Some thoughts on choosing a proper cam.
http://www.camcraft-cams.com/index.p...electing-a-cam
Section 1: for Stock Cars
1- To many racers the best cam is the one in the car when they get the chassis sorted out and the driver finally gets the track figured out.
2- Short duration cams with wider lobe separations usually yield much flatter torque curves
3- Longer rod motors prefer a shorter duration cam with wider lobe separation
4- Longer duration cams require tighter lobe separation to have any power off the corner. (not usually a preferable combination in 2 bbl classes.)
5- Stock exhaust manifolds or a highly restricted exhaust usually respond well to shorter exhaust duration and wider lobe separation. Power increases are most evident at higher rpm where exhaust backpressure is greatest and reversion is most prevalent.
6- Most unported heads approach 85 or 95 % of peak flow at .400 to .450 lift and do not need or want a maximum valve lift over .540 to.555. Often a low cam lift with 1.65 or 1.7 ratio rockers is very helpful on the intake side as long as lift is kept to about .550. Exhaust is less critical with 1.5 or 1.55 being the most popular.
7- Dyno testing doesn’t test drivability or throttle response of the engine.
8- The important numbers on a dyno sheet are about a thousand RPM above and below peak torque and peak horsepower. Peak numbers are for bragging purposes and high peak numbers do not win races.
9- The benefit of high ratio rockers is faster valve movement and the added lift is frequently detrimental in unported heads. It often helps to utilize a lower cam lift with high ratio rockers.
10- Changing the valve lash is a good way to get an indication of which way to go for your next cam change. You won’t hurt anything by going too tight but too loose will let the valves slam shut, causing damage to valves and seats. .004 to .006 loose is usually OK.
11- Look at the Major Intensity numbers to get an idea as to how radical the profile is.( major intensity is the difference between the .020 duration and the .050 duration.) Lower numbers are more radical but anything less than 27 or 28 degrees may be very hard on the valve train. Our 26-degree SXTL profiles are a notable exception to this.
12- Glowing exhaust pipes may be an indication of over scavenging by the exhaust. A shorter exhaust duration. Smaller headers, or even a restrictor plate at the header may help. The problem is often mixture burning in the exhaust rather than in the cylinder. Many people think a lean mixture causes it. Be sure to ascertain which problem you have as the lean mixture is a much more serious problem and can cause quick meltdown.
Camshaft intensity is a measurement term coined by Harvey Crane to compare ramp characteristics of camshafts.
Hydraulic Intensity is the difference between the .004 duration and the .050 duration.
Minor intensity is the difference between the .010 duration and the .050 duration.
Major intensity is the difference between the .020 duration and the .050 duration
Lower numbers indicate more radical profiles but too low can be too radical and lead to noisy valve train and even to broken parts.
Section 2: For drag cars
1- Cars with small tires or poor suspension may need a larger cam to kill some power off the line in order have a consistent launch without overpowering the suspension.
2- Hard tail dragsters usually run quicker with shorter duration cams but then lose consistency because the harder launch overpowers the track. Soft tail cars can adjust for the harder launch and remain consistent.
3- Be absolutely sure of your gear ratio when selecting a cam. Just cause your buddy told you that trick new rear is a 3.73 or 4.10 doesn’t mean it really is. If you pick a cam to go with a 3.73 rear and it turns out you have a 2.73 The difference would be 3529 rpm with 2.73 gears and 26” tires @ 100mph or 4822 rpm with 3.73 gears. The same cam won’t work real well in both scenarios.
4- The first thing to do in selecting a cam is to determine top speed in high gear. If 6,000 rpm = 150 mph in high gear and you will never see that speed then you definitely don’t need a cam with 6,000 rpm power. Tuning for power at too high an rpm is a very common mistake.
5- You need to make good power below the converter stall speed (not flash rpm) in order to hit the converter hard. This becomes more critical as car weight goes up. Peak power should be slightly below the rpm through the lights.
Section 3: For street cars
Be absolutely sure of your gear ratio when selecting a cam.
1- The first thing to do in selecting a cam is to determine top speed in high gear (not overdrive). If 6,000 rpm = 150 mph in high gear and you will never see that speed then you definitely don’t need a cam with 6,000 rpm power. Tuning for power at too high an rpm is a very common mistake.
2- A car with 3.08 gears will go 150+ mph @ 6,000 rpm. 5,000 rpm still = 125+ mph while 2,600 = 70 mph. It makes a lot more sense to tune the engine for power in the 2,000 to 4,500 rpm range. You can get good fuel economy as well as better performance. Any cam bigger than stock will make more power at high rpm. It just doesn’t make sense to select a cam with max power at 5,000 rpm if it will spend most of its life below 3,000 rpm. Below are common rpm=speed ratios. All are based on a 26-inch diameter tire.
2.73 @3000 = 85 mph, 3.55 @3000 = 66 mph, 4.10 @3000= 57 mph
3.08 @3000 = 76 mph, 3.73 @3000= 62 mph
3.23 @3000 = 72 mph, 3.90 @3000= 60 mph
3- If you have big heads you definitely need a smaller cam than if you used the stock heads. Failure to realize this will result is a big loss in low end and midrange throttle response. It will likely make great power at rpm levels you don’t even want to see.
4- Be careful of using big carbs. A 650 cfm carb is plenty for all but the most radical 350 engines, even for strictly racing. A 750 may make more top end power but it gives up a lot in low-end throttle response and midrange torque. This is especially true when you have big heads. The fact that your buddy ran faster with your 750 than his 650 only means there was something wrong with his 650.
cam shaft selection. How to pick a cam!
I've had some funny looks from people when I do this, I guess I'm just a big kid almost 52 and 355 pounds... 
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