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| phatba |
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hey guys,
have an fg xr6 n/a here my housemate just bought, thats running a full twin 2.5inch exhaust. the current exhaust also has no front muffler, it goes straight through from cat to the rear muffler. car otherwise is completely stock. took it for a drive and it seems to be slow to haul itself off the line, but once its moving its all good. my guess is that the exhaust is to big for the car? if we downgrade would a single 3 inch still be too big or should we go single 2.5inch? or would it suffice to chuck in a replacement twin 2.5 front half with a muffler in it? ive rung two different exhaust shops, one told me i would need a whole new system in 2inch at an over the top price, and another told me to get the cars ecu checked by ford. sounds to me like the first place wanted a sale and the second place couldnt be bothered. any ideas or suggestions? |
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| bry40l |
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I know that performance workshops who specialize in the barra motor recommend the 3 inch exhaust as it gives a few kw over the single 2.5inch on a dyno. As for a dual 2.5 it seems rather big for an otherwise standard car.
_________________ BF XR6 |
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| phatba |
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some calculations ive been looking at of the internal pipe size differences. i know stock is 2 inch but still not sure if there is a limit to the bigger is better idea.
single 2 inch = radius of 25.4mm so 3.14 x (25.4mm squared) = 2025.8024mm squared single 2.5inch = radius 31.75mm so 3.14 x (31.75mm squared) = 3163.31625mm squared single 3 inch = radius 38.1 so 3.14 x (38.1 squared) = 4558.0554mm squared twin 2.5 inch = radius 31.75 so 2(3.14 x (31.75 squared)) = 6330.6325mm squared |
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| WARMACHINE |
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Last edited by WARMACHINE on Mon Dec 02, 2013 4:52 am, edited 1 time in total. |
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| Matt_jew |
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Exhaust is miles too big.
For top end power yeah I agree with warmachine that you cant go too big on the exhaust. For torque you can definitely go too big. I run a twin 2.5 on my 5.0 and its too big for it. When I put the rear tailpipes and hotdogs on it after my little neighbour incident , it actually picked up grunt off the mark.
_________________ xr6turnip wrote: More people paid for a ride in a VT commodore then an AU Falcon so the VT is superior.
Based on that fact my Mum is the best around! |
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| WARMACHINE |
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Last edited by WARMACHINE on Mon Dec 02, 2013 4:52 am, edited 1 time in total. |
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| WARMACHINE |
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Last edited by WARMACHINE on Mon Dec 02, 2013 4:53 am, edited 1 time in total. |
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| wrongwaynorris |
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WARMACHINE wrote: The reason why the car feels slow to get going is because it is a ford falcon, it weights near on 2 tonne with people in it... The exhaust is fine theres nothing wrong with it, myths about exhaust being too big are just that. Wrong , ever heard of back pressure ? Companies like Caterpillar spend millions of dollars on computer programs to calculate things such as back pressure and dew point . Exhaust size is critical . |
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| WARMACHINE |
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| wrongwaynorris |
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I don't know as I have never bothered to calculate what is an ideal size exhaust for that specific car at whatever specific R.P.M. It may be 3 " it may not . What I was responding to was the fact that you made an exceptionally stupid statement saying " The exhaust is fine theres nothing wrong with it, myths about exhaust being too big are just that. "
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| WARMACHINE |
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op go and buy the smaller exhaust (meaning go waste your time and money ) to try and fix a problem that isnt caused by it/ if infact it actually has a problem.
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| fiend |
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Interesting calculation regarding exhaust gas speed, header length and pipe size is more to the point. You *can* get more power buy using smaller diameter but longer headers. Rev range dependant and whole heap of other variations thrown in.
You may also get more power from a straight 2.5" pipe with hot dog at the end than a straight 3" pipe without. Gas expulsion speed contains momentum. A smaller pipe has faster moving gases, which have more "pull" between cylinders firing, thus creating lower pressure zones to actually help "suck" the next load of cylinder expulsion into the header pipe. This presumes you have half decent long headers and not a standard manifold, of course. Standard manifold is complete fail in all regards (unless it has a turbo on it, in which case it is barely passable and most tuners will laugh at you...) |
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| WARMACHINE |
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fiend wrote: Interesting calculation regarding exhaust gas speed, header length and pipe size is more to the point. You *can* get more power buy using smaller diameter but longer headers. Rev range dependant and whole heap of other variations thrown in. You may also get more power from a straight 2.5" pipe with hot dog at the end than a straight 3" pipe without. Gas expulsion speed contains momentum. A smaller pipe has faster moving gases, which have more "pull" between cylinders firing, thus creating lower pressure zones to actually help "suck" the next load of cylinder expulsion into the header pipe. This presumes you have half decent long headers and not a standard manifold, of course. Standard manifold is complete fail in all regards (unless it has a turbo on it, in which case it is barely passable and most tuners will laugh at you...) Geez mate your more than a pretty face around here lol. |
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| GeZza200 |
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phatba wrote: hey guys, have an fg xr6 n/a here my housemate just bought, thats running a full twin 2.5inch exhaust. the current exhaust also has no front muffler, it goes straight through from cat to the rear muffler. car otherwise is completely stock. took it for a drive and it seems to be slow to haul itself off the line, but once its moving its all good. my guess is that the exhaust is to big for the car? if we downgrade would a single 3 inch still be too big or should we go single 2.5inch? or would it suffice to chuck in a replacement twin 2.5 front half with a muffler in it? ive rung two different exhaust shops, one told me i would need a whole new system in 2inch at an over the top price, and another told me to get the cars ecu checked by ford. sounds to me like the first place wanted a sale and the second place couldnt be bothered. any ideas or suggestions? If the standard exhaust manifold is still fitted theirs not much point increasing the exhaust size. The biggest restriction is right there. Twin 2.5" is way too big for a stock 4L. I would be putting a 2.5" exhaust if a note is what he is after. When you start changing cams and changing the rev limiter is when you should be look at different exhaust sizes.
_________________ EL Futura: CVE head, Wolf V500, ICE Ignition and Coil, 36lb injectors, Walbro 255lb, Paci comps, 3" exhaust, T5, Harrop Truetrac with 3.9s. Now with 198.9rwkw, (~185rwkw and 13.80 @99.1mph) with more power to come |
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| fiend |
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Sure I just said that? Ah well.
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