What overlap is and why the cam card hides it
Overlap is the number of crank degrees during which the exhaust valve has not yet closed and the intake valve has already opened. It sits around top dead centre at the end of the exhaust stroke, and it is the single most consequential thing about a camshaft that the cam card does not print.
During overlap the cylinder is open to both the header and the intake port at the same time. At high rpm that is exactly what you want: a well-tuned exhaust system pulls a negative pressure wave past the open exhaust valve just as the intake cracks open, and that suction starts the intake charge moving before the piston has begun the intake stroke. The cylinder gets scavenged and then over-filled. At idle there is no wave energy to work with, so overlap simply lets exhaust gas back into the intake and fresh charge out of the header. That is the whole explanation for the lopey idle and the low vacuum reading of a big cam.
Cards print duration and lobe separation because those are what the grinder controls. Overlap falls out of them, and you have to calculate it before you can compare two cams that quote different numbers.
The algebra of a cam card
Start with the two centrelines. The lobe separation angle is the angle between the peaks of the intake and exhaust lobes, ground into the camshaft and unchangeable by installation. Advancing the cam by A crank degrees rotates the whole cam relative to the crank, so the intake peak arrives A degrees sooner and the exhaust peak A degrees sooner too:
ICL = LSA − A (degrees after TDC) ECL = LSA + A (degrees before TDC)
Each lobe is symmetrical about its own peak, so half the duration lies on each side. Working from the centrelines:
- IVO = Dint/2 − ICL, degrees before TDC
- IVC = Dint/2 + ICL − 180, degrees after BDC
- EVO = Dexh/2 + ECL − 180, degrees before BDC
- EVC = Dexh/2 − ECL, degrees after TDC
Overlap is the sum of the two events that straddle TDC: IVO + EVC. Substitute and the advance term cancels exactly:
Overlap = (Dint/2 − LSA + A) + (Dexh/2 − LSA − A) = (Dint + Dexh)/2 − 2·LSA
That cancellation is worth pausing on, because it is the most commonly misunderstood point in cam selection. Advancing a camshaft does not change overlap. It moves every event earlier by the same amount, which shifts the torque curve down the rev range and closes the intake sooner, but the window in which both valves are open is fixed by duration and lobe separation alone. If you want less overlap you need a wider LSA or less duration, not a different installed centreline.
Notice too that LSA carries a factor of two. Widening lobe separation by one degree removes two degrees of overlap, because it moves the intake opening one degree later and the exhaust closing one degree earlier.
Worked example: 224/230 on a 110° LSA, installed 4° advanced
A typical street hydraulic roller: 224° intake and 230° exhaust duration at 0.050 in tappet lift, ground on a 110° lobe separation angle, and installed 4° advanced as most cams are dowelled from the factory.
- Intake centreline. 110 − 4 = 106° ATDC.
- Exhaust centreline. 110 + 4 = 114° BTDC.
- Half durations. 224 ÷ 2 = 112°; 230 ÷ 2 = 115°.
- Intake opens. 112 − 106 = 6° BTDC.
- Intake closes. 112 + 106 − 180 = 38° ABDC.
- Exhaust opens. 115 + 114 − 180 = 49° BBDC.
- Exhaust closes. 115 − 114 = 1° ATDC.
- Overlap. IVO + EVC = 6 + 1 = 7°. Cross-check with the short form: (224 + 230) ÷ 2 − 2 × 110 = 227 − 220 = 7°. They agree.
Seven degrees of overlap at 0.050 in lift is a mild figure: this cam will idle cleanly, hold good vacuum for power brakes, and make its torque low in the range. That 38° intake closing point is also the number you feed into the dynamic compression ratio calculator to find out what compression the engine really runs.
Now install the same cam 4° retarded instead. ICL becomes 114° ATDC, IVO becomes 112 − 114 = −2° — the intake now opens 2° after TDC — and EVC becomes 115 − 106 = 9° ATDC. Overlap: −2 + 9 = 7°. Unchanged, exactly as the algebra promised, but the intake now closes at 46° ABDC instead of 38°, which moves the torque peak up and drops dynamic compression.
Reading the overlap figure
Judge overlap against what you want the engine to do, and always on the same checking-height basis. These bands are for timing measured at 0.050 in tappet lift, which is the convention this calculator defaults to.
