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Power difference of 2 - 2 vs 2 - 1

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  • dicko
    dicko
    4 years ago
    Quoting Jay-Dee on 18 Apr 2020 12:20 AMedited: 18 Apr 2020 12:22 AM

    Exhaust gas velocity/cylinder scavenging matters.

    No engine will make more power or torque at any RPM with positive pressure on top of the piston during the exhaust stroke.

    Quoting leachy on 21 Apr 2020 08:39 PM

    Yes I completely understand that but I can see how the correct tuned pipe lengths of a 2 into 1 would work and the gas flow of the first cylinder acts like a venturi or vacuum on the second cylinder and drags the exhaust out of the other pipe. I just wasn't sure how well that effect happened and like a lot of things in the real world is it that noticable.

    Quoting tussuck on 21 Apr 2020 09:46 PMedited: 21 Apr 2020 09:47 PM

    I have seen modelling of this (Bub website I think).  The focus of that particular exercise was in scavenging at full throttle/peak RPM to get more fuel into an normally asperated engine so the 'power band' for want of a better word was narrowish and in the RPM range they needed (ie top end vs midrange)

    Its complicated and the end result was quite interesting in that while it does indeed work it tends to be within a certain narrow RPM band ie its not a gain across the RPM board.  And where it gives a boost in one area it has a downside of removing from another.

    Also the length of the pipes is critical as all it is si a tuned waveguide (my Radio background actually has a use!). and its the reflections of the exhaust pressure pulse creating the vacum at the exhaust port.

    My experience with the 2006 road king is that the 2 into 1 system gave me more low down torque , but removed the extra from the top end. Screamin eagle air filter fitted with ECM map)
    A 2 into 2 system had a bit extra low and top end with a bit less in the middle.
    The very best of all worlds was the standard headers with rush slipons and  and ecm map
    ( the 2006 had the "Y" crossover pipe on the rear and standard was the cat in the mufflers.)
    later models have the exhaust crossover down lower. and cat in the headers.
    I believe the crossover is the part that allows the system to sort itself out at a wider range of rpm.

    So the best for all round on stock engines remove cat , fit screamin eagle air filter, slip on mufflers. and ecm mapping.
  • leachy
    leachy
    4 years ago
    So possibly in the real world, with a stock 103 that runs over a wide RPM range there may not be that big a difference between a well designed  2 into 2 vs the stock headers with cat and SE muffler.
  • tussuck
    tussuck
    4 years ago
    Quoting leachy on 21 Apr 2020 11:22 PM

    So possibly in the real world, with a stock 103 that runs over a wide RPM range there may not be that big a difference between a well designed  2 into 2 vs the stock headers with cat and SE muffler.

    Correct....  You will most like see a bigger impreovement with an ECU reflash or the like.  No point in have all singing all dancing pipes if your still running a lean factory fuel mix.
    Ideally a wideband O2 tuner with autotune should be step 1 as that can make a good difference.
  • Jay-Dee
    Jay-Dee
    4 years ago
    Quoting fatbat on 17 Apr 2020 08:11 AMedited: 17 Apr 2020 08:14 AM

    The radius style exhausts aren’t great performing. Compared to another crappy designed exhausts like short shots there probably isn’t a noticeable difference. Compared to a well designed 2-2 or 2-1 you will feel a difference in the seat of the pants particularly down low and in midrange where back pressure matters.  Some of the best performing systems for street are long AND have a collector or cross over pipe (and are designed with no cat). 

    Quoting Jay-Dee on 18 Apr 2020 12:20 AMedited: 18 Apr 2020 12:22 AM

    Exhaust gas velocity/cylinder scavenging matters.

    No engine will make more power or torque at any RPM with positive pressure on top of the piston during the exhaust stroke.

