Post by Garrett on Apr 25, 2005 14:49:46 GMT -5
Cam Lobe Center lines are really pretty easy to figure and I'll give you a couple of ways to go about it. Lobe Center lines give you a relative perspective of how advanced or retarded a cam is in relation to top dead center(TDC). In a Briggs motor under the WKA/IKF cam profiles it is possible to have an intake center line from 98 to 103 degrees. Exhaust center lines run from 114 to 109. An intake center line of 98 is considered to be the most advanced and generally gives the most torque. The 103 center line will give power in the upper rpm range.
Exhaust centerline of 114 is the most advanced while the 109 is the most retarded. Again an advanced lobe will give power in the lower rpm range while the retarded lobe will have it's power range extended in the rpm range. For practical terms, most Briggs ground cams are in the range of 100 to 103 on intake and 110 to 112 on the exhaust.
To figure the intake lobe center line do the following: While profiling the intake lobe rotate the motor until you are at TDC and read the dial indicator over the intake valve(always use a dead valve or one with 0 clearance). It should be somewhere between .055 and .065 lift. Write this number down. Now rotate the motor until you reach the same lift before the valve closes. Note the degrees on the degree wheel. Now add 180 to the degrees noted and divide the sum by 2. This will give the intake center line.
Example: Rotating the motor you find that at TDC the dial indicator reads .058. After continuing to rotate the motor and stopping at .058 on the down side of the lobe you find the degree wheel reading 22 degrees ABDC. 180 + 22 = 202/2 = 101. This is the center line.
To figure the exhaust center line do the following: Right after the valve passes through the .050 lift point on the closing side and before reaching .020 lift you will pass TDC. At TDC read the dial indicator and mark this number down. It will probably be somewhere around .026 and .034 lift. Now continue to rotate the motor until the valve closes and begins to open again. Stop the motor when you reach the lift noted above. Read the degree wheel. Take this reading and add it to 180 and divide by 2. This will be the exhaust center line.
Example: While the exhaust valve is closing the lift noted at TDC is .034. Continuing to rotate the motor and when the valve starts it's opening cycle stop the motor at .034 lift. You read 44 BBDC. Adding 180 +44 = 224/2 = 112. 112 is the exhaust center line.
If you want a really simple and slightly less accurate way of finding your center line do the following. After profiling the cam to compare it against the WKA/IKF specs look at the .220 lift numbers. On the intake simply take the .200 number going up such as 34 and subtract it from 180 and add the .200 number going down such as 56. Divide the total by 2 and you will get a quick look at your center line. You can do the same on the exhaust but in reverse. Always subtract the smaller number and add the larger.
Cam duration is simply the opening point of the valve + 180 + the closing point. A cam opening at 22 degrees BTDC and closing at 75 degrees BTDC, would give a duration of 22+180+75 or 277 degrees. Most cam builders will tell you to calculate the intake closing at the point where the easy spin starts. The easy spin was developed by Briggs to help make cranking the motor easier. WKA says the easy spin must start between 45 and 60 degrees ABDC. Rules also say the lift at the point of easy spin must be between .013 and .019. The easy spin must be a minimum of 30 degrees in duration. Most Cams today have around 33 - 39 degrees.
Specific Recommendations:
For rookie motors and any restricted class keep the overlap to a minimum. Opening less than 20 degrees BTDC and closing around 80 degrees BTDC on the intake and exhaust openings around 58 degrees with a closing around 15 degrees ATDC will give good torque and minimum overlap. The intake center line should be 99 -100 with the exhaust around 112. For purple motors keep the rpm to around 4500 and for gold plate don't go above 5400.
For seniors or stock heavy, use a very similar cam to the rookie profile with the exception of possibly opening the intake earlier and closing the exhaust later. Keep the rpm in the 56-5700 rpm range.
Stock medium and stock light can take advantage of a higher rpm cam with center lines around 101 on the intake with 110 on the exhaust. Intake opening can be as much as 30 degrees but current practice is to limit this to around 25. intake closing should be around 75 degrees BTDC. Exhaust opening should be around 60 degrees with a closing around 20. This will allow the motor to rev better and produce power in a higher rpm range. Rpm range should not go above 6000 as you simply don't have enough air through the stock carburetor.
Super Stock motors can use intake openings above 30 degrees, but you will be very surprised how well the stock light cam profiled above will run.
Large tracks of 1/4 mile or so like cam intake center lines around 99 with a exhaust line of around 110. This cam will have a broad power band. Smaller tracks can use rather peaky power bands with the key to success being gearing. Peaky cams are generally ones that have both retarded intake and exhaust center lines.
