2003-04 Mach 1 Registry Owners Club - View Single Post

tmhutch · 05-13-2010, 06:10 AM

Intake Valve Opening

Determining where to open the intake valve is critical to the success of the entire induction cycle for the same reason advancing the intake lobe centerline is beneficial to power production. Much like an advanced intake lobe centerline, an earlier intake valve opening is preferred in performance applications because it gets the valve further off the seat and into a more functional range when the piston begins to draw on the intake port. In addition to the valve being further off the seat earlier in the induction phase, the earlier opening increases overlap. Both elements enhance flow in the critical early stages of the induction cycle. Of course the benefits are not without cost and there is such thing as too much of a good thing. As in most everything camshaft related the added benefits of an earlier intake opening can create issues elsewhere in the RPM range.

The Downside

The earlier the intake valve is opened on the exhaust stroke the more exhaust is pushed up into the intake tract by the piston coming up the bore (intake reversion). This dilutes the intake charge and contributes to rough running at low RPM and if excessive enough, the narrowing of the RPM band and loss of power at low to mid RPM. Also contributing to poor low speed operation is increased overlap. At low RPM, overlap can over scavenge the cylinder and pull the intake charge out the exhaust, or it can allow exhaust to flow backwards into the cylinder (exhaust reversion) and pollute the intake charge.

Another potential issue with overlap is that in common plenum intakes a cylinder can pull exhaust in from the next cylinder in the firing sequence as it begins its induction stroke and overlap period. This is known as “cross talk” and it occurs when the throttle is in the closed or near closed position causing high manifold vacuum. When this happens it is easier for cylinder 1 to pull air from cylinder 2 than to try to pull it past the throttle blade. The result is lowered signal response to the throttle body or carburetor and a diluted intake charge causing rough low speed operation.

The Debate

According to Harold Brookshire, IVO is one of the top two most important events in cam timing because the amount of exhaust pressure that exists at intake opening impacts how well the port flows in the early stages of the induction cycle. But not all the experts agree. Ron Iskendarian also has an opinion on early IVO:

“When the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little (virtually no) exhaust gases remain in the cylinder. It is a "myth" that an earlier opening of the intake valve (even by a mere 2 or 3 degrees) causes the phenomenon known as "reversion". Nothing could be further from the truth! This misconception not only defies common sense, it also establishes a false premise from which other, incorrect conclusions can be drawn. Simply put, those who focus on overlap are on the wrong end of the cam-timing diagram!” Ron Iskendarian

On the other hand, Racer Brown has this to say:

“When the intake valve opens early, the engine will usually immediately respond by being rough and balky at low engine speeds. This occurs because of the greater dilution effect the exhaust gases have on the air/fuel mixture charge as the mixture attempts to enter the cylinder. As engine speed increases, the velocity and inertia of the mixture charge overcomes most (not all) exhaust gas dilution and helps power output at higher engine speeds. Very early intake valve opening points will kill performance in the low and mid-range speeds, making engine power and response acceptable only in the highest engine speed ranges.” Racer Brown

Once again we have some of the industry’s most knowledgeable cam designers disagreeing. Racer Brown is known by many as the Father of the Camshaft because of his pioneering work in the field. Harold Brookshire’s track record of success is indisputable, and Ron is the son of famous cam grinder Ed Iskendarian. Each of these individuals has vast knowledge of camshaft design and should be studied a little further for a complete understanding of their perception. For now we’ll go with the prevailing opinion that opening the intake valve early can create some level of reversion.

IVO Nuts and Bolts

Opening the valve earlier can be accomplished by either adding duration to the lobe or advancing it. As discussed earlier, the primary purpose for adding duration to a lobe is to extend the RPM range for additional power by allowing the valve to stay open for a longer period of time and to close the valve later after bottom dead center. Adding duration for the purpose of an earlier intake valve opening can only be beneficial up to a point because it has the potential to extend the cams functional range beyond the engines capability.

If RPM limitations of certain components of the engine are preventing the builder from adding enough duration to achieve the desired IVO point we can simply advance the entire lobe. The earlier opening is good for getting the valve further off the seat earlier in the induction stroke and aligning the lobe center closer to maximum piston draw. As an added benefit, the earlier closing (not earlier opening) of the valve actually brings the RPM range back down a bit. Some engine builders subscribe to the philosophy that adding as much duration as the combination needs to get the job done at the appropriate RPM is the first priority followed by advancing the intake lobe as far as possible within the constraints dictated by early IVO, IVC, and overlap. But don’t forget, changes on the opening side create changes on the closing side.

