" GRUMPY? I recently swapped from 1 .625" shorty headers and a 2.75" dual exhaust to 1.75" full length headers and 3" exhaust with an (X) pipe to reduce the cars effective back pressure, the car runs noticeably better once the rpms exceed about 5000rpm but my 60 foot times got slower and I seem to have lost some lower rpm torque, do I need to make a change to increase my exhaust back pressure?"
first lets point out a few things, headers are designed to increase cylinder scavenging, increasing cylinder scavenging in theory reduces the level of previously burnt exhaust gases , and increases the percentage of fresh fuel/air entering the combustion chamber and being compressed , thus increasing the potential power the engine can make. but in most cases this also means some of the new fuel/air charger is drawn out thru the exhaust before the valves seat on the compression stroke. the diameter of the primary pipes and the length of those pipes plus the collector they feed all effect the exhaust gas velocity as does the cars displacement, compression ratio and cam timing, its also common that changes to the cylinder scavenging will require changes to the jetting, or intake design to maximize the cylinder fill efficiency. and the intake design will also effect how efficiently the cylinders get fed and scavenged by the exhaust gas inertial drawing in the next intake runner charge of fuel/air mix.
back pressure is always bad, but if you increased the header size too much you reduced the exhaust gas velocity thus reducing the cylinder scavenging, this has zero to do with back pressure, but a good deal to do with effectively using the velocity of the gases flowing out of the cylinder to draw in the next fuel fuel air charge.
in many cases adding an extended collector to the headers will get you back the previous headers efficiency, in many case shorty headers use the exhaust system they are matched with as an extended header collector, swapping to a full length header and a larger exhaust effectively reduces the working length of the exhaust.
BACK PRESSURE will ALWAYS restrict the engines power levels IF the engines properly tuned, the cam timings correct and youve got the car geared correctly, if you do some careful investigation Im reasonably sure youll find the increased exhaust header primaries and lower restriction exhaust slowed the exhaust gasses and may have EITHER reduced or increased cylinder scavenging , as youve changed the pressure levels in the cylinder and/or the change in scavenging changed the effective fuel air ratio, in the engine, this is VERY COMMON, and in many cases just using a vacuum/pressure gauge on the intake and header collectors and reading the plugs condition or ideally using a fuel/air ratio meter and an infrared temp gun on the headers where they exit the cylinder heads,will show you that change. in many cases a tighter LSA cam will help increase the power levels as the slightly change overlap tends to offset the increased scavenging and increase torque due to the valves closing a bit earlier.
in some cases retarding the cam will allow the engine to breath at the slightly increased rpm levels the better scavenging allows.
in most cases youll need to make changes to the tuning to try to get as close to 12.5:1 -13:1 on the fuel air ratio as you can, as thats where most engines make the best power levels, in some cases a cam change to more closely match the ideal cylinder scavenging will be required or a rear gear change to match a change in the effective rpm range where your now making power.
Ive seen several cars where cheaper "LONG TUBE HEADERS" like these in this picture, failed to provide the intended power until an extended section like the one posted just below was added, which markedly increased the effectiveness of the header scavenging.
keep in mind most of the less expensive headers are designed with low cost and ease of installation as far more important goals in the fabrication process than maximizing power levels, plus the header manufacturer has zero idea as to the cam timing compression, displacement or other factors in the engine they will be matched with.
you might also consider that longer and larger diameter headers will more effectively handle increased exhaust gas flow, if you feed the flow into a common (X) pipe that smooths and blends the exhaust flow from both sides of the engine
READ THE THREAD THRU TO THE END AND BE AWARE IT, AND ALL OTHER THREADS ARE CONSTANTLY UPDATED WITH NEW LINKS AND INFO
http://www.summitracing.com/parts/FLO-C ... /?rtype=10
This is not, or at least should not be, random guess work, its a process of, testing, observing and correcting how the engine runs from observed test results. while most guys rely on a system of trail and error, the results can be calculated, the physics involved are well known, so you,ll have too spend far less time swapping random parts to find the ideal component config. if you read the links and do some calculations and testing
related info, the factors effecting your engines current power level can be tested, observed and if necessary corrected,
http://maxracesoftware.com/pipemax36xp2.htm
http://www.circletrack.com/enginetech/c ... ewall.html
http://www.bing.com/search?q=how%20head ... owAppsUI=1
http://perfweldheaders.com/headerscavengetech.html
http://blog.racingarticles.com/2008/02/ ... aders.html
http://www.hondatuningmagazine.com/tech ... ewall.html
http://www.carcraft.com/techarticles/he ... ewall.html
first lets point out a few things, headers are designed to increase cylinder scavenging, increasing cylinder scavenging in theory reduces the level of previously burnt exhaust gases , and increases the percentage of fresh fuel/air entering the combustion chamber and being compressed , thus increasing the potential power the engine can make. but in most cases this also means some of the new fuel/air charger is drawn out thru the exhaust before the valves seat on the compression stroke. the diameter of the primary pipes and the length of those pipes plus the collector they feed all effect the exhaust gas velocity as does the cars displacement, compression ratio and cam timing, its also common that changes to the cylinder scavenging will require changes to the jetting, or intake design to maximize the cylinder fill efficiency. and the intake design will also effect how efficiently the cylinders get fed and scavenged by the exhaust gas inertial drawing in the next intake runner charge of fuel/air mix.
