(This is a final version of something that I thought would be a useful technical artical for people. Thanks to Camgrinder who straightened me out on a few things and was kind enough to share his thoughts on a subject at which he makes his living. What do you guys think? - John)
How to Select a Cam
Camshafts are one of the defining elements of an engine’s performance and often one of the least understood. As a result the camshaft is often selected as an afterthought which can result unintentional compromises and an engine that doesn’t perform to its full potential. In this article I will be outlining a process that can help you to select the right camshaft for your air cooled normally aspirated 911’s engine. Please keep in mind that camshaft design and the interactions between the camshaft and the engine is a very complex subject which could never be covered in a few paragraphs. My intention is to introduce the key facets so that reader can be confident that they are “in the ball park” and have a meaningful conversation with their engine builder or cam designer.
At it’s simplest, the function of a camshaft is to open and close an engine’s valves in synchronicity with the pistons cycles. How it does this has a fundamental impact on the speed with which each cylinder is filled, the acoustical tuning of the exhaust and intake systems and the functional compression ratio of the engine. By the careful management of these factors it is possible to select a camshaft that will work in concert with the other components in the engine and optimize its performance.
Step 1) Define the Performance Parameters
Before you buy anything it is important to define a set of goals for your engine.
• What is the desired rev range? For the sake of discussion it would be best to pick a 2000 RPM range where you want the engine to be strongest. The desired rev range is really a function of the intended gear box and the gap between gears. If you are running a close ratio gearbox you’ll be able to optimize the engine’s rev range for a smaller drop in rev’s after every shift. If you are running the relatively wide gearbox from a 4 speed 911T on the other hand, you might actually consider targeting a wider rev range.
• Where are you going to use it? On the street? Track? Race? Are emissions important?
• What sort of induction system are you planning on using? Carbs? MFI? CIS? EFI or Individual Throttle Bodies?
• What sort of fuel will be used – specifically what octane?
All of the factors of a cam’s design do only one thing when it comes to engine performance. That is to define the shape of the torque curve. The shape and rev range of the torque curve will by definition define the engine’s horsepower since the two are related by the following equation:
Horsepower = (RPM * torque)/5252
Generally, an engine’s peak horsepower engine speed will be about 1500 to 2000 RPM above the engine’s peak torque engine speed. So the first parameter that we need to identify is where in the engine’s rev band you want the peak torque to occur. I’ve included a copy of the 2.2 liter 911S’s torque and horsepower curves below as an example.
On it you can see that see that the engine develops its peak torque of 20.3 mkp at 5200 RPM (the graph isn’t perfectly accurate). This is the “sweetspot” for the engine where the engine is pulling the strongest as a result all of the pieces being in tune and the cylinders getting the maximum charge. As the revs increase from that point the cylinder will no longer have enough time to fill completely and as a result the torque will begin to decrease. As long as the revs are increasing at a faster rate then the torque is dropping, the horsepower will increase. At 6500 RPM in the example, the torque begins to decay at a rate greater then 1 mkp (note the scale on the right) per 1000 RPM. At that point the torque is dropping off faster then the revs are increasing and so the HP begins to drop.
Step 2) Cam Parameters
From a manufacturing perspective there are many subtle facets of the cam’s design that need to be optimized, none of which I’m going to cover. When selecting a cam for your 911 from a number of designs, the task is somewhat simpler and you really only need to worry about 4 things.
1. Duration: In general the intake duration of the camshaft will determine where in the rev range the peak torque will occur. Unfortunately camshafts often have their duration listed based on different measurement methods. Porsche’s factory camshafts were specified based on 0.1 mm (.0039 inches) valve clearance. A reasonable rule of thumb for a 911 engine (based on the lifts being measured with 0.1 mm of valve clearance) is that the peak torque engine speed will be a function of the following equation:
a. Peak torque engine speed = -3151+(Duration * 32.53)
b. In the case of 911 engines, the following rule of thumb (once again based on a .1 mm valve clearance) can give you an indication of how the exhaust duration will affect a potential engine’s peak HP engine speed. While hardly exact, it can help you to understand the magnitude of the impact that exhaust duration can have.
a. Peak HP Engine Speed = (exhaust duration degrees * 66.62) - 9083
2. Lift: Lift has a more subtle influence on an engine’s performance and is closely tied with the intake porting and all of the trade-offs involved in that subject. In general the greater the lift, the easier it will be for mixture to flow into the cylinders – limited by the flow in the rest of the induction system. So in general if your cam has excess lift, it won’t create any more torque and HP then a cam with the ideal lift for your engine. It will on the other hand generate higher valve accelerations (see below). If on the other hand your camshaft’s lift is insufficient for the engine’s requirements, it will limit the high RPM horsepower as the torque will drop off faster then if you had used an “ideal” camshaft. The best way to determine how much lift you should spec for your camshaft is to have your heads flowed. Below is an example of some flow data for some sampled early 911 heads.
Note that the CIS 2.4TK heads don’t flow more then 150 CFM at .4 inches of lift. In general a camshaft that provides more then .4 inches of lift will not perform much better then a camshaft with .4 inches of lift in a TK head. The 2.2 S head on the other hand (also used in the 2.7RS) keeps flowing more air all the way up to .5 inches. If you were to use an E cam which only lifts to about .4 inches with an S head, you won’t even be using the last 25 CFM of flow that the cam and heads can provide. A chart of your heads’ flows such as this is very useful for comparing the valve lifts defined by the camshaft. Ideally you want a situation where the cam has opened far enough to allow maximum flow when the pistons are undergoing their maximum acceleration. Depending on the rod-stroke ratio, this generally occurs around 75-80 degrees of crank shaft angle. Now compare the head flows with the valve lift graph for the 911 S camshaft below.
Note that the valve has opened to a point that allows maximum flow by the time that the crankshaft has reached maximum acceleration. In some cases in order to achieve this condition of full flow at maximum piston accelerations, the cam designer does have to “over-lift” the valve past the head’s peak flows just to manage the valve accelerations.
3. Overlap: Overlap can be great for extracting those last few ponies from a well tuned engine, but it’s murder if you need to pass any sort of emissions testing. The reason is that when an engine is on-cam, overlap allows a well-tuned exhaust to draw the new charge into the cylinder actually making an engine more efficient then its capacity would suggest. The downside is that when the engine is off-cam, unburned fuel can go out the exhaust causing high emissions and poor drivability and mileage. With a lot of overlap it’s also possible to have the exhaust push back into the cylinder and in extreme situations back up the intake and cause reversion. In general overlap is desirable for high RPM track and race engines, but not desirable for smooth low rev’ing street and autocross engines.