cam lobe aceleration rates


Staff member
If you've ever hit a speed bump in the road thats meant to slow traffic, at several different speeds, you know theres a very noticeable relationship between the speed somethings moving and time it takes for inertia and mass of a moving object to change, direction when force is applied thru the use of a ramp changing its path of movement, and the resulting increase in shock to the components as the rate of that change over a shorter time is applied.
a speed bump is similar to a cams lobe acceleration ramp, in that it lifts the lifter/valve against spring resistance similar to your cars wheel hitting the speed bump.
a cams lobe design can rather gradually or rather suddenly impart a change in direction to the lifter in its bore and effect the speed at which the valve lifts off its seat or returns to a closed position, but that ramp design has a huge effect on the stress the valve train is subjected to as the speed of engine rotation is increased.
just like your car hitting the speed bump in the road, theres a big difference at 7 miles per hour vs 70 miles per hour, and a cam lobe that pops the valves open at idle speed of 700rpm,imparts far less stress than the same lobe spinning at 7000rpm.
naturally every choice is a compromise, lower lifts and smoother low acceleration rates make for long life and lower stress but restrict the potential area open under the valve per degree of rotation, which restricts potential breathing of the engine.
rapid ramp acceleration rates tend to increase potential hp but impart higher stress
Isky claims that the Comp XE cams violate the 47.5% rule. The 47.5% rule applies to flat tappet cams for SBCs with 1.5 rockers but the concept is still the same for other configurations where the designs are "on the edge" or "over the edge" for lobe intensity. For 1.5 ratio SBCs, the duration at .50 must exceed 47.5% of the total valve lift or your asking valve train problems. For example, take a Comp Cams Magnum 280H, with 230 duration and, 480 lift...230/.480 = 47.9% which exceeds 47.5% therefore would not pose a threat to components. We do not regularly hear about the older, safer HE and Magnum designs rounding off lobes anywhere near as often as the XE cam designs. Unfortunately, some of the Comp Cams XE dual pattern lobes break this 47.5% rule on the intake side so they are likely to be problematic. The design has "steeper" ramps that are too quick for durability and reliability according to other cam manufacturers. They will wipe lobes in a heart beat especially if you have not followed the proper break-in procedure. Other designs are more forgiving during break-in and less likely to fail.

one factor I will mention is that each manufacturer tends to look at durability, ramp speeds and max lifter acceleration very differently, one reason I tend to prefer CRANE & CROWER is that they both company's in general realize the engine must finish the race to win and a busted valve train is a HUGE problem,they both realize, and design valve train components and cam lobes with DURABILITY and reliable valve control as top priority,s that are far more important than squeezing every possible potential HP from a cam lobe design at the expense of long term durability

BTW if your running a flat tappet cam INSIST ON A P55 core, to have it ground on as they are far more durable than the cheaper cores

keep in mind in most cases higher valve spring pressures don,t tend to make the engine significantly harder to spin because theres an equal number of valves closing at the same time there are valves opening ,thus much of the increased force or load is off set, but the stress on the lifters and cam lobes, and lifter contact points on the lobes IS increased, so a billet cam core is a nearly mandatory choice at above about 400 ft lbs of valve spring pressure for long term durability and with the faster acceleration rates on most roller cam lobe designs the higher pressures become mandatory to maintain high rpm valve train stability, if you loft a roller lifter valve train,at high rpms, under valve float conditions it frequently and rapidly leads to valve train failures, so verify the required valve springs and clearances with the cam and cylinder head manufacturers before installation


these are both cast core cams (look between the lobes) the dark surface is a flat tappet cam lobe coating, the polished is a roller cam