    Quoting leachy on 21 Apr 2020 08:39 PM

    Yes I completely understand that but I can see how the correct tuned pipe lengths of a 2 into 1 would work and the gas flow of the first cylinder acts like a venturi or vacuum on the second cylinder and drags the exhaust out of the other pipe. I just wasn't sure how well that effect happened and like a lot of things in the real world is it that noticable.

    My comment was more in response to fatbat talking about back pressure.
     
    I've never read so much about engines wanting back pressure than since I've started looking at different Harley forums, back pressure doesn't equal performance, exhaust sizing and design does. A stock exhaust with stock mufflers creates back pressure to meet noise restrictions, they seem to be sized quite well but restricted heavily by the mufflers/cats. I suspect not too many aftermarket exhausts are really designed with overall performance in mind, they're more about aesthetics and noise.

    As you've mentioned in posts since, a stock engine (and even a reasonably modified one) with a stock exhaust, free flowing mufflers and other restrictions removed with a decent tune will likely be your winner overall. And if your building something for high RPM performance, no exhaust is going to give you heaps of bottom end power, it's a trade off.

    I have a bunch of mods to do to my Breakout and I'll be using the slightly modified stock headers (ceramic coated) with straight through modified factory mufflers and see how it works. I have a D&D Bobcat but probably won't use it if I don't have to. They're known for being one of the best but looking at it there's nothing exotic about it, especially at the merge and tubing size is about the same as the stocker.
  • fatbat
    fatbat
    4 years ago
    Back pressure is important for street use. Compare a short open flowing exhaust without back pressure to an exhaust with back pressure. What’s going to perform best from idle through low revs and up to midrange where most people ride for most of the time? Why do some exhausts step their headers?
    The factory exhaust system is for noise and back pressure. Same with cars and all other combustion engine vehicles. 
    Lots of other principles and variables effect exhaust scavenging; equal length headers are not the be all and end all. To get equal length headers on a Harley Vtwin, how many curves and how sharp are the curves required to be in the rear header pipe? Everything changes when there are curves. Equal length with different curves are not tuned. 

    I’m glad it’s been mentioned a few times but on some Harley models, among the very best performing exhausts is the factory Harley header pipes (with cross over and no cat was ever in the headers) and free flowing but well designed mufflers. The tapered turbo muffler type previously made by s&s and still made by Khrome werks work very well and they’re not obnoxiously loud.  The tapered tip plays its part and i doubt its coincidence that it’s the same tapered end design on oem mufflers. 
  • Jay-Dee
    Jay-Dee
    4 years ago
    Quoting fatbat on 22 Apr 2020 03:39 AM

    Back pressure is important for street use. Compare a short open flowing exhaust without back pressure to an exhaust with back pressure. What’s going to perform best from idle through low revs and up to midrange where most people ride for most of the time? Why do some exhausts step their headers?

    The factory exhaust system is for noise and back pressure. Same with cars and all other combustion engine vehicles. 
    Lots of other principles and variables effect exhaust scavenging; equal length headers are not the be all and end all. To get equal length headers on a Harley Vtwin, how many curves and how sharp are the curves required to be in the rear header pipe? Everything changes when there are curves. Equal length with different curves are not tuned. 

    I’m glad it’s been mentioned a few times but on some Harley models, among the very best performing exhausts is the factory Harley header pipes (with cross over and no cat was ever in the headers) and free flowing but well designed mufflers. The tapered turbo muffler type previously made by s&s and still made by Khrome werks work very well and they’re not obnoxiously loud.  The tapered tip plays its part and i doubt its coincidence that it’s the same tapered end design on oem mufflers. 

    To answer your two questions in paragraph one:

    1) An appropriately sized/designed exhaust to maximise exhaust gas velocity and minimise restriction (back pressure) for that RPM range. The short open flowing exhaust if it's well designed may just be the correct size and length to work in that range once it's optimally tuned.

    2) I've never really looked into it too much but I assume the idea for stepped headers is to act similar to a venturi (carburetor) to optimise exhaust gas velocity and reduce reversion pulses to in theory increase power all through the RPM range, not just one end (or part) of it. They very likely won't be restricted enough or designed to create any back pressure at low to mid RPM though.