Intake Lobe Centerline

In a perfect world the ideal intake lobe for any engine would be full valve lift coinciding with the point in the induction cycle where the piston is creating the most suction on the intake port. At low RPM this occurs between the point of maximum piston speed and peak valve lift, but it changes with the RPM of the engine. This is one of the reasons we can only tune for a limited RPM range. At low RPM the greatest pressure differential occurs around 30* ATDC. By mid RPM it is somewhere between 70 – 80 degrees ATDC, and at high RPM it occurs around 100 degrees ATDC. According to David Vizard, once maximum pressure disparity occurs past 115* ATDC the engine has reached its maximum potential and will not produce any additional power no matter how high the engine is spun. This number is not exact for every engine but it represents a good approximation.

To get a better understanding of the critical placement of the ICL we have to re-visit the relationship between the movement of mechanical parts and air mass. Simply stated; as RPM increases, the air mass becomes less cooperative. At low engine speeds the air can move fast enough to keep up with the piston but air and fuel mixture carries mass that takes time to get moving. Where mechanical parts can be forced to move instantaneously, air will take its time no matter how we try to shove it around. Consequently, as RPM increases, the piston makes its way further down the bore before the air begins to follow it. As this delayed reaction increases, the useful distance in the pistons downward stroke that becomes shorter. At a certain point (around 115 ATDC) the piston can no longer pull in enough air to properly feed the engine. Picture the piston only pulling on the air/fuel for 35* of the downward stroke and you can see how the engine would run out of breath. The RPM at which this occurs is largely dependent on how restrictive the intake path is. A more restrictive path causes this to happen at a lower RPM.

It’s amazing an engine can run at all when the intake charge doesn’t begin moving until the piston is almost to the bottom of the bore. Where 35* is not enough for the engine to continue making additional power, 55* of piston travel is enough to make tremendous power. How is that possible? It is initially made possible through an effective overlap period that can jump start the air entering the cylinder without the help of the piston. As the air front makes its way into the bore it encounters a powerful suction created by the large distance between itself and the fast moving piston. Up to a point this more powerful pressure differential and subsequent faster moving air mass can compensate for the reduction in piston draw. Think of the air as a ball with a rubber band attached. The piston is like the hand pulling the ball (column of air) by the rubber band (pressure differential) between the two. If we use the rubber band to slowly pick the ball up off the floor, it will begin to move almost instantaneously with the hand movement and it will follow the hand at roughly the same speed. Now picture jerking the ball off the floor with the rubber band. Obviously the hand will travel some distance stretching the rubber band before the ball begins moving. However, once the ball starts to react to the force acting against it by the stretched rubber band, it begins moving very quickly and with a lot of inertia. So much so that even though the hand stops just as the piston does at the bottom of the bore, the ball will utilize the kinetic energy created by the rubber band to continue moving. It is the same with the piston, the further it moves away from the air mass, the greater the pressure differential is. When the air finally starts moving, it is playing serious catch up and literally acts like a ball being pulled by an outstretched rubber band. It moves fast and makes up for lost time. Between the time the delayed air front starts moving and the piston travels the final 55* to the bottom of the bore, the column of air has picked up enough speed to not only catch up to the piston but to overcome the force of the piston pushing back up against it and to overfill the cylinder through inertial ramming.

Advancing and Retarding the Intake Lobe Centerline

You might notice that vehicles that compete in stock class racing where cylinder head ports cannot be altered from their original casting usually have extremely advanced intake lobes and a lot of overlap. This is done to make up for the engines inability to RPM. The advanced intake centerline reduces power at the top of the RPM scale and concentrates it lower in the RPM range. Unrestricted classes like pro-stock don’t require near as much advance and overlap because they utilize monster ports and valves relative to the cubic inch dimensions of the engine.

Advancing ICL

Advancing the intake lobe opening point can help performance in a few ways. Opening the valve sooner increases overlap and allows the exhaust to have a greater influence on the intake charge as the intake valve is opening. Advancing the entire lobe also moves the centerline and effective lift of the opening ramp closer to the pistons highest point of velocity where it produces the most suction or pressure differential. Both of these attributes help kick start airflow allowing a weaker inlet path to appear bigger to the cylinder. As David Vizard said; what happens at the beginning of the intake valve opening determines the success of the entire intake event.