roller lifters can be run on much more aggressive ramp rates, because the edge of a flat tappet lifter will dig into the lobe ramp at a far lower angle than a roller wheel will jam at as either is,lifted on a steep cam lobe as its rotated under a lifter, that trapped in a lifter bore thus the roller cams wheel design allows the valves to be forced open at faster rates and held open longer, increasing air flow rates thru the intake and exhaust ports
look at this diagram, you'll see with a bit of time examining it that even a cam with the same duration and lift can have a very different ramp acceleration rate, and that would have a big effect on both the total time the valves off its seat (which effects potential power produced)and the stress imparted to the valve train.(which effects the engines stress levels and potential longevity) reducing speed of rotation, spring load rates or the mass(WEIGHT) of the components tends to reduce the stress induced thru rapid changes in direction, of the valve train
ok heres the cliff note version
a roller cam and matched valve train generally costs $700-$1200 MORE than a flat tappet valve train, and cam, and PROVIDED you select the valve train, heads, and intake to match the flow and rpm potential,, and having matched those components correctly, the extra cost, will allow you to pop the valves open and hold them open longer, potentially increasing the air flow into the cylinders, given equal duration and lift on the cams, selected for either application.
theres nothing wrong with a flat tappet solid lifter in most mild-to mid power performance applications but a roller cam has advantages in some cases where you want to maximize the port flow rates and the roller cam usually requires better springs with higher load rates, because the roller lifters weight more and are a bit harder to control (force to remain in constant direct contact with the cam lobes) at higher rpms
you might also keep in mind that you reduce friction,with a roller valve train,reduce heat of the oil, slightly and have a lower tendency to wipe cam lobes, during the normal break-in procedure that's not necessary with a roller cam like it is with the flat tappet cams lifters

READ ... s/5952.pdf




high spring loads don,t play well with roller cams over long term use, heres a very clear example of why you should only use Billet cam cores with roller cams having over about 320 lbs of spring pressure and why you MUST verify valve train geometry and clearances.









RELATED BITS OF INFO, yeah theres a great deal of info in the links youll want to read thru ... esign.html ... ive-3.html ... kStart.pdf ... index.html ... index.html ... index.html ... _stock.htm ... ticleID=51 ... basics.htm ... s-883.html ... cobar1.pdf ... index.html


Staff member
Its always a good idea to discuss your potential valve train changes during the engine planing and build stages with several different cam manufacturers, and your engine builder as each will provide a slightly different perspective, just keep in mind all components used should match the intended power band and rpm range and air flow rates, it would be rather foolish to spend a great deal on a killer roller cam and valve train that will spin 7600rpm, and reach .700 lift, all day if your heads, intake, exhaust, etc. stop effectively flowing at 5500rpm and .450 lift,on your combo

Cam Design Terms And Definitions

Lobe Lift- Total difference from base circle ("0" reference value) to maximum displacement height of the lobe, usually expressed in thousandths of an inch (.433").
Base Circle- Region of the lobe that is not supposed to have any lift, or the reference starting point. Base Circle diameter is also measured for calculating stress loads and determining how the lobe duration will change during grinding.
Lash Ramp- Usually defined as the lash region, or the event that occurs between the first movement of the lifter and the point where valve movement is supposed to start.
Ramp- Different cam designers have different versions of where they define the beginning or end of the ramp, some look at the maximum acceleration point, some the full POSITIVE portion of the acceleration pulse. There is no consensus on what defines the "ramp", just that it is the opening or closing portion of the lobe and not the nose region.
Nose- The upper portion of the lobe, mostly defined by the NEGATIVE portion of the acceleration curve.
Velocity- Rate of change in LIFT, or speed of the lifter. Usually expressed as lift/degree (.00735"/deg). The velocity number is useful in determining the maximum speed that a flat tappet lifter may achieve with a certain diameter. An example is speed limit like on the highway or your rev limiter of the engine, both limit how fast you can go.
Acceleration- Rate of change in VELOCITY, usually expressed as lift/deg 2 (.00038"/deg 2 ). High acceleration rates place greater loads on valve train components, but get the valve open or closed faster usually leading to more power. Lower acceleration rates are easier on components, but usually yield lower power. An example would be to think of dropping the clutch at high rpm in your vehicle, as opposed to letting the clutch out gently at a low rpm and then standing on the gas.
Jerk- Rate of change in ACCELERATION, expressed in lift/deg 3 (.00003/deg 3 ). High jerk values mean that the acceleration rate is changing rapidly, usually an indication of a cam that is noisy to the ear, and probably harder on valve train parts than either the peak acceleration or velocity rates would indicate. An example would be to think of accelerating your car (as in the previous definition) over some railroad tracks instead of a smooth road. These kind of quick load reversals are just as damaging to valve train as they are to your drive train in similar circumstances.
Snap- Believe it or not, rate of change in JERK is sometimes measured. This measurement is in lift/deg 4 , which is not easy to derive from data taken from cam measurements. This is usually looked at during the design stage, where the equations being used are in 7 to 8 place accuracy (.00000001"), nearly impossible to actually measure on a finished product.


