322

1moparman,
Single pattern cams are ok but when running a single pattern cam as the centerline gets lower and the overlap becomes greater, thus needing alot more compression and not very streetable. Along with more converter and steeper gears. This is why cam grinders grind cams with lobe seperations around 108-110-112 then they advance the intake centerline 2-4 degrees, most grinders advance them 4* so you would have a intake centerline of 106 and a LSA of 110 and exh centerline of 114. Generally when they get lower than 110 they are single pattern grinds. Cams ground on a 108 CL is generally used on both sides, like 108 int. CL and 108 exh CL so this would be a straight pattern cam.

The main reason that they do this is for either racing or street drivability, the further you move the centerline back the more drivability you'll have. On a single pattern cam, the lower the centerline the more racey it becomes. Dual pattern cams also allow you to have better bottom end and more top end with the same cam. By moving the int. CL down gives better low rpm performance, and with the exh CL being higher gives better scavageing. This cuts the overlap down and will add cylinder pressure which helps pull in a fuel/air charge and helps to push out the burned gasses. They use these on heads that don't have real good ports, either in design or flow. This brings us to the next question about duration.

Now on duration, this value is how long the valve is off the seat. They have 2 different terms for duration. One is seat to seat or any value less than .020 from the seat. Some are as .006 some at .015 this is all considered seat to seat or advertised specs.. Then you have duration at .050, this value is generally less than advertised and is more widely used for cams with more than 230* duration.
Cams with less than 230* duration are ment for street and have slower ramp speeds and more driveability. These cams usually have 110 or 112 centerlines and are ment for more Tq. and low rpms HP. These type of cams generally have very good mid range Tq and HP with flat or broad power curves. Cams like these generally are 2000/5800 rpm range cams.

The more duration at .050 the more radical the cam becomes and the faster the ramps get for opening the valves. Generally these are Race only designs, and too the lifter has to change in order to not cut the lobe off the cam. As durations at .050 get larger the taper or face of the lifter is ground differently so the edge of the lifter doesn't cut into the ramp of the lobe. Then when the ramp gets so fast they have to use a roller lifter design, as this is the only effective way that they could make a lifter live. Thus the new engines have fast ramp cams and use hyd. roller lifters. These make more Tq. and low rpm HP, along with smoother engine operation.

As for the lift question, lift is just a value that comes from how small or how large the base circle is. Most street cams have a large base circles and this allows the cam grinder to smoothen out the lobe or slow the ramp speed down and give you more advertised duration, but the actual time the valve is really open is less. Because the time that the valve really starts to open is later. Most race cams have small base circles and fast lobes thus creating more duration @ .050 and more lift.

From the cam grinders that I've talked to, lift is just a value to them and not really important. To them duration @ .050 and lobe centerlines are most important, as this tell them when and how long the valve will be open. You can have a cam that has only .450 lift and have 270+ duration @ .050 and 104 centerline. A cam like this will have the same effects as a very large roller cam but because of the large base circle can use a hyd. or solid lifter. And this is the type of cams that they use in Stock Eliminator Racing. So you can see that lift doesn't really mean much. Thats why they say it's just a value.

Cam grinders will tell you that if you pick a cam by lift you'll almost pick the wrong cam everytime. Or at least this is what they told me many many years ago. Since then I've bought cam profiling programs where I can put in the head flows and port sizes, rod lengths, and strokes, and bore sizes. These all have a adverse effects on the type of cam used. Along with the type of transmission and gears and weight of the vehicle. So you can see it becomes a science of more than just what cam to buy.

As for the specs. or who's specs.. I use Cam Motions and Comp Cams, and Crane Cams. Those specs. are just some that I threw out to give you a general idea as to what to look for. For street use in a 318 I like the 6901 cam from Summit. This cam is actually ground by Crane Cams, it's .441 lift on both sides and has a split duration like 218/224. Sorry for being so long but I hope that I stated it in such a way that you can understand it.