Choose a Cam
Obviously, designing a camshaft isn’t for the week of heart but most cam grinders have done all the hard work to get you in the ball park. The area they usually fudge is LSA in terms of less overlap and a later intake centerline because they are willing to sacrifice some power for drivability. It typically makes for a happier customer. As a result, advancing the intake lobe a bit and increasing overlap can usually pick up a little power but don’t expect miracles with the DOHC.
That leaves the builder with choosing the appropriate duration for the RPM the combination is designed to support, then adjust LSA/overlap appropriate to the RPM level and within the confines of the individual drivability tolerance.
To simplify (blasphemy) things we can assume that most cam manufacturers have figured out the hard stuff. As mentioned above, pick an off the shelf cam with the appropriate duration for your RPM range and if you want the last bit of horsepower, consult with the manufacturer for a custom grind which will probably give it a little more overlap and advance in the intake lobe centerline.
How do we choose duration? Well, if we’re lucky we have dyno sheets to refer to for the purpose of identifying how our particular combination responds to duration. Doesn’t matter if you’re a Chevrolet guy, Mopar or Ford, the internet forums are filled with dyno sheets from like minded enthusiasts.
If we’re talking about the 4.6L DOHC Ford we have to be careful because the restricted factory intake skews the results. It’s easy to look at the cylinder head architecture and start drooling but reality is they need a good intake to support them.
Cams with anywhere between 218* - 234* duration at .050 will peak at 6300 - 6400 RPM with the stock intake while stock and mild performance cams in the 209* - 212* range (FR500’s, GT’s Cobra R’s) will peak around 6100 RPM. Shorten the runners in the 5” – 7” range and a little 209* cam wont peak until at least 6800 RPM and it will carry the curve flat well beyond that. Once again, cylinder head airflow is the least of our worries. Prepare to sacrifice up to 75 horsepower in the 5000 RPM range and not make as much power as the stock intake until 6000 RPM if running a 5” runner. For most people, somewhere between 7” - 10” runners are ideal.
Here are some comparisons between different intake and camshaft combinations. Use it as a general guideline to give an idea of each ones characteristics.
It is OK to over cam the DOHC as long as your power curve within your racing RPM range is better than the next mildest cam down. If it’s not than you can pull the cams RPM range down some by advancing the intake lobe if you don’t mind even rougher low speed operation.
For a basic 4.6 DOHC that runs to 6800 RPM in stock configuration we can figure an RPM drop of roughly 2000 between gear shifts, we would like the most aggressive cam that begins to overpower the next milder cam starting at 4800 RPM. Doesn’t matter how it performs below that if we are pursuing a combination that we want to accelerate as quickly as possible. Notice were not talking about drivability.
Now we must determine a reference point. We know that stock Mach 1’s peak horsepower around 5200 RPM with 184* of duration and 99/01 Cobra’s with basically the same engine and 200* of intake duration peak at around 6100 RPM. In this particular case, 15 degrees of duration add almost 1000 RPM to the point at which peak power occurs. We must maintain perspective of runner length and airflow restriction which are both elements lowering the RPM characteristics of the above combinations. Adding another 15* of duration will not move the horsepower peak to 7000 RPM with the stock intake manifold but with short enough runners and unrestricted airflow, it will.
The stock intake starts restricting airflow around 5200 RPM and by 6100 RPM it is pretty much finished. This means that no matter how large the camshaft is, it will not peak power much higher. Maximum is about 6400 with a very aggressive cam. As a result, we have to shift focus from peak numbers to average numbers and what a bigger cam will do is hold power at the peak level longer and fall off slower after peak has been reached. This is why it isn’t futile to “over cam” this particular engine. A Comp Cams 106400 with 234* duration will peak around 6300 – 6400 and hold power longer than a smaller cam. However, if this cam is given enough air and shorter runners it will make considerably more power and peak well into the mid 7000 RPM range. When camming this engine we are typically working within the constraints of the shortblock and the intake manifold.
I would say the 106400 at 234* duration is about maximum for a stock or ported stock intake equipped engine. It does sacrifice low RPM operation but has decent mid range and gives excellent power within the limitations of the stock short block and intake. Also, it offers plenty of potential for any aftermarket or modified intake. On the other hand, if a person is willing to sacrifice racing performance for some mid range torque and smoother low speed operation, cams in the 218* - 226* range provide similar power with a little more docile behavior.