Camshaft Selection

 

"What cams should I use?" This is a question we get asked a LOT. While we've got packages that are designed and proven to work well together, some people like to understand a little more about the theory behind their parts, so here goes.

 

First and foremost, I want to debunk a major myth: your camshaft determines your powerband. Simply not true. It's ONE component of a series of components that determines your powerband. Others have a BIG influence, in particular your exhaust system has a big influence (or it can, if the cams let it, more on that in a minute). The most successful motors are the ones that get the cams working together with the ports, the compression, the motor size, and especially the exhaust. Entirely too many people look for cam miracles when trying to get more out of their motor. It ain't gonna happen. Get all the players on the same page, that's how you make power.

 

A tale of four cams, from mild (left) to wild (right). You can see how much taller and broader

the lobes get. These cams vary greatly in their power characteristics, not to mention the other

engine modifications required to make them work.

 

Most people look at the "lift" and "duration" specs of a cam to get an idea of what it's suitable for. I want you to look a little deeper, at the timing specs, and pretty much ignore duration. Duration is just a "quick reference" figure to give you a rough idea of how much cam it is. All it really tells you is the number of crankshaft degrees between the point where the valve crosses. .053 lift while opening and again crosses .053 lift while closing. So it tells you how long it's open. But it doesn't tell you when it opened and closed, and that's actually very critical information.

The timing figures are referenced to the stroke of interest i.e. the intake timing tells you the open and close points relative to the intake stroke, and the exhaust timing tells you the open and close points relative to the exhaust stroke. For example, if a cam opens the intake valve at 32 degrees (like the Andrews N8 does), that means it opens it 32 degrees before the intake stroke begins, or 32 degrees before the intake stroke's top dead center point (BTDC). Likewise if the intake close point is 44 degrees, that means 44 degrees after the intake stroke ends, or 44 degrees after bottom dead center (ABDC).

Since the duration of the intake stroke itself is 180 degrees, this cam has 32 degrees of duration BTDC, plus the 180 degrees of the intake stroke, plus 44 more degrees of duration ABDC, equals 256 degrees of intake duration. So you see, the timing figures tell you the duration, but they also give you a whole lot more information because instead of just telling you how long the valve is open they also tell you when the valve is open. And that's important information!

The intake close timing is the most critical timing spec, in fact it's the most critical spec of the whole grind, it affects the rpm range of the cams more than all the other specs put together. It's also critical for understanding the appropriate compression ratio for the motor. Basically what's happening is that the motor has completed it's intake stroke, and started it's compression stroke, but we still have the valve hanging open. We don't actually start compressing the charge until we close that valve! So if you're closing it later, it stands to reason that you can run a higher compression ratio, because you're using less of the compression stroke to actually compress the charge. Likewise if you're closing the valve early you may want to run a lower compression ratio to avoid pinging because you're using more of the compression stroke to compress the charge. Hence the intake close point and the compression ratio you want to choose go hand in hand.

The reason closing the intake valve later can help is that the incoming fuel has inertia, and the cylinder is still filling even though the piston is coming up. If we close the valve too early, we slam the door while fuel is still coming in, and that hurts power. Likewise if we slam the door too late, we push some of the intake charge back out and that hurts power. You have to choose very wisely when to quit trying to fill the cylinder and instead start compressing the charge. Well, as you can imagine, the best spot to close the door depends heavily on the rpm of the engine. Part of this is because the velocity of the intake charge changes with rpm, and a big part of it is that a crankshaft degree is a different amount of time depending on the rpm. So we select an intake close point largely depending on the rpm where we care most about cylinder fill.

This is why the intake close event is actually the single most critical cam spec as it relates to the rpm range of the cams. It's always what you should look at first when you're trying to decide whether the cams you're looking at will put the power where you want it. It's importance is also the reason that advancing the cams tends to move the power down in rpm and retarding them tends to move it up in rpm. Even though the advancing the exhaust timing would try to move the powerband the opposite direction, the intake close event has the dominant effect.

Typical street cams are in the mid 40's range for intake close event. In the 50's is the hotter street cams and some of the race cams. In the 60's you find the race cams, some even get into the 70's. Below 40 degrees is where you find the high torque/stock compression type street cams.

The next cam timing event to look at closely is actually two of them together: the intake open (degrees before top dead center) and exhaust close (degrees after top dead center) events. This period of time is called "overlap". It's the window in time when the exhaust cycle is finishing and the next intake cycle is beginning. During that small window of time, both valves are open at the same time. What that does is basically connect the intake and exhaust tracts together, and this gives the exhaust system a HUGE opportunity to affect the intake flow.

