1badfox
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- February 11, 2012
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- Texas
- Year, Model & Trim Level
- 93 exploder 6 shooter
For you 5.0 guys and possibly some 4.0's 
WHAT GIVES A CAM ROUGH OR LOPEY IDLE?
The reason most want the lopey idle is because race cars usually have a rough idle and we want our cars to look and sound like race cars. The reason race cars have that rough idle is because the engines used are designed for high-rpm operation. To get the air/fuel mixture in the engine and the exhaust out at high rpm, race cars need a camshaft with as much lobe lift and as many degrees of duration as possible(as calculated from the port size camshaft duration equation explained later) and a narrow lobe-separation angle(LSA), which results in valve overlap. A cam like this has extremely poor idle quality the engine wont like run below 2000 rpm, but idle quality doesnt really matter on a race car. Also keep in mind that generally speaking, the more overlap present in the camshaft, the higher the desired compression ratio.
what effect does overlap have on flow/velocity flow reversion and intake plenum:#Within the plenum (engine at idle speed), when the intake and exhaust valves are open at the same time (overlap), the exhaust gas bleeds into the intake, interupting the flow, and also diluting the intake air with exhaust gas. The immediate effect is to cause that cylinder to "stumble". This will disrupt the smoothness of the engine by having random cylinders getting a poisened intake charge, either by flow interuption, or gas dilution. In the days of carbs, the flow interuption would actually have the fuel droplets drop out of the air stream to the bottom of the port.
Now, we all know that high duration leads to weaker lower rpm performance and stronger higher end performance. But some questions may rise like why? and is there a limit to how far we go? Cam is to the intake as pb is to j, so it can be argued that part of the gains could be from intake tuning but even with the intake factors absent it is seen that the relation between high and low rpms still holds.#
The key is inertia. Air is like a very heavy weight bus traveling down the road, the faster this bus is traveling the more it will take to stop it. Like the bus, when air is traveling fast(higher rpm) It will not want to stop and will continue to fill the cylinder well into the compression stroke. But at lower rpms the air will stop easier/sooner and begin to reverse back the other way if held open to long causing the lope sound as described
Overlap
is the period that the intake and exhaust valves are open at the same time.#
CALCULATING OVERLAP
The amount of overlap is what causes your "rough idle". You can figure this with this equation: Intake duration + Exaust duration, divided by 4, - LSA, x 2. The more overlap, the rougher the idle. So for these mustang 3.8 cams for example you can predict how they will sound or perform for the most part, although tuning, compression, ci, and runner length do play a factor...
calculated at .050"
210/226 compcam#44-703-9= -12
218/224 ssms billet cam= -3
218/226 compcam#44-704-9= -2
224/230 ssms billet cam= -1
224/232 ssms regrind cam= 0
236/244 ssms billet cam= 12
WHAT IS LIFT?
Lift refers to maximum valve lift. This is how much the valve is "lifted" off its seat at the cam lobe's highest point. The intake and exhaust valves need to be open to let air/fuel in and exhaust out of the cylinders. Generally, opening the valves quicker and further will increase engine output. Increasing valve lift, without increasing duration, can yield more power without much change to the nature of the power curve. However, an increase in valve lift almost always is accompanied by an increase in duration. This is because ramps are limited in their shape which is directly related to the type of lifters being used, such as flat or roller. Flat tapper lifters have a limited ramp rates due to the nature of friction, wear, and longevity reasons. I won't go to deep into this yet, let's keep this post simple..
WHAT IS DURATION?
Duration is the angle in crankshaft degrees that the valve stays off its seat during the lifting cycle of the cam lobe. Increasing duration keeps the valve open longer, and can increase high-rpm power. Doing so increases the RPM range that the engine produces power. By increasing duration without a change/or decrease in lobe separation angle will result in increased valve overlap
WHAT IS LSA?
LSA is the Lobe Separation Angle or the number of degrees between the intake and exhaust lobe centerlines. The LSA has a direct relationship to amount of overlap on a cam. Cams with identical duration and lift specifications can have very different LSAs. Generally speaking, a wide LSA will produce greater low end torque and a narrow separation angle will produce better top end power. For any given LSA, an engine will give similar torque curves and peak torque RPMs even with different overlaps.#
This is not an end all to everything camshafts, I have better things to do than post a thousand pages picking apart every piece of camshaft theory.
