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

tmhutch · 05-13-2010, 06:12 AM

Overlap

Overlap is the period when both the intake and exhaust valves are open at the same time. It starts when the piston is approaching TDC on the exhaust stroke and lasts until just after TDC. In the 1950’s racer and camshaft designer Ed Iskendarian was blowing away the competition with his new 5 cycle cams and as a result the term 5th cycle is now common vernacular in camshaft terminology. It refers to the addition of a new cycle to the traditional induction, compression, combustion and exhaust grouping.

The power producing potential of overlap is considered second only to duration. Under the right conditions a cam utilizing the proper amount of overlap can easily pull 20 additional horsepower over an almost identical cam on wider lobe centers. If you look at the difference between cams of progressively more aggressive performance levels you will see that the additional duration is primarily used to increase overlap by opening the intake valve sooner and closing the exhaust valve later. In other words, the intake valve opening point is the most aggressively altered timing event when designing performance camshafts and it is followed closely by EVC whose primary function is dialing in the proper amount of overlap.

Of course, the extra power doesn’t come without a cost. Additional overlap often results in a rough idle, increased emissions, low vacuum and poor throttle response until the RPM’s are sufficient enough to overcome intake and exhaust reversion.

Let’s take an in depth look at overlap. In a performance application, the primary purpose of overlap is to allow the velocity of the escaping exhaust gasses to pull on the intake port while both valves are open. As the piston is rising in the bore and pushing exhaust out of the engine it will encounter a slowly opening intake valve about a half inch from TDC. Some race applications will start opening the intake valve earlier than 50* BTDC while some mild street applications can be as late as 3* BTDC. The exhaust valve closing ATDC has a similar range, it can be very early or quite late. If we add the number of degrees before TDC that the intake valve opens and the number of degrees ATDC that the exhaust valve closes, we get the number of degrees of overlap. Typical street engines have between 30* - 50* of overlap and race oriented engines can have more than 100*.

In performance applications the intake valve will start to open early enough that the piston will push some exhaust gas into the intake port (intake reversion) and some will continue to flow out the exhaust pipe. To the extent this reversion occurs depends primarily on how early the intake valve opens and how much back pressure exists at the valve and in the exhaust. As the piston proceeds up the bore and reaches TDC, the inertia created by the velocity of the escaping exhaust gasses pulls exhaust from the combustion chamber where the piston can’t reach. This tiny pocket of air between the top of the piston and the roof of the combustion chamber is referred to as “clearance volume”. Removing this pocket of left over exhaust gas is the minimum expectation of the escaping exhaust gasses even for the mildest of combinations. If a slightly more aggressive combination with an earlier opening intake valve is being built than the goal for the escaping exhaust gasses during the overlap period is to not only rid the chamber of exhaust, but to pull out the exhaust that was pushed up into the intake port so the induction stroke gets a clean, un-diluted intake charge. When we start talking about high performance applications, overlaps responsibilities become even bigger because the inertia of the escaping exhaust gasses is expected to initiate the intake charge and kick off the induction cycle.

The goal is to run enough overlap so that the exhaust has time to not only pull spent gasses from the combustion chamber where the piston cannot reach, to not only clear the intake port of reversion, but to also give the fresh intake charge a yank. This phenomenon where the exhaust is used to pull the intake charge into the cylinder is also known as flow through and it is the holy grail of overlap tuning.

Here is how David Vizard regards this aspect of overlap:
“With a well-tuned exhaust, we find that the strongest draw on the intake port is brought about by the negative pressure created by the exhaust - not, as is so often supposed, the piston going down the bore.”

You might want to read that twice. I know I did when I first read it.

And this is why it is so important:
“We find from port and cylinder pressure measurements, that success in the first half of the induction, dictates the success in the second half. If the first half is not optimal, there is nothing that can be done in the second half to rectify the problem.”

When building for maximum power the challenge is to leave the exhaust valve open long enough so that the escaping gasses can pull the fresh intake charge into the cylinder but snap shut just before it pulls the charge out the exhaust pipe. Some considerations when tuning overlap are the intended RPM range, the exhaust system, and drivability. Since this is a discussion built around performance applications we won’t spend time on drivability except to say that if rough idle and poor throttle response at low RPM are a concern, don’t run a lot of overlap.

RPM is the primary consideration when tuning overlap because it imparts the same time constraints that have to be considered when specifying other aspects of cam timing. As RPM increases, the overlap period has less time to evacuate exhaust and pull intake into the cylinder. Mechanically the amount of overlap remains the same but the amount of time the two valves have to create effective cross-flow is reduced. Consequently overlap has to be increased to keep both valves open long enough to be effective. If both valves aren’t held open long enough the intake won’t be jump started and exhaust gasses will not be effectively evacuated.

