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I am quoting directly from page 50 of Edelbrock's "The Great Manifold Bolt-On!" , published in1982, ISBN No. 0-9608740-0-3.
The chapter is entitled "Exhaust Systems: Understanding & Selecting" I am going to quote paragraphs in the direct order that they appear in the book, relating to the subject of exhaust and back pressure as it relates to the ongoing debate, (This is uncut, unedited)...
" An exhaust system can be "too efficient." By this we mean that during the period of camshaft overlap(discussed in a previous chapter), there is direct communication from induction system inlet to exhaust system exit. It was stated that the intake system could "see" the exhaust system at this particular point in time. Near the end of the overlap period as exhaust gas continues to pass out of the exhaust system, incoming fresh air/fuel mixtures can be drawn directly through the engine, unburned. This is mixture that could have remained in the engine for the production of heat (power) but did not because of an OVERLY EFFICIENT EXHAUST SYSTEM. (emphasis by me) If we were to assume that by the creation of back-pressure (resistance of exhaust gas flow out of the engine) raw fuel would not be passed from the engine during overlap, then back pressure can be detrimental to good engine performance and economy. (I'm not sure that makes sense entirely, considering the context -Matt)
For example, as engine rpm increases, some practical limit to the amount of exhaust gas a given system can handle is reached. At this point, resistance to exhaust gas flow through the system can build rapidly. Subsequent increase in engine rpm will cause pressure at the exhaust ports to be sufficiently high as to reduce the volume of exhaust gas ABLE (emphasis -theirs) to pass from the engine's cylinders. Resulting residual exhaust gas in the cylenders will then mix with fresh incoming air and fuel, thereby causing dilution and reduction of net combustion temperature. this cooling effect can be related to the fact that less burnable mixture gets into a cylinder partially filled with exhaust gas from excessive back pressure. The results are often measured in lost power and economy. Part throttle operation of an engine doesn't recognize back pressure as significantly as near or wide-open-throttle operation simply because of reduced net air flow through the engine at the smaller throttle openings. But the condition exists anyway.
This is also why the installation of street-driven headers may or may not show noticeable gains in engine performance at lower ranges of RPM. But as air flow through the engine increase, the tendancy for headers to reduce back pressure improves. then the benefits become more evident.
As mentioned briefly in an earlier chapter, such pressure conditions in an engine's exhaust system can be "read" by the induction system. REDUCED BACK-PRESSURE (emph.- me) tends to lower actual cylinder pressure at the time incoming air/fuel mixtures first enter a cylinder. Less exhaust gas is present at this time, allowing for an attending increase in the amount of fresh mixture in the cylinder at the time of combustion. "
Now this makes a couple points we have been debating; 1- headers don't really show a great deal of flow improvement at low rpm, but will still make some difference. 2- back pressure increases as rpm increases, if you have Little BP at low rpm, you will have MORE BP at higher rpm. this directly affects the amount of air/fuel allowed into the cylenders during valve overlap to be combusted at the compression/power stroke.. This also brings up the point of cam choice, less restrictive exhaust is way more noticeable on high overlap or long duration cammed engines than on short duration, short overlap cammed engines. 3- NO WHERE does it say back pressure makes low rpm torque! Not once. it simply stated - some BP blocks fuel/air mixture from being pulled into and then out of the cylinder during valve overlap at the intake/compression phase of piston movement. with a moderate cam, this only happens at VERY low rpm and with an overly efficient exhaust system. UNFORTUNATELY, they don't specify how effecient or what rpm or how much of an effect the raw fuel in the exhaust sytem there is and for how long. this possibly/probably just results in stinky, rich exhaust and black smoke when punched off-idle. Valve overlap (and other cam specs)is set up to allow the maximum air/fuel to be sucked through the intake system and into the cylinder to be burned at a certain rpm. since the air is not sped up by anything at high rpms, the cam must be setup to take advantage of atmopheric pressure as well as the down force of the piston to fill the cylinders at high rpm. since we don't use blowers, or variable valve timing, we are stuck with cams that work well at a given rpm range. tune the engine for low rpm or high rpm or a compromise of both. if you wanna run at highway speeds and you don't like slowing down going up steep hills at 65 to 75 mph, hauling the old bass boat, then mid to high rpm cams are the way to go. But then low rpm efficiency suffers.
This isn't mentioned in the quoted text, but is mentioned later in the same chapter- most Naturally Aspirated V8 engines are lucky to see 80% volumetric efficiency at 3000 to 4000 rpm. You can alter this by tuning your intake system AWAY from your exhaust system. This means, basically,- If you want all your power below 2000 rpm and nothing at 3000, then run a dinky carb , dinky cam, lowrise duel plane ,and dinky pipes... and idle every where. if you want to make power below 2000 and above 2000 into the 3000 to 4000 rpm range, go with a small to medium(small primaries and large secondaries) carb, decent mid range (idle to 4500 rpm) cam, highrise duel plane, and larger exhaust. if you want virtually no power below 3000 rpm but tons above and into 7000 rpm, go with a highrise single plane mainfold, large cam(300to 330* duration, large overlap cam, large carb, and larger exhaust.
A final note: If you run duel pipes and want better throttle response and low rpm power you can try to fit H or X pipe under the rig, preferably close to the collectors. If you decide single is for you and you like highway speeds AND low rpm power, run single 3" pipe and easy breathing muffs and cat.
-Matt W
Sorry for all the shouting and such, but it really isn't like everyone says- you don't make power with restrictive exhaust at low rpm. you just don't notice it the engine gagging on spent exhaust fumes during the start of the intake stroke at low rpm. you do however, allow more power at mid to high rpm with open exhaust.
