RB04Av

Ain't happenin with the 4.8 or 5.3. That's fantasy land.

Torque is the measure of how hard the motor twists the crank. It's determined by (a) how much fuel the engine can burn (and therefore how much energy can be released) per cyl fill, and (b) how efficiently it can take that energy and turn it into twisting force.

The first obviously, is determined by the induction system How much air can be moved in & out. Moving fuel is trivial by comparison; butt fuel won't burn without oxygen. So, barring any form of enrichment of the air (The Force, nitrous, special oxygenated fuel, etc.), you can pull a deep-space vacuum inside the cyl as the piston moves down and let atmospheric pressure push air back in to replace it; and THAT'S ALL the oxygen you will EVER have, therefore that's the limit to ALL the fuel you can EVER burn. All this, at low RPMs of course, where the air is moving slowly and things take a long time to happen; at high RPMs, the inertia of the air becomes a significant factor, and can provide a small amount of extra enrichment within some narrow band of circumstances. Butt for an engine pinned to a very low RPM range by crappy gears and a stock torque converter, the RPMs don't get high enough for any of that to matter, until the vehicle is moving MUCH faster than a dead stop.

The 2nd is a matter of what you can call "thermal efficiency", if you will. The fuel burns, raising the temp in the cyl, which as a consequence raises the pressure in there; some of the heat energy escapes into the cooling system, some escapes out the exhaust, and some pushes the piston down. Just the portion of the combustion energy required to keep the water vapor (the combustion product resulting from the hydrogen in the hydrocarbons) in vapor form as it leaves the cyl, is close to 10%, depending on the exact formulation of the fuel. See https://en.wikipedia.org/wiki/Heat_of_combustion The 3 "destinations" of the energy are roughly (VERY roughly) equal at peak horsepower RPM, although this relationship doesn't automatically hold at peak torque RPM; that is, about 1/3 each of the total available energy.

The push on the piston is equal to the psi in the cyl times the piston area; the twisting on the crank is then this force, times the "lever arm" aka STROKE of the crank. Note that this relationship involves piston area time stroke; this quantity is conveniently known as "displacement", aka CID aka liters. The average pressure being exerted on the piston is known as BMEP, "brake mean effective pressure" https://en.wikipedia.org/wiki/Mean_effective_pressure. Things that affect BMEP include compression, the degree of leakage caused by the cam's properties (specifically, during how much of the engine strokes when the cyl is supposed to be sealed off from the atmosphere, it holds them open), the thermal conductivity of the cyl surfaces (dominated by the heads), the ring seal, and so on.

It's all pretty simple, really. Just numbers. Utterly dominated by the CID, though there's a few areas where you can nibble at around the edges. For an approximate "rule of thumb", about the max possible torque a gasoline engine can make, when fed typical highway-taxed pump fuel, is around 1.25 times the CID, or maybe slightly higher in cases of careful optimization and ideal circumstances. For the 4.8 this means you're gonna be REAL LUCKY to ever reach 375 ft-lbs at whatever RPM the peak torque occurs at, NO MATTER what else you do; and for an engine that's expected to be heavily loaded (low power-to-weight ratio, poor gearing, extra towing load) it will probably need to be quite a bit less than that, to avoid engine damage due to detonation and so forth. Which is why the factory's output is a shade below 300 or so for the various iterations of 4.8 (closer to 1.0 times the CID), and up to around 320 for the 5.3.

All of this is even before considering any of the engine's internal losses. Friction, pumping loss, oil viscosity, accessories (even the minimum ones you can't avoid like water pump, alternator), etc. all take their bite out of the combustion heat, and lower the mechanical output accordingly.

There's nothing magical about "small cubic inch engines" any more than there is about large ones. It's all numbers. Plug the inputs into the cruncher, crank the handle, the output comes out. From there, it's all about how truthful and accurate and complete the input numbers are. Honesty in, honesty out; fantasy in, reality out; garbage in, garbage out.

212/218 is more than the ideal cam for a 4.8 in tow duty in a 5000 lb vehicle with crappy gears and a stock torque converter. The "Stage 1" is more suited to the application butt is still probably more than ideal for your stated goals and situation.

Cherrybomb

4-500 hp and tq somewhere in that range boosted
i’ll keep digging👍🏽

RB04Av

He already said he's not boosting it. This is his tow barge. He wants it N/A. Good choice, vs turbo. Screw-type blower might be different butt he doesn't want that. He wants cheeeeeep and thinks he's gonna get it with a cam swap.

400-500 ft-lbs out of a 4.8 or 5.3 N/A is pie-in-the-sky. Physics is physics. Chemistry is chemistry. Numbers are numbers. Fuel molecules contain however much energy they contain, and no more. "Dreams", goals, wishful thinking, won't change any of it. No cam will get a N/A 4.8 or 5.3 ANYWHERE CLOSE to 400 ft-lbs within a "towing" envelope, let alone 500. The sooner he realizes this, the better, and quicker, and less trial-and-error (mostly error), his decision making will be.

By all means keep digging.

_zebra

iTrader: (5)

i gambled on a 219/224 .551/.551 113° in my 5.3, and the only reason i didn't wish i'd stuck with a 212/218 is because i already had 4.30 gears.

Marky Dissod

Stop with the 4.8L. Even GM eventually gave up on that displacement.
Get a 6.0L. If it has iron heads, and/or pistons that are NOT FLAT, put 243 / 799 heads on it.
Do whatever you want to it after that.

With the 4.8L, to get the peak numbers you think you want, you will be sacrificing everything under 4500RpM.
Even with the most aggressive transmission shift table, your engine will spend most of its time UNDER 4500RpM in the real world.

If you want performance the moment you step on the gas pedal, regardless of RpM, 6.0L.
Far more bang for your buck in the long run than whatever you're gonna do to a 4.8L.