    A factory exhaust is built to meet noise restrictions/limitations first, with the maximum performance that can be achieved while managing that assuming it's a vehicle that the manufacturer actually care about it's performance of course i.e. HSV, Porsche, sports bike, etc and maybe Harley. On most vehicles though it's pretty much just a way to get exhaust gas to the rear of the vehicle, keep it quiet and not much else. In nearly all instances though it can be improved on (less flow restriction) if noise limits aren't a consideration.

    A stock Harley exhaust on a stock engine with free flowing mufflers and no cats (stage one if you want) will have next to zero restriction or back pressure at low to mid range RPM where most people ride. But I'm willing to bet it in 99.9% of instances it will make more power and torque right through the RPM range over the restrictive stock mufflers once tuned correctly.
  • fatbat
    fatbat
    4 years ago
    Quoting fatbat on 22 Apr 2020 03:39 AM

    Back pressure is important for street use. Compare a short open flowing exhaust without back pressure to an exhaust with back pressure. What’s going to perform best from idle through low revs and up to midrange where most people ride for most of the time? Why do some exhausts step their headers?

    The factory exhaust system is for noise and back pressure. Same with cars and all other combustion engine vehicles. 
    Lots of other principles and variables effect exhaust scavenging; equal length headers are not the be all and end all. To get equal length headers on a Harley Vtwin, how many curves and how sharp are the curves required to be in the rear header pipe? Everything changes when there are curves. Equal length with different curves are not tuned. 

    I’m glad it’s been mentioned a few times but on some Harley models, among the very best performing exhausts is the factory Harley header pipes (with cross over and no cat was ever in the headers) and free flowing but well designed mufflers. The tapered turbo muffler type previously made by s&s and still made by Khrome werks work very well and they’re not obnoxiously loud.  The tapered tip plays its part and i doubt its coincidence that it’s the same tapered end design on oem mufflers. 

    Quoting Jay-Dee on 22 Apr 2020 12:17 PMedited: 22 Apr 2020 01:26 PM

    To answer your two questions in paragraph one:

    1) An appropriately sized/designed exhaust to maximise exhaust gas velocity and minimise restriction (back pressure) for that RPM range. The short open flowing exhaust if it's well designed may just be the correct size and length to work in that range once it's optimally tuned.

    2) I've never really looked into it too much but I assume the idea for stepped headers is to act similar to a venturi (carburetor) to optimise exhaust gas velocity and reduce reversion pulses to in theory increase power all through the RPM range, not just one end (or part) of it. They very likely won't be restricted enough or designed to create any back pressure at low to mid RPM though.


    A factory exhaust is built to meet noise restrictions/limitations first, with the maximum performance that can be achieved while managing that assuming it's a vehicle that the manufacturer actually care about it's performance of course i.e. HSV, Porsche, sports bike, etc and maybe Harley. On most vehicles though it's pretty much just a way to get exhaust gas to the rear of the vehicle, keep it quiet and not much else. In nearly all instances though it can be improved on (less flow restriction) if noise limits aren't a consideration.

    A stock Harley exhaust on a stock engine with free flowing mufflers and no cats (stage one if you want) will have next to zero restriction or back pressure at low to mid range RPM where most people ride. But I'm willing to bet it in 99.9% of instances it will make more power and torque right through the RPM range over the restrictive stock mufflers once tuned correctly.

    Maybe revisit back pressure - and I think we agree on what’s being discussed. 
    And I’m taking about street use and have all along. And my response to this thread was about those short radius pipes. 
  • leachy
    leachy
    4 years ago
    This certainly has been an interesting read, shame we dont just have a few dynos to play with. 
    There has to be something in having a certain amount of back pressure in the lower RPM for smoothness and driveabilty, (is that such a word) that is necessary. One of my previous bikes was a YZF 1000 Thunderace, Its was had 5 valves per cylinder and pumped out 150hp not bad for a 90's model carby bike. That exhaust had a gate that would open up depending on the RPM. People reported that when that was disconnected or removed for a full system the bike was a pig, got worse fuel economy and was only good at high RPM and WOT.

