andersbrink
hasenfrasen
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William H. Carpenter writes:
Wednesday, August 30, 2000 - 04:40 pm
Basic theory on a 4-valve engine says because the air can enter easier than a 2-valve it can also leave easier. This means that the "supercharge" effect that you get on a 2-valve motor with big numbers(110+ on intake)only pumps the mixture back into the intake tract. The end result is a motor that runs good up high but really doesn't make the best power. The motor cannot begin to breathe until the intake valve is fully off the seat. To get maximum effect the valve should be open as far as the valve cutouts in the piston will allow before the piston starts down on the intake stroke. The overlap should be split at this point,if the intake open number is say 20 degrees then the exhaust closing number should also be 20 degrees.
I have found that using short numbers on the exhaust say 15 degrees allow the exhaust valve to clear the just beginning to open intake. You also get some piston safety for valve float. These twisted numbers also give some more blowdown for the exhaust. All cam timing is subjective,that is give some get some, but this arrangement will give a nice feeling motor. My latest motors use 5 degrees stagger in the overlap split. Short closing the intake valve also allows the motor to begin compression sooner.
I only use the lobe center method after I have seen the total numbers. The standard method of mixing cams should be a high lift,short duration intake combined with a lesser lift,longer duration exhaust. The extra duration on the exhaust is used as openning the exhaust sooner to get more blowdown. The reason that the exhaust is closed sooner on the overlap is to allow the motor to begin to draw. The piston can't suck any intake in with the exhaust valve open. The overlap effect works best if the numbers are bigger on the intake than exhaust with split overlap being the end of the range of adjustment in cam timing.
It is very rare that any engine will work better with the intake lobe center larger than the exhaust. Using the 108 intake 100 exhaust combination is dangerous because the exaust valve will "hang" on closing interfering with the intake openning. There is also less piston to ex valve clearance so if the engine is over revved you get bent valves. The key to power is the intake lobe center with the exhaust moved to keep the torque curve as flat as can be made or with an increasing torque peak. If you use 108 100 the effect will be a late "hit" to get on the power band plus it will die right at the top because of the short blowdown.
The effect of overlap only helps in the midrange. The reason is the exhaust system returns a wave just as the exhaust valve is closing which pulls across the top of the piston to help start the intake flow. The ex valve closes and the piston which is going down now continues to draw as the intake finishes opening. The point of max piston speed occurs when the con rod is at 90 degrees to the throw which is like 75 to 78 degrees after top dead center.
The biggest help is to get the intake open as far as possible before the piston reaches max speed. When the lobe center is big like 110 the piston is pulling against a valve that is not open very far. The use of lobe center in a discussion is decieving unless you know the total numbers ie IO at 20 BTDC IC at 50 ABDC and IO at 30 BTDC IC at 60 ABDC both are 105 lobe center. The cams are degreed using a number like 1mm lift(.040). The reason this is done is to be sure that valve motion has started.
The reason for .040 is assuming that flow cannot take place until the valve has moved that far off the seat. Using stock cams the valve motions are kind of slow but race cams have a quicker action meaning the accelerations on the valves are more abusive. At some point the valves will collide with something, either each other or the piston. If the cams are mild then the duration is less than what the engine can use,but when radical cams are used they can be degreed wrong on either end. This means that blowback will occur in the intake tract either at overlap(piston near the top)or at the bottom.
If you ride a bike with short numbers they have a cammy sound, rough idle, etc. If you move them to big numbers(110+) the idle will be really smooth because at slow speeds the mixture is being pushed back out by the rising piston. As speed increases the high flow in the intake continues coming in even after the piston is rising giving a "supercharge" effect. This works really well in a 2 valve design because the port is similar to a funnel. On a 4 valve design because the valve areas become large quickly the mixture is pumped back out just as easy as it was pumped in. You must consider all of the valve timing events NOT just lobe center.
If you only look at overlap the amount the intake opens before top center should be equal to the amount the exhaust closes after top center. This is called split overlap or the cams are installed straight up or square. I prefer using numbers that make the ex number 1 or 2 degees less than the intake number this will smooth the idle. If the ex number is bigger than the intake then the overlap effect is wasted.
All cam timing is subjective only you can decide if you like the feel. If the cams are the same total numbers ie 250 degrees of duration then the numbers should be square IO 20 IC 50 EO 50 EC 20 at 105 lobe center. I would prefer these numbers become like IO 20 IC 50 EO 52 EC 18 at 105 intake and 107 ex lobe center. Lately my stagger numbers are more like IO 20 IC 50 EO 55 EC 15 for 105 intake and 110 ex lobe center. The next choice to move the power band would be IO 25 IC 45 EO 50 EC 20 for 100 intake and 105 ex lobe center.
All engines feel similar ie a 110 lobe center motor will want to run up high and be dead at the bottom a 100 lobe center motor will run strong at the bottom and tend to die up high. My bikes are Honda 4 valve motors and I have many different engine sizes and cam shaft combinations. It really suprised me when a larger intake cam slowed the bike down regardless of lobe center this means the mixture was being pumped out either at the top or at the bottom.
