A different approach to choosing a cam
When choosing a cam most people look at the duration and LSA then compare that to the RPM range the cam grinder specs. This is a vary narrow view and does not take into account the heads, intake, displacement, stroke etc.... A cam for the 5.0 will also install in a 351. So this cam could be used in anything from 302 (actually 289) to 408 CID engine. How is it possible that one cam will act the same in each of these engines? The answer is it won't.
Ok but wait the cam manufactures tells you a cam is designed for a 5.0 and requires an aftermarket intake, 5 speed and 4.10 gears. So that must mean this cam will work well with any aftermarket head on a 347. That's simply not the case
The reality is what matters are the valve events, the duration and LSA are a RESULT of those events.
There are many myths about cam specs that are perpetuated by magazine articles and the cam grinders themselves. A commonly held belief is there are certain LSAs for certain applications, such as a wide LSA is required for computer controlled or power adder engines and a narrow LSA is required for a N/A high RPM engine.
If this is the case then why does Ford list an F-303 as a turbo/SC camshaft due to its 114 LSA even though the E-303 with a 110 LSA is listed as a "good" SC cam profile? The answer is marketing and the fact that these cams are a compromise at best.
In a recent magazine article two AFM camshafts for the Ford 5.0 were compared. The article recommended a N-41 cam for great street manners with 373 gears and a RPM limit of 6200. For stroker nitrous engines the N-71 Hi-Rev cam is recommended to push the RPM limit to 7000. Both of these cams have a 110 LSA. Again these cams break the very rules these cam grinders have set. Wide LSA for boost, Narrow LSA for RPM
Another myth regarding LSA is it's affect on overlap. A tighter LSA creates more overlap and a wider LSA creates less. Lets look at some examples.
a 224/224 cam with a 110 LSA will have 4 degrees of overlap.
a 228/228 cam with a 112 LSA will have 4 degrees of overlap
a 232/232 cam with a 114 LSA will have 4 degrees of overlap
So where is the change of overlap when only looking at LSA? Now it is true that for a given duration a change in LSA will affect overlap but you can't make a judgement on the motors behavior based solely on LSA.
Duration is another spec that is often used to generalize a cam characteristics. A small duration cam is said to be a street friendly cam that will produce good idle but a low RPM range. Alternatively the larger the duration the higher the RPM range and the rougher the idle. This is generally true given a fixed combination of parts but it falls apart when different induction pieces are used. For example a 5.0 with factory heads will need a rather large cam (250 duration) to rev to 7000 but put a set of RHS200 heads on that same motor and a cam with as little as 214 duration will rev to the same level. Use that same 215 duration cam in a 408 with the same RHS heads and the peak RPM will drop well below 6000.
So picking a cam is not about choosing the LSA and duration, it's about tailoring the valve events to the the CID, flow characteristics of the engine and the RPM. For a given displacement and RPM an unrestricted engine will require less duration than a restricted engine. Keep in mind that an engine that is free flowing at 5000 RPM might not be at 7000 RPM so the whole package has to be considered. The goal is to tailor the valve events to the flow characteristics of the engine and the movement of the piston. Two engines with identical displacements and induction parts will use different valve events if their bore and stokes differ.
Ok but wait the cam manufactures tells you a cam is designed for a 5.0 and requires an aftermarket intake, 5 speed and 4.10 gears. So that must mean this cam will work well with any aftermarket head on a 347. That's simply not the case
The reality is what matters are the valve events, the duration and LSA are a RESULT of those events.
There are many myths about cam specs that are perpetuated by magazine articles and the cam grinders themselves. A commonly held belief is there are certain LSAs for certain applications, such as a wide LSA is required for computer controlled or power adder engines and a narrow LSA is required for a N/A high RPM engine.
If this is the case then why does Ford list an F-303 as a turbo/SC camshaft due to its 114 LSA even though the E-303 with a 110 LSA is listed as a "good" SC cam profile? The answer is marketing and the fact that these cams are a compromise at best.
In a recent magazine article two AFM camshafts for the Ford 5.0 were compared. The article recommended a N-41 cam for great street manners with 373 gears and a RPM limit of 6200. For stroker nitrous engines the N-71 Hi-Rev cam is recommended to push the RPM limit to 7000. Both of these cams have a 110 LSA. Again these cams break the very rules these cam grinders have set. Wide LSA for boost, Narrow LSA for RPM
Another myth regarding LSA is it's affect on overlap. A tighter LSA creates more overlap and a wider LSA creates less. Lets look at some examples.
a 224/224 cam with a 110 LSA will have 4 degrees of overlap.
a 228/228 cam with a 112 LSA will have 4 degrees of overlap
a 232/232 cam with a 114 LSA will have 4 degrees of overlap
So where is the change of overlap when only looking at LSA? Now it is true that for a given duration a change in LSA will affect overlap but you can't make a judgement on the motors behavior based solely on LSA.
Duration is another spec that is often used to generalize a cam characteristics. A small duration cam is said to be a street friendly cam that will produce good idle but a low RPM range. Alternatively the larger the duration the higher the RPM range and the rougher the idle. This is generally true given a fixed combination of parts but it falls apart when different induction pieces are used. For example a 5.0 with factory heads will need a rather large cam (250 duration) to rev to 7000 but put a set of RHS200 heads on that same motor and a cam with as little as 214 duration will rev to the same level. Use that same 215 duration cam in a 408 with the same RHS heads and the peak RPM will drop well below 6000.