    Theres a big difference between a max power drag bike which operates in a very narrow RPM range Vs an all round street / touring bike. 

    Does any one know what you would expect to pay to put a bike on a dyno to get it tuned properly. APL Performance are not too far from me but I havent called them yet until I work out what exhaust to go with. 
  • paulybronco
    paulybronco
    4 years ago
    Quoting leachy on 22 Apr 2020 08:28 PM

    This certainly has been an interesting read, shame we dont just have a few dynos to play with. 

    There has to be something in having a certain amount of back pressure in the lower RPM for smoothness and driveabilty, (is that such a word) that is necessary. One of my previous bikes was a YZF 1000 Thunderace, Its was had 5 valves per cylinder and pumped out 150hp not bad for a 90's model carby bike. That exhaust had a gate that would open up depending on the RPM. People reported that when that was disconnected or removed for a full system the bike was a pig, got worse fuel economy and was only good at high RPM and WOT.

    Theres a big difference between a max power drag bike which operates in a very narrow RPM range Vs an all round street / touring bike. 

    Does any one know what you would expect to pay to put a bike on a dyno to get it tuned properly. APL Performance are not too far from me but I havent called them yet until I work out what exhaust to go with. 

    Harley tourers also have the exhaust butterfly but its fitted really as an emmisions device.
  • Jay-Dee
    Jay-Dee
    4 years ago
    Quoting fatbat on 22 Apr 2020 03:39 AM

    Back pressure is important for street use. Compare a short open flowing exhaust without back pressure to an exhaust with back pressure. What’s going to perform best from idle through low revs and up to midrange where most people ride for most of the time? Why do some exhausts step their headers?

    The factory exhaust system is for noise and back pressure. Same with cars and all other combustion engine vehicles. 
    Lots of other principles and variables effect exhaust scavenging; equal length headers are not the be all and end all. To get equal length headers on a Harley Vtwin, how many curves and how sharp are the curves required to be in the rear header pipe? Everything changes when there are curves. Equal length with different curves are not tuned. 

    I’m glad it’s been mentioned a few times but on some Harley models, among the very best performing exhausts is the factory Harley header pipes (with cross over and no cat was ever in the headers) and free flowing but well designed mufflers. The tapered turbo muffler type previously made by s&s and still made by Khrome werks work very well and they’re not obnoxiously loud.  The tapered tip plays its part and i doubt its coincidence that it’s the same tapered end design on oem mufflers. 

    Quoting Jay-Dee on 22 Apr 2020 12:17 PMedited: 22 Apr 2020 01:26 PM

    To answer your two questions in paragraph one:

    1) An appropriately sized/designed exhaust to maximise exhaust gas velocity and minimise restriction (back pressure) for that RPM range. The short open flowing exhaust if it's well designed may just be the correct size and length to work in that range once it's optimally tuned.

    2) I've never really looked into it too much but I assume the idea for stepped headers is to act similar to a venturi (carburetor) to optimise exhaust gas velocity and reduce reversion pulses to in theory increase power all through the RPM range, not just one end (or part) of it. They very likely won't be restricted enough or designed to create any back pressure at low to mid RPM though.


    A factory exhaust is built to meet noise restrictions/limitations first, with the maximum performance that can be achieved while managing that assuming it's a vehicle that the manufacturer actually care about it's performance of course i.e. HSV, Porsche, sports bike, etc and maybe Harley. On most vehicles though it's pretty much just a way to get exhaust gas to the rear of the vehicle, keep it quiet and not much else. In nearly all instances though it can be improved on (less flow restriction) if noise limits aren't a consideration.

