This is the Damage Formula
Damage = ((((2 * Level / 5 + 2) * AttackStat * AttackPower / DefenseStat) / 50) + 2) * STAB * Weakness/Resistance * RandomNumber / 100
All calculations will be made considering max possible damage. This is a Life Orb Modest Heatran with
Flash Fire and Sunny Day in effect so therefore…
(((42) * (394 * Life Orb 1.2) * (120 or 140) / DefenseStat) / 50) + 2) * 1.5 * Weakness/Resistance * (Sunny Day * 1.5) * (Flash Fire * 1.5)
We can simplify this further to...
(((42) * (472.8) * (120 or 140) / DefenseStat) / 50) + 2) * 3.375 * Weakness/Resistance
-- 3.375 is (1.5 * 1.5 * 1.5)
Is this equation correct? I'm pretty sure Life Orb's description says increases attack and sp. attack by 20%, so I'm pretty sure it should be a direct modifier to Heatran's Sp. Attack.
We’ll start out by listing the damage a supposed switch-in Pokemon in Uber or OU in alphabetical order, all Pokemon are at max IVs, +Sp.Def Nature, 252 HP, and 252 Special Defense for calculation purposes. This is in exception to Blissey as she 99.99% of the time runs at a Bold nature and 148 HP and 252 Def EVs.
Blissey (688 HP)
-- Fire Blast: 532 Damage
-- Overheat: 619 Damage (It’s still plenty alive to Softboil up and live easily through a half-power Overheat)
Bronzong (338 HP) (Heatproof Ability)
-- Fire Blast: 448 Damage (Dead)
-- Overheat: Overkill
Cresselia (444 HP)
-- Fire Blast: 414 Damage
-- Overheat: 484 Damage (Dead)
Deoxys (304 HP) (Defense)
-- Fire Blast: 356 Damage (Dead)
-- Overheat: Overkill
Dialga (404 HP)
-- Fire Blast: 497 Damage (Dead)
-- Overheat: Overkill
Dragonite (386 HP)
-- Fire Blast: 248 Damage
-- Overheat: 289 Damage
Gyarados (394 HP)
-- Fire Blast: 248 Damage
-- Overheat: 289 Damage
Ho-Oh (416 HP)
-- Fire Blast: There’s no way in hell Heatran will be doing anything worthy versus Ho-Oh
-- Overheat: 416 HP, 447 Sp. Def, Fire Resist, and Ground Immune.
Kyogre (404 HP)
-- Fire Blast: There’s no way in hell Heatran will be doing anything worthy versus Kyogre
-- Overheat: 404 HP, 416 Sp. Def, Fire Resist, and Water > Fire/Steel.
Latias (364 HP)
-- Fire Blast: There’s no way in hell Heatran will be doing anything worthy versus Latias
-- Overheat: 364 HP, 394 Sp. Def, Fire Resist, and Ground Immune.
-- I’ll stop here…
I’m pretty sure I’ve made myself clear here. Even Pokemon that are rarely used like Dragonite can take an Overheat and live. I believe the only Pokemon that can die only from Overheat’s increased power is Cresselia, but that doesn’t even take into consideration the 85% - 100% random damage (All damage was calculated at 100%) or the fact that Cresselia could have a CM up.
In most situations, both Overheat and Fire Blast come close to 1HKO’ing, so they both need to use two hits to kill a Pokemon.
Fire Blast + Fire Blast > Overheat + 50% Overheat
There isn’t anything mind-blowingly noteworthy that Overheat kills to insist on keeping it in. There are some things that Dark Pulse or Dragon Pulse can do that can counter Pokemon that can wall you badly. Ho-Oh walls both your Tangrowth and Heatran combo, easily. Dark Pulse/Dragon Pulse for example can hit SE and likely KO Lati@s.
If nothing else, Ho-Oh will become an instant marine airforce landing base sized wall if you don't consider something to use against it. Hell, it got a lot worse since Stone Edge and Stealth Rocks were added to the game, but it still walls those 2 Pokemon with ease.
Fire Blast is much better in my opinion. After the opponent falls for it once, they're not going to get baited again so stupidly. You'll likely have 1 chance to set up this set, and it would be wise to Fire Blast anything that comes near it until it runs out of PP. 1 Overheat Sp. Attack loss completely ruins any huge power Overheat had, so Overheat will be killing sometimes maybe ONE more Pokemon? I'd rather take a chance and 2 hit any Special Wall that comes near it, because after the Special Walls are down, you can turn right around and destroy any and all sweepers the enemy has.
I think the main weakness of Heatran is the very low speed. I'd really recommend Timid instead of Modest. It's packing enough firepower and it can sacrifice overkill damage to outspeed a few Pokemon to 1HKO them.
20 base power * Sunny Day (1.5) * Flash Fire (1.5) * STAB (1.5) =
30 * 1.5 * 1.5 =
45 * 1.5 =
67.5
If my calculations are correct... That 67.5 has drastically lower effects the higher the Sp. Def the opponent has and much lower effects with Resistances. Especially Palkia/Kingra's 4x Fire Resist which craps all over any Overheats.