How I Selected my next Cam.
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Joined: Nov 2002
Posts: 1,471
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From: Berkeley, California
How I Selected my next Cam. In response to the never-ending question, I offer my humble method.
The first step must be to clearly define your requirements. In my case, the requirements are:
1) More Power – Better dragstrip performance than my current XR265HR 212/218 .558/.563 115
2) Preserve drivability – Car will be driven to strip and will occasionally be used for transportation.
3) Avoid needing valve reliefs (flycutting pistons).
I have been very impressed with my current cam and I want my next cam to be “similar but better”.
Requirement 1 can be met by a combination of increased duration and lift.
Requirement 2 can be met by avoiding or minimizing overlap.
When considering piston-to-valve clearance, Requirement 3, the two most critical specifications are the point at which the intake valve begins to open (IVO), and the point at which the exhaust valve closes (EVC). These events must occur when the piston is near top-dead-center (TDC). Piston-to-Valve clearance is reduced whenever the intake valve opens before TDC, or the exhaust valve closes after TDC. Both of these conditions are characteristic of most high-performance camshafts.
Common camshaft specifications consist of: Intake and exhaust Duration and Lift, Lobe-Separation-Angle (LSA), and Intake Centerline (ICL). Valve events such as IVO and EVC are not usually specified explicitly but they can be derived from the common specs by simple calculations. I created an Excel spreadsheet to calculate valve events and compare camshaft specifications.
My spreadsheet includes specifications of Thunder racing’s T-Rex cam which was designed to produce the most power possible with stock (unmilled) heads and pistons without valve relief. The T-Rex therefore provides limits for IVO and EVC that I must not exceed. Comparison of my current cam with the T-Rex showed that I had considerable leeway. My spreadsheet is attached. Anyone who cares to, can probably guess my final choice.
I cannot yet claim that my selection has been validated. My new cam has arrived but I will not install it for several weeks. I will post results when I have them.
The first step must be to clearly define your requirements. In my case, the requirements are:
1) More Power – Better dragstrip performance than my current XR265HR 212/218 .558/.563 115
2) Preserve drivability – Car will be driven to strip and will occasionally be used for transportation.
3) Avoid needing valve reliefs (flycutting pistons).
I have been very impressed with my current cam and I want my next cam to be “similar but better”.
Requirement 1 can be met by a combination of increased duration and lift.
Requirement 2 can be met by avoiding or minimizing overlap.
When considering piston-to-valve clearance, Requirement 3, the two most critical specifications are the point at which the intake valve begins to open (IVO), and the point at which the exhaust valve closes (EVC). These events must occur when the piston is near top-dead-center (TDC). Piston-to-Valve clearance is reduced whenever the intake valve opens before TDC, or the exhaust valve closes after TDC. Both of these conditions are characteristic of most high-performance camshafts.
Common camshaft specifications consist of: Intake and exhaust Duration and Lift, Lobe-Separation-Angle (LSA), and Intake Centerline (ICL). Valve events such as IVO and EVC are not usually specified explicitly but they can be derived from the common specs by simple calculations. I created an Excel spreadsheet to calculate valve events and compare camshaft specifications.
My spreadsheet includes specifications of Thunder racing’s T-Rex cam which was designed to produce the most power possible with stock (unmilled) heads and pistons without valve relief. The T-Rex therefore provides limits for IVO and EVC that I must not exceed. Comparison of my current cam with the T-Rex showed that I had considerable leeway. My spreadsheet is attached. Anyone who cares to, can probably guess my final choice.
I cannot yet claim that my selection has been validated. My new cam has arrived but I will not install it for several weeks. I will post results when I have them.
Last edited by Gary Z; Jan 5, 2008 at 10:02 AM.
Teching In
Joined: May 2007
Posts: 41
Likes: 0
From: Long beach Ca
In response to the never-ending question, I offer my humble answer.
The first step must be to clearly define your requirements. In my case, the requirements are:
1) More Power – Better dragstrip performance than my current XR265HR 212/218 .558/.563 115
2) Preserve drivability – Car will be driven to strip and will occasionally be used for transportation.
3) Avoid needing valve reliefs (flycutting pistons).
I have been very impressed with my current cam and I want my next cam to be “similar but better”.
Requirement 1 can be met by a combination of increased duration and lift.
Requirement 2 can be met by avoiding or minimizing overlap.
When considering piston-to-valve clearance, Requirement 3, the two most critical specifications are the point at which the intake valve begins to open (IVO), and the point at which the exhaust valve closes (EVC). These events must occur when the piston is near top-dead-center (TDC). Piston-to-Valve clearance is reduced whenever the intake valve opens before TDC, or the exhaust valve closes after TDC. Both of these conditions are characteristic of most high-performance camshafts.
Common camshaft specifications consist of: Intake and exhaust Duration and Lift, Lobe-Separation-Angle (LSA), and Intake Centerline (ICL). Valve events such as IVO and EVC are not usually specified explicitly but they can be derived from the common specs by simple calculations. I created an Excel spreadsheet to calculate valve events and compare camshaft specifications.
My spreadsheet includes specifications of Thunder racing’s T-Rex cam which was designed to produce the most power possible with stock (unmilled) heads and pistons without valve relief. The T-Rex therefore provides limits for IVO and EVC that I must not exceed. Comparison of my current cam with the T-Rex showed that I had considerable leeway. My spreadsheet is attached. Anyone who cares to, can probably guess my final choice.
I cannot yet claim that my selection has been validated. My new cam has arrived but I will not install it for several weeks. I will post results when I have them.
The first step must be to clearly define your requirements. In my case, the requirements are:
1) More Power – Better dragstrip performance than my current XR265HR 212/218 .558/.563 115
2) Preserve drivability – Car will be driven to strip and will occasionally be used for transportation.
3) Avoid needing valve reliefs (flycutting pistons).
I have been very impressed with my current cam and I want my next cam to be “similar but better”.
Requirement 1 can be met by a combination of increased duration and lift.
Requirement 2 can be met by avoiding or minimizing overlap.
When considering piston-to-valve clearance, Requirement 3, the two most critical specifications are the point at which the intake valve begins to open (IVO), and the point at which the exhaust valve closes (EVC). These events must occur when the piston is near top-dead-center (TDC). Piston-to-Valve clearance is reduced whenever the intake valve opens before TDC, or the exhaust valve closes after TDC. Both of these conditions are characteristic of most high-performance camshafts.
Common camshaft specifications consist of: Intake and exhaust Duration and Lift, Lobe-Separation-Angle (LSA), and Intake Centerline (ICL). Valve events such as IVO and EVC are not usually specified explicitly but they can be derived from the common specs by simple calculations. I created an Excel spreadsheet to calculate valve events and compare camshaft specifications.
My spreadsheet includes specifications of Thunder racing’s T-Rex cam which was designed to produce the most power possible with stock (unmilled) heads and pistons without valve relief. The T-Rex therefore provides limits for IVO and EVC that I must not exceed. Comparison of my current cam with the T-Rex showed that I had considerable leeway. My spreadsheet is attached. Anyone who cares to, can probably guess my final choice.
I cannot yet claim that my selection has been validated. My new cam has arrived but I will not install it for several weeks. I will post results when I have them.
