As to the bottom end; here's my take;
The biggest contributors to soft power at lower rpm, in a given engine, are;
1) lack of effective cylinder pressure or the
2) peak cylinder pressure not occurring at the optimum crank position
Lets talk about #1
causes of low effective cylinder pressure can be;
by design
by a late closing intake
by the induction of hot low-density air
by a too small carb,
by a loss of help from the headers, or
by too restrictive an exhaust system
Typically we measure the CCP with a compression tester. But this gives us a measurement that is only applicable at cranking speed with the hood up and usually with the air-filter house removed and the throttle set to WOT. And we crank it until no additional pressure is read.
But this number is constantly changing in a running engine!
At idle, the throttle is nearly closed. With a big cam having a late-closing intake, and the pistons all pushing just-inducted air back up into the plenum before the intake actually closes, there is no way that engine will produce the measured pressure. It will idle all day at whatever it takes to overcome internal friction. Ima guessing down to 60psi.
Now, as the rpm increases, and at WOT;
there comes a time measured in rpm, at the which, there is no longer enough time for the just-inducted mixture to back up into the intake, and so the pressure may be peaking at 100% VE.
And
there may come another time, measured in rpm, when, by inertial tuning, the engine actually inducts and traps more air than it's cubic inches and so the pressure would peak at a higher number than it's CCP, indicating a VE higher than 100% .
This condition may exist over just a few hundred rpm, at which time, the design of the engine, at rpm, will begin to lose efficiency because of the lack of time to induct air, thus the torque curve begins to fall.
The Power numbers continue to increase because of the way the formula works. But eventually as the torque continues to fall, so will the power.
Now lets go back to low-rpm.
One can only design for so much pressure before the engine will detonate itself to death. Especially as the rpm rises because of the ever-decreasing time to lift off the throttle.
Pressure translates to heat, which translates to Torque, which translates to Power. So then, if you can control the temperature in the chamber, thus avoiding detonation, then you can exploit the pressure. Or you can just keep throwing anti-knock at her until detonation stops.
So now, how do we control the chamber temperature?
Well;
step #1 is to;
straighten the path to the airhorn, AND induct the coldest air possible AND to keep it cold all the way to the sparkplug, AND to keep the velocity up all the way there so it has a harder time backing up into the Plenum. The more successful you are at this, the higher the effective pressure of the design, can be.
Step #2 is to
create that max effective pressure of the burning gasses, at the exact right time in the rotation of the crank, to transfer the most energy of the expanding gasses into the flywheel. As I understand this, this occurs in the window of 25 to 28 degrees AFTER TDC, regardless of rpm. So ALL your timing systems need to attempt to achieve this.
If the timing is wrong, the whole team takes a hit. If she gets into detonation, that robs power big time. If the timing is late, power just goes soft.
Another thing that happens with late timing is more heat goes into the cylinder walls, because of the position of the piston steadily dropping. Of course, the cooling system is right there so the water temp rises...... which may raise the chamber temperature, which we don't want to see.
Another thing that happens with late timing and short power duration, is that the combustion event continues into the exhaust system, which heats up the exhaust port, and pressurizes the header, which upsets the scavenging during overlap. This can be a good thing at idle, but you sure don't want this to continue after stall speed.
But if you are running
log-style exhaust manifolds, you want NUNUVIT, because those hi temp still-burning gasses can back up into an adjacent cylinder on it's overlap cycle! When this happens, Late timing, the exhaust pressure is artificially increased, while the plenum pressure is very low, so then, the exhaust gasses are
forced to scoot across the piston and maybe some of it gets into the intake plenum before the intake closes.
>This has several consequences;
The First is that, with the piston now close to TDC, but falling, the chamber is already full of hot inert gasses from another cylinder that came in thru the still-open exhaust valve, and
the Second is that, this occurrence delays the plenum air from getting started towards the intake valve. and
the Third is that, those hot gasses contribute to a higher initial chamber temperature, thus defeating all your attempts to keep it down.
Log manifolds with large overlaps, are a bad idea, in every way. This was Chrysler's primitive form of EGR.
the Fourth is that, with the Air/Fuel charge now being infused with inert EGR, 1) the molecules are far-spread thruout the chamber, and 2) the heat of compression is reduced, and 3) complete combustion may not occur with typical timing settings. Thus un-burned fuel molecules can enter the log-manifolds and continue burning there, aggravating an already bad situation. Eventually, with continuing rpm, this all goes away.......... unless the large volume of high-pressure gasses thus created cannot get away, in too small or restrictive, an exhaust system.
BTW
the 284/292/108 cam has 72 degrees of overlap, 28 more than the stock 340 cam, so that's a lot of street overlap; more typical is 55>60..
Chrysler must have figured out that 44* was already not good with log manifolds, and so gave 340 cars special exhaust system considerations. and that is also partly why the 340 cam was on a 114LSA.
Ok so, that's about all I got;
Happy HotRodding
EDITS;
with long-tube headers tuned for midrange, the low-rpm can be affected by what you bolt to your collectors, due to a change of the secondary scavenge signal.
