Valve Train

What the valve train actually does

An engine that doesn't breathe doesn't work. To make power on each cycle, a four-stroke engine has to ingest a fresh charge of air-fuel mixture (intake stroke), trap it (compression stroke), burn it (power stroke), and expel the spent exhaust (exhaust stroke). Two of those four strokes — intake and exhaust — depend on opening a valve at exactly the right crank angle, holding it open for exactly the right number of crank degrees, and slamming it shut again before the next event. Mismatch any of those numbers by more than a few degrees and the engine loses compression, mis-times combustion, or breathes badly enough to lose ten percent of its power.

The valve train is the mechanical program that delivers those events. It is not optional, it is not adjustable in operation (except via variable-valve-timing actuators), and it cannot fail without taking the engine with it. The chain runs as follows:

1. The crankshaft turns. Power piston drives it.
2. A timing belt, chain, or gear train links the crank to the camshaft at a 2:1 reduction — the cam turns at half crank speed because the four-stroke cycle takes two crank revolutions and each valve opens once per cycle.
3. The camshaft rotates. It carries one shaped lobe per valve. As the cam turns, the lobe's high point sweeps past each valve in turn at exactly the right crank position.
4. A lifter (also called a tappet or follower) rides on the cam lobe. When the lobe's high point arrives, the lifter is pushed away from the cam centreline.
5. (In OHV layouts only) a pushrod transmits the motion from the lifter up to the cylinder head.
6. A rocker arm pivots on a fulcrum and converts the upward motion of the pushrod (or direct cam motion in OHC) into downward motion on the valve stem.
7. The valve opens by being pushed down off its seat. Spring pressure on the valve stem keeps it closed when the cam isn't lifting it.
8. The valve spring returns the valve to its seat as the cam lobe rotates past.

That is the OHV (overhead-valve, pushrod) layout — the simplest, lightest, and the architecture used by every American V8 from the 1955 Chevrolet small-block to the current GM LT-series and Hellcat Hemi. OHC layouts skip the pushrod (the camshaft is now in the cylinder head, above the valves); DOHC skips the rocker arm in some implementations (the cam acts directly on a bucket tappet sitting on the valve stem). The chain shortens, the valve-train mass drops, the rev capability rises.

OHV vs SOHC vs DOHC — the three architectures

The location of the camshaft (or shafts) is the defining architectural choice of an engine. Three layouts dominate.

Property	OHV (pushrod)	SOHC	DOHC
Camshaft location	In the block (between the cylinder banks on a V)	In the head, one per bank	In the head, two per bank — one intake, one exhaust
Valves per cylinder (typical)	2	2 or 4	4 or 5
Timing drive	Short chain or gear	Long chain or belt	Long chain or belt, often with intermediate shaft
Rev limit (typical)	6,000–7,500 rpm	7,000–9,000 rpm	9,000–11,000 rpm (street); 18,000 (F1)
Cost	Lowest	Middle	Highest
Packaging	Most compact (narrow head)	Wider head	Widest head
Typical use today	American V8s, large truck diesels	Compact entry-level cars (e.g. early Civic)	Almost every modern gasoline engine
Famous example	GM LS3, Hellcat Hemi, Cummins B-series	Honda Civic D-series, BMC A-series	Toyota 2JZ-GE, Honda K20, BMW S65

Worked example — designing a cam for a 3.0 L straight-six

Let's size the cam events for a notional naturally aspirated 3.0 L DOHC inline-six gasoline engine targeting 220 kW at 7,000 rpm.

Target operating range:    idle 800 rpm → red line 7,500 rpm
Volumetric efficiency tgt: 95 % at 5,500 rpm peak torque
Number of valves:          24 (4 per cylinder, 2 intake + 2 exhaust)
Intake valve diameter:     33 mm
Exhaust valve diameter:    28 mm

Cam events (intake):
  Opening (IVO):    14° BTDC  — opens just before piston reaches top
  Closing (IVC):    52° ABDC  — stays open into compression stroke
  Duration:         246° crank
  Lift:             10.2 mm at the valve (with 1.65 rocker ratio,
                                            6.18 mm at the cam)
  Peak velocity:    ~5.5 m/s at the valve

Cam events (exhaust):
  Opening (EVO):    48° BBDC  — opens before bottom dead centre for blowdown
  Closing (EVC):    12° ATDC  — closes just after TDC

Overlap (both valves open simultaneously):
  IVO 14° BTDC to EVC 12° ATDC = 26° crank
  Promotes scavenging at peak power, hurts idle quality

Cam-to-crank ratio:        1:2  (cam turns once per two crank turns)
Timing chain pitch:        9.525 mm (3/8" silent chain)
Chain length:              ~500 mm round trip from crank to two cams

VVT range:
  Intake cam phaser:   25° advance / 5° retard (continuous)
  Exhaust cam phaser:  5° advance / 25° retard
  At idle:    minimum overlap (smooth idle, low residual)
  At peak power: maximum overlap (scavenging at WOT high rpm)
  At cruise:  intermediate overlap (low residual, low pumping loss)

The intake's 246° duration and 10.2 mm lift produce roughly 95 percent volumetric efficiency at 5,500 rpm peak torque and degrade to ~85 percent at 7,500 rpm red line. The variable-cam phasers give the engine three different cam-timing personalities: a sleepy short-overlap personality at idle, an aggressive long-overlap personality at peak power, and a moderate cruise personality in between. The same hardware delivers all three.

