. #12
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    Quote Originally Posted by 90sc35thann View Post
    There are many reasons no mufflers hurt the engine performance but none of which had anything to do with the loss of back pressure in the exhaust system. IMO


    It probably had to do more with the afr and spark going out of whack due to the change in exhaust.

    Probably that, no I know u would want the least back pressure possible but im saying there will be some backpressure unless u go with an overkill pipe that will hurt low end torque but little as possible pressure while retaining the low end is the best wouldnt that be ?

  13. #13
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    Serious drag racers run open headers. I don't think it is hurting their torque.

  14. #14
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    Quote Originally Posted by 90sc35thann View Post
    Serious drag racers run open headers. I don't think it is hurting their torque.
    Because they have high hp and take off on high rpm they dont see low end at all they stall and take off at mid to high rpms ?

    The bigger pipe less back pressure helps on high rpms because there is little to no back pressure but that car doesnt see low rpms its going WOT since start

    Exhaust Velocity is just to aid the daily drivers

    I think im gonna do more researching and see if I find anything else related but for now I gotta sleep a bit :/

    Talk to you later man ! Nice talking to you

  15. #15
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    Quote Originally Posted by 90sc35thann View Post
    Serious drag racers run open headers. I don't think it is hurting their torque.
    Don't be so sure.

    A properly sized exhaust system is beneficial. But to actually have a properly sized exhaust system means you'll have to throw out some of your favorite ideas first which is something most people refuse to do.

    Maintaining velocity is important in any case. Maximum scavenging is not always the rule. It's too complex to really get into here, but the article above is a very good start.
    Quote Originally Posted by Miller View Post
    Ya thats why i tape mine down. People think its bc i dont have a moonroof seal (which is true) but its really to keep my roof from ripping off .
    Email me here.

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