 

Overlap happens as the piston is passing through top dead center, finishing it's exhaust stroke and beginning it's intake stroke. If the exhaust system can pull right then, it'll get the next intake charge moving before the piston can even start pulling on it. This can greatly help cylinder fill! Likewise, if the exhaust pushes back right then, it shoves the intake charge (that's sitting in the ports and manifold and carb) right back out the carb. Then the piston goes down and sucks it in again. Not only does this hurt cylinder fill, but it triple carburetes the intake charge, because it got sucked in, pushed back out, and sucked in again. We call this condition a "reversion" and it's easily identified by a dip in the torque curve accompanied by a rich condition and you'll generally see a fog out the mouth of the carburetor at that rpm, referred to as a "stand-off".

 

Why you get positive or negative pressure waves gets into a whole bunch of exhaust theory, but the main thing to know is that these pressure waves in the exhaust system are traveling up and down the pipe, reflecting off each end, and they're moving at a pretty constant speed (the speed of sound) regardless of the rpm of the motor. So as you can imagine, as the motor changes rpm, so does the type and size of the wave that's arriving during overlap. Diffusion devices in the exhaust system (baffles, megaphones) can broaden and time the wave to work over a wider rpm range, albeit with a somewhat smaller wave intensity. The real key here, though, as it relates to the cams, is that the more overlap you have in your cams (exhaust close point plus intake open point), the more opportunity you give the exhaust system to influence the powerband. What you're after is what we call a "happy marriage" between the pipes and the cams: enough overlap in the cams that the exhaust system can play a role, and an exhaust system that's pulling during overlap at the same rpm that the intake close point is timed for. Now you've got the exhaust system helping on the front end of the intake cycle and the fuel's inertia helping on the tail end. That's a recipe for power.

 

One thing to be cautious of with respect to overlap: overlap manifests itself in the form of higher TDC lift figures, i.e. how much the valves are open at TDC between the exhaust and intake strokes. Well, high TDC lift figures make cams difficult to install, as they compromise valve to valve and valve to piston clearance. Make sure the person preparing your heads knows the specs on your cams and adjusts the valve to valve clearance accordingly. And clay your valve pockets and do mock assemblies to make sure you've got plenty of valve to piston clearance. Look for .050 around the perimeter of the valve, .060 minimum vertical clearance on the intake, and .090 minimum vertical clearance on the exhaust. The reason that the exhaust needs more vertical clearance is that the piston chases it closed; conversely, the intake valve chases the piston down. Valve float may cause piston to exhaust valve contact, but not piston to intake valve contact.

 

The final exhaust timing spec to look at is the exhaust open point. The exhaust valve actually opens during the power stroke, while the piston is still moving down. Why? Because to overcome pumping losses in the exhaust system, it's necessary to use some of the pressure from combustion to help push the exhaust out. The faster the motor is turning, the less time is available to remove the exhaust, and therefore the earlier the exhaust valve should be opened. So earlier exhaust open points are associated with higher rpm ranges.

 

Often you'll see cams with more exhaust duration than intake duration. This is known as a "dual pattern" grind, and it's done largely to overcome the pumping losses of typical street exhaust systems that use a backpressure inducing device (like a baffle) to provide wave diffusion. A zero backpressure diffusion device, like a reverse cone megaphone, won't benefit nearly as much, if at all, from a dual pattern cam grind.

 

The lift specs of the cam tell how far each valve gets opened. The main thing to know about lift is that it's useful to match it to the flow characteristics of the heads. An engine of a given size at a given rpm has an airflow requirement that can be calculated. What you're after with the cams and the heads is to meet that requirement, and the flow characteristics of the ports along with the lift of the cams plays a big part in that. Also important is when that lift is achieved. In the ideal world, you want the port flowing it's maximum (i.e. valve open to the point of max flow) by the time the piston is pulling it's hardest on it. The lobe center, which is generally halfway in between the open and close events, tells you where that point is. You want the ports sized to not only deliver the required airflow at this point, but also to provide the optimum velocity for the charge; too much or too little velocity will reduce cylinder fill, and the velocity through the ports also needs to be adjusted with consideration for the intake close timing.

 

Flow data like this should be carefully matched to the set of cams used. This is a totally

stock Thunderstorm head. Notice how the flow does not increase beyond about .500 lift.

An NRHS Stage 3 head not only flows substantially more, but it keeps increasing flow all

the way to .700 lift and beyond.

 

Confused? Don't be. We've got proven porting and camshaft combinations that we know work well together. In fact, our CNC porting process specifically sizes ports for the bore, stroke, rpm, and camshafts of the target application, and it does so without human error.

 

 

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