A Cam Timing Primer

I was originally going to just post the cam specs my dad and I have measured here on the site without any context but I think an brief explanation of what all the numbers mean would be helpful. The most important thing to keep in mind in selecting a camshaft, in general, is the characteristics of the cylinder head you are working with. A higher flowing head in general can support more camshaft duration without becoming “over-cammed”, losing throttle response and costing power in the process. This is a common mistake to make, because frankly big numbers are exciting and we’re all here because we want to go fast. A turbo engine also tends to be more easily over cammed than a non turbo engine due to the lower static compression ratio found in turbo engines as well as the negative effect excessive valve overlap can have on spool time.

In my experience over about 240 degrees of duration at .050” (i.e. the valve is lifted over .050” for 240 crankshaft degrees) is too much for an untouched 8V Volvo head. This is around the duration of the largest factory cam grinds. Possibly not a coincidence.

Camshaft Duration

Camshaft duration is generally the biggest determinant of where peak torque occurs in an engine, along with cylinder head flow and sizing of the turbocharger if equipped. A cam with more duration will generally make peak torque at higher RPM if no other changes occur. A good rule of thumb is that the engine’s RPM range will be moved up or down proportionally to the change in intake duration. For example, if your current cam has 200 degrees of intake duration and peak power at 5000 RPM, switching to a cam with 220 degrees of intake duration will make the engine make peak power at approximately 5500 RPM.

Exhaust duration in a naturally aspirated application should be sized such that the exhaust airflow is approximately 75% of the intake airflow, which depending on the application may require more or less duration than the intake port. When tested with the standard B230F intake manifold the stock intake port flows almost the same as the stock exhaust port, requiring a larger intake lobe on the camshaft for the right flow bias.

In a turbocharged application, the normal intake to exhaust flow ratio should be taken as a bare minimum, and in fact closer to 100% is preferable. This is because the exhaust flow is not pressurized the way the intake flow is. For a practical example you can look at the factory Volvo VX cam, as found in later non-turbo applications with a larger intake lobe, versus the T cam, as found in nearly all turbo applications. The VX cam features a much larger intake than exhaust lobe to compensate for the more restrictive B230F intake manifold design, whereas the T cam features the same intake and exhaust lobe.

Camshaft duration has two parts, degrees of duration AND a particular lift threshold, and many cam grinders give an incomplete picture of lobe design by only providing a number of degrees the valve is open for. You must know both in order to compare cams across manufacturers. I have included cam duration at .050” and 1 mm lift as those are the most common ways to measure in the US and Europe.

Valve Lift

Valve lift is fairly simple: As much as your cylinder head and valve springs can take is the best amount for maximum power. All things being equal, more valve lift will move the power curve straight up, not changing the engine’s operating range at all. However, the lifters can only accelerate at a certain rate and this means that there is always a theoretical maximum valve lift for a given camshaft duration. Slower lifter acceleration is also kinder to the valvetrain and can help with the service life of some parts (think slamming a door open or closed vs opening it gently like a normal person, whose door frame will last longer?)

In addition, some cylinder heads can actually flow less beyond a certain amount of valve lift as turbulence occurs inside the port. I haven’t seen this very often on Volvo heads but it does happen on occasion and should be kept in mind if building a serious engine.

Standard Volvo valve springs come in three sizes but in general are all good to ~6700 rpm and a little north of 12 mm of lift before binding (in the case of valve lift) or losing valve control (in the case of RPM). Both of these have disastrous consequences.

Valve Event Timing

There is a lot to valve event timing but I will keep things simple in the interest of brevity. Camshafts are installed at a certain intake centerline (and exhaust centerline if you have two, but I don’t have two). This is the midpoint of the intake lobe and usually is somewhere around 110 crankshaft degrees after the piston reaches top dead center. The exhaust centerline is generally around 110 degrees before top dead center. This is what you are adjusting with an adjustable cam gear.

Advancing the cam causes the intake lobe to open earlier and can improve lower RPM performance at the cost of power at higher RPM, and retarding the cam does the opposite. In general, a cam that is heavily advanced or retarded from where it is originally ground (probably around 5 degrees either way) is a sign of an inappropriate cam choice and you would be better off with a smaller or larger one, respectively.

The intake centerline plus the exhaust centerline divided by two is the lobe separation angle, or LSA. This determines valve overlap which has a major effect on idle quality and RPM range. A cam with a tighter (lower number) LSA will idle rougher but make peak torque at a lower RPM.

Hopefully this explainer allows you to better choose and compare camshafts. These measurements were taken by my dad and I using cams we had as well as ones that were generously donated by Turbobricks members. If you have one not listed here that you would like measured please contact me.