AJ/FormS said

But ok lets do it!

The given will be a 340 cuber at 9.61 Scr; and lets stick to a known cam, the Mopar 340cam, that everyone so loves.
the specs are;
268 intake duration/ 276 exhaust duration/ ground on a 114LSA.
This cam has advertised overlap of 44*, and therefore the duration left for compression plus power is;
(360 x2+44) less (268+276) = 220* , which is a reasonably good number.
Lets install it at "straight up, and 4* in either direction, plus 8* advanced. So, the events are:
( in at/intake/comp/power/exhaust/Effective-overlap/Ica)
@-4*/268/108/112/276/32/72.....142/104
@+0*/268/112/108/276/40/68....148/113
@+4*/268/116/104/276/40/64....155/123
@+8*/268/120/100/276/32/60....161/132


Notice the 4* increasing march in the compression degrees, and
the 4* decreasing march in the power duration. These two on this cam, always add up to the 220(in blue), and are fixed in this relationship by the intake/exhaust/overlap.
Notice also the changes in Effective overlap(in red). Overlap is the time at near TDC when both valves are open, the intake is opening while the exhaust is closing. In this cam, the overlap by the math is 44*. But when you install it, about half goes to each side (split overlap), so the Effective overlap COULD be 44* .. But when you install it at some position other than split overlap, then the Effective overlap is less than 44*, namely, it is double the smallest number. Split overlap for this cam would be, in at 112*LSA.
If you don't know how overlap works, see note-1

Now, lets see what happens to the predicted cylinder pressures, at sealevel in the 9.61Scr 340 . Check out the final two columns which are pressure and VP. Notice the approximately increasing 6psi march in pressure. But also notice the more rapidly increasing march in VP. The VP is what you feel , from idle to stall, perhaps to as high as 3500rpm. Small numbers indicate small performance. Big numbers point to big performance.
For comparison to your favorite stock engines; the slanty makes a VP of around 87, the 273 around 100, the Smoggerteen is about 116, the 360 was about 126, the 318M is about 129, and the 10.5-340 comes in around 137.... by the numbers.
So then, as can be seen, the VP numbers that you need to target with a manual trans are about 130 or more. But with an automatic, you can get away with less, if you increase your stall-rpm. Personally, I feel that my combo with a VP over 159(at sealevel) is waaaay overkill..... but it is soooo much fun.

But back to the chart; look what happens to the Effective overlap, when you advance thechit out of this cam. You get the pressure, and the VP, but lose overlap big-time. Furthermore, the power extraction is down to 100* so the engine is leaving a lot of energy in the exhaust, energy that could have been used to propel the vehicle. This translates to excessive fuel consumption...........
Also; the high pressure translates to what makes the engine throttle response snappy around town, allowing you to run a bigger carb without the low-rpm sluggishness that often accompanies that choice.

Ok, gotta go to work, so EDIT; this was only half-finished at 7.30 this morning,lol.

note-1
Overlap is the Fifth Cycle, and works almost exclusively with headers. What happens in the Fifth Cycle, is that the headers by their design, "yank" on the intake plenum during this time by the fact that the plenum is connected to the exhaust system during overlap. Log-manifolds to NOT pull on the intake. This yank/tug/ negative pressure; whatever you want to call it, gets the A/F mixture moving from the plenum, down the runners, and hopefully into the chambers, SOONER than is possible without overlap, and so begins the cylinder-filling process, sooner. The more overlap you have, the higher up into the rpm operating range, this process can continue. So for power production, the more overlap you have, the more power you will make while this process is in full swing. And this is no minor process.

By the way the cam is ground, a given set of lobes will, by way of the Fifth Cycle and headers, make more power the tighter you LSA them, ....... at the expense of Powerband. Therefore, you can tune a given cam to suit your trans or your weight, etc, by fudging with the LSA.
The A999/A500 have gearsplits of .56/.65
The A904/A727 have gearsplits of .59/.69
The A833 , ....has splits averaging .735
The GVod operating as a splitter, has splits averaging .79 thru the first two gears of the Commando box.

So as you can see, each of these transmissions has a different powerband requirement; the autos have only three gears to the A833s 4 gears; so naturally the powerband will need to be wider for it.................. right up until you install a very-high stall TC. For instance; if you shift at 6000 with a 4500TC, then your powerband requirement is just 1500 rpm or 4500/6000= .75, Aha!.

Click to expand...