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Drum roll.....................
Before we dig into this any deeper, there are a couple things we need to place in context.
FIRST: OHC "follower" type cams play by different rules than the traditional OHV V8 design. In the OHV design, you have the lifter that is captured by the lifter bore in the block. The lifter can only travel in one plane, and it's contact point with respect to the centerline of the camshaft never changes. This makes measuring and mapping an OHV camshaft relatively easy. The stylus of the dial indicator can be positioned on the pushrod, or in the pushrod seat of the lifter, and measurements can be derived with a minmum of geometrical error. A .050" lift at the tappet (lifter) equates directly to a .050" lift at the pushrod end of the rocker arm. The rocker arm ratio does not come into play except for measuring maximum valve lift. In the case of the OHC "follower" design, the cam lobe acts directly on the pad of the rocker arm. The lobe ramp makes contact with the follower pad on the leading edge and continues across the face of the pad until exiting at the opposite end. The contact area changes in relation to the rocker arm shaft (pivot point). The result is a variable rocker ratio.....from high to low or low to high....depending on if it's the Intake or Exhaust (which are on different sides of the cam).
This makes it extremely difficult to measure duration at a given value of lobe lift and be able to correlate it directly to the "standard .050" value that we use on OHV V8 engines. You can not get a good reading by setting the dial indicator on the rocker arm, since the motion of the rocker arm pad is in an arc, which introduces geometrical errors as well. Running the dial indicator directly on the cam lobe can be used for determining duration, but there are two problems with this method:
A) The lobe profile geometry does not correlate to that of an OHV lobe.
B) The valve timing events can not be accurate. They change, depending on the angle and position on the cam at which you measure.
Because of this, most measurements were taken on the face of the valve (from the combustion chamber side). To derive the duration and timing events at ".050" lobe lift", I converted it to .087" valve lift, based on a constant rocker arm ratio of 1.75:1 which is the ratio at max valve lift. This is not 100% accurate, but I figure should be pretty darn close. I spoke with Engineers from both Crane & Comp Cams this morning, and they confirmed my theory and method.
I also found that Sealed Power advertises their stock replacement cams as being 205* duration at .050" lobe lift. When I measured the stock cam, I found that I had to be at ~.0325" valve lift to achieve 205*. For reference and comparison, I measured the FMS cam at .0325" valve lift as well. I also took measurements for "seat-to-seat" and .004" valve lifts to represent what is commonly listed as "advertised" duration......which means absolutely nothing. Which brings us to....
SECOND: Duration without any kind of lift specification at which it was taken means nothing. Lazy lobe ramp angles, especially at the transition from the base circle, have a huge effect on "advertised" duration with little effect on actual flow. Comparing the seat-to-seat duration and .050" duration can give you good indication of how aggressive a cam profile is. The point is this...do not judge a cam's performance based on "advertised" duration. For a detailed explanation, you can follow the link I gave earlier in this thread.
Before we dig into this any deeper, there are a couple things we need to place in context.
FIRST: OHC "follower" type cams play by different rules than the traditional OHV V8 design. In the OHV design, you have the lifter that is captured by the lifter bore in the block. The lifter can only travel in one plane, and it's contact point with respect to the centerline of the camshaft never changes. This makes measuring and mapping an OHV camshaft relatively easy. The stylus of the dial indicator can be positioned on the pushrod, or in the pushrod seat of the lifter, and measurements can be derived with a minmum of geometrical error. A .050" lift at the tappet (lifter) equates directly to a .050" lift at the pushrod end of the rocker arm. The rocker arm ratio does not come into play except for measuring maximum valve lift. In the case of the OHC "follower" design, the cam lobe acts directly on the pad of the rocker arm. The lobe ramp makes contact with the follower pad on the leading edge and continues across the face of the pad until exiting at the opposite end. The contact area changes in relation to the rocker arm shaft (pivot point). The result is a variable rocker ratio.....from high to low or low to high....depending on if it's the Intake or Exhaust (which are on different sides of the cam).
This makes it extremely difficult to measure duration at a given value of lobe lift and be able to correlate it directly to the "standard .050" value that we use on OHV V8 engines. You can not get a good reading by setting the dial indicator on the rocker arm, since the motion of the rocker arm pad is in an arc, which introduces geometrical errors as well. Running the dial indicator directly on the cam lobe can be used for determining duration, but there are two problems with this method:
A) The lobe profile geometry does not correlate to that of an OHV lobe.
B) The valve timing events can not be accurate. They change, depending on the angle and position on the cam at which you measure.
Because of this, most measurements were taken on the face of the valve (from the combustion chamber side). To derive the duration and timing events at ".050" lobe lift", I converted it to .087" valve lift, based on a constant rocker arm ratio of 1.75:1 which is the ratio at max valve lift. This is not 100% accurate, but I figure should be pretty darn close. I spoke with Engineers from both Crane & Comp Cams this morning, and they confirmed my theory and method.
I also found that Sealed Power advertises their stock replacement cams as being 205* duration at .050" lobe lift. When I measured the stock cam, I found that I had to be at ~.0325" valve lift to achieve 205*. For reference and comparison, I measured the FMS cam at .0325" valve lift as well. I also took measurements for "seat-to-seat" and .004" valve lifts to represent what is commonly listed as "advertised" duration......which means absolutely nothing. Which brings us to....
SECOND: Duration without any kind of lift specification at which it was taken means nothing. Lazy lobe ramp angles, especially at the transition from the base circle, have a huge effect on "advertised" duration with little effect on actual flow. Comparing the seat-to-seat duration and .050" duration can give you good indication of how aggressive a cam profile is. The point is this...do not judge a cam's performance based on "advertised" duration. For a detailed explanation, you can follow the link I gave earlier in this thread.
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