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A few weeks ago I was calculating the BV for a custom design and noticed some differences with HMPro and Tactical Operations and Maximum Tech regards engine modifiers. The modifiers in Tactical Operations match those in Maximum Tech.

Engine Type           Modifier(TM)(TO)   HMPro Modifier
Standard                1.0                    1.0 
Light I.S.              0.75                   0.75
Compact                 1.0                    1.0
I.S. XL                 0.5                    0.5
Clan XL                 0.75                   0.75
Large                   1.125                  0.75
XXL I.S.                0.5                    0.33333
XXL Clan                0.75                   0.5           
Large XXL I.S.          0.375                  0.25
Large XXL Clan          0.5                    0.33333

I can usually follow BV calculations and understand what they are reflecting in a designs capabilities but the engine modifiers in Tactical Operations and Maximum Tech do not seem to reflect a units capabilities compared to the values in HMPro, it looks like there was supposed to be some errata to Maximum Tech but when Tactical Operations was released it was missed

As far as I can tell the engine modifier reflects the ability of a 'Mech to withstand damage, for example a 'Mech with a I.S. XL engine reduces the defensive BV because the loss of a side torso will remove the 'Mech from battle.

With a large fusion engine Tactical Operations gives a modifier of 1.125 compared to HMPro's 0.75. I do not know about Super Heavy 'Mechs but I can not design a standard 'Mech with a large fusion engine but I do not see how having two extra engine crits in the center torso would lead to an increase of the defensive BV. If any of the engine's should have a 1.125 modifier I would have thought it would be the compact fusion engine, but for any extra survivability that would depend on mounting other equipment in the center torso

With the XXL I.S. engine I am undecided as to which is the best modifier, 0.5 in Tactical Operations or 0.33333 in HMPro. The risk from the loss of a side torso is the same as the XL I.S. engine and the chance of the extra critical hits to the engine depends on what other equipment mounted in the same location. And 0.33333 is not a nice number if you are using pen and paper.

With the Clan XXL engine the 0.75 modifier in Technical Manual compared with 0.5 in HMPro seems wrong considering that it seems more vulnerable to damage than a I.S. XL engine though I am not sure how Clan CASE affects these things

With the Large XXL engines, 0.375 seems a better number than 0.33333.

1/3 is quite round isn't it?

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typos and spelling-mistakes are property of the finder. english is not my mother-tongue.

I just remember there being a lot of numbers after the decimal point