    A stock Harley exhaust on a stock engine with free flowing mufflers and no cats (stage one if you want) will have next to zero restriction or back pressure at low to mid range RPM where most people ride. But I'm willing to bet it in 99.9% of instances it will make more power and torque right through the RPM range over the restrictive stock mufflers once tuned correctly.

    Quoting fatbat on 22 Apr 2020 08:03 PMedited: 22 Apr 2020 08:11 PM

    Maybe revisit back pressure - and I think we agree on what’s being discussed. 

    And I’m taking about street use and have all along. And my response to this thread was about those short radius pipes. 

    It's good healthy discussion no matter what and I was going to write in my last post that we agree from paragraph four of your previous post, but I'm not sure what you're referring to as back pressure. I think you mean having header pipes that are wrongly sized or designed killing exhaust gas velocity at low RPM requiring some restriction to regain low end performance. A balance pipe/chamber in this instance isn't to create back pressure, as far as I know it's to assist scavenging and even out the exhaust pulses, and both mufflers/silencers to share the exhaust pulses.
    As I see it (and hopefully I'm not the only one) back pressure is positive pressure in the exhaust tract due to restricted flow acting against the piston and intake charge during the the exhaust stroke and overlap. It makes no difference if it's only for street use or not, that won't improve performance at any RPM.

    Also in case there was some confusion previously when I was talking about tuning. I wasn't meaning pipe lengths/diameters as in equal length headers, I was referring to air/fuel ratios and timing to optimise the set up.

    As mentioned previously in the thread, the Freedom (and also Cobra) seem to be a well designed short 2 into 2's with some interaction between the two pipes/cylinders and tuned properly would easily outperform a stock, fully restricted system. Will they be better than a well designed two into one, maybe, maybe not, tuning and a dyno will tell but I don't imagine there'd be a massive difference overall.

    I had a Freedom Radical Radius (for sale in the classifieds) on my Breakout for a little while with a canned tune and low RPM performance was fine, it was just too loud for me.
  • Jay-Dee
    Jay-Dee
    4 years ago
    Quoting leachy on 22 Apr 2020 08:28 PM

    This certainly has been an interesting read, shame we dont just have a few dynos to play with. 

    There has to be something in having a certain amount of back pressure in the lower RPM for smoothness and driveabilty, (is that such a word) that is necessary. One of my previous bikes was a YZF 1000 Thunderace, Its was had 5 valves per cylinder and pumped out 150hp not bad for a 90's model carby bike. That exhaust had a gate that would open up depending on the RPM. People reported that when that was disconnected or removed for a full system the bike was a pig, got worse fuel economy and was only good at high RPM and WOT.

    Theres a big difference between a max power drag bike which operates in a very narrow RPM range Vs an all round street / touring bike. 

    Does any one know what you would expect to pay to put a bike on a dyno to get it tuned properly. APL Performance are not too far from me but I havent called them yet until I work out what exhaust to go with. 

    A dyno tune is usually somewhere around $500 bucks I think if you're keen.
  • dicko
    dicko
    4 years ago
    Quoting Jay-Dee on 22 Apr 2020 12:17 PMedited: 22 Apr 2020 01:26 PM

    To answer your two questions in paragraph one:

    1) An appropriately sized/designed exhaust to maximise exhaust gas velocity and minimise restriction (back pressure) for that RPM range. The short open flowing exhaust if it's well designed may just be the correct size and length to work in that range once it's optimally tuned.

    2) I've never really looked into it too much but I assume the idea for stepped headers is to act similar to a venturi (carburetor) to optimise exhaust gas velocity and reduce reversion pulses to in theory increase power all through the RPM range, not just one end (or part) of it. They very likely won't be restricted enough or designed to create any back pressure at low to mid RPM though.