Negative or near zero. The exhaust valve shuts before the intake opens. Idle vacuum is high, the idle is smooth enough for an automatic transmission and power brakes, and the engine makes its torque low. Every factory economy cam lives here.
Roughly 5° to 30°. Street performance. There is a slight lope, vacuum drops but usually stays adequate, and the torque peak moves up a few hundred rpm.
Roughly 30° to 60°. The idle becomes clearly audible and vacuum falls far enough that brake boosters and vacuum-operated accessories often need a dedicated pump. Power moves distinctly to the top of the range.
Above 60°. Race territory. The engine may not idle below 1,000 rpm at all, and the exhaust system has to be tuned to make the overlap pay.
Lobe separation angle carries information that overlap alone does not. Two cams can produce identical overlap — a short-duration cam on a narrow LSA and a long-duration cam on a wide one — and behave differently, because duration also sets when the intake closes and therefore the rpm at which cylinder filling peaks. Read overlap and the intake closing point together, never overlap on its own.
Overlap and valve events across lobe separation angles
| LSA (°) | IVO (° BTDC) | IVC (° ABDC) | EVC (° ATDC) | Overlap (°) |
|---|---|---|---|---|
| 104 | 12 | 32 | 7 | 19 |
| 106 | 10 | 34 | 5 | 15 |
| 108 | 8 | 36 | 3 | 11 |
| 110 | 6 | 38 | 1 | 7 |
| 112 | 4 | 40 | −1 | 3 |
| 114 | 2 | 42 | −3 | −1 |
| 116 | 0 | 44 | −5 | −5 |
Overlap falls by two degrees for every one degree of extra lobe separation. A negative EVC means the exhaust valve is already closed before top dead centre.
Advertised and 0.050 inch numbers are not interchangeable
Advertised duration is measured near the seat — commonly 0.006 in for hydraulic lobes and 0.020 in for solid ones — while 0.050 in timing is the industry checking standard used for degreeing. Advertised duration on the same lobe is typically 40 to 55 degrees longer, which through the overlap formula produces an overlap figure tens of degrees larger. Both are legitimate; comparing one against the other is not. Set the selector on this page to match your card, keep every cam you compare on the same basis, and state which basis you used whenever you quote an overlap number to someone else.
Mistakes that produce wrong valve events
- Confusing lobe separation angle with intake centreline. LSA is ground into the cam and never changes. The centreline is where you install it. They are equal only when the cam is installed straight up, with zero advance.
- Entering advance in cam degrees. Every figure on this page is in crank degrees, which are twice cam degrees. Four degrees of advance on a cam card means four crank degrees.
- Assuming advance fixes an overlap problem. The algebra above shows it cannot. Advancing moves every event earlier and lowers the intake closing point, but the overlap window is unchanged.
- Ignoring piston-to-valve clearance after a timing change. Advancing the cam moves the intake closer to the piston at TDC and retarding moves the exhaust closer. Check clearance physically after any change of more than a couple of degrees.
- Choosing on overlap alone. The intake closing point drives cylinder filling and dynamic compression, and the exhaust opening point drives blowdown. Overlap tells you about idle and scavenging, not about where peak power lands.
Where this sits in choosing and degreeing a cam
Use this calculator twice: once when you are choosing a cam, and again when you have degreed the one you fitted.
When choosing, work backwards from the engine you want. Fix the intake closing point first, because it and the static compression ratio between them decide the dynamic compression ratio and therefore the fuel you can run — that is the constraint most likely to bite. Then check the overlap the required duration and LSA produce, and confirm it is compatible with the idle quality, vacuum and converter you have. If overlap comes out too high, ask the grinder for a wider lobe separation rather than less duration; you keep the breathing and lose the roughness.
When degreeing, measure the installed intake centreline with a degree wheel and dial indicator, and enter it against the LSA to recover the actual advance: advance = LSA − measured ICL. Cam cards, keyways and timing sets all carry tolerance, and a chain that stretches shifts everything retarded over time.
Overlap only pays if the head and exhaust can use it. A large-overlap cam on a head that runs out of flow at 5,500 rpm gives you the rough idle without the top end; check the airflow the head can actually supply with the cylinder head airflow horsepower calculator before ordering duration, and if you are also changing rockers, work the real lift out with the rocker arm ratio and valve lift calculator because a higher ratio adds a little effective duration at the checking height as well as lift.