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Pontiacman8 Pontiacman8
Moderator | Posts: 5778 | Joined: 02/08
Posted: 12/02/13
09:52 AM

So, which cam do you use? A car that idles rough, stalls in gear, is a bear to drive in traffic, and gets poor gas mileage, is still cool if it rocks when you floor the gas pedal. An engine that suffers these driveability headaches and still gets blown away by a Ricer is the worst possible experience. The most prevalent reason for engines that don't run as they should is an improper camshaft for the engine and vehicle combination, or a camshaft that wasn't degree'd when it was installed.
We'd all like to have a 10-second car with a great stereo, air conditioning, 30-mpg fuel economy and 1.2g handling capability, but reality says that isn't going to happen unless your surname is "Superman". In the real world a street car is built for either ultimate straight-line quickness, driveability with some performance, or a combination of both, which ultimately means a compromise of both. Do you want the car to run low ET's or get you to work every day? Does the car have an automatic or manual transmission? How rough an idle can you put up with? These are things to consider when you make your cam selection.
Everything about the cars combination and intended function must be decided upon before choosing the cam. Once this is determined, the following important details must be taken into account to get exactly the right camshaft: the engine's compression ratio, the basic power range of the heads, intake-manifold-carb and header combination, the car's weight, the transmission type (and/or torque converter stall speed), the rear gear ratio and the rear tire size. Once you've decided how you want the car to behave, you must build the entire engine and drive-train accordingly.
The problem is that there are many things to consider when choosing a cam, and it involves much more than just the other engine parts. The entire vehicle and the sum of its parts are just as important. Below is an outline of what all those specs and numbers mean and a general explanation of how they affect performance, followed by a rundown of the areas that must be addressed at cam selection time.
Understanding Camshaft Specifications
There are many numbers and terms used when describing a camshaft's design that must be understood when choosing a cam. It's good to know exactly how each of these specs affects the engine's performance, but one of the most important is duration, so pay special attention to that one.

The cam's basic function is to open the valves. Lift refers to how far the valve is opened, or lifted off it's seat. A street performance cam will usually have between .450 and .550 inches of lift. More lift increases power, and increased lift without changing the duration, increases power without affecting the point of peak power on the rpm band. The rocker arms have a direct effect on lift because they don't have a 1:1 lever ratio. A cam that has .318 inch of lobe lift (that's how far it lifts the lifter) will open the valve .477 inch with 1.5:1 rocker arms (.318 x 1.5 = .477) and .508 inch with 1.6:1 rockers. Generally, a stock engine will tolerate .500-inch lift before the valves hit the pistons or the valve springs coil bind, but any time lift is increased, these clearances should be checked.

A hydraulic lifter cam is the best choice if the car is to be a daily driver, because it doesn't need periodic lash adjustments. A solid lifter cam is beneficial in high rpm applications (6500 rpm and higher), but it requires a lash adjustment every few months. Hydraulic and solid lifter cams, without a roller lifter, are called flat tappet cams. Hydraulic and solid lifter cams, with a roller lifter, are called roller cams. You also need to use the proper lifter for the camshaft, as you can't mix and match cams and lifters. Because they are ground differently, a solid-lifter cam must use solid lifters and a hydraulic cam must use hydraulic lifters.

Duration is how long the cam holds the valves open, and is expressed in degrees of crankshaft rotation (remember, the cam rotates at half the speed of the crank). A 280-degree-duration cam holds the valves open longer than a 260-degree-duration cam. Holding the valves open longer allows more air and fuel into the engine and allows more to get out through the exhaust. A longer duration (higher number) cam will improve top-end power, but almost always sacrifices low-end torque. A low duration cam improves low-end torque and the idle quality, however it limits top-end power. Plus you can only get so much valve lift with a short duration cam, due to the rate-of-lift limitations of the lifter. Roller cams have the advantage of allowing higher rates of lift, with a relatively short duration.  

The confusing thing about duration is the difference between "advertised" and "at .050-lift" duration. At .050-lift duration is measured from the point where the cam moves the lifter up .050 inch until .050 inch before the lifter is all the way back down. Most cam manufacturers differ in where they start and finish measuring for advertised duration. Some start at .004-inch lift, some at .008-inch and some measure it somewhere in between. That's why the .050-lift numbers are the best to go by. A 280 cam (advertised duration) from one manufacturer could actually have less at .050 duration than a 278 cam from another, due to the different points at which the companies measure advertised duration.

Listed below is a general group classification for aftermarket cam profiles. The durations shown are based on .050 cam lift with a 112° lobe center. The descriptions within each group show the characteristics of the cams in that group as well as any recommended modifications to the car or engine that will help get the desired performance.