back pressure is always bad, but if you increased the header size too much you reduced the exhaust gas velocity thus reducing the cylinder scavenging, this has zero to do with back pressure, but a good deal to do with effectively using the velocity of the gases flowing out of the cylinder to draw in the next fuel fuel air charge.
in many cases adding an extended collector to the headers will get you back the previous headers efficiency, in many case shorty headers use the exhaust system they are matched with as an extended header collector, swapping to a full length header and a larger exhaust effectively reduces the working length of the exhaust.
BACK PRESSURE will ALWAYS restrict the engines power levels IF the engines properly tuned, the cam timings correct and youve got the car geared correctly, if you do some careful investigation Im reasonably sure youll find the increased exhaust header primaries and lower restriction exhaust slowed the exhaust gasses and may have EITHER reduced or increased cylinder scavenging , as youve changed the pressure levels in the cylinder and/or the change in scavenging changed the effective fuel air ratio, in the engine, this is VERY COMMON, and in many cases just using a vacuum/pressure gauge on the intake and header collectors and reading the plugs condition or ideally using a fuel/air ratio meter and an infrared temp gun on the headers where they exit the cylinder heads,will show you that change. in many cases a tighter LSA cam will help increase the power levels as the slightly change overlap tends to offset the increased scavenging and increase torque due to the valves closing a bit earlier.
in some cases retarding the cam will allow the engine to breath at the slightly increased rpm levels the better scavenging allows.
in most cases youll need to make changes to the tuning to try to get as close to 12.5:1 -13:1 on the fuel air ratio as you can, as thats where most engines make the best power levels, in some cases a cam change to more closely match the ideal cylinder scavenging will be required or a rear gear change to match a change in the effective rpm range where your now making power.
Ive seen several cars where cheaper "LONG TUBE HEADERS" like these in this picture, failed to provide the intended power until an extended section like the one posted just below was added, which markedly increased the effectiveness of the header scavenging.
keep in mind most of the less expensive headers are designed with low cost and ease of installation as far more important goals in the fabrication process than maximizing power levels, plus the header manufacturer has zero idea as to the cam timing compression, displacement or other factors in the engine they will be matched with.
you might also consider that longer and larger diameter headers will more effectively handle increased exhaust gas flow, if you feed the flow into a common (X) pipe that smooths and blends the exhaust flow from both sides of the engine
READ THE THREAD THRU TO THE END AND BE AWARE IT, AND ALL OTHER THREADS ARE CONSTANTLY UPDATED WITH NEW LINKS AND INFO
http://www.summitracing.com/parts/FLO-C ... /?rtype=10
This is not, or at least should not be, random guess work, its a process of, testing, observing and correcting how the engine runs from observed test results. while most guys rely on a system of trail and error, the results can be calculated, the physics involved are well known, so you,ll have too spend far less time swapping random parts to find the ideal component config. if you read the links and do some calculations and testing
related info, the factors effecting your engines current power level can be tested, observed and if necessary corrected,
http://maxracesoftware.com/pipemax36xp2.htm
http://www.circletrack.com/enginetech/c ... ewall.html
http://www.bing.com/search?q=how%20head ... owAppsUI=1
http://perfweldheaders.com/headerscavengetech.html
http://blog.racingarticles.com/2008/02/ ... aders.html
http://www.hondatuningmagazine.com/tech ... ewall.html
http://www.carcraft.com/techarticles/he ... ewall.html
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