Please give Mike Bordeaux at DYNO cams a call and ask for his free catalog. This will give you a great chance to look at many different profiles and combinations. It will become a good reference for profiling any cam. DYNO cams can be reached at 910-655-9035. p.s. Don't be afraid to ask them to grind a cam for you with any combination of intake and exhaust profile from any of their cams. tough to beat the 95-3 for a good all aroudn cam for almost any application. I like it in the plate classes as well as hte light classes. The 99-3 is a killer heavy cam as is the 96-3
Thanks To The Author
Exhaust centerline of 114 is the most advanced while the 109 is the most retarded. Again an advanced lobe will give power in the lower rpm range while the retarded lobe will have it's power range extended in the rpm range. For practical terms, most Briggs ground cams are in the range of 100 to 103 on intake and 110 to 112 on the exhaust.
To figure the intake lobe center line do the following: While profiling the intake lobe rotate the motor until you are at TDC and read the dial indicator over the intake valve(always use a dead valve or one with 0 clearance). It should be somewhere between .055 and .065 lift. Write this number down. Now rotate the motor until you reach the same lift before the valve closes. Note the degrees on the degree wheel. Now add 180 to the degrees noted and divide the sum by 2. This will give the intake center line.
Example: Rotating the motor you find that at TDC the dial indicator reads .058. After continuing to rotate the motor and stopping at .058 on the down side of the lobe you find the degree wheel reading 22 degrees ABDC. 180 + 22 = 202/2 = 101. This is the center line.
To figure the exhaust center line do the following: Right after the valve passes through the .050 lift point on the closing side and before reaching .020 lift you will pass TDC. At TDC read the dial indicator and mark this number down. It will probably be somewhere around .026 and .034 lift. Now continue to rotate the motor until the valve closes and begins to open again. Stop the motor when you reach the lift noted above. Read the degree wheel. Take this reading and add it to 180 and divide by 2. This will be the exhaust center line.
Example: While the exhaust valve is closing the lift noted at TDC is .034. Continuing to rotate the motor and when the valve starts it's opening cycle stop the motor at .034 lift. You read 44 BBDC. Adding 180 +44 = 224/2 = 112. 112 is the exhaust center line.
If you want a really simple and slightly less accurate way of finding your center line do the following. After profiling the cam to compare it against the WKA/IKF specs look at the .220 lift numbers. On the intake simply take the .200 number going up such as 34 and subtract it from 180 and add the .200 number going down such as 56. Divide the total by 2 and you will get a quick look at your center line. You can do the same on the exhaust but in reverse. Always subtract the smaller number and add the larger.
Cam duration is simply the opening point of the valve + 180 + the closing point. A cam opening at 22 degrees BTDC and closing at 75 degrees BTDC, would give a duration of 22+180+75 or 277 degrees. Most cam builders will tell you to calculate the intake closing at the point where the easy spin starts. The easy spin was developed by Briggs to help make cranking the motor easier. WKA says the easy spin must start between 45 and 60 degrees ABDC. Rules also say the lift at the point of easy spin must be between .013 and .019. The easy spin must be a minimum of 30 degrees in duration. Most Cams today have around 33 - 39 degrees.
Specific Recommendations:
For rookie motors and any restricted class keep the overlap to a minimum. Opening less than 20 degrees BTDC and closing around 80 degrees BTDC on the intake and exhaust openings around 58 degrees with a closing around 15 degrees ATDC will give good torque and minimum overlap. The intake center line should be 99 -100 with the exhaust around 112. For purple motors keep the rpm to around 4500 and for gold plate don't go above 5400.
For seniors or stock heavy, use a very similar cam to the rookie profile with the exception of possibly opening the intake earlier and closing the exhaust later. Keep the rpm in the 56-5700 rpm range.
Stock medium and stock light can take advantage of a higher rpm cam with center lines around 101 on the intake with 110 on the exhaust. Intake opening can be as much as 30 degrees but current practice is to limit this to around 25. intake closing should be around 75 degrees BTDC. Exhaust opening should be around 60 degrees with a closing around 20. This will allow the motor to rev better and produce power in a higher rpm range. Rpm range should not go above 6000 as you simply don't have enough air through the stock carburetor.
Super Stock motors can use intake openings above 30 degrees, but you will be very surprised how well the stock light cam profiled above will run.
Large tracks of 1/4 mile or so like cam intake center lines around 99 with a exhaust line of around 110. This cam will have a broad power band. Smaller tracks can use rather peaky power bands with the key to success being gearing. Peaky cams are generally ones that have both retarded intake and exhaust center lines.
Please give Mike Bordeaux at DYNO cams a call and ask for his free catalog. This will give you a great chance to look at many different profiles and combinations. It will become a good reference for profiling any cam. DYNO cams can be reached at 910-655-9035. p.s. Don't be afraid to ask them to grind a cam for you with any combination of intake and exhaust profile from any of their cams. tough to beat the 95-3 for a good all aroudn cam for almost any application. I like it in the plate classes as well as hte light classes. The 99-3 is a killer heavy cam as is the 96-3
Thanks To The Author