Valve Lift Calculator converts cam base circle, peak height, rocker ratio, and lash into gross and net valve lift, showing the mechanical gain and loss introduced at each step.
Finding Net Valve Lift with the Valve Lift Calculator
The Valve Lift Calculator follows valve lift through the whole mechanical chain — lobe, rocker arm, and lash — rather than stopping at gross lift like a simple rocker-ratio multiplier does. Give it base circle, peak height, rocker ratio, and lash, and it separates out how much each step adds or subtracts.
Base Circle and Peak Height Come From a Cam Checking Fixture, Not a Guess
Base circle and peak height are numbers read off the actual lobe with a dial indicator on a cam-checking fixture, not estimated from a catalog listing. Rocker ratio and lash come from the valvetrain spec sheet or a direct check at the valve tip, and results convert automatically between inches and millimeters.
From Lobe Lift to Net Valve Lift: the Valve Lift Calculator’s Math
Cam lift is the raw distance the lobe pushes the lifter, found by subtracting base circle from peak height.
$$ CamLift = PeakHeight – BaseCircle $$
This is the same subtraction built into a cam-checking fixture: zero the dial indicator on the base circle, then read the difference at peak lobe height, per the method sold by COMP Cams’ own checking tools.
$$ GrossLift = CamLift \times RockerRatio $$
Multiplying lobe lift by rocker ratio to project valve lift is a documented convention across cam-degreeing references and engine forums, not a fixed geometric law — several builders note rocker ratio isn’t perfectly constant through the full lift event, since friction and roller rockers can shift slightly with pushrod length and arc geometry.
A mix-up worth flagging: a cam card’s advertised lift figure is sometimes already multiplied by a stock rocker ratio, so re-multiplying that number by a different ratio here effectively counts the rocker twice.
$$ NetLift = GrossLift – Lash $$
Net lift subtracts lash the same way cam-degreeing walkthroughs pair gross lift with a hydraulic or mechanical lash figure. This calculator doesn’t add a separate pushrod-deflection term the way some worked examples do, so a heavily loaded or unusually long pushrod can run slightly less real lift than the net figure shown here.
Peak height has to exceed base circle or there’s no lift profile to speak of, rocker ratio can’t drop below 1.0 since a rocker arm amplifies rather than reduces motion, and lash reaching or exceeding gross lift means the valve never opens at all — the calculator blocks each of these outright rather than returning a nonsense number.
What Cam Lift Actually Measures on the Lobe
What a 1.5-to-1.6 Rocker Swap Actually Buys You
The calculator always checks one step above your entered ratio — plus 0.10 — against the same lobe lift and lash, which mirrors the specific 1.5-to-1.6 rocker swap discussed repeatedly in engine-building references.
At this calculator’s own default 0.300 inch lobe lift, a 1.5 ratio nets 0.450 inch gross lift and a 1.6 ratio nets 0.480 inch, a 0.030 inch gain that matches a worked example found independently across several sources rather than a number derived from theory. The same references that describe this swap also flag it as the reason to recheck valve spring travel and piston-to-valve clearance before committing to it.
The 0.060 Inch Behind the Spring Travel Number
Spring travel required adds a fixed margin on top of net lift.
$$ SpringTravel = NetLift + 0.060in $$
That 0.060 inch isn’t an arbitrary buffer — it’s the minimum coil-to-coil clearance at full lift that spring manufacturers including Lunati and Clay Smith Cams publish as the floor below which coil bind risks valvetrain failure, with some references pushing to 0.100 inch for high-rpm builds. Piston-to-valve clearance and hot lash aren’t estimated here, since both require direct engine measurement rather than a formula.
Reading the Wrong Numbers Off a Cam Card or Dial Indicator
Taking lobe lift from a quick caliper measurement across the lobe’s widest point, without zeroing against the true base circle first, can be off by several thousandths compared to a proper dial-indicator check.
Entering the rocker ratio stamped on the rocker body without confirming it against the actual measured ratio is another. Forum measurements on the same engine family have turned up rockers closer to 1.6:1 or 1.7:1 than the round 1.5:1 printed on the box.
Mixing up cold lash and hot running lash is the third. The two figures for the same valvetrain can differ enough to change the net lift result by a meaningful amount.
Straight Answers on Cam Lift, Rocker Ratio, and Net Lift
Why doesn’t my calculated net lift match the number on the cam card?
Cam card figures sometimes come from the grinder’s own testing rather than simple gross-minus-lash arithmetic, and small differences in rocker geometry can shift the real number away from a straight multiplication.
Does upgrading from a 1.5 to a 1.6 rocker ratio always add the same amount of lift?
No. The gain scales with lobe lift — at 0.300 inch lobe lift the jump adds 0.030 inch, but a bigger lobe lift gains more from the same ratio increase.
Can I use the lift number printed on a cam card in place of measuring lobe lift myself?
Only if you confirm whether that printed figure is raw lobe lift or already multiplied by a rocker ratio. Engine-building forums document builders getting this distinction wrong in both directions.
What’s a safe minimum clearance before valve springs hit coil bind?
Spring manufacturers commonly publish 0.060 inch as the floor at maximum lift, with high-rpm builds often targeting 0.100 inch instead.
Does valve lash subtract directly from valve lift?
Yes, in the simplified version used here. A more complete calculation some builders use also nets out pushrod deflection, which this calculator doesn’t include.
Is rocker ratio the same at every point of valve travel?
Not exactly. Ratio can vary slightly through the lift event depending on rocker geometry, so treating it as one constant number is a useful approximation rather than an exact model.