Overlap, or valve overlap, is the period when the intake and exhaust valves are both open at the same time. It happens around top dead center, right at the transition from the exhaust stroke to the intake stroke. ENEM, one of Sweden's most established camshaft manufacturers, describes it like this: "Overlap is the point where the exhaust valve is about to close and the intake valve has begun to open. Both valves are open at the same time and the piston is essentially at its highest position."
What determines the amount of overlap?
Overlap is not a standalone camshaft property. It is the result of duration and lobe separation angle working together.
- Longer duration: the longer the valves are held open, the more of that window overlaps around TDC.
- Tighter lobe separation: the closer the intake and exhaust lobe centerlines sit to each other, the more overlap you get.
Comp Cams lobe designer Billy Godbold illustrates this with numbers: three camshafts with an identical 110° lobe separation angle but different durations produced widely different overlap, from 53° with the shorter duration to 77° with the longer. So lobe separation alone does not determine how much overlap you end up with.
Why overlap exists
In a naturally aspirated engine, overlap creates a scavenging effect. The kinetic energy of the outgoing exhaust leaves a low-pressure region behind it, and that effect helps pull fresh charge in through the intake port before the piston has even started moving down. At high rpm, this exhaust scavenging can noticeably improve cylinder filling and therefore power.
That is why racing cams for naturally aspirated engines often combine a tight lobe separation angle (104 to 108°) with long duration, which produces a lot of overlap. It maximizes peak power but costs idle quality and low-end torque.
Overlap and idle
At low rpm and small throttle openings, overlap causes problems. Residual exhaust gases in the combustion chamber get pushed backwards into the intake system, a phenomenon known as reversion. The more overlap, the more reversion:
- Intake pressure rises (vacuum drops)
- Fresh mixture gets diluted with inert exhaust residue
- The idle turns rough and choppy
- Fuel consumption and emissions increase
Tests on the same engine with three different lobe separation angles (106°, 112° and 114°) at identical duration showed that 112° gave the best compromise: smooth idle, 2 inches more vacuum than 106°, four miles per gallon better economy, and essentially the same power up to 4,000 to 4,500 rpm. Only above that did the tighter lobe separation pull ahead.
Overlap in turbo engines
In a turbocharged engine the rules change. The turbo creates positive pressure in the intake (boost) and, at the same time, backpressure in the exhaust system. The relationship between those two pressures decides what happens during overlap:
- If exhaust backpressure is higher than boost pressure, exhaust gases get pushed back into the cylinder and out into the intake system. That acts as internal EGR at full throttle and costs power.
- If boost pressure is higher, fresh charge air blows straight through the cylinder and out into the exhaust. That wastes boost and can overheat the exhaust valves and the turbine.
ENEM puts it plainly: "Boosted engines almost without exception make more power with a wide lobe separation angle", in other words a wider lobe separation with less overlap. That is why turbo cams have traditionally been designed with 112 to 114° lobe separation, compared with 106 to 110° for naturally aspirated engines.
PF Racing, who design turbo cams specifically for Volvo's B230, consistently use what they call a "dual lobe design", meaning different duration on the intake and exhaust sides. Shorter exhaust duration closes the exhaust valve earlier and shrinks the overlap window without giving up intake duration. The result is faster spool and cleaner cylinder filling under boost.
Modern turbos change the picture
Turbo technology has advanced dramatically. Modern, efficient turbos run lower exhaust backpressure relative to boost pressure than older designs did. Bonneville record holder Kenny Duttweiler has shown that with a properly sized modern turbo you can run a tighter lobe separation angle (109°) and still keep a smooth idle with 16 inches of vacuum, something that was unthinkable with older turbo designs.
This does not mean you should run a tight lobe separation on just any turbo engine. But it shows that what matters most is not whether the engine is turbocharged, but how efficient the turbo is and how well the exhaust side flows.
The bottom line
Overlap is the product of duration and lobe separation angle, and it affects everything: idle, vacuum, midrange, peak power and emissions. There is no universal rule. The right amount of overlap depends on whether the engine is naturally aspirated or turbocharged, which rpm range matters most, and how efficiently the exhaust system flows.
Want to understand how your cam's overlap affects your engine? Contact Meksta and we will help you find the right balance.