    A factory exhaust is built to meet noise restrictions/limitations first, with the maximum performance that can be achieved while managing that assuming it's a vehicle that the manufacturer actually care about it's performance of course i.e. HSV, Porsche, sports bike, etc and maybe Harley. On most vehicles though it's pretty much just a way to get exhaust gas to the rear of the vehicle, keep it quiet and not much else. In nearly all instances though it can be improved on (less flow restriction) if noise limits aren't a consideration.

    A stock Harley exhaust on a stock engine with free flowing mufflers and no cats (stage one if you want) will have next to zero restriction or back pressure at low to mid range RPM where most people ride. But I'm willing to bet it in 99.9% of instances it will make more power and torque right through the RPM range over the restrictive stock mufflers once tuned correctly.

    Quoting fatbat on 22 Apr 2020 08:03 PMedited: 22 Apr 2020 08:11 PM

    Maybe revisit back pressure - and I think we agree on what’s being discussed. 

    And I’m taking about street use and have all along. And my response to this thread was about those short radius pipes. 

    Quoting Jay-Dee on 23 Apr 2020 12:54 AMedited: 23 Apr 2020 01:52 AM

    It's good healthy discussion no matter what and I was going to write in my last post that we agree from paragraph four of your previous post, but I'm not sure what you're referring to as back pressure. I think you mean having header pipes that are wrongly sized or designed killing exhaust gas velocity at low RPM requiring some restriction to regain low end performance. A balance pipe/chamber in this instance isn't to create back pressure, as far as I know it's to assist scavenging and even out the exhaust pulses, and both mufflers/silencers to share the exhaust pulses.

    As I see it (and hopefully I'm not the only one) back pressure is positive pressure in the exhaust tract due to restricted flow acting against the piston and intake charge during the the exhaust stroke and overlap. It makes no difference if it's only for street use or not, that won't improve performance at any RPM.

    Also in case there was some confusion previously when I was talking about tuning. I wasn't meaning pipe lengths/diameters as in equal length headers, I was referring to air/fuel ratios and timing to optimise the set up.

    As mentioned previously in the thread, the Freedom (and also Cobra) seem to be a well designed short 2 into 2's with some interaction between the two pipes/cylinders and tuned properly would easily outperform a stock, fully restricted system. Will they be better than a well designed two into one, maybe, maybe not, tuning and a dyno will tell but I don't imagine there'd be a massive difference overall.

    I had a Freedom Radical Radius (for sale in the classifieds) on my Breakout for a little while with a canned tune and low RPM performance was fine, it was just too loud for me.

    https://www.youtube.com/watch?v=eOg1sZhUNcg
    This guy here reckons he knows how it works.
  • fatbat
    fatbat
    4 years ago
    Quoting Jay-Dee on 22 Apr 2020 12:17 PMedited: 22 Apr 2020 01:26 PM

    To answer your two questions in paragraph one:

    1) An appropriately sized/designed exhaust to maximise exhaust gas velocity and minimise restriction (back pressure) for that RPM range. The short open flowing exhaust if it's well designed may just be the correct size and length to work in that range once it's optimally tuned.

    2) I've never really looked into it too much but I assume the idea for stepped headers is to act similar to a venturi (carburetor) to optimise exhaust gas velocity and reduce reversion pulses to in theory increase power all through the RPM range, not just one end (or part) of it. They very likely won't be restricted enough or designed to create any back pressure at low to mid RPM though.


    A factory exhaust is built to meet noise restrictions/limitations first, with the maximum performance that can be achieved while managing that assuming it's a vehicle that the manufacturer actually care about it's performance of course i.e. HSV, Porsche, sports bike, etc and maybe Harley. On most vehicles though it's pretty much just a way to get exhaust gas to the rear of the vehicle, keep it quiet and not much else. In nearly all instances though it can be improved on (less flow restriction) if noise limits aren't a consideration.

    A stock Harley exhaust on a stock engine with free flowing mufflers and no cats (stage one if you want) will have next to zero restriction or back pressure at low to mid range RPM where most people ride. But I'm willing to bet it in 99.9% of instances it will make more power and torque right through the RPM range over the restrictive stock mufflers once tuned correctly.

