The camshaft is the brain of the engine — it controls when the valves open, how far they open, and how long they stay open. These timing events determine the engine's personality: where it makes power, how it idles, how it responds to throttle, and what RPM range it's happiest in.
Cam specs can be intimidating at first, but once you understand what the numbers mean, you can read a cam card like a blueprint for the engine's behavior.
Duration: How Long the Valve Is Open
Duration is measured in degrees of crankshaft rotation. It tells you how long the valve stays off its seat. Cam specs typically list two duration numbers:
- Advertised duration: Measured at a very small lift point (usually 0.006" for hydraulic cams, 0.004" for solid cams). This number varies between manufacturers because there's no universal standard for the measurement point.
- Duration at 0.050": Measured at 0.050" of valve lift. This is the industry standard comparison point. When comparing cams from different manufacturers, always use the 0.050" number.
What Duration Affects
- More duration (longer): Shifts the power band higher in the RPM range. More top-end power, less low-end torque. Rougher idle. Reduced vacuum at idle (which affects power brakes and other vacuum accessories).
- Less duration (shorter): Better low-end torque, smoother idle, stronger vacuum signal. Less peak HP potential but more usable power for street driving.
Typical Duration Ranges (at 0.050")
- 190–210°: Stock replacement / RV cam. Smooth idle, good low-end, excellent vacuum. Doesn't wake up the engine much.
- 210–220°: Mild street cam. Noticeable improvement in power with a slightly lopey idle. Good compromise for daily-driven cars.
- 220–230°: Aggressive street cam. Distinct idle lope, needs stall converter with automatics. Power moves noticeably to mid-range and up.
- 230–240°: Street/strip cam. Rough idle, weak vacuum, poor low-RPM manners. Needs supporting mods (heads, intake, exhaust, converter).
- 240°+: Race cam. Not streetable without significant supporting modifications. Virtually no vacuum at idle.
Calculate valve timing events, overlap, and cam character from duration at 0.050, lobe separation angle, and advance.
Lift: How Far the Valve Opens
Valve lift is measured in inches (or mm) and describes the maximum distance the valve moves off its seat. Higher lift flows more air — up to a point.
Lift is a function of the cam lobe's shape multiplied by the rocker arm ratio:
Valve lift = Lobe lift × Rocker ratio
A cam with 0.325" lobe lift and 1.5:1 rockers produces 0.488" valve lift. Swapping to 1.6:1 rockers bumps it to 0.520" — a free lift increase without changing the cam.
How Much Lift Do You Need?
More lift helps, but only if the cylinder heads can flow the additional air. Every head has a point where additional valve lift stops producing more airflow. Stock small-block Chevy heads typically max out around 0.450–0.480". Aftermarket performance heads may flow well to 0.600"+ on the intake side.
Running high lift also increases valvetrain stress, may require upgraded valve springs, and can cause piston-to-valve interference issues.
Lobe Separation Angle (LSA)
LSA is the angle in camshaft degrees between the intake lobe centerline and the exhaust lobe centerline. It's ground into the cam and cannot be changed after manufacturing.
- Tight LSA (106–110°): More valve overlap. Results in a lopey, aggressive idle with more exhaust reversion. Stronger mid-range power but peaky. Good for NA engines where scavenging helps fill the cylinders.
- Wide LSA (112–116°): Less overlap. Smoother idle, broader power band, better vacuum. Preferred for forced induction (less blowthrough to the exhaust at low RPM) and street cars that need manners.
Why LSA Matters for Forced Induction
Turbocharged and supercharged engines generally want wider LSA (112–116°). Tight LSA creates more overlap, and under boost, that overlap pushes pressurized intake charge straight out the exhaust port. This wastes boost, burns fuel, and heats the catalytic converter. Wider LSA reduces overlap and keeps the charge in the cylinder where it belongs.
Overlap: When Both Valves Are Open
Overlap is the period (in crankshaft degrees) when both the intake and exhaust valves are open simultaneously. This happens around TDC on the exhaust stroke — the exhaust valve is closing and the intake valve is opening.
Overlap is determined by duration and LSA together:
Overlap = (Intake duration + Exhaust duration) / 2 − LSA
(Using duration at 0.050" gives you overlap at 0.050", which is the meaningful measurement.)
What Overlap Does
- More overlap: Better exhaust scavenging at high RPM (the outgoing exhaust helps pull fresh charge in). But at low RPM, exhaust gases dilute the incoming charge, causing a rough idle and poor throttle response.
- Less overlap: Cleaner idle, better emissions, stronger vacuum signal. Less scavenging benefit at high RPM.
Cam Advance / Retard
Cam timing can be advanced or retarded relative to the crankshaft by using an adjustable timing gear or cam phaser:
- Advancing the cam (2–4°): Intake valve opens and closes earlier. Improves low-end and mid-range torque. Most street cam grinds come with 2–4° advance ground in.
- Retarding the cam: Intake valve opens and closes later. Shifts power higher in the RPM range. Used for race applications targeting peak HP over drivability.
An adjustable timing set lets you fine-tune the installed centerline to optimize the power curve for your specific combination.
Reading a Cam Card
A typical cam card looks like this:
Duration @ 0.050": 224° intake / 230° exhaust
Valve lift: 0.510" intake / 0.520" exhaust
LSA: 112°
Installed centerline: 108° (4° advance)
This tells you:
- Moderate-to-aggressive street cam (224/230 duration)
- Good lift that needs decent heads to take advantage of it
- Wide LSA (112°) for smoother idle and good vacuum
- 4° advance for improved low-end response
Matching the Cam to the Build
The cam doesn't work in isolation. It needs to match:
- Cylinder heads: The heads need to flow enough air to use the cam's lift and duration. Big cam + stock heads = wasted potential.
- Compression ratio: Longer duration bleeds off effective compression. A big cam on a low-compression engine may feel flat. A mild cam on a high-compression engine may knock.
- Intake and exhaust: The cam determines airflow demand. The intake manifold and exhaust need to support it.
- Torque converter (automatics): Bigger cams need higher stall converters so the engine can get into the powerband off the line. A 230° cam behind a stock 1,800 RPM converter will feel terrible.
Calculate static compression ratio from bore, stroke, chamber volume, head gasket, deck clearance, and piston dome or dish volume.
Calculate engine volumetric efficiency from displacement, RPM, and measured airflow. Supports MAF and MAP-based calculations.
Common Mistakes
- “Biggest cam that fits” mentality: A cam that's too big for the heads, compression, and intake will make less power than a properly matched smaller cam. The cam has to match the whole package.
- Comparing advertised duration across brands: One company's “280° cam” is not the same as another's. Always compare at 0.050".
- Ignoring LSA for boosted builds: Tight LSA cams in turbo engines cause boost blowthrough, wasted fuel, and poor spool. Go 112°+ for forced induction.
- Not upgrading valve springs: Higher lift and faster ramp rates demand stiffer springs. Running the stock springs with an aggressive cam causes valve float, loss of control, and potential valve-to-piston contact at high RPM.
- Skipping the torque converter upgrade: An automatic with a stock converter behind a big cam will be sluggish off the line and feel worse than stock until the RPM climbs. Match the stall speed to the cam's effective powerband.