The Heat Sink mod decreases the time between each hack. This mod is cools down your portal faster, so you can hack it multiple times in a row without the long wait of 5 minutes. This mod can be used perfectly in combination with the Multi-hack to create bigger and better farms.
Heat Sink stats
Every deployed Heat Sink will decrease the cool-down between each hack (not the Burnout time!) of the portal. This mod comes in 3 different classes; common, rare and very rare. These are the precise values on every class:
Common: -20% cool-down time
Rare: -50% cool-down time
Very rare: -70% cool-down time
That means that if you hack a portal without this mod twice, it would take you 5 minutes between the hacks. If you hack a portal with a rare Heat Sink deployed on it twice, it would take you 2.5 minutes between the hacks!
Penalty for using this mod multiple times
As calculated, the first deployed rare Heat Sink, gives you a 2.5 minute decrease in cool-down but when we deploy another Heat Sink the cool-down will not go to 0, it will give you another 2.5 * 0.5 = 1.25 minute decrease.
This means it’ll still take you 1 minute and 15 seconds to hack a portal twice when there are 2 rare Heat Sinks deployed. This is a way to make sure the portals don’t get too powerful.
The highest decrease in cool-down is 99.19% and this is accomplished by deploying four very rare Heat Sinks: 300 * 0.3⁴ = 2.43 seconds cool-down between hacks, which is a decrease of 99.19%.
When 1 very rare Heat Sink and 1 common Heat Sink are deployed, the algorithm becomes:
300 * 0.3 = 90
90 * 0.8 = 72 seconds cool-down between each hack
Meaning that only the first one is really significant, because you get a decrease of max 70%.
eXact Matter Usage
Common: 400 XM
Rare: 800 XM
Very Rare: 1000 XM