An Explanation of PGI Heat Management

Most people don't know what the "Heat Mgmt" number in the image at left means. We can guess that 2 / 2 means a 'Mech dissipates as much heat as it makes, that 1 / 2 is half as good as that, or that quirks and ghost heat have an effect on it. In reality the first guess isn't always true, and the last two are just wrong. So what's it mean?

The formula for PGI's heat management is the square root of five multiplied by the heat efficiency. That's the heat dissipated per second divided by the heat generated per second, times five and then square rooted. This number is also capped at 2, so a heat efficiency of 200% (you dissipate heat twice as fast as you generate it) is 2 / 2, and one of 100% is also 2 / 2. Because this function is exponential, 1 / 2 heat management is not half as good as 2 / 2; instead it corresponds to a heat efficiency of 20%. This means that 2 / 2 isn't twice, but five times as good as 1 / 2. Most mechwarriors agree that a heat management of 1.3 - 1.6 is ideal. That's an efficiency between 34 and 72 percent.

Confused? There's More

The heat management stat is flawed for a number of reasons. First, its exponential nature and lack of explanation makes it hard to understand and only a vague indicator of a 'Mech's true heat. Pilots usually have to take their 'Mech into a game or testing grounds to get a real feel of their heat output. Second, it's wrong most of the time anyway. The MWO heat management stat doesn't take laser durations into account for the heat efficiency. Lasers as we know start their cooldown after their full burn is complete. This means that a laser with a cooldown of 4 seconds and a duration of 1 second can be fired every 5 seconds, but the MWO heat will treat it as 4 seconds. Here are some numbers to illustrate that:

First we take a test loadout without any burn-time weapons, purely SRMs. I've chosen this Marauder because it has no quirks that may interfere with our calculations. First, we find the heat dissipation. Double heat sinks dissipate 0.2 heat per second when in the engine, and 0.15 heat outside. We have six internal heat sinks and none external (see heat sink numbers in the 'Mech stats at the bottom right), so our dissipation is 0.2 * 6 = 1.2 heat per second. SRM6s generate 4 heat every 4 seconds, or 1 HPS each for a total of 2. Our heat efficiency is thus 1.2 / 2 = 0.6, or 60%. We multiply this by 5 and take the square root to get 1.732, which matches the PGI heat stat.

Now let's try lasers. We take a Marauder 3R with 4 medium lasers. I've switched variants to avoid the energy heat quirk on the last Marauder. Our heat dissipation is the same, but our heat gen is 4 medium lasers * (3.4 heat every 3.5 + 0.9 = 4.4 seconds) = 3.09 heat per second. Our efficiency is then 1.2 / 3.09 = 38.8%, heat management 1.39 / 2. But that's not right, it should be 1.24 / 2 (see the image below). What went wrong? The laser duration. If we take that out, we get a heat generation of 3.89 heat per second and the "correct" heat management of 1.24 / 2. But recall the 3.4 second cooldown doesn't begin until the 0.9 second duration is over, so med lasers don't actually fire every 3.4 seconds and the MWO heat stat is wrong.

For this reason, the MWO heat will always say your 'Mech is hotter than it really is if you mount lasers. To make matters worse, the heat stat doesn't take quirks into account either. Try that same loadout on the Marauder 5D, the one whose energy quirk we avoided earlier. The heat management is the same 1.24, even though it should be 1.27. Try the same loadout on a Commando 1B and an Awesome Pretty Baby. Their energy quirks are the same aside from the Commando getting a dissipation quirk. The heat management is still the same.

There's Still More

If you're not yet convinced the MWO heat management stat is confusing and useless, there's even more weirdness under the hood. Clan heat management is calculated differently than InnerSphere heat. To illustrate this, take a look at the two loadouts below:

The first time I made this comparison I made the mistake of putting single heat sinks on the Assassin with the usual doubles on the Marauder IIC, as you can see above. Notice though that even with singles, the Assassin's heat management is much better than the Marauder IIC's! I switched the Assassin to doubles and the heat management went even higher to a whopping 1.73. The Marauder has the exact same loadout, but its heat management is as low as 1.02! SRM6s and C-SRM6s both have 4 heat and a 4 second cooldown, and both InnerSphere and Clan internal double heat sinks dissipate 0.2 heat every second (you can see the values in MWO's game files below to confirm this). So why are the numbers so vastly different? I don't actually know. Clan heat appears to always be worse than InnerSphere even when the heat generations and dissipations are the same. Could it be different heat capacities? No, both factions have a base heat capacity of 30 and the effects from heat sinks on it are the same (ten times the dissipation for each heat sink). Also, heat capacity isn't even used in the formula anyway. My only guess is that Clan has a reduced heat factor (square root of a number lower than the usual five, for whatever reason), or that the Clan heat does in fact use the heat capacity while the InnerSphere heat demonstrably does not. Both of those don't make any sense. In summary, I believe the MWO heat stat is worse than useless.

MechDB's Heat Stats

Currently MechDB has a PGI Heat Mgmt field you can look at for convenience. This site includes laser durations in the calculation so if you mount lasers on your build my heat stat won't match the one you see in-game; that's intentional. Additionally, I have no idea how the clan heat management is calculated, so all clan mechs will use the regular formula and not match the in-game stat. Even further, after MechDB's quirk update, heat will take quirks into account and due to the large amount of InnerSphere quirks, MechDB will reach the point where its PGI Heat Mgmt stat virtually never matches the in-game stat. Because of this, I think it would be best to remove the PGI heat stat from the site entirely, as it's misleading, incorrect, and confusing by its very nature. I'm convinced the heat efficiency and Alpha heat as percentages are a far better and more intuitive measure of your 'Mech's heat. Let me know what you think by voting in this strawpoll, and thank you for reading! https://strawpoll.com/5216r3ed