**TL;DR SUMMARY:**

Based on extensive math, there is an optimal range of engine sizes for each weight class of 'Mech. This isn't a hard and fast rule, but 'Mechs will generally feel stronger if they fall within the optimal speed range.

These tables are expressed as an optimal engine size range with base speed in parentheses. This does not include Speed Tweak. The upper speed limit assumes that you are using EndoSteel to save weight. The modified "NoEndo" upper speed limit does not.

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**OPTIMAL STD ENGINE SIZE:**

20t: STD170 (137.7kph) - N/A 25t: STD190 (123.1kph) - STD270 (174.96) 30t: STD225 (121.5kph) - STD270 (145.8) 35t: STD240 (111.1kph) - STD285 (131.9) 40t: STD245 (99.2kph) - STD295 (119.5) 45t: STD260 (93.6kph) - STD300 (108) 50t: STD260 (84.2kph) - STD300 (97.2) 55t: STD275 (81.0kph) - STD305 (89.8) 60t: STD280 (75.6kph) - STD305 (82.3); NoEndo = STD300 (81.0) 65t: STD280 (69.8kph) - STD325 (81.0); NoEndo = STD310 (77.3) 70t: STD285 (66.0kph) - STD340 (78.7); NoEndo = STD335 (77.5) 75t: STD285 (61.6kph) - STD340 (73.4); NoEndo = STD335 (72.4) 80t: STD285 (57.7kph) - STD340 (68.9); NoEndo = STD335 (67.8) 85t: STD285 (54.3kph) - STD340 (64.8); NoEndo = STD340 (64.8) 90t: STD285 (51.3kph) - STD355 (63.9); NoEndo = STD350 (63.0) 95t: STD295 (50.3kph) - STD360 (61.4); NoEndo = STD350 (59.7) 100t: N/A (0.0kph) - STD360 (58.3); NoEndo = STD350 (56.7)

**OPTIMAL XL ENGINE SIZE:**

20t: XL245 (198.5kph) - N/A 25t: XL255 (165.2kph) - XL300 (194.4) 30t: XL260 (140.4kph) - XL310 (167.4) 35t: XL265 (122.7kph) - XL320 (148.1) 40t: XL280 (113.4kph) - XL350 (141.8) ; NoEndo = XL320 (129.6) 45t: XL280 (100.8kph) - XL350 (126.0) ; NoEndo = XL325 (117.0) 50t: XL295 (95.6kph) - XL360 (116.6) ; NoEndo = XL350 (113.4) 55t: XL295 (86.9kph) - XL365 (107.5) ; NoEndo = XL360 (106.0) 60t: XL300 (81.0kph) - XL365 (98.6) ; NoEndo = XL360 (97.2) 65t: XL330 (82.2kph) - XL380 (94.7) ; NoEndo = XL370 (92.2) 70t: XL340 (78.7kph) - XL380 (87.9) ; NoEndo = XL370 (85.6) 75t: XL340 (73.4kph) - XL380 (82.1) ; NoEndo = XL375 (81.0) 80t: XL340 (68.9kph) - XL390 (79.0) ; NoEndo = XL380 (77.0) 85t: XL345 (65.8kph) - XL390 (74.3) ; NoEndo = XL380 (72.4) 90t: XL360 (64.8kph) - XL390 (70.2) ; NoEndo = XL380 (68.4) 95t: XL360 (61.4kph) - XL390 (66.5) ; NoEndo = XL380 (64.8) 100t: N/A (0.0kph) - XL395 (64.0) ; NoEndo = XL395 (64.0)

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**First of all**, let's define what "Optimal Engine Size" means:

**I) A 'Mech is "Too Slow" for its tonnage if you could go up in weight class and upgrade its engine to maintain equal or greater speed, without losing any payload.**

Let's use a stock HBK-4G as an example. It's a 50-tonner that moves at 64.8kph. The total payload is 50 tons minus 5 tons of internals minus 13.5 tons of engine and required heatsinks = 31.5 tons.