    Quoting fatbat on 22 Apr 2020 08:03 PMedited: 22 Apr 2020 08:11 PM

    Maybe revisit back pressure - and I think we agree on what’s being discussed. 

    And I’m taking about street use and have all along. And my response to this thread was about those short radius pipes. 

    Quoting Jay-Dee on 23 Apr 2020 12:54 AMedited: 23 Apr 2020 01:52 AM

    It's good healthy discussion no matter what and I was going to write in my last post that we agree from paragraph four of your previous post, but I'm not sure what you're referring to as back pressure. I think you mean having header pipes that are wrongly sized or designed killing exhaust gas velocity at low RPM requiring some restriction to regain low end performance. A balance pipe/chamber in this instance isn't to create back pressure, as far as I know it's to assist scavenging and even out the exhaust pulses, and both mufflers/silencers to share the exhaust pulses.

    As I see it (and hopefully I'm not the only one) back pressure is positive pressure in the exhaust tract due to restricted flow acting against the piston and intake charge during the the exhaust stroke and overlap. It makes no difference if it's only for street use or not, that won't improve performance at any RPM.

    Also in case there was some confusion previously when I was talking about tuning. I wasn't meaning pipe lengths/diameters as in equal length headers, I was referring to air/fuel ratios and timing to optimise the set up.

    As mentioned previously in the thread, the Freedom (and also Cobra) seem to be a well designed short 2 into 2's with some interaction between the two pipes/cylinders and tuned properly would easily outperform a stock, fully restricted system. Will they be better than a well designed two into one, maybe, maybe not, tuning and a dyno will tell but I don't imagine there'd be a massive difference overall.

    I had a Freedom Radical Radius (for sale in the classifieds) on my Breakout for a little while with a canned tune and low RPM performance was fine, it was just too loud for me.

    I’m not smart enough to explain my thinking well but some restriction, for lack of a better word, can be more efficient at certain gas pressures than no restriction. I don’t know if restriction is the right word to use but when I’m thinking about restriction I’m thinking about header pipe diameter and length among other things. It even includes things people deliberately add to their exhaust to assist low pressure efficiency such as torque cones and lollipops. 
     I like the example about blowing air through a straw. Blowing air through the least restrictive straw (think short and fat) is not as efficient as when you blow air through a longer and skinnier straw. That might change if you can blow massive amounts of air like a top fuel drag car but our lungs can’t do that. 

    In relation to the topic, I’ve had a similar short, fat non restrictive radius style exhaust on a Harley twin cam 107 cube engine. It was dyno tuned and performed ok at wide open throttle in the upper revs but not so well at partial throttle openings at lower revs which is how I rode the bike most of the time. I changed over to two other exhausts that were longer and more restrictive for lack of a better word and borne performed better at partial throttle openings at lower-mid revs. One of the replacement exhausts didn’t match the wide open throttle Hp at high revs compared to the first exhaust but wasn’t far off. The second replacement exhaust was simply better everywhere. Testing was done on a dyno and yes you could noticeably feel the difference between the first and other exhaust after the different exhausts were fitted and dyno tuned. The difference was most pronounced at partial throttle openings at lower to mid revs. 

    Collectors and cross overs I agree often help with scavenging. 
  • Jay-Dee
    Jay-Dee
    4 years ago
    Quoting fatbat on 22 Apr 2020 08:03 PMedited: 22 Apr 2020 08:11 PM

    Maybe revisit back pressure - and I think we agree on what’s being discussed. 

    And I’m taking about street use and have all along. And my response to this thread was about those short radius pipes. 

