Understanding Cam Shafts
Cam shaft specifications can be very confusing, and it seems there is little information out there to explaining them in easy terms. Therefore, I am going to make an attempt to explain what I can from information I have gathered. I would truly welcome anyone's comments who has further information to add. Please email me at zeener96@yahoo.com with any suggestions.
To begin, we will look at the specifications for the ISKY Max #1 cam:
| Grind | Type | Intake | Exhaust | Lift | Valve Lash | Duration |
| Max #1 | 3/4 | 17-52 | 57-12 | .364 | .014 | 249 |
Type: I know absolutely nothing about why this cam is considered a 3/4, some are considered full race, etc. Obviously, a full race is more aggressive than a 3/4. I would appreciate anyone's input as to how these cams are "rated".
Intake: This is the time the intake valve is open is degrees of rotation of the crank shaft. In this example of 17-52, the intake valve is open 17 degrees before TDC and closed 52 degrees after BDC.
Exhaust: This is the time the exhaust valve is open in degrees of rotation of the crank shaft. In this example of 57-12, the exhaust valve opens 57 degrees before BDC and closes 12 degrees after TDC.
Lift: This is the lift of the cam. There are two types of lift, Cam lift and Valve lift. Cam lift is measured with a dial indicator on the the top of the lifter, set at zero with the lifter on the heel of the cam. Rotating the cam till the lifter is on the peak of the cam will give you the Cam lift.
The second type of lift is Valve lift, this lift takes the cam lift and multiplies it by the rocker arm ratio to get the actual distance the valve is lifted from its seat. Of course, flatheads don't have rockers....so the two figures are basically the same for a flatty.
Valve lash: Speaking in flathead terms, this is the clearance between the top of the lifter and the valve stem. To measure this the lifter needs to be on the heel of the cam, then you can put a feeler gage between the lifter and valve. Normally this is measured with the engine hot, but this is to my knowledge basically impossible on a flathead.
Duration: This is the time, measured in degrees of crankshaft rotation, that a valve is open. In this example the valve is open for 249 degrees. You can find this by doing the math:
Intake: 17+180+52= 249
Exhaust: 57+180+12= 249
It just happens that the duration of the intake and exhaust for the flathead are the same duration. However, this might not always be the case, and you may see both a intake duration and an exhaust duration.
So.......
What does all of this mean????? I will attempt to explain, but once again, if anyone out there has some better info I would be glad to hear it.
In a perfect world you would want a duration of 180 degrees, since that is the time the piston is moving down on an intake stroke or moving up on an exhaust stroke. However, as we all know these engines are far from perfect so allowances have to be made.
When a piston moves down on the intake stroke, it creates a void of low pressure inside the engine. Air/fuel mixture rushes in to fill this void and balance the pressures. However, because of the speed of the fuel/air inrush and the speed of the engine, fuel is still flowing into the cylinder after the piston reaches BDC. Therefore the intake valve will stay open 52 degrees after the piston starts going up again.
The same with the exhaust, which open us 57 degrees before the piston reaches BDC.
Now for the tricky part. If you are doing the math, by now you have realized that the intake and exhaust are BOTH open at the same time for a period of time, this is called overlap. Why do you want overlap????
If you take what I just wrote, you see that it is difficult to get the fuel/air in and the exhaust out in the small amount of time the engine allows. Therefore you overlap to give the valves more time to do their job.
Naturally, the faster the engine turns, the worse this problem becomes. The limiting factor is how fast can the engine turn and still allow adequate time for the valves to do their job. Which in turn determines your engines top RPM.
This fosters the need for aftermarket cams. The max #1 used in this example is relatively mild, if you do the math (17+12) you will see it has an overlap of 29 degrees.
In general:
more aggressive cam= more overlap = higher RPM's
The "lope" you hear in an engine is a result of valve overlap. The more lope, the higher RPM the engine is capable of.
Therefore, when selecting a cam, keep in mind that although an aggressive lopey idle sounds cool, it is not particularly suited to street driving and you will probably be disappointed in the performance.
That's all I have. If you can offer any insight, email me!!!!