The chapter is entitled "Exhaust Systems: Understanding & Selecting" I am going to quote paragraphs in the direct order that they appear in the book, relating to the subject of exhaust and back pressure as it relates to the ongoing debate, (This is uncut, unedited)...
" An exhaust system can be "too efficient." By this we mean that during the period of camshaft overlap(discussed in a previous chapter), there is direct communication from induction system inlet to exhaust system exit. It was stated that the intake system could "see" the exhaust system at this particular point in time. Near the end of the overlap period as exhaust gas continues to pass out of the exhaust system, incoming fresh air/fuel mixtures can be drawn directly through the engine, unburned. This is mixture that could have remained in the engine for the production of heat (power) but did not because of an OVERLY EFFICIENT EXHAUST SYSTEM. (emphasis by me) If we were to assume that by the creation of back-pressure (resistance of exhaust gas flow out of the engine) raw fuel would not be passed from the engine during overlap, then back pressure can be detrimental to good engine performance and economy. (I'm not sure that makes sense entirely, considering the context -Matt)
For example, as engine rpm increases, some practical limit to the amount of exhaust gas a given system can handle is reached. At this point, resistance to exhaust gas flow through the system can build rapidly. Subsequent increase in engine rpm will cause pressure at the exhaust ports to be sufficiently high as to reduce the volume of exhaust gas ABLE (emphasis -theirs) to pass from the engine's cylinders. Resulting residual exhaust gas in the cylenders will then mix with fresh incoming air and fuel, thereby causing dilution and reduction of net combustion temperature. this cooling effect can be related to the fact that less burnable mixture gets into a cylinder partially filled with exhaust gas from excessive back pressure. The results are often measured in lost power and economy. Part throttle operation of an engine doesn't recognize back pressure as significantly as near or wide-open-throttle operation simply because of reduced net air flow through the engine at the smaller throttle openings. But the condition exists anyway.
This is also why the installation of street-driven headers may or may not show noticeable gains in engine performance at lower ranges of RPM. But as air flow through the engine increase, the tendancy for headers to reduce back pressure improves. then the benefits become more evident.
As mentioned briefly in an earlier chapter, such pressure conditions in an engine's exhaust system can be "read" by the induction system. REDUCED BACK-PRESSURE (emph.- me) tends to lower actual cylinder pressure at the time incoming air/fuel mixtures first enter a cylinder. Less exhaust gas is present at this time, allowing for an attending increase in the amount of fresh mixture in the cylinder at the time of combustion. "
Now this makes a couple points we have been debating; 1- headers don't really show a great deal of flow improvement at low rpm, but will still make some difference. 2- back pressure increases as rpm increases, if you have Little BP at low rpm, you will have MORE BP at higher rpm. this directly affects the amount of air/fuel allowed into the cylenders during valve overlap to be combusted at the compression/power stroke.. This also brings up the point of cam choice, less restrictive exhaust is way more noticeable on high overlap or long duration cammed engines than on short duration, short overlap cammed engines. 3- NO WHERE does it say back pressure makes low rpm torque! Not once. it simply stated - some BP blocks fuel/air mixture from being pulled into and then out of the cylinder during valve overlap at the intake/compression phase of piston movement. with a moderate cam, this only happens at VERY low rpm and with an overly efficient exhaust system. UNFORTUNATELY, they don't specify how effecient or what rpm or how much of an effect the raw fuel in the exhaust sytem there is and for how long. this possibly/probably just results in stinky, rich exhaust and black smoke when punched off-idle. Valve overlap (and other cam specs)is set up to allow the maximum air/fuel to be sucked through the intake system and into the cylinder to be burned at a certain rpm. since the air is not sped up by anything at high rpms, the cam must be setup to take advantage of atmopheric pressure as well as the down force of the piston to fill the cylinders at high rpm. since we don't use blowers, or variable valve timing, we are stuck with cams that work well at a given rpm range. tune the engine for low rpm or high rpm or a compromise of both. if you wanna run at highway speeds and you don't like slowing down going up steep hills at 65 to 75 mph, hauling the old bass boat, then mid to high rpm cams are the way to go. But then low rpm efficiency suffers.
This isn't mentioned in the quoted text, but is mentioned later in the same chapter- most Naturally Aspirated V8 engines are lucky to see 80% volumetric efficiency at 3000 to 4000 rpm. You can alter this by tuning your intake system AWAY from your exhaust system. This means, basically,- If you want all your power below 2000 rpm and nothing at 3000, then run a dinky carb , dinky cam, lowrise duel plane ,and dinky pipes... and idle every where. if you want to make power below 2000 and above 2000 into the 3000 to 4000 rpm range, go with a small to medium(small primaries and large secondaries) carb, decent mid range (idle to 4500 rpm) cam, highrise duel plane, and larger exhaust. if you want virtually no power below 3000 rpm but tons above and into 7000 rpm, go with a highrise single plane mainfold, large cam(300to 330* duration, large overlap cam, large carb, and larger exhaust.
A final note: If you run duel pipes and want better throttle response and low rpm power you can try to fit H or X pipe under the rig, preferably close to the collectors. If you decide single is for you and you like highway speeds AND low rpm power, run single 3" pipe and easy breathing muffs and cat.
-Matt W
Sorry for all the shouting and such, but it really isn't like everyone says- you don't make power with restrictive exhaust at low rpm. you just don't notice it the engine gagging on spent exhaust fumes during the start of the intake stroke at low rpm. you do however, allow more power at mid to high rpm with open exhaust.
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1978 Wagoneer
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