It is pretty safe bet to say that everyone has their own opinion of what camshaft dynamics should be used for a particular engines style and function … kind of like cooks in a kitchen … take 15 cooks and ask them to all cook the same thing and you will get 15 different tastes, some really good and some that flat out suck! So, with that being said, these articles are not intended to tell you what camshaft to use or install in your particular application, but are intended only to give you as much information on how camshafts work so that you can make a conscious informed decision on just which type / style of camshaft will work for your particular application.
It is also pretty common for an individual to want the best of both worlds … good strong bottom end power as well as great top end, high RPM power … but, it is practically impossible to have them both. It is a very true rule of thumb that to gain in power and performance in one area you must give up some in another … don’t blame me; it’s just basic physics folks! This is where the 85% rule that we discussed in the last couple of articles comes more into play when designing your engine.
“Lift, Duration, Overlap, and Valve Timing” are probably the most important camshaft dynamics that should be used in our camshaft selection. Most good engine builders have spent many long hours figuring out what combinations work best together, yet the majority of “Novice” engine builders will simply purchase a camshaft that they think is correct without understanding these basic factors and end up with disappointing results.
“Lift” The height that the camshaft lifts the valve has a direct influence of the airflow through the cylinder head. The higher the camshaft lifts the valve, the greater the airflow through the cylinder head will be, but, only to a given point. At the lower lifts of the camshaft, when the valve is just off of its seat, the airflow becomes restricted between the valve head and the valve seat. But as the valve lift increases a conical shaped airflow is formed around the valve head circumference. This conical shaped airflow, if obstructed or interfered with by objects such as the combustion chamber, will drop off through the valve opening. This is where matching the camshaft to the cylinder head characteristics becomes critical.
The amount of time that the camshaft keeps the valve off of its seat, “Duration”, and as we discussed in the previous articles, has a direct impact on the “Mechanical Compression” of the engine. We also discussed previously that “Duration” is simply a function of engine displacement and RPM’s and as the displacement of the engine increases so can the camshafts “Duration”. Now here comes the trade off … or sacrifice if you want … as the duration of the camshaft increases so will the “Cylinder Fill” at higher RPM’s, but, the longer that the valve is off of its seat, the less time there is to build / maintain cylinder pressure at lower RPM’s which typically results in less power down on the bottom or what is commonly referred to as a “Soft Feel” in the lower RPM spectrum … so, what is gained up top is given up on the bottom. This is where the Realistic matching of the camshaft comes into play in the “85% rule”.
As we discussed in the last article “Overlap” is simply the amount of time that both the intake and exhaust valves are off of their seats at the same time, measured in crankshaft degrees, and that this typically occurs when the piston is at or near Top Dead Center (TDC). We also discussed that the amount of “Overlap” and where it occurs plays a major roll on the engines ability to make power.
The camshafts “Overlap” can have some very beneficial characteristics, but at the same time have some that are less desirable or detrimental.
Due to the fact that valves cannot be lifted instantaneously, the camshaft “Overlap” allows for a more gradual opening and closing of the valves. Here is where the exhaust system and induction system being used comes into the equation when choosing the correct camshaft for a particular application. If matched correctly, the camshaft overlap can greatly assist the engine in evacuating the exhaust gases when the piston is at or near Top Dead Center (TDC) while also assisting the induction system with the fresh “Incoming Charge”. But, at lower RPM’s, the inertial effects of the gases are to low to be beneficial which will allow for a great exhaust gas “Reversion” back into the cylinder, contaminating the fresh incoming charge which will result in lower power and poor performance down low.
Previously we discussed that “Valve Timing” is the point / location in crankshaft degrees that the valve events occur (Opening & Closing), and that it is a dynamic that is just as important as the others we have discussed so far … and that when and where they occur will determine where the engine will make its power. To actually figure out the true performance differences between specific camshafts you simply must install them and run them, either on the street, track, or on a dynamometer … this is the only true, real life way to collect this kind of data. But, using the advertised opening and closing degrees of a camshaft along with the advertised “Duration” you can get a pretty good idea of it performance characteristics and a closer point in which to start your camshaft selection.
Of the four opening and closing events, the closing of the intake valve is the most important followed by the closing of the exhaust valve. The reason that these events, in that order, are so important is that they will determine when the cylinder will become sealed and start to create pressure against the piston.
There are many different camshafts on the market that are available for your engine projects. Many of them will work in a broad spectrum of applications and the engines will run and perform so-so, but, if you want to truly maximize your engines performance, take the time to do your research when it comes to choosing the correct camshaft for YOUR engine. Talk to your local engine builder and the camshaft manufacturers technical support people. They are in the business because they are not only good at what they do, but because they love what they do!
I have been in the “Game” for more years that I care to think about anymore, and the great people that I have had the pleasure to work with in this industry are in it way more for the passion and the love of the way of life than the money any day … call them up, talk to them about your projects and they will get you going in the right direction, right out of the gate. A few minutes talking to a knowledgeable person will save you time, money and tons of aggravation. Until the next time … shiny side up!
By Steve “Posie” Pfaff