You've got it pretty close, Jetstang. One of the main variables is the cam shaft. Keep in mind that everything about an engine is in constantly changing motion. It's tempting to think about intake flow and exhaust flow as 'contants' at first blush but that's not what's going on. Every time a valve opens and closes, the air flow associated with it has to start and stop. Air/fuel mixture and exhaust gasses both have mass, therefore they have inertia, and therefore they neither start nor stop instantaneously. It takes time for that gas to get moving so it can either flow into a cylinder or out of it. As engine rpm increases, the length of time that those valves are open gets shorter and shorter, giving the gas less and less time to get moving. That's even true in an artifically aspirated engine. So, to optimize volumetric efficiency you have to get that cylinder as full of air/fuel mixture as you can get it. As rpm goes up, the only way to do that is to open the intake valve earlier in the cycle and keep it open longer so you can fill the cylinders, and open the exhaust valve earlier and hold it open longer so you can empty them. It's possible to optimize for more than 100% VE at a given camshaft's peak torque range by taking advantage of the intertia of the moving intake flow. Just like it doesn't want to start instantaneously, it doesn't want to stop instantaneously either so if you can get that flow velocity up you can actually 'cram' more into the cylinder before that valve shuts. On the exhaust side, getting the flow velocity up helps you do a better job of emptying the cylinders. A common mistake the people make is putting on headers that have "too big" primary tubes for the amount of exhaust flow their engine really needs. The bigger cross-sectional area means you've got more volume of gas to 'get moving' and if the engine's exhaust 'blow down' can't move that much volume then the engine has to work (pump) to push it out (instead of taking advantage of flow velocity) and you loose energy that could have otherwise gone into making torque out the crankshaft. It just so happens that peak torque production usually occurs at or very near peak volumetric efficiency (VE) because that's where you're doing the best job of filling and emptying the cylinders, so there's more potential energy in there to release through combustion. So why does it usually work out that peak horsepower and peak torque occur at different RPM? It's because rpm, being a relatively large number (3000 to 6000 or so in our case) has more effect on the horsepower formula (HP = (torque * rpm)/5252) than does torque (around 300 to around 500 or so). RPM is almost always going up faster than torque is coming down - up to a point - and that shows up in the calculation.
But... (you knew there'd be a 'but' right?) here's the down side. Unless we're talking about a modern engine that has some sort of "variable valve timing -VVT" technology, that camshaft profile is going to be fixed - the valves are going to open/close at the same points, regardless of RPM. If we time the valve events such that we move peak VE (and peak torque) higher in the rpm range so we get more horsepower, the down-side of that is the engine becomes less and less efficient at low rpm, so it makes less and less torque at low rpm. That "nasty" cam sound at idle that we all know and love? What's really going on there is the engine is stumbling all over itself just trying to keep running. We're holding the valves open so long that during the overlap period (when both valves are open and that we love so much at higher rpm to get above 100% VE) we're allowing the gas flow through the engine to actually stop and reverse, sucking exhaust back into the cylinders and polluting the intake charge. We associate that nasty idle with a powerful engine, but in reality when it's running like that it's anything but.
The real purpose of a 'loose' torque converter is to allow the engine to quickly spin up to an rpm where it's "on the cam" and running efficiently. That's also why launch technique for a manual trans involves spinning the engine up and sidestepping the clutch.
That's what I meant way back in my original post where I said Pontiacs aren't Chevys and it's a mistake to treat them as if they were. Small block Chevys, even with low rpm street cams, just don't make a whole lot of torque - period - so to get power out of them you have to move what torque they have way up in the rpm range. They respond to that pretty well, but to take advantage of it you then have to resort to shorter gears and loose converters to let the engines get "up there" at launch - and those are the kinds of changes that make a car pretty unpleasant for street driving. Pontiacs, on the other hand, make mountains of torque and they make it down low. For example, the 400 stroker (461) I've built for my 69 GTO "only" makes 492 peak horsepower at 5800 rpm, and it's all done by 6500 or sooner. However, it also makes 543 lb. ft. of torque at only 3100 rpm (!) and it stays above 500 lb.ft. all the way through 5100 rpm. This is a 9.46:1, 93 octane pump gas motor with a moderate solid roller cam, iron heads, factory iron intake, and a 455 SD Qjet. I'm running 3.50 rear gears and a 3200 stall converter, both of which are very streetable even for the occasional road trip, but if I can get it to hook (which will be a challenge with that much torque) it should also run mid 11 second quarter mile et's - maybe low 11's with slicks. If I put a 'bigger' cam in it to move VE higher in the rpm range, it will be mostly because I want to take some low end torque OUT of it even though I do have the crank/rods/rotating assembly that ought to be plenty safe to 7000. Small block Chevy guys should be turning green with envy right about now...
A car is more than just a motor. You have to consider how you're going to drive it, what you're going to do with it, and then choose/build the entire drive train so it's optimized for that. The reality is that there's no such thing as a 'one size fits all' car. A competitive and class legal NHRA Stocker or Super Stocker would be miserable to downright impossible to drive on the street for more than a couple of miles at a time, especially in traffic, and you can't build a street car that will run those kinds of e.t.'s without going *way* outside what would be class legal, and also hideously expensive.
It's the thinking and the planning what you want to do, then making it all come together that makes all this such fun stuff.
arty:
Even more so when you do it with something other than a bow-tie....
Bear