Variable valve timing — the architectural revolution since 1989

A fixed cam profile is always a compromise. Long duration and high overlap give good high-rpm breathing but ruin idle and low-end torque. Short duration is the opposite. Until 1989, every production gasoline engine ran a single static cam profile and made the compromise. Then Honda introduced VTEC.

• Cam phasing (Toyota VVT-i, BMW VANOS, GM Continuously Variable Cam Phasing, Ford Ti-VCT). An actuator at the front of the cam — typically a vane-type hydraulic mechanism, more recently an electric phaser on premium engines — rotates the cam relative to its drive sprocket. Range is typically 25–60 degrees of cam angle. The cam profile itself doesn't change; only when it happens relative to the crank changes. Simple, cheap, but limited because the duration is still fixed.
• Profile switching (Honda VTEC, Mitsubishi MIVEC, Audi AVS). Two or three different cam lobes per valve, with a hydraulic pin that locks a low-rpm rocker arm to a high-rpm rocker arm at a switchpoint. The engine literally has two different cam profiles available; the changeover is famous — a sudden character change at 5,800 rpm on a 1990s B16A. Gives much more useful range than phasing alone but only has 2 or 3 discrete settings.
• Lift continuously variable (BMW Valvetronic). An eccentric shaft between the cam and the rocker arm rotates to scale the cam lift from near zero to full. Removes the throttle butterfly entirely — engine load is controlled by valve lift instead. Reduces pumping loss at part throttle. Used on every modern BMW gasoline engine since the N62.
• Electrohydraulic per-valve (Fiat MultiAir, Schaeffler UniAir). The cam drives a hydraulic pump that pressurises an oil column; a solenoid in the column releases pressure into the valve actuator, opening the valve. By varying when the solenoid opens and closes, the valve event is completely decoupled from the cam profile — any duration, any lift, any timing. Production on Fiat MultiAir engines since 2010.
• Camless (Freevalve, Koenigsegg). The cam is eliminated entirely. Pneumatic or electromechanical actuators driven by the ECU open each valve independently. Allows arbitrary valve events, cylinder deactivation, Miller cycle, Atkinson cycle, and skip-firing all in one engine. Production in the Koenigsegg Gemera (2023) — the first production camless engine.

Failure modes — how valve trains break

• Timing-belt failure. Rubber belts deteriorate from heat, oil contamination, and age regardless of mileage. Failure is usually catastrophic — the cam stops, the crank keeps turning, valves meet pistons in interference engines, and the entire head needs rebuild. Manufacturer interval is typically 100,000–150,000 km; never let it lapse. Cars with timing chains (most modern engines) escape this failure mode because chains last the life of the engine when properly oiled.
• Worn camshaft lobe. Common on flat-tappet OHV engines using older zinc-deficient oils (post-2005 motor oil). The cam lobe rounds off; valve lift drops; the engine loses 10–20 percent power and runs rough. Diagnosis: comparison between actual lift and spec. Cure: replace cam and lifters as a matched set.
• Bent valve stem. If a timing belt jumps a tooth (or breaks) on an interference engine, the piston meets the valve at high closing rate; the valve bends and won't seal. Symptom: zero compression on the affected cylinder, no metal noise. Cure: remove head, replace bent valves, check head for damage.
• Burnt exhaust valve. The exhaust valve runs at 600–800 °C and dissipates heat through contact with its seat in the head. If the valve doesn't fully close (valve clearance too tight, weak spring, carbon on the seat), local hotspots form and the valve face burns away in a notch. Symptom: low compression on one cylinder, blue smoke under load. Cure: head off, replace valve and seat-grind.
• Valve float. At very high rpm the valve spring can't decelerate the valve fast enough; the valve lifts off the cam, floats above it, and crashes back. In interference engines this causes valve-to-piston contact. Cures: stiffer springs, lighter valves (titanium, sodium-filled stem), pneumatic springs (F1).
• Hydraulic-lifter collapse or pump-up. Old or contaminated oil causes hydraulic lifters to either bleed down completely (no lift) or fail to bleed (valve held slightly open). Symptom: clattering at startup (cold collapse, normal for a few seconds) or persistent rough running (pump-up). Cure: oil change, lifter replacement.
• Rocker-arm failure. Rocker arm pivot wear or stud breakage on OHV engines, particularly with high-lift cam upgrades. Symptom: severe clatter and loss of valve lift. Cure: stronger rockers, hardened studs, or roller-bearing rockers.

