A cam profile that works beautifully in a naturally aspirated engine can wreck the drivability of a turbo engine, and the reverse is just as true. Choosing a turbo cam is about understanding how boost pressure and exhaust backpressure affect cylinder filling, rather than simply reaching for a "milder" grind.
Why turbo and NA engines need different cams
In a naturally aspirated engine, valve overlap creates a scavenging effect: the kinetic energy of the exhaust gases helps pull in fresh charge. More overlap improves cylinder filling at high rpm.
In a turbo engine, two pressures compete: boost pressure in the intake system and backpressure in the exhaust. What happens during overlap depends on the balance between them. When exhaust backpressure exceeds boost pressure, exhaust gas gets pushed back through the cylinder and into the intake, diluting the fresh charge with inert exhaust residue, essentially internal EGR at full throttle. When boost pressure exceeds backpressure, charge air blows straight through the cylinder and out the exhaust valve, wasting the turbo's work and potentially overheating the exhaust valves and the turbine.
This is why turbo cams have traditionally been designed with wider lobe separation angles (112 to 114°) than NA cams (106 to 110°). A wider lobe separation shrinks the overlap window and guards against boost leakage.
Asymmetric duration: different intake and exhaust
Many turbo cams run different duration on the intake and exhaust lobes. PF Racing, who design cams specifically for Volvo's B230 turbo, call it a "dual lobe design" and treat it as a central part of their turbo cam philosophy.
Two common strategies:
- Shorter exhaust duration closes the exhaust valve earlier, reduces overlap and prevents boost leakage. The result is cleaner cylinder filling and smoother boost build. It works well for street cars focused on midrange and turbo response.
- Longer exhaust duration opens the exhaust valve earlier and feeds exhaust gas to the turbine sooner. That improves spool-up and peak power, but it also adds overlap and can cost midrange. It suits racing applications running large, efficient turbos.
Modern turbos complicate the old rule
The wide-lobe-separation rule for turbo cams dates from an era when turbos were less efficient and generated high exhaust backpressure relative to boost. Bonneville record holder Kenny Duttweiler has shown that with modern, efficient turbochargers, where the backpressure ratio is lower, you can run a tighter lobe separation (109°) and still keep a smooth idle with good vacuum.
Kurt Urban, who builds street cars whose turbo engines cover more than 160,000 km (100,000 miles), has shown that a relatively mild cam (227/223° at 0.050") with a reverse split (more intake duration than exhaust) and a well-matched 72 mm turbo delivers 1,200 Nm at 3,500 rpm in everyday driving, in a car that runs the quarter mile in 11.4 seconds.
The takeaway: how well the turbo is sized and how efficiently the exhaust side flows matters far more than the simple fact that the engine runs boost.
Common mistakes when choosing a turbo cam
- "Everyone runs this one." A cam that works in one specific recipe of turbo, manifold, exhaust flow and mapping is not automatically right in yours. PF Racing, for example, offers seven different turbo cams from the PF-12T to the PF-5T, each designed for a specific power window.
- Too hot a cam on a chipped engine. PF Racing explicitly warns that their sharper cams (PF-4T, PF-5T) are not intended for a lightly tuned engine such as a chip-only build. An oversized cam in an otherwise stock engine costs more than it delivers.
- Ignoring the springs. PF Racing's mildest turbo cams (PF-1T, PF-2T) run on stock springs. The PF-3T and up, with 13 mm of lift, require uprated single springs. Running a hot cam on tired stock springs invites valve float and failure.
- Skipping the degreeing. A turbo amplifies every degree of misadjustment. A couple of degrees of cam timing error that would barely register in an NA engine can cost noticeable boost and response in a turbo engine.
What information should you give us?
Kelford Cams, one of the world's leading camshaft manufacturers, uses these questions to size a turbo cam:
- Intended use: street car, track, drag racing?
- Power target: 300 hp and 1,700 hp call for completely different cams
- Rev range: broad midrange or narrow top-end power?
- Turbo size and type: a small unit with fast spool or a big unit chasing peak power?
- Fuel: gasoline, E85, methanol?
- Other modifications: head, intake, exhaust, compression
The answers determine duration, lift, lobe separation and whether an asymmetric profile is called for.
Summary
A turbo engine makes different demands than a naturally aspirated one. Overlap, lobe separation and asymmetric duration all play a part in how quickly boost builds, how cleanly the cylinder fills, and where in the rev range the power arrives. The right cam choice, combined with correct turbo sizing, the right valve springs and proper degreeing, is what separates an engine that pulls the way you hoped from one that never quite gets there.
Contact Meksta and we will help you find the right turbo cam.