When overlap is spec’d for high RPM, the time quotient will be more than enough at low RPM. The most notable consequences being that either the intake charge is pulled out the exhaust or in the case of big primary tube headers that lack velocity, exhaust gasses will reverse direction and be sucked back in by the descending piston (exhaust reversion). Both conditions result in lower torque and a rough idle.

Aggressive overlap requires a good exhaust system. With the exhaust valve being held open for so long, any backpressure can reduce the effectiveness of the overlap period and result in reversion and polluting of the intake charge. The primary tube header needs to be sized big enough so there is no back pressure in the intended RPM range but not so big that it doesn’t have enough velocity to scavenge the combustion chamber and intake port. As stated in the EVC section, any primary tube big enough to handle 6800 RPM operation is going to be lazy enough at low RPM to fail at scavenging and likely have some exhaust reversion with a big cam. But when the RPM’s come up, hold on.

The theme continues; what works for high RPM doesn’t work for low RPM. We must choose.

There are a few things beside RPM that will have an influence on how much overlap we should run: Compression, combustion chamber size, crankshaft stroke, rod length, rod to stroke ratio, low lift port flow, rocker arm ratio and engine displacement.

1) Higher compression equals higher cylinder pressure which causes higher exit speeds for escaping gasses. This causes a more powerful scavenging ability requiring a shorter overlap period.

2) A smaller combustion chamber is evacuated more quickly. This equals less need for overlap.

3) Crankshaft stroke, rod length and rod to stroke ratio all affect how long the piston dwells at TDC and how many feet per second the piston travels before and after. This in turn affects the amount of time overlap has to get the job done.

4) Low lift port flow and rocker arm ratio both affect how well the intake port moves air at the beginning of the induction stroke. Improved low lift flow resulting from a well designed port and valve layout or higher ratio rockers reduce required overlap.

5) As engine displacement goes up, the same size intake port will need more help to feed the engine so overlap must be increased.
Now that we have a pretty good understanding of overlap we can move on to lobe separation angle.

Now that we have a pretty good understanding of overlap we can move on to lobe separation angle.

