the higher the engine goes, the less oxygen there is to oxidize the fuel, and so the engine loses power.
If your 318 pumps 140psi at 100ft elevation, which is not uncommon; then by 4000ft, it will be down to 120psi, and by 8000ft, down to 100psi.
But it's the exact same engine.
Plus, the higher it goes the richer the carburation gets...... which also kills power.
At 4000 ft elevation, the Scr would need to be bumped up to 9/1 from 8/1 just to get the 140psi pressure back, then the carb would need to be rejetted and the ignition timing might also be different.
From 100ft to 8000ft, the Scr would need to climb up to nearly 10/1 again just to maintain the paltry 140psi.
That's why I started asking people for elevations, if I can't get your address from the avatar., for which I can then go to the internet.
This is also why if a guy says he's gonna hop his 318 up to operate at 5000 ft; I tell him to forget about it, just get a bigger engine. You can't hardly get enough Compression ratio at sea-level and by 5000, you better just supercharge it.
Here is an example
stick a 340 cam into an 8/1, 318.
Right away, at sealevel the pressure drops from 140 to 122psi. So it's already a dog on the bottom end.
Now take it to 5000 and the pressure has dropped to 97psi, that thing is a dog from bottom to top.
To get the pressure back to 140, would take an Scr of 10.2, and
and to get it up to 160 where it needs to be, would take 11.2.
Now, how are you gonna get a 318 up to 11.2 ?
Well firstly, throw away the 72plus cc heads. then throw away the 8/1 pistons.
To get to 11.2 Scr, with 3.94 pistons, would require a total cylinder volume of , less than or equal to 64.9cc.
So, the pistons have to be up at the deck, the Quench is gonna be tight, the gasket to use for 11.2/1 is the FelPro 039s the pistons need eyebrows now for the 340 cam, and the heads have to be closed chambers.
Can it be done? sure, but at what cost!
Just get a bigger engine.
The higher the elevation gets, the bigger the problem gets to be.
You said your engine was soft on the bottom. the short answer is; That's because it ain't getting enough air, and/or, it ain't compressing it enough.
Pressure makes heat, makes Power.
No pressure= no power.
Sure you can band-aid the situation with a higher stall and bigger gears....... but the engine is still soft.
Sure you can make it quicker in FIRST gear with a higher stall, and gears. But in Second with a TF transmission, the typical street convertor is no longer a tool. Now the gears come into play. But when you get into Third, and start winding it up, now it's on the engine. That soft engine is now huffing and puffing and wheezing itself half to death, cuz it's, um, soft. and if you put too much gear in it, maybe your lifters pump up and maybe they drive the valves down into the pistons, and yur heads are done.
Butum, if you just want a lil more go in first Gear, I really like my 2800, lol.