Possibly the most beneficial component of advancing the ICL is the elevated dynamic compression ratio resulting from the earlier intake valve closing point. Raising the DCR from 8.0:1 to 9.0:1 is just like raising the static compression ratio because they both have a direct affect on cylinder pressure. However, unlike static compression the benefits of advancing the intake centerline and increased dynamic compression diminish as RPM increases. The reason for this is that as RPM increases an engine depends more on inertial charging to fill the cylinder. It is one of the primary elements of achieving volumetric efficiencies beyond 100%. Closing the intake valve sooner inhibits inertial charging. The specific RPM at which the benefits of inertial charging begin to out way the benefits of higher dynamic compression can be as low as 4,000 RPM or as high as 7,000 RPM depending on the characteristics of the cylinder head and its' capabilities.

As noted earlier, the biggest problem with creating the perfect lobe is that maximum pressure differentials occur too late in the induction stroke to align with the lobes centerline. As we advance the lobe further to attempt parity we encounter some negative side effects that limit how far we can go. The first is limited RPM from the intake valve closing sooner and hindering inertial ramming at the end of the induction cycle. The second is intake reversion gets worse as the intake valve is opened earlier in the exhaust cycle causing rough running at low RPM and requiring more engine speed to get into the cams comfort zone. These two elements combined can narrow the power band. The final problem we have with advancing the intake lobe is excessive overlap. This can over scavenge the cylinder, pull intake charge out the exhaust and allow exhaust reversion back into the cylinder. Excessive overlap can also create cross scavenging between adjacent cylinders in the firing order. Advancing the intake lobe can be beneficial but care must be taken not to overdo it.

Retarding ICL

At this point it is probably somewhat obvious what happens when retarding the intake lobe. We lose the benefit of a more efficient opening event and increased dynamic compression but it delays the closing point and provides improved inertial charging for high RPM operation. It also reduces overlap which improves low RPM drivability but can hinder high RPM operation. Around in circles we go but as usual each one of these conditions occurs to more or less extremes based on the engine combination. If the engine is seriously starving for more RPM then retarding the lobe up to a certain point will help remedy the problem. If the engine cannot tolerate much RPM than advancing the lobe would be the preferred adjustment.

Again a balance must be struck between opposing benefits and consequences and the extent to which each attribute contributes to or hinders power production depends on several other factors in the combination.

05-13-2010, 06:10 AM	#4
tmhutch 4v>3v>2v Join Date: Jun 2004 Location: Pacific Northwest Posts: 727	Re: Cam Science 101 and Beyond Intake Valve Opening Determining where to open the intake valve is critical to the success of the entire induction cycle for the same reason advancing the intake lobe centerline is beneficial to power production. Much like an advanced intake lobe centerline, an earlier intake valve opening is preferred in performance applications because it gets the valve further off the seat and into a more functional range when the piston begins to draw on the intake port. In addition to the valve being further off the seat earlier in the induction phase, the earlier opening increases overlap. Both elements enhance flow in the critical early stages of the induction cycle. Of course the benefits are not without cost and there is such thing as too much of a good thing. As in most everything camshaft related the added benefits of an earlier intake opening can create issues elsewhere in the RPM range. The Downside The earlier the intake valve is opened on the exhaust stroke the more exhaust is pushed up into the intake tract by the piston coming up the bore (intake reversion). This dilutes the intake charge and contributes to rough running at low RPM and if excessive enough, the narrowing of the RPM band and loss of power at low to mid RPM. Also contributing to poor low speed operation is increased overlap. At low RPM, overlap can over scavenge the cylinder and pull the intake charge out the exhaust, or it can allow exhaust to flow backwards into the cylinder (exhaust reversion) and pollute the intake charge. Another potential issue with overlap is that in common plenum intakes a cylinder can pull exhaust in from the next cylinder in the firing sequence as it begins its induction stroke and overlap period. This is known as “cross talk” and it occurs when the throttle is in the closed or near closed position causing high manifold vacuum. When this happens it is easier for cylinder 1 to pull air from cylinder 2 than to try to pull it past the throttle blade. The result is lowered signal response to the throttle body or carburetor and a diluted intake charge causing rough low speed operation. The Debate According to Harold Brookshire, IVO is one of the top two most important events in cam timing because the amount of exhaust pressure that exists at intake opening impacts how well the port flows in the early stages of the induction cycle. But not all the experts agree. Ron Iskendarian also has an opinion on early IVO: “When the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little (virtually no) exhaust gases remain in the cylinder. It is a "myth" that an earlier opening of the intake valve (even by a mere 2 or 3 degrees) causes the phenomenon known as "reversion". Nothing could be further from the truth! This misconception not only defies common sense, it also establishes a false premise from which other, incorrect conclusions can be drawn. Simply put, those who focus on overlap are on the wrong end of the cam-timing diagram!” Ron Iskendarian On the other hand, Racer Brown has this to say: “When the intake valve opens early, the engine will usually immediately respond by being rough and balky at low engine speeds. This occurs because of the greater dilution effect the exhaust gases have on the air/fuel mixture charge as the mixture attempts to enter the cylinder. As engine speed increases, the velocity and inertia of the mixture charge overcomes most (not all) exhaust gas dilution and helps power output at higher engine speeds. Very early intake valve opening points will kill performance in the low and mid-range speeds, making engine power and response acceptable only in the highest engine speed ranges.” Racer Brown Once again we have some of the industry’s most knowledgeable cam designers disagreeing. Racer Brown is known by many as the Father of the Camshaft because of his pioneering work in the field. Harold Brookshire’s track record of success is indisputable, and Ron is the son of famous cam grinder Ed Iskendarian. Each of these individuals has vast knowledge of camshaft design and should be studied a little further for a complete understanding of their perception. For now we’ll go with the prevailing opinion that opening the intake valve early can create some level of reversion. IVO Nuts and Bolts Opening the valve earlier can be accomplished by either adding duration to the lobe or advancing it. As discussed earlier, the primary purpose for adding duration to a lobe is to extend the RPM range for additional power by allowing the valve to stay open for a longer period of time and to close the valve later after bottom dead center. Adding duration for the purpose of an earlier intake valve opening can only be beneficial up to a point because it has the potential to extend the cams functional range beyond the engines capability. If RPM limitations of certain components of the engine are preventing the builder from adding enough duration to achieve the desired IVO point we can simply advance the entire lobe. The earlier opening is good for getting the valve further off the seat earlier in the induction stroke and aligning the lobe center closer to maximum piston draw. As an added benefit, the earlier closing (not earlier opening) of the valve actually brings the RPM range back down a bit. Some engine builders subscribe to the philosophy that adding as much duration as the combination needs to get the job done at the appropriate RPM is the first priority followed by advancing the intake lobe as far as possible within the constraints dictated by early IVO, IVC, and overlap. But don’t forget, changes on the opening side create changes on the closing side. Intake Lobe Centerline In a perfect world the ideal intake lobe for any engine would be full valve lift coinciding with the point in the induction cycle where the piston is creating the most suction on the intake port. At low RPM this occurs between the point of maximum piston speed and peak valve lift, but it changes with the RPM of the engine. This is one of the reasons we can only tune for a limited RPM range. At low RPM the greatest pressure differential occurs around 30* ATDC. By mid RPM it is somewhere between 70 – 80 degrees ATDC, and at high RPM it occurs around 100 degrees ATDC. According to David Vizard, once maximum pressure disparity occurs past 115* ATDC the engine has reached its maximum potential and will not produce any additional power no matter how high the engine is spun. This number is not exact for every engine but it represents a good approximation. To get a better understanding of the critical placement of the ICL we have to re-visit the relationship between the movement of mechanical parts and air mass. Simply stated; as RPM increases, the air mass becomes less cooperative. At low engine speeds the air can move fast enough to keep up with the piston but air and fuel mixture carries mass that takes time to get moving. Where mechanical parts can be forced to move instantaneously, air will take its time no matter how we try to shove it around. Consequently, as RPM increases, the piston makes its way further down the bore before the air begins to follow it. As this delayed reaction increases, the useful distance in the pistons downward stroke that becomes shorter. At a certain point (around 115 ATDC) the piston can no longer pull in enough air to properly feed the engine. Picture the piston only pulling on the air/fuel for 35* of the downward stroke and you can see how the engine would run out of breath. The RPM at which this occurs is largely dependent on how restrictive the intake path is. A more restrictive path causes this to happen at a lower RPM. It’s amazing an engine can run at all when the intake charge doesn’t begin moving until the piston is almost to the bottom of the bore. Where 35* is not enough for the engine to continue making additional power, 55* of piston travel is enough to make tremendous power. How is that possible? It is initially made possible through an effective overlap period that can jump start the air entering the cylinder without the help of the piston. As the air front makes its way into the bore it encounters a powerful suction created by the large distance between itself and the fast moving piston. Up to a point this more powerful pressure differential and subsequent faster moving air mass can compensate for the reduction in piston draw. Think of the air as a ball with a rubber band attached. The piston is like the hand pulling the ball (column of air) by the rubber band (pressure differential) between the two. If we use the rubber band to slowly pick the ball up off the floor, it will begin to move almost instantaneously with the hand movement and it will follow the hand at roughly the same speed. Now picture jerking the ball off the floor with the rubber band. Obviously the hand will travel some distance stretching the rubber band before the ball begins moving. However, once the ball starts to react to the force acting against it by the stretched rubber band, it begins moving very quickly and with a lot of inertia. So much so that even though the hand stops just as the piston does at the bottom of the bore, the ball will utilize the kinetic energy created by the rubber band to continue moving. It is the same with the piston, the further it moves away from the air mass, the greater the pressure differential is. When the air finally starts moving, it is playing serious catch up and literally acts like a ball being pulled by an outstretched rubber band. It moves fast and makes up for lost time. Between the time the delayed air front starts moving and the piston travels the final 55* to the bottom of the bore, the column of air has picked up enough speed to not only catch up to the piston but to overcome the force of the piston pushing back up against it and to overfill the cylinder through inertial ramming. Advancing and Retarding the Intake Lobe Centerline You might notice that vehicles that compete in stock class racing where cylinder head ports cannot be altered from their original casting usually have extremely advanced intake lobes and a lot of overlap. This is done to make up for the engines inability to RPM. The advanced intake centerline reduces power at the top of the RPM scale and concentrates it lower in the RPM range. Unrestricted classes like pro-stock don’t require near as much advance and overlap because they utilize monster ports and valves relative to the cubic inch dimensions of the engine. Advancing ICL Advancing the intake lobe opening point can help performance in a few ways. Opening the valve sooner increases overlap and allows the exhaust to have a greater influence on the intake charge as the intake valve is opening. Advancing the entire lobe also moves the centerline and effective lift of the opening ramp closer to the pistons highest point of velocity where it produces the most suction or pressure differential. Both of these attributes help kick start airflow allowing a weaker inlet path to appear bigger to the cylinder. As David Vizard said; what happens at the beginning of the intake valve opening determines the success of the entire intake event. Possibly the most beneficial component of advancing the ICL is the elevated dynamic compression ratio resulting from the earlier intake valve closing point. Raising the DCR from 8.0:1 to 9.0:1 is just like raising the static compression ratio because they both have a direct affect on cylinder pressure. However, unlike static compression the benefits of advancing the intake centerline and increased dynamic compression diminish as RPM increases. The reason for this is that as RPM increases an engine depends more on inertial charging to fill the cylinder. It is one of the primary elements of achieving volumetric efficiencies beyond 100%. Closing the intake valve sooner inhibits inertial charging. The specific RPM at which the benefits of inertial charging begin to out way the benefits of higher dynamic compression can be as low as 4,000 RPM or as high as 7,000 RPM depending on the characteristics of the cylinder head and its' capabilities. As noted earlier, the biggest problem with creating the perfect lobe is that maximum pressure differentials occur too late in the induction stroke to align with the lobes centerline. As we advance the lobe further to attempt parity we encounter some negative side effects that limit how far we can go. The first is limited RPM from the intake valve closing sooner and hindering inertial ramming at the end of the induction cycle. The second is intake reversion gets worse as the intake valve is opened earlier in the exhaust cycle causing rough running at low RPM and requiring more engine speed to get into the cams comfort zone. These two elements combined can narrow the power band. The final problem we have with advancing the intake lobe is excessive overlap. This can over scavenge the cylinder, pull intake charge out the exhaust and allow exhaust reversion back into the cylinder. Excessive overlap can also create cross scavenging between adjacent cylinders in the firing order. Advancing the intake lobe can be beneficial but care must be taken not to overdo it. Retarding ICL At this point it is probably somewhat obvious what happens when retarding the intake lobe. We lose the benefit of a more efficient opening event and increased dynamic compression but it delays the closing point and provides improved inertial charging for high RPM operation. It also reduces overlap which improves low RPM drivability but can hinder high RPM operation. Around in circles we go but as usual each one of these conditions occurs to more or less extremes based on the engine combination. If the engine is seriously starving for more RPM then retarding the lobe up to a certain point will help remedy the problem. If the engine cannot tolerate much RPM than advancing the lobe would be the preferred adjustment. Again a balance must be struck between opposing benefits and consequences and the extent to which each attribute contributes to or hinders power production depends on several other factors in the combination. Last edited by tmhutch; 05-22-2010 at 12:53 AM.