Join Date: May 2011
Location: Oshawa, Ontario, Canada
Re: More Valve Adjustment Questions
just to confuse things further - pulled from an online article
Ideally, you could slam a valve open, hold it open, and then slam it closed, and many drag race camshafts attempt to perform this feat, but this harsh action is incredibly abusive on valvetrain parts, especially valves and springs. To make these parts live over hundreds of thousands of miles, the cam lobe lift curve must be gentler. The easiest way to measure the amount of time the lobe is creating lift is with degrees of duration. A long time ago, our cam-building forefathers decided to use crankshaft degrees to measure cam lobe duration. So a typical performance camshaft may have a duration of 280 crankshaft degrees. Keep in mind that a camshaft actually spins at half engine speed.
But this created confusion because all the different cam companies measured the beginning of the lift curve at different points a few thousandths of an inch above the cam's base circle. This is called advertised duration and it can get complicated because, for example, Comp Cams begins measuring its advertised duration for hydraulic lifter cams when the lifter rises 0.006 inch off the base circle. Crane uses 0.004 inch, which would make the same lobe "appear" a few degrees longer in duration because the duration would be measured over a 0.004-inch-longer distance (0.002 inch on the opening and closing sides added together). This difference between companies eventually led to the selection of 0.050 inch as the standard checking point where all the different companies' lobes can be compared. So now we have both advertised duration and duration at the 0.050-inch checking figure. Now that you have the basics, we can dive into a few more items that make camshafts complex, but also fun.
The best way to install a cam is to degree it into the engine so you know exactly "where"
For each lobe there is an opening and closing point. Let's say you are measuring an intake lobe on a camshaft in the engine using a dial indicator and a degree wheel. Once the lifter rises off the base circle 0.006 inch, let's say that the degree wheel reads 20 degrees before top dead center (BTDC) and closes 90 degrees after bottom dead center (ABDC), then you can add those two numbers together along with 180 degrees and come up with the advertised duration: 20 + 180 + 90 = 290 degrees. This same formula can also be used to determine duration at 0.050 inch tappet lift.
Duration is a major contributor to the torque curve and where it occurs in the engine's rpm band. Generally speaking, as you increase the amount of intake lobe duration, this makes for an earlier-opening and later-closing intake valve. This additional duration also extends the rpm point where peak torque occurs. This tends to increase peak hp (depending upon the other components used on the engine) while sacrificing low- and mid-range torque. Conversely, a very short duration camshaft opens the intake valve later and closes it sooner, reducing the potential for high-rpm horsepower but increasing torque at a lower engine speed.
2000 V Star 650/Velorex sidecar
1960 AJS 500 single