Originally posted by Bosswrench:
I've always thought that in a 351-C, what cam duration etc worked well was also dependent on whether the engine used stock iron heads with their curled-up exhausts, or modern high-port heads with more flowing exhaust ports. Seem intuitive that modern heads would NOT need the extra cam timing traditionally used on 351-C exhausts to make power. Yes?
Not really ...
Forgive me Jack if I explain things too basically, I don't mean to insult your intelligence, but there are many readers of varying levels of understanding, and I want to make this explanation as understandable as possible to the greatest number of readers.
Between the "piston dwell period" at BDC and the "piston dwell period" at TDC there is about 120 degrees of crankshaft rotation for the piston to "push" exhaust gases out of the cylinder, no amount of additional exhaust duration can change that. BUT that is a moot point anyway because if a cylinder hasn't purged itself of exhaust gases by the time the piston begins moving upward during the exhaust stroke, it will fall on its face. This is due to the fact that the engine will have to work harder "pushing" the exhaust gases out as the piston rises on the exhaust stroke. The problem worsens as engine speed increases, the torque curve will fall-off like a brick at higher rpm. There is at least 100° of crankshaft rotation between the time the exhaust valve opens and the time when the piston begins moving upward during the exhaust stroke, this is the period in which most of the exhaust gases must be purged from the cylinder ... without the aid of piston motion
Keep in mind that the exhaust valve is not fully open until about 115° BTDC, which is more than 65° too late! This reality is reflected in the trend these days to use low-ratio rocker arms (~1.6:1) on the exhaust valves of race engines, yet the intake valves are equipped with high ratio rocker arms (~1.8:1 to 2.0:1). The performance of the exhaust port when the valve first comes off the seat is most important, not air flow at peak valve lift. The 351C 4V exhaust port's performance is aided by that big 1.7" exhaust valve which exposes more valve curtain area for a given amount of lift than cylinder heads equipped with smaller valves. Cam timing must take into consideration not only the low-lift performance of the exhaust port itself, but also the scavenging performance of the exhaust system. You cannot consider them separately, they combine to make a system
With a race engine the "early scavenging" of the exhaust gases is accomplished with a low back pressure, free flowing, and properly tuned exhaust system that presents a negative pressure wave at the exhaust port just as the exhaust valve opens. Engine designers try to grind the cam so the exhaust valve opens in harmony with the arrival of the negative pressure wave. However, the same is not true of a street engine equipped with mufflers, small tail pipes, long tail pipes, etc. The exhaust system is not going to provide a strong scavenging pulse when the exhaust valve opens. More than likely there will be some amount of pressure in the exhaust system (i.e. back pressure), and that pressure shall rise as engine speed rises. It is important to understand that as a piston descends within a cylinder during the power stroke the pressure within the cylinder decreases. In order for exhaust gases to begin flowing from the cylinder and into the exhaust system the exhaust valve must open early enough that the cylinder pressure is greater than the pressure in the exhaust system. Since exhaust system pressure increases with engine speed, the higher you want a street engine to rev, the earlier the exhaust valve must open. Opening the exhaust valve earlier is accomplished either by changing the exhaust lobe's centerline or by employing an exhaust lobe with longer duration.
I have good luck with most applications getting 351C 4V street engines to rev freely to 7000 rpm by opening the exhaust valve around 80° BBDC, these are engines equipped with headers and decent mufflers. But in most applications if the exhaust valve is opened just 5° later, around 75° BBDC, the engine will tend to loose steam after 6000 rpm.
That's the high rpm reason for a longer duration exhaust lobe. There's a low rpm reason too.
Overlap softens low rpm torque. The big intake valve and canted valve geometry of the 351C 4V increases the interaction between the exhaust port, the combustion chamber, piston motion, and the intake port during the overlap period. To put this another way, the big intake valve AMPLIFIES the effects of overlap.
Piston motion provides the energy during the overlap period that causes gases to flow in directions we don't want them to flow. There is a window however at top dead center in which piston motion virtually stops, its called the "dwell period". I've learned empirically that in regards to the 351C this window is about 60° wide, extending 30° on either side of TDC. If I can keep the overlap period within that window, overlap shall have less effect upon low rpm torque. If a cam has 50° of overlap, we have 10° of "wiggle room", but if a cam has 60° of overlap we have no wiggle room. With performance cams ... as overlap increases, it becomes more important to center the overlap period at TDC, if we wish to keep low rpm performance and drivability at its best. To put this into understandable numbers, a cam with 286° of "average duration" ground on 112° lobe centers has 62° of overlap.
If I time a 286°/286°, 112° LSA, single pattern camshaft to open the exhaust valve at 80° BBDC (for reasons I explained above) and to close the intake valve at 70° ABDC (for reasons I haven't explained), the overlap period shall be centered not at TDC, but at 5° BTDC.
By subtracting 5° duration from the intake lobe, and adding 5° duration to the exhaust lobe, the intake duration shall be 10° less than the exhaust duration but the resulting dual pattern camshaft (281°/291°) shall have the same average duration as the single pattern camshaft. The lobe centerlines of the dual pattern camshaft are "adjusted" to again establish opening the exhaust valve at 80° BBDC and closing the intake valve at 70° ABDC. With the lobe centerlines thus adjusted the overlap period shall also be centered on TDC, thus minimizing the effects of overlap and holding on to as much low rpm torque as possible.
Centering the overlap period on TDC is the other reason for a longer duration exhaust lobe ... and keeping the overlap period thus centered works very well. In terms of the "Cobra Jet" cam spec (the subject of this thread) combined with the 351C 4V cylinder heads the result is an engine that operates from 700 rpm to 7000 rpm; exhibiting good low rpm power, factory-type drivability, strong mid-range acceleration and a willingness to rev.
For those of you who like to experiment, or do things on your own, here's some math:
The camshaft for the M code version of the 351C 4V had 274° average duration (268°/280°), it was ground on 118.5° lobe centers and thus had 37° overlap. The exhaust valve opened (EVO) at 81° BBDC and the intake valve closed (IVC) at 70° ABDC. Thus the overall valve event duration encompassed 511° of crankshaft rotation. With roughly 10° difference between intake duration and exhaust duration (12° to be precise) the overlap period was very well centered on TDC.
In order to open the exhaust valve at 80° BBDC and close the intake valve at 70° ABDC a camshaft's overall valve event duration must encompass at least 510° of crankshaft rotation. To arrive at that figure is as simple as adding 80° + 360° + 70°.
A camshaft having 280° average duration, ground on 115° lobe centers, is the smallest cam we can have ground today that meets this 510° valve event criteria. Cam grinders cannot or will not grind a cam with lobe centers wider than 115°. However in some cases it is difficult, if not impossible, to get a cam grinder to grind a cam with 115° lobe centers. Therefore I've adopted a self-imposed limitation of 114° lobe centers. Based upon that limitation a cam with 282° average duration and 114° lobe centers is the smallest cam that achieves 510° overall valve event duration. Centering the overlap period at TDC results in a camshaft having approximately 277° intake duration, 287° exhaust duration, 54° overlap, and 114° lobe separation. Such a "street camshaft" should employ lobes having no less than 54° lobe intensity, thus overlap as measured at 0.050" lobe lift shall be zero or less.
Here's a formula for determining a camshaft's overall valve event duration: overall valve event duration = (advertised intake duration + advertised exhaust duration) - overlap
Computing overlap is easy:overlap = average advertised duration - (LSA x 2)
and average advertised duration ... average advertised duration = (advertised intake duration + advertised exhaust duration) ÷ 2
Have fun with this ...