Class I (200° - 215°)
Good idle quality. Low rpm torque and mid range performance.Will work with
stock or slightly modified engine. Manual or auto transmission. Good vacuum.
Class II (215° - 230°)
Fair idle quality. Good low to mid range torque and horsepower. Will work with
stock or modified engine. For use with manual or automatic transmission with mild stall converter. Lower vacuum than stock.
Class III (230° - 245°)
Rough idle quality. Good mid to high rpm torque and horsepower. For use with manual transmission or high stall automatic. Requires improved induction, exhaust system, and ignition system. Good street/strip cam, but has low vacuum.
Class IV (245° - UP)
Rough idle quality. Good high rpm torque and horsepower. For serious racing.
Need proper selection of rear axle ratio and improvements in carburetion and exhaust systems. For use with manual transmission or automatic with very high
stall converter. Will not have enough vacuum for power accessories.
@ .050
@ .050

A dual-pattern cam is one that has different duration and/or lift specs for the intake and exhaust, usually with the exhaust lobes having more duration and lift than the intakes. Depending on the engine, this is said to be beneficial for engines with poor exhaust port flow or otherwise restricted exhaust systems. There is some debate as to weather dual pattern cams actually have any advantage over single pattern cams, however they seem to be more common now days. Due to the restrictive exhaust flow of the log head, we feel a dual pattern cam is especially beneficial when the induction is increase by adding multiple or larger carbs on the log intake.

Lobe Center or Separation
While most of our customers know what lift and duration are, most have no idea what the term Lobe Center means, or how it effects the engines performance. Therefore the biggest obstacle to overcome when selecting a cam profile, is selecting the lobe center. While there are plenty of articles on the internet that give a good explanation, you'd need a masters degree in physics to understand most of them, therefore I decided to see if I can simplify it.

Lobe Center (or Separation) is simply the distance between the peak opening points on the intake and exhaust lobes. A 110-degree lobe separation means that the peak opening points of the intake and exhaust lobes are 110 degrees apart. Lobe Center is another way of expressing the Valve Overlap, which was the term formerly used by cam manufacturers. Overlap is the amount of time that both valves are open in the same cylinder. While all three terms are similar, as they all relate to the relationship between the centerline of the intake lobe and the centerline of the exhaust lobe, we'll focus on Lobe Centers, as it is the term most commonly used by cam manufacturers when describing their cam profiles.

Simply put, the Lobe Center of a cam controls where the power curve is applied. The tighter the lobe center, the lower the rpm range; the wider the lobe center, the higher the rpm range. The rpm range is also known as, or refer to as the power band. In general, two degrees of lobe center is equal to approximately 500rpm. Therefore, if a cam with a 110* lobe center has a power range between 2500-6000 RPM, the same cam with a 112* lobe center would have a power range of 3000-6500 RPM. Conversely the same cam with a 108* lobe center would have a power range of 2000-5500 RPM.

A cam with a 112* lobe center will generally have a smooth idle. A cam with a 110* lobe center will be a bit lopey, while a cam with a 108* lobe center will have a rough or choppy idle. Therefore, a cam with a 112 lobe center will idle smoother than a cam with a 110* lobe center, while a cam with a 108* lobe center will idle rougher.

A cam with a 112* lobe center works great with automatic or manual transmissions. A cam with a 110 lobe center will also work with automatic and manual transmissions, however an automatic transmission may be a bit temperamental in city traffic. A cam with a 108* lobe center requires a manual transmission, however an automatic can be used with a matched stall converter.

Longer Durations
(higher number)
Tighter Lobe Centers
(lower number)
increase peak power
reduces peak power
decreases maximum torque increases maximum torque
rougher idle
rougher idle
less bottom end torque
more bottom end torque
peak power occurs later
peak power occurs sooner
widens the power band
narrows the power band
raises the power band
lowers the power band
higher emissions
higher emissions
higher fuel consumption
higher fuel consumption
decreases manifold vacuum decreases manifold vacuum
decreases compression increases compression

Tests have shown that, for a given cam profile, a tighter (smaller) lobe center will produce more average horsepower and a quicker revving engine, however it is at the expense of a small portion of the peak power and the idle quality, due to the slightly increased overlap. In reality, a cam with 220º at 0.050" and a lobe center angle of 112º will have the exact same mechanical overlap as a cam with 217º at 0.050" and a lobe center angle of 109º.  If you compared these two cams side by side in identical engines, the first cam would actually have a slightly better idle and would produce slightly more peak power at upper RPM's. The second cam would have a rougher idle, produce more torque in the 2000-5000 range and would rev quicker.

