So whatever torque is coming out of the crank, gets multiplied by these numbers.
Say you have a stock stall of 2000ish rpm, and say your engine makes 200 ftlbs at 2000. Therefore 200 x 7.86x 1.8=2830ftlbs briefly, diminishing to 200 x 7.86 x 1.1=1730 ftlbs as the rpm starts to build. These are lackluster numbers. And you are not a happy guy. So you swap out that convertor for a 3000, and at this rpm say your engine is now making 327 ftlbs. Your new numbers are 4626 diminishing to 2827ftlbs which is a chitload better than 2830/1730. In fact, as you can see, the max at zero mph and 2000 of 2830 ftlbs, is now about equal to the minimum achieved by the 3000 stall TC@2827.
2827 is not that great, but not so bad either, and for a DD I would gladly be happy with that. Now, I'm not saying that you need a 3000 stall Convertor. I'm just presenting a thought-process.
Lets move on; With 27" tires and 3.21s, 65=2600 at Zero slip, could be 100/150 either way on the tach. So for optimum fuel usage, the thought is to have a stall no higher than this. But if your leaning more towards performance, I can tell you that I really really liked my 2800 Turbo-Action, which I purchased back in the late 70s and at that time, it had the name of Dirt-Jerker. Great TC. I still have that TC today, still installed, having been thru many combos.
If yur still reading;
Now we have the basic combo worked out which is ;
318/904/3.21s/2800 stall.
Next up is the gas, what octane are you willing to pay for. This will determine NOT the compression ratio to run, but the maximum cylinder pressure.
Here are the typical pressures to octane numbers, for an iron open-chamber :
87 at 150psi
89 at 155psi
91 at 160psi.
A Quench-design will usually tolerate about 5 more psi.
For max-torque, you would build to the highest pressure
For max-Power at high rpm, whatever octane is sufficient to suppress detonation.
For max fuel economy, you would build to the cheapest gas
Lets say you choose 89, and 155psi, as a happy medium.
Now we're getting into the nitty gritty.
Now is the time to pick the cam.
Getting back to the 3.21s being a handicap, and the 2800 stall, we are looking at a power peak of ;
2800 plus about 1500=4300 This is just about exactly equal to the stock 318 cam.
But this is a DD, so it doesn't have to be a super gas-sipper.
Lets stretch the power band out to fill up all of First gear and if it falls on it's face on the 1-2 shift, we won't lose any sleep over it....... cuz it's a DD; ok?
So then you gotta chose an rpm limit, at which the engine will survive say 100,000 miles, cuz longivity also plays into fuel-economy. Ok then, we know that Mopar small-blocks tend to throw rod bearings with stock oiling systems, after 6000, so lets sneak in at a 5500 rpm shift.
Allowing just 300 rpm past peak for the factory heads, this gives us a power peak of around 5200. Oh well that's just swell, this allows a cam of up to
[email protected] which for a 318 is really big and NOT suitable for getting fuel mileage.