William H. Carpenter writes:
Wednesday, August 30, 2000 - 04:40 pm
Basic theory on a 4-valve engine says because the air can enter easier than a 2-valve it can also leave easier. This means that the "supercharge" effect that you get on a 2-valve motor with big numbers(110+ on intake)only pumps the mixture back into the intake tract. The end result is a motor that runs good up high but really doesn't make the best power. The motor cannot begin to breathe until the intake valve is fully off the seat. To get maximum effect the valve should be open as far as the valve cutouts in the piston will allow before the piston starts down on the intake stroke. The overlap should be split at this point,if the intake open number is say 20 degrees then the exhaust closing number should also be 20 degrees.
I have found that using short numbers on the exhaust say 15 degrees allow the exhaust valve to clear the just beginning to open intake. You also get some piston safety for valve float. These twisted numbers also give some more blowdown for the exhaust. All cam timing is subjective,that is give some get some, but this arrangement will give a nice feeling motor. My latest motors use 5 degrees stagger in the overlap split. Short closing the intake valve also allows the motor to begin compression sooner.
I only use the lobe center method after I have seen the total numbers. The standard method of mixing cams should be a high lift,short duration intake combined with a lesser lift,longer duration exhaust. The extra duration on the exhaust is used as openning the exhaust sooner to get more blowdown. The reason that the exhaust is closed sooner on the overlap is to allow the motor to begin to draw. The piston can't suck any intake in with the exhaust valve open. The overlap effect works best if the numbers are bigger on the intake than exhaust with split overlap being the end of the range of adjustment in cam timing.
It is very rare that any engine will work better with the intake lobe center larger than the exhaust. Using the 108 intake 100 exhaust combination is dangerous because the exaust valve will "hang" on closing interfering with the intake openning. There is also less piston to ex valve clearance so if the engine is over revved you get bent valves. The key to power is the intake lobe center with the exhaust moved to keep the torque curve as flat as can be made or with an increasing torque peak. If you use 108 100 the effect will be a late "hit" to get on the power band plus it will die right at the top because of the short blowdown.
The effect of overlap only helps in the midrange. The reason is the exhaust system returns a wave just as the exhaust valve is closing which pulls across the top of the piston to help start the intake flow. The ex valve closes and the piston which is going down now continues to draw as the intake finishes opening. The point of max piston speed occurs when the con rod is at 90 degrees to the throw which is like 75 to 78 degrees after top dead center.
The biggest help is to get the intake open as far as possible before the piston reaches max speed. When the lobe center is big like 110 the piston is pulling against a valve that is not open very far. The use of lobe center in a discussion is decieving unless you know the total numbers ie IO at 20 BTDC IC at 50 ABDC and IO at 30 BTDC IC at 60 ABDC both are 105 lobe center. The cams are degreed using a number like 1mm lift(.040). The reason this is done is to be sure that valve motion has started.
The reason for .040 is assuming that flow cannot take place until the valve has moved that far off the seat. Using stock cams the valve motions are kind of slow but race cams have a quicker action meaning the accelerations on the valves are more abusive. At some point the valves will collide with something, either each other or the piston. If the cams are mild then the duration is less than what the engine can use,but when radical cams are used they can be degreed wrong on either end. This means that blowback will occur in the intake tract either at overlap(piston near the top)or at the bottom.
If you ride a bike with short numbers they have a cammy sound, rough idle, etc. If you move them to big numbers(110+) the idle will be really smooth because at slow speeds the mixture is being pushed back out by the rising piston. As speed increases the high flow in the intake continues coming in even after the piston is rising giving a "supercharge" effect. This works really well in a 2 valve design because the port is similar to a funnel. On a 4 valve design because the valve areas become large quickly the mixture is pumped back out just as easy as it was pumped in. You must consider all of the valve timing events NOT just lobe center.
If you only look at overlap the amount the intake opens before top center should be equal to the amount the exhaust closes after top center. This is called split overlap or the cams are installed straight up or square. I prefer using numbers that make the ex number 1 or 2 degees less than the intake number this will smooth the idle. If the ex number is bigger than the intake then the overlap effect is wasted.
All cam timing is subjective only you can decide if you like the feel. If the cams are the same total numbers ie 250 degrees of duration then the numbers should be square IO 20 IC 50 EO 50 EC 20 at 105 lobe center. I would prefer these numbers become like IO 20 IC 50 EO 52 EC 18 at 105 intake and 107 ex lobe center. Lately my stagger numbers are more like IO 20 IC 50 EO 55 EC 15 for 105 intake and 110 ex lobe center. The next choice to move the power band would be IO 25 IC 45 EO 50 EC 20 for 100 intake and 105 ex lobe center.
All engines feel similar ie a 110 lobe center motor will want to run up high and be dead at the bottom a 100 lobe center motor will run strong at the bottom and tend to die up high. My bikes are Honda 4 valve motors and I have many different engine sizes and cam shaft combinations. It really suprised me when a larger intake cam slowed the bike down regardless of lobe center this means the mixture was being pumped out either at the top or at the bottom.