You just need to decide which profile works best for your application. If you want a smooth idle and do a lot of highway driving, where passing power is of concern, the 112* might be a better choice. If you do a lot of light-to-light driving, don't mind (or prefer) a lopey idle, the 110* may be better suited.  But if you want to eat V8's off the line and can live with a choppy idle, a 108-109* lobe center might be just the ticket. Basically, it comes down to a compromise between performance and idle quality, base on your driving requirements.

Inches of vacuum may be of concern if you have vacuum assisted accessories, such as power bakes or steering. A cam with a 112* lobe center will pull approximately 16-20" of vacuum, which is good for power brakes. A cam with 110* lobe center approximately 14-18", is borderline. However a cam with 108* will only pull 12-16", which is not enough  for power assisted accessories. These numbers can vary a few inches one way or the other, depending on several other factors and your individual engine specs, so you may want to check your vacuum once the cam is installed. If you don't have enough vacuum you can install a vacuum canister or reservoir, which works by storing the peak vacuum in the reservoir. If that's still not enough, then as a last resort you can use a vacuum pump.

The final step in selecting a cam profile, is learning about Dynamic Compression Ratios, how it effects engine performance, fuel requirements, and ultimately your cam selection.

A last word of advise. Once you get your cam and are ready to install it, be sure to pick up a Cam Degree Kit and degree the cam to the manufacturers specs when installing. When you degree the cam, you are making sure the intake centerline is precisely where the cam manufacturer intended it to be. This is very critical to vacuum, throttle response, emissions and especially gas mileage. If the intake valve opens too early, it will push the new charge into the intake manifold. If it occurs too late, it will lean out the cylinder and greatly hinder the performance of the engine. If the exhaust valve closes too early it will trap some of the spent gases in the combustion chamber, and if it closes too late it will over-scavenge the chamber and take out too much of the charge, again creating an artificially lean condition. If the overlap phase occurs too early, it will create an overly rich condition in the exhaust port, severely decreasing the gas mileage.

Many manufacturers suggest installing the cam three or four degrees advanced. This also lowers the power curve of the cam and increases low-end torque. At the same time, it sacrifices top end power, but more is gained on the bottom end than is lost on the top end. It should also be noted that some manufacturers, such as Comp Cams, grind their cams with a 4-degree advance automatically ground in, while others do not. So you need to verify this prior to degreeing the cam. All Classic Inlines cams are ground straight up, so we highly recommend degreeing the cam 3-4 degrees advanced for optimum performance.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races


69GTOby 69GTOby
Guru | Posts: 1118 | Joined: 02/09
Posted: 12/05/13
06:48 PM

I am loving all the camshaft threads, thank you!

On the topic, I've been a bit obsessive and quite thirsty for more knowledge.  I have read a lot about cams, and I have learned a lot, but I still have questions, and have not yet reached the depth of understanding I desire.

For the sake of discussion, I'll share my thoughts.

I have received a lot of mixed signals about the performance of Pontiac factory cams.  Some say the old cams are obsolete.  New computer technology has allowed us to advance on our cam designs, making them more efficient and better overall performers compared to the old designs.  Others swear by the old factory Pontiac cams.  Jim Hand for one, has written an elaborate article on testing different cams, old and new, and his results suggest that the factory Pontiac cams are right at home in the old school Pontiac engines.  People like him claim that the further they deviate from the original Pontiac engine designs, the worst off they are, and that the engineers really did their homework and knew what worked best.  So, is it generally just a matter of preference?  I know there are endless variables and considerations in the building of an engine, but generally speaking... simply put, if you have an average 400 a little hopped up, which is better, a new fancy fast ramp cam on a 110 LSA, or an old Pontiac design with a wide LSA and lots of duration?  
1969 GTO resto in progress. '76 350+.060, #13 heads, 9.2:1 CR, Lunati cam, Edelbrock Performer, 750 Holley, HEI, Ram Air manifolds, 2 1/2" exhaust. TH400 trans w/shift kit and 2400 stall.  235/60/15 front, 275/60/15 rear on Rally II 15X7s.  GM 12 bolt posi with 3.73's.

thinman56 thinman56
User | Posts: 112 | Joined: 04/13
Posted: 12/18/13
06:13 AM

Good, in-depth write-up, P-man, thanks for posting.  

Pontiacman8 Pontiacman8
Moderator | Posts: 5778 | Joined: 02/08
Posted: 12/18/13
07:16 AM

your welcome thinman,and thanks.  
Engine builder,self taught auto body guy.
Horsepower sells engines and torque wins races