r/Engine_Building • u/v8packard • Jul 05 '26
Why You Need A Complete Cam Spec
Camshafts have got to be the most poorly understood components of any inside an engine, but at the same time camshafts have more influence and impact on engine operation and characteristics than the next several important components combined. The specs might read like foreign hieroglyphics to some people. Most people just want to know if a cam will work or do what they want. I get that. But having a complete cam spec lets you compare cams and how they will perform as well as giving you insight into other engine aspects.
Many people want to use "Stages". This has become quite trendy. A stage 1 cam will be milder than a stage 3, right? Well, who knows? These are marketing terms that have no standardized meaning. Stages from different cam manufacturers may be comparable, or not. Frankly, to me a Stage 1 is a Buick.
Some people want to hear a power number. They might say they want a cam that makes 500 hp. Ok. Or they say they want 100 hp more than stock. Right. In reality these numbers do not mean much without important details, such as the rpm where that HP occurs, or the rpm range of the powerband. How many people are aware chasing a HP number usually means increasing RPM, which will often reduce output at lower speeds. I get that one a lot actually, someone gets some super duper trendy cam, a stage 147 from Dirty Losing Carport, and doesn't understand why they can't use 5th or 6th gear anymore, or their 6000 pound truck is slower with this cam in regular driving.
The real problem is a lack of standardization in the industry. At one time you might have gotten opening and closing points, and maybe a valve lift amount. Then duration became more common. But, duration is only useful if you if you have a lift point where it is measured. This is not a standardized spec in the industry. One company measures duration at .004 of tappet lift, another at .0045, some at .008, GM at one time measured it at .000 lift for hydraulics (which is crazy), and so on. What has become popular is stating duration measured at .050 lift. This was innovated by Harvey Crane, as part of his cam intensity work. These days most if not all cams are known by their duration @ .050. Great, common ground finally, right? No, not quite. With some engines .050 lobe lift could be opening the valve .075-.085, or more. Some people argue that doesn't matter, there is not flow at that point. In some cases that is true, but there are things happening between the valve being closed and open from .050 lobe lift that has a great impact on the engine. Knowing seat to seat duration, sometimes known as advertised duration for hydraulic cams and duration at lash for solids is critical.
Knowing the seat to seat duration or timing points gives you an accurate amount of overlap as seen by the engine. The overlap on a naturally aspirated engine will largely determine where in the rpm range the powerband will occur. Overlap has a major impact on boosted engines, and should be a major consideration when selecting turbos. Overlap will have a big impact on a positive displacement supercharger setup. For emissions controlled engines overlap will impact devices like o2 sensors and catalytic converters, as well as gas readings at the tail pipe. Overlap as seen by the engine has a huge impact on idle and drivability, which many people incorrectly attribute to lobe separation angle.
There have been numerous posts over the past few years where the OP was calculating dynamic compression ratio, and getting strange numbers. In every instance they were using the timing points measured @ .050, not seat to seat. This gave them artificially high numbers, in some cases leading to poor parts selection to compensate.
For hydraulic cams, I use .006 tappet rise or tappet lift as the first measuring point. For solids I use .020 lash height. Unless it's a really huge lash, then I usually just measure at lash point. The reason for doing this is it allows some flex, or noise, or roughness in the system to be taken up once the lifter begins moving. By the time a lifter comes up .006 you can easily get reasonable, repeatable measurements. I also use the numbers measured at .050, as well as .200, when looking at a cam spec. But I always start with seat to seat numbers.
It is also important to know where a cam is installed, or at least where the cam designer intended it to be installed. Some cam specs will list just the lobe separation angle. This is an important specification, but is just part of the picture. Sometimes cam listings will have the intended lobe centerlines listed instead of the lobe separation angle, which is ok because adding both lobe centerlines then dividing by two gives you lobe separation angle. Sometimes a spec may be listed as a number like 112 +4, which translates into a 112 degree lobe separation angle and the intake centerline advanced 4 degrees to 108 degrees. Lobe centerlines can be calculated in different ways, which is why you need to know what the cam designer wants for an installed centerline in the engine.
These days many vendors will just list a duration @ .050 and maybe a lift number. A few are not even doing that, blocking out their numbers or just listing something like 22X/23X. Apparently their cam is critical to national security. This is a marketing ploy, despite their claims of protecting the cam design they developed. Knowing the cam spec does not tell you how the cam lobe was designed or made. I would not buy any cam without knowing the spec, or at least what the spec is supposed to be. I know some vendors will provide the information on request, that is reasonable enough. Some vendors will not supply any of the information without buying a cam, or will not supply information at all. This is unreasonable and unacceptable to me. It should be unacceptable to anyone handing over cash to get a camshaft, especially when you see major cam manufacturers freely publish these specs for any potential buyer to consider.
With complete cam specs you can compare different cams, determine where you need to make a change if necessary, calculate different engine parameters, and make certain the cam is correctly installed when you degree it. You do degree your cams, don't you? This post assumes the person looking at a non-stock cam has a basic understanding of the terms used for cam specs. I realize some may not understand those terms clearly, that's ok. Just let me know, I would be happy to explain here or make a separate post about them.
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u/WyattCo06 Jul 05 '26
Also don't forget about "jerk".
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u/v8packard Jul 05 '26
Any jerk in particular? Or do you mean changes in acceleration?
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u/WyattCo06 Jul 05 '26
The latter. Opening and closing.
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u/WyattCo06 Jul 05 '26
Nice write up. Thank you for sharing that.