Just to clarify how many "Engine Heat Sinks" there are: the number of engine heat sinks is dependent on the size of the engine, you get 1 heat sink (single or double) per 25 engine size, so 250 engines (XL or regular) will hold 10 Heat Sinks, 300 will hold 12 Heat Sinks. So take that into account when purchasing XL engines, I picked up the 255XL and 280XL engines since they get me past the 25 barrier for one more heat sink and weigh the same as the 250XL/275XL equivalent.
Here is my summary as I understand it and from testing from Vlad's informal testing as well.
Heat thresold=
30+ Heat Sink add-ons
1.0 per standard heatsink
1.4 per non-engine double heatsink
2.0 per engine double heat sink
Heat dissipation=
.1 per second for standard heatsinks
.14 per second for external double heatsinks
.20 per second for engine double heat sinks
-S
Edited by Splinters, 29 January 2013 - 04:18 PM.