Variants — desmodromic, sleeve-valve, rotary-valve

• Desmodromic valves (Ducati). The cam closes the valve as well as opens it — two cam lobes per valve, two rocker arms, no return spring. Eliminates valve float entirely because there is no spring to overrun. Used on every Ducati street motorcycle since the late 1960s; the brand's defining mechanical signature. Cost: complex valve adjustment every ~12,000 km.
• Sleeve-valve engines. A reciprocating-and-rotating cylinder sleeve carries the ports; as it moves, ports align with the intake and exhaust tracts. No poppet valves at all. Used on Bristol Hercules and Centaurus radial aero engines (WWII), Bristol Brabazon Mk.II prototype. Extinct in current production.
• Rotary valves (Aspin head). A rotating drum or disc in the head carries intake and exhaust ports that periodically align with the cylinder. Lighter than poppet valves but sealing is difficult; production never reached significance.
• Free-piston engines. No camshaft, no crankshaft — pistons move under combustion pressure alone, driving a linear alternator or hydraulic pump. Toyota and Sandia experimented with prototypes. No production yet.
• Two-stroke port-scavenged engines. No valves at all — intake and exhaust are controlled by ports cut into the cylinder wall that the piston uncovers near BDC. The 'no valve train' architecture; entire reason a chainsaw can run in any orientation.

Notable production engines by valve-train architecture

• OHV V8. Chevrolet small-block (1955–present, multiple generations to LS3 and LT4), Ford Windsor (1962–2001), Chrysler Hemi (current 6.4 and 6.2 Hellcat), GM Vortec / LS / LT, Cummins B/X-series diesels. Pushrod survives in trucks for cost, packaging, and torque-curve characteristics.
• OHV inline-four. Largely extinct from cars but still found in industrial engines (Briggs & Stratton, Kohler) and many small diesels. Mazda's MZR-CD diesel uses a unique chain-driven OHC and pushrod combination.
• SOHC inline-four. Honda Civic 1.3L (1973 first-gen), 1980s small Japanese cars, Ford Pinto OHC 2.0L, GM Ecotec earlier variants. Mostly replaced by DOHC since 2000.
• DOHC inline-six. Toyota 2JZ-GE / GTE (Supra), BMW S54 (E46 M3), Mercedes M104 and M256, Nissan RB-series, Cosworth YB. The architecture's home — six cylinders, twin cams, four valves, and a long timing chain.
• DOHC V8/V10/V12. Ferrari F140 V12 (Enzo, FXX, Aperta), BMW S65 V8 (E92 M3), Lamborghini Aventador V12, Porsche 9R6 V8 (RS Spyder), Cosworth DFV V8 (Lotus 49 onward). Every modern high-rpm performance V engine.
• DOHC with VVT. Effectively universal on every modern passenger car. Toyota 2GR-FE / 8AR-FTS (VVT-iE), BMW B58 (Valvetronic + double VANOS), Honda L15B / K20C, Hyundai Theta II (CVVD continuously variable valve duration).

Valve count and breathing — does 4 valves really beat 2?

Configuration	Total valve area	Peak flow at 7000 rpm	Notes
2-valve, 1 in + 1 ex, large valves	Baseline	100 %	Simple OHV, packaging-friendly
3-valve, 2 in + 1 ex	+10 to 15 %	112 %	Honda VTEC-E, Yamaha 1990s
4-valve, 2 in + 2 ex	+30 to 40 %	135 %	Modern DOHC standard
5-valve, 3 in + 2 ex	+38 to 48 %	140 %	Ferrari F355, Audi 20V (now extinct)

Common pitfalls when working on valve trains

• Wrong timing-mark alignment. A single tooth off on the timing belt or chain costs ~17 crank degrees of valve timing — enough to wreck both idle and power. Always align all marks (crank, intake cam, exhaust cam) before final tensioning.
• Treating an interference engine like a non-interference engine. A non-interference engine survives a broken timing belt; an interference engine doesn't. Check before changing the belt; if interference, the timing-belt service is mandatory.
• Using the wrong valve clearance spec. Solid lifters need clearance set per spec at the right valve temperature. Cold and hot specs differ by ~0.05 mm; getting them confused costs power or causes valve burn-out.
• Mixing engine oil grades on flat-tappet OHV engines. Modern API SN/SP oils have low zinc/phosphorus (ZDDP) levels to protect catalytic converters. Older flat-tappet cams need high-ZDDP oil (or a ZDDP additive) to avoid lobe wear.
• Ignoring the valve-spring fatigue limit. Springs lose preload over ~150,000 km even with no obvious failure. Performance engines need spring inspection at scheduled intervals; rebuilds always include springs.
• Forgetting that VVT actuators need clean oil. Variable-valve-timing phasers are precise hydraulic devices and clog with carbon or oil sludge. Manufacturer oil-change intervals are not optional on VVT engines.