Compare this to a stock GHR-5H, a 70-tonner that also moves at 64.8kph. The total payload is 70t - 7t internals - 22t engine = 41 tons.

This means that the GHR-5H can carry 9.5 tons more armor+weapons+equipment while having exactly the same speed and agility as a HBK-4G (ignoring quirks).

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**II) A 'Mech is "Too Fast" for its tonnage if you could go down one weight class and downgrade its engine while maintaining equal or greater speed, without losing out on any payload.**

Let's use the infamous BNC-3E as an example, a 95-tonner that moves at 64.8kph. The total payload is 95t - 9.5t internals - 48t engine = 37.5 tons.

Amazingly, the Assault BNC-3E carries less payload than the Heavy GHR-5H! It's no wonder the stock 'Mech is so bad. The Banshee is too fast. If you wanted to go that fast you should run a lighter 'Mech. If you want to run a 95-tonner, you will need to go slower.

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**III) For any tonnage class of 'Mech, the Optimal Speed Range is defined as the entire range of engine sizes that do not meet the criteria of "Too Slow" or "Too Fast".**

In order to produce a generous range of speeds, we will use the lowest possible estimate of "too slow" and the highest possible estimate of "too fast":

If we count payload as "weapons and equipment only" (not including Armor), 'Mechs will have to slow down because they have less tonnage to split between engines and payload.

**Therefore, the lower-bound Slow estimate assumes that all 'Mechs equip maxed armor (~20% of gross tonnage).**

If we count payload as "armor, weapons, and equipment", and we allow 'Mechs to equip Endo-Steel internals, we will allow the greatest possible tonnage to mount large engines. This produces the highest possible "fast" estimate.

**Therefore, the upper-bound Fast estimate ignores armor and assumes that all 'Mechs use EndoSteel.**

A modified upper-bound estimate may be used for heavier 'Mechs since they are fairly unlikely to have room for Endo-Steel.

**The Semi-Fast estimate ignores armor but does not use EndoSteel**.

I made a spreadsheet to do all of these calculations and you can find the sheet at the end of this forum post.

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**IV) Light Discussion**

Based on the math, 20- and 25-ton 'Mechs have an optimal XL-engine size that is greater than the maximum allowed engines in game. This makes a lot of sense when you look at Light 'Mechs in game. A 145-kph Arctic Cheetah outclasses a 154-kph Locust because going up from 20 to 30 tons gives you so much more room for Armor and Weapons, and within the 140-150kph range the extra tonnage doesn't slow you down. Locusts are incredibly heavily quirked because they'd otherwise be useless.

In order to fully utilize the move-speed advantage of the 20-ton weight class, one would need to mount a XL245 and run at 200kph. While there are tabletop 'Mechs that can move at these insane speeds (12/18/0 and up), in MWO lag and desyncs would be a big problem at such speeds.

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**V) Exceptions to the Rules**

Optimal speed ranges are certainly not hard-and-fast rules. There are many times when you may want to build a 'Mech slower than its optimal speed range. Some 'Mechs simply cannot mount engines large enough to put them in the optimal speed range, including all 20- and 25-ton 'Mechs in the game.

Otherwise, downgrading an engine is usually done to take advantage of hardpoints or Quirks. While a spreadsheet may say that a 70-ton, 60-kph Archer cannot carry as much payload as a 90-ton, 60-kph Mauler... we all know that an Archer has 9 missile hardpoints and a Mauler does not.

There are also plenty of "Turret Builds" (such as Urbanmechs) that intentionally move very slowly and carry as much firepower as possible. Since Optimal Engine Size is used to maximize 'Mech speed for a given amount of weapons payload, it really doesn't apply to 'Mechs that are intentionally slow.

On the other hand, there is no good reason why you would ever want to build a 'Mech faster than its optimal speed range. You can't take advantage of quirks or hardpoints if your 'Mech has no tonnage left over for weapons, like the BNC-3E. If