    Quoting Jay-Dee on 23 Apr 2020 12:54 AMedited: 23 Apr 2020 01:52 AM

    It's good healthy discussion no matter what and I was going to write in my last post that we agree from paragraph four of your previous post, but I'm not sure what you're referring to as back pressure. I think you mean having header pipes that are wrongly sized or designed killing exhaust gas velocity at low RPM requiring some restriction to regain low end performance. A balance pipe/chamber in this instance isn't to create back pressure, as far as I know it's to assist scavenging and even out the exhaust pulses, and both mufflers/silencers to share the exhaust pulses.

    As I see it (and hopefully I'm not the only one) back pressure is positive pressure in the exhaust tract due to restricted flow acting against the piston and intake charge during the the exhaust stroke and overlap. It makes no difference if it's only for street use or not, that won't improve performance at any RPM.

    Also in case there was some confusion previously when I was talking about tuning. I wasn't meaning pipe lengths/diameters as in equal length headers, I was referring to air/fuel ratios and timing to optimise the set up.

    As mentioned previously in the thread, the Freedom (and also Cobra) seem to be a well designed short 2 into 2's with some interaction between the two pipes/cylinders and tuned properly would easily outperform a stock, fully restricted system. Will they be better than a well designed two into one, maybe, maybe not, tuning and a dyno will tell but I don't imagine there'd be a massive difference overall.

    I had a Freedom Radical Radius (for sale in the classifieds) on my Breakout for a little while with a canned tune and low RPM performance was fine, it was just too loud for me.

    Quoting fatbat on 23 Apr 2020 09:24 AMedited: 23 Apr 2020 09:25 AM

    I’m not smart enough to explain my thinking well but some restriction, for lack of a better word, can be more efficient at certain gas pressures than no restriction. I don’t know if restriction is the right word to use but when I’m thinking about restriction I’m thinking about header pipe diameter and length among other things. It even includes things people deliberately add to their exhaust to assist low pressure efficiency such as torque cones and lollipops. 

     I like the example about blowing air through a straw. Blowing air through the least restrictive straw (think short and fat) is not as efficient as when you blow air through a longer and skinnier straw. That might change if you can blow massive amounts of air like a top fuel drag car but our lungs can’t do that. 

    In relation to the topic, I’ve had a similar short, fat non restrictive radius style exhaust on a Harley twin cam 107 cube engine. It was dyno tuned and performed ok at wide open throttle in the upper revs but not so well at partial throttle openings at lower revs which is how I rode the bike most of the time. I changed over to two other exhausts that were longer and more restrictive for lack of a better word and borne performed better at partial throttle openings at lower-mid revs. One of the replacement exhausts didn’t match the wide open throttle Hp at high revs compared to the first exhaust but wasn’t far off. The second replacement exhaust was simply better everywhere. Testing was done on a dyno and yes you could noticeably feel the difference between the first and other exhaust after the different exhausts were fitted and dyno tuned. The difference was most pronounced at partial throttle openings at lower to mid revs. 

    Collectors and cross overs I agree often help with scavenging. 

    I know what you're trying to say and I think we agree but differently, lower speed exhaust efficiency (maybe smaller with more airspeed, not necessarily back pressure) as opposed to having a big pipe that just dumps the exhaust gas but loses velocity. Sometimes though a bigger but well designed exhaust will net more power, even lower down depending on it's application. The stock header pipes on Harleys these days are hardly small, I'm pretty sure my Breakouts are 1 3/4".
    I personally can't see how a Vance and Hines big radius (the almost factory fitted aftermarket pipe for Breakouts) would perform better than the stock exhaust with free flowing mufflers and other restrictions removed, it will for certain be louder and sound worse though, each to their own of course.

    The Freedom Radical Radius felt pretty good to ride but by butt dyno I couldn't tell you if it made more power anywhere and I've never had it on an actual dyno to know, it was only download tuned. I thought it was too loud (although not obnoxiously) but when I put the stocker back on initially with Rinehart mufflers my ears couldn't notice a massive difference in volume but it definitely sounded better. The stock mufflers modified with perforated glass packed cores are a little bit quieter though I think.
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