So picking a cam is not about choosing the LSA and duration, it's about tailoring the valve events to the the CID, flow characteristics of the engine and the RPM. For a given displacement and RPM an unrestricted engine will require less duration than a restricted engine. Keep in mind that an engine that is free flowing at 5000 RPM might not be at 7000 RPM so the whole package has to be considered. The goal is to tailor the valve events to the flow characteristics of the engine and the movement of the piston. Two engines with identical displacements and induction parts will use different valve events if their bore and stokes differ.
(A 3.25 stroke with a 4.125 bore is 347 CID. A 3.4 stroke with a 4.030 bore is also 347 CID.)
In general a free flowing engine will benefit from a later intake opening event as this keeps reversion down and allows the piston to build a bit of a head on the valve. Once it opens a very fast ramp rate will allow the cylinder to fill rapidly and the resulting inertia will keep the cylinder filling longer without the need to delay the intake closing event. The intake closing event must be set to keep the compression pressure in check for the fuel and static compression ratio being used. Close the intake too early and compression pressures can quickly reach a point where pump gas is not useable even on a 10:1 motor. Close it too late and you've given away considerable power.
Due to it's free flowing characteristics the need to open the exhaust valve early, to fully evacuate the cylinder, is lessened. The engine will benefit from a delayed exhaust opening event as the cylinder pressure is put to work longer and will broaden the torque curve. The exhaust closing event can also happen sooner as the free flowing inlet (coupled with the correct valve timing) does not need the "suction" created by the exhaust to get the air charge moving. This reduces overlap and helps to eliminate reversion and the corresponding "spoiling" of the intake charge. This also helps eliminate the incoming air charge being sucked out the exhaust and wasted.
A few last words about camshaft myths.
Piston to valve clearance. Max lift, duration and LSA are not what determine whether you will have PTV clearance issues. The events at overlap (exhaust closing and intake opening) is all that matters as this is when the piston is closest to TDC.
Valve springs good for .xxx lift. This is only one parameter when a spring is speced. The ramp rates do more to determine what spring you must run as spring pressures is what controls the valve. Running a spring that is good for .600 lift on a cam that only has .500 may seem like a good idea but if the spring is too weak valve float will result. Each cam requires a specific spring the more aggressive the lobe the more spring pressure is necessary.
Low and midlift flow is all important. The reason for this is said to be that the cam spends very little time at max lift compared to the amount of time at low and mid lift. This is a throw back to the days of restrictive heads and flat tappet cams. Modern roller cams have very aggressive ramp rates. They accelerate the valve very quickly and hold the valve up near max lift for a very long time compared to their flat tappet counterpart. Consequently the valve spends a considerable amount of time near max lift. In fact minimizing low lift flow is another tool in combating power robbing reversion.
So by now you can see why these generalizations exist. Any OTS cam is a compromise to fit a wide range of combinations. It would be impossible for a cam manufacturer to list a camshaft that is optimized for every possible combination. Not to say that an OTS cam is not perfect for a particular application, it may be, but it takes trial and error to determine what any particular combination needs. This is why a custom cam is what is needed for anyone trying to optimize a combination. For less than the cost of trying two different cams you can have one that is tailor made to extract the most out of your particular combination. When ordering a custom cam a lot of information is required. Not only does the grinder need to know the normal things such as bore, stroke, RPM range but also what the heads flow, what type of intake and carb is to be used, the weight of the car, type of transmission, gear ratio and many other specs as these all affect the design of the cam.
Due to it's free flowing characteristics the need to open the exhaust valve early, to fully evacuate the cylinder, is lessened. The engine will benefit from a delayed exhaust opening event as the cylinder pressure is put to work longer and will broaden the torque curve. The exhaust closing event can also happen sooner as the free flowing inlet (coupled with the correct valve timing) does not need the "suction" created by the exhaust to get the air charge moving. This reduces overlap and helps to eliminate reversion and the corresponding "spoiling" of the intake charge. This also helps eliminate the incoming air charge being sucked out the exhaust and wasted.
A few last words about camshaft myths.
Piston to valve clearance. Max lift, duration and LSA are not what determine whether you will have PTV clearance issues. The events at overlap (exhaust closing and intake opening) is all that matters as this is when the piston is closest to TDC.
Valve springs good for .xxx lift. This is only one parameter when a spring is speced. The ramp rates do more to determine what spring you must run as spring pressures is what controls the valve. Running a spring that is good for .600 lift on a cam that only has .500 may seem like a good idea but if the spring is too weak valve float will result. Each cam requires a specific spring the more aggressive the lobe the more spring pressure is necessary.
Low and midlift flow is all important. The reason for this is said to be that the cam spends very little time at max lift compared to the amount of time at low and mid lift. This is a throw back to the days of restrictive heads and flat tappet cams. Modern roller cams have very aggressive ramp rates. They accelerate the valve very quickly and hold the valve up near max lift for a very long time compared to their flat tappet counterpart. Consequently the valve spends a considerable amount of time near max lift. In fact minimizing low lift flow is another tool in combating power robbing reversion.
So by now you can see why these generalizations exist. Any OTS cam is a compromise to fit a wide range of combinations. It would be impossible for a cam manufacturer to list a camshaft that is optimized for every possible combination. Not to say that an OTS cam is not perfect for a particular application, it may be, but it takes trial and error to determine what any particular combination needs. This is why a custom cam is what is needed for anyone trying to optimize a combination. For less than the cost of trying two different cams you can have one that is tailor made to extract the most out of your particular combination. When ordering a custom cam a lot of information is required. Not only does the grinder need to know the normal things such as bore, stroke, RPM range but also what the heads flow, what type of intake and carb is to be used, the weight of the car, type of transmission, gear ratio and many other specs as these all affect the design of the cam.