Staff member
read this first
keep in mind you can,t use .050 lift duration specs to calculate overlap [/color] and to maximize the results youll want to use a compression ratio in the 10.5:1-10.8:1 plus range
so thin head gaskets, milled heads,domed piston will help
and an intake like the RPM AIR GAP , with a 2" 4 hole carb spacer MAY HELP, due to the smaller displacement

If the seat to seat opening and closing times are not known, the overlap can be calculated using the advertised duration and the Lobe Separation Angle (LSA). This formula works for both single and dual pattern cams.

Add the intake and exhaust adv durations
Divide the results by 4
Subtract the LSA
Multiply the results by 2

Using the example cam above:
The overlap is 54º

Here's another example:
Comp Cams XE294H
Adv Dur: int 294º, exh 306º
110 LSA, 106 ILC (4º advance)
Valve timing @ .006"
Int opens @ 41º BTDC
Int closes @ 73º ABDC
Exh opens @ 87º BBDC
Exh closes @ 39º ATDC

Adding the int open and exh close times (41+39) the overlap is 80º

Using the formula above:
The overlap is 80º

read thru these links



my computer says you want this crower cam i linked below,

Ill save you the math, its got 77 degrees of seat-to-seat overlap, yes you want the wide lsa as it traps cylinder volume and results in more high rpm tq

crower 00323 with 1.6:1 rockers, verify clearances and geometry, and spring load rates as that results in a .563/.581 lift,,but you may want to use the 1.5:1 ratio rockers as the gains with the higher ratio rockers are not all that large, and a rocker stud girdle seems like a good idea for valve train stability


as the ratio of displacement to valve diam. is reduced the lsa needs to get wider to maximize cylinder fill efficiency
or put a different way, as the valve size is limited in a 265 to about a 1.90 valve ,by the small bore,and as the displacement is reduced by the short stroke the wider lsa improves high rpm cylinder filling ... index.html

using .050 lift to compare overlaps about as valid as selecting a girl friend based on the color of hair brush in her purse


overlap is very important and its measured from valve seat to valve seat, so ALL the flows a factor, cam lobe ramp designs differ a great deal and the flow during the overlap varies as a result, with similar advertised duration lobes ... basics.htm ... index.html

0505phr_exh_02_z.jpg ... index.html ... index.html

you should also keep in mind that a roller cam valve train with the same lift and duration can provide a good deal more port flow and resulting power.



your cams lift is the result of the lifter movement distance from the cams base circle, where the valves seated to the point where its fully up on the nose of the cam lobe where the valves at full lift.
lets say in this case we compare two imaginary cams
a standard cams base circle is 1.125" and
your cams running on a .900 base circle
both cams have a .560 valve lift and run with 1.5:1 rockers
so both cams will need to move the lifter .374"
that means the standard cam lobe will be 1.125"+.374" or 1.499" from the cams base to the cam lobe nose
that means the small base cam lobe will be .900"+.374" or 1.274" from the cams base to the cam lobe nose
which is significantly smaller


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