This post is 100% original and does not infringe upon any copyrights.
WHAT GIVES A CAM ROUGH OR LOPEY IDLE?
The reason most want the lopey idle is because race cars usually have a rough idle and we want our cars to look and sound like race cars. The reason race cars have that rough idle is because the engines used are designed for high-rpm operation. To get the air/fuel mixture in the engine and the exhaust out at high rpm, race cars need a camshaft with as much lobe lift and as many degrees of duration as possible(as calculated from the port size camshaft duration equation explained later) and a narrow lobe-separation angle(LSA), which results in valve overlap. A cam like this has extremely poor idle quality the engine wont like run below 2000 rpm, but idle quality doesnt really matter on a race car. Also keep in mind that generally speaking, the more overlap present in the camshaft, the higher the desired compression ratio.
what effect does overlap have on flow/velocity flow reversion and intake plenum:#Within the plenum (engine at idle speed), when the intake and exhaust valves are open at the same time (overlap), the exhaust gas bleeds into the intake, interupting the flow, and also diluting the intake air with exhaust gas. The immediate effect is to cause that cylinder to "stumble". This will disrupt the smoothness of the engine by having random cylinders getting a poisened intake charge, either by flow interuption, or gas dilution. In the days of carbs, the flow interuption would actually have the fuel droplets drop out of the air stream to the bottom of the port.
Now, we all know that high duration leads to weaker lower rpm performance and stronger higher end performance. But some questions may rise like why? and is there a limit to how far we go? Cam is to the intake as pb is to j, so it can be argued that part of the gains could be from intake tuning but even with the intake factors absent it is seen that the relation between high and low rpms still holds.#
The key is inertia. Air is like a very heavy weight bus traveling down the road, the faster this bus is traveling the more it will take to stop it. Like the bus, when air is traveling fast(higher rpm) It will not want to stop and will continue to fill the cylinder well into the compression stroke. But at lower rpms the air will stop easier/sooner and begin to reverse back the other way if held open to long causing the lope sound as described
Overlap
is the period that the intake and exhaust valves are open at the same time.#
CALCULATING OVERLAP
The amount of overlap is what causes your "rough idle". You can figure this with this equation: Intake duration + Exaust duration, divided by 4, - LSA, x 2. The more overlap, the rougher the idle. So for these mustang 3.8 cams for example you can predict how they will sound or perform for the most part, although tuning, compression, ci, and runner length do play a factor...
calculated at .050"
210/226 compcam#44-703-9= -12
218/224 ssms billet cam= -3
218/226 compcam#44-704-9= -2
224/230 ssms billet cam= -1
224/232 ssms regrind cam= 0
236/244 ssms billet cam= 12
WHAT IS LIFT?
Lift refers to maximum valve lift. This is how much the valve is "lifted" off its seat at the cam lobe's highest point. The intake and exhaust valves need to be open to let air/fuel in and exhaust out of the cylinders. Generally, opening the valves quicker and further will increase engine output. Increasing valve lift, without increasing duration, can yield more power without much change to the nature of the power curve. However, an increase in valve lift almost always is accompanied by an increase in duration. This is because ramps are limited in their shape which is directly related to the type of lifters being used, such as flat or roller. Flat tapper lifters have a limited ramp rates due to the nature of friction, wear, and longevity reasons. I won't go to deep into this yet, let's keep this post simple..
WHAT IS DURATION?
Duration is the angle in crankshaft degrees that the valve stays off its seat during the lifting cycle of the cam lobe. Increasing duration keeps the valve open longer, and can increase high-rpm power. Doing so increases the RPM range that the engine produces power. By increasing duration without a change/or decrease in lobe separation angle will result in increased valve overlap
WHAT IS LSA?
LSA is the Lobe Separation Angle or the number of degrees between the intake and exhaust lobe centerlines. The LSA has a direct relationship to amount of overlap on a cam. Cams with identical duration and lift specifications can have very different LSAs. Generally speaking, a wide LSA will produce greater low end torque and a narrow separation angle will produce better top end power. For any given LSA, an engine will give similar torque curves and peak torque RPMs even with different overlaps.#
This is not an end all to everything camshafts, I have better things to do than post a thousand pages picking apart every piece of camshaft theory.
This post is 100% original and does not infringe upon any copyrights.
