Engines have 1 internal heatsink per 25 rating up to 10, so a 100 rated engine will have 4 internal heatsinks while a 250 rated engine will have 10 internal heatsinks.
275 rated and above have 10 internal heatsinks and space for you to add external heatsinks, so a 300 rated will give you 10 internal and have room for you to add another 2 external heatsinks to the mech crit free.
With the double heatsinks upgrade, internal DHS work at 0.2h/s while external DHS work at 0.14h/s, so if you upgrade to DHS you'll want at least a 250 rated engine to maximise the amount of internal heatsinks you get.
Single heatsinks are much simpler, they work at 0.1h/s reguardless of where they are.
And Ferro is a weight saving upgrade, just like Endo, it doesn't raise the maximum amount of armour you can take but rather lowers your armours weight by 12% at the cost of 14 crit slots. It's a fairly bad upgrade because it's pretty much just flat out inferior to Endo-Steel, and taking both as you found out doesn't leave you enough crit slots to really do anything fun with the saved weight.
Edited by CheeseThief, 06 April 2013 - 07:14 PM.