05-13-2010, 06:12 AM	#6
tmhutch 4v>3v>2v Join Date: Jun 2004 Location: Pacific Northwest Posts: 727	Re: Cam Science 101 and Beyond Overlap Overlap is the period when both the intake and exhaust valves are open at the same time. It starts when the piston is approaching TDC on the exhaust stroke and lasts until just after TDC. In the 1950’s racer and camshaft designer Ed Iskendarian was blowing away the competition with his new 5 cycle cams and as a result the term 5th cycle is now common vernacular in camshaft terminology. It refers to the addition of a new cycle to the traditional induction, compression, combustion and exhaust grouping. The power producing potential of overlap is considered second only to duration. Under the right conditions a cam utilizing the proper amount of overlap can easily pull 20 additional horsepower over an almost identical cam on wider lobe centers. If you look at the difference between cams of progressively more aggressive performance levels you will see that the additional duration is primarily used to increase overlap by opening the intake valve sooner and closing the exhaust valve later. In other words, the intake valve opening point is the most aggressively altered timing event when designing performance camshafts and it is followed closely by EVC whose primary function is dialing in the proper amount of overlap. Of course, the extra power doesn’t come without a cost. Additional overlap often results in a rough idle, increased emissions, low vacuum and poor throttle response until the RPM’s are sufficient enough to overcome intake and exhaust reversion. Let’s take an in depth look at overlap. In a performance application, the primary purpose of overlap is to allow the velocity of the escaping exhaust gasses to pull on the intake port while both valves are open. As the piston is rising in the bore and pushing exhaust out of the engine it will encounter a slowly opening intake valve about a half inch from TDC. Some race applications will start opening the intake valve earlier than 50* BTDC while some mild street applications can be as late as 3* BTDC. The exhaust valve closing ATDC has a similar range, it can be very early or quite late. If we add the number of degrees before TDC that the intake valve opens and the number of degrees ATDC that the exhaust valve closes, we get the number of degrees of overlap. Typical street engines have between 30* - 50* of overlap and race oriented engines can have more than 100*. In performance applications the intake valve will start to open early enough that the piston will push some exhaust gas into the intake port (intake reversion) and some will continue to flow out the exhaust pipe. To the extent this reversion occurs depends primarily on how early the intake valve opens and how much back pressure exists at the valve and in the exhaust. As the piston proceeds up the bore and reaches TDC, the inertia created by the velocity of the escaping exhaust gasses pulls exhaust from the combustion chamber where the piston can’t reach. This tiny pocket of air between the top of the piston and the roof of the combustion chamber is referred to as “clearance volume”. Removing this pocket of left over exhaust gas is the minimum expectation of the escaping exhaust gasses even for the mildest of combinations. If a slightly more aggressive combination with an earlier opening intake valve is being built than the goal for the escaping exhaust gasses during the overlap period is to not only rid the chamber of exhaust, but to pull out the exhaust that was pushed up into the intake port so the induction stroke gets a clean, un-diluted intake charge. When we start talking about high performance applications, overlaps responsibilities become even bigger because the inertia of the escaping exhaust gasses is expected to initiate the intake charge and kick off the induction cycle. The goal is to run enough overlap so that the exhaust has time to not only pull spent gasses from the combustion chamber where the piston cannot reach, to not only clear the intake port of reversion, but to also give the fresh intake charge a yank. This phenomenon where the exhaust is used to pull the intake charge into the cylinder is also known as flow through and it is the holy grail of overlap tuning. Here is how David Vizard regards this aspect of overlap: “With a well-tuned exhaust, we find that the strongest draw on the intake port is brought about by the negative pressure created by the exhaust - not, as is so often supposed, the piston going down the bore.” You might want to read that twice. I know I did when I first read it. And this is why it is so important: “We find from port and cylinder pressure measurements, that success in the first half of the induction, dictates the success in the second half. If the first half is not optimal, there is nothing that can be done in the second half to rectify the problem.” When building for maximum power the challenge is to leave the exhaust valve open long enough so that the escaping gasses can pull the fresh intake charge into the cylinder but snap shut just before it pulls the charge out the exhaust pipe. Some considerations when tuning overlap are the intended RPM range, the exhaust system, and drivability. Since this is a discussion built around performance applications we won’t spend time on drivability except to say that if rough idle and poor throttle response at low RPM are a concern, don’t run a lot of overlap. RPM is the primary consideration when tuning overlap because it imparts the same time constraints that have to be considered when specifying other aspects of cam timing. As RPM increases, the overlap period has less time to evacuate exhaust and pull intake into the cylinder. Mechanically the amount of overlap remains the same but the amount of time the two valves have to create effective cross-flow is reduced. Consequently overlap has to be increased to keep both valves open long enough to be effective. If both valves aren’t held open long enough the intake won’t be jump started and exhaust gasses will not be effectively evacuated. When overlap is spec’d for high RPM, the time quotient will be more than enough at low RPM. The most notable consequences being that either the intake charge is pulled out the exhaust or in the case of big primary tube headers that lack velocity, exhaust gasses will reverse direction and be sucked back in by the descending piston (exhaust reversion). Both conditions result in lower torque and a rough idle. Aggressive overlap requires a good exhaust system. With the exhaust valve being held open for so long, any backpressure can reduce the effectiveness of the overlap period and result in reversion and polluting of the intake charge. The primary tube header needs to be sized big enough so there is no back pressure in the intended RPM range but not so big that it doesn’t have enough velocity to scavenge the combustion chamber and intake port. As stated in the EVC section, any primary tube big enough to handle 6800 RPM operation is going to be lazy enough at low RPM to fail at scavenging and likely have some exhaust reversion with a big cam. But when the RPM’s come up, hold on. The theme continues; what works for high RPM doesn’t work for low RPM. We must choose. There are a few things beside RPM that will have an influence on how much overlap we should run: Compression, combustion chamber size, crankshaft stroke, rod length, rod to stroke ratio, low lift port flow, rocker arm ratio and engine displacement. 1) Higher compression equals higher cylinder pressure which causes higher exit speeds for escaping gasses. This causes a more powerful scavenging ability requiring a shorter overlap period. 2) A smaller combustion chamber is evacuated more quickly. This equals less need for overlap. 3) Crankshaft stroke, rod length and rod to stroke ratio all affect how long the piston dwells at TDC and how many feet per second the piston travels before and after. This in turn affects the amount of time overlap has to get the job done. 4) Low lift port flow and rocker arm ratio both affect how well the intake port moves air at the beginning of the induction stroke. Improved low lift flow resulting from a well designed port and valve layout or higher ratio rockers reduce required overlap. 5) As engine displacement goes up, the same size intake port will need more help to feed the engine so overlap must be increased. Now that we have a pretty good understanding of overlap we can move on to lobe separation angle. Now that we have a pretty good understanding of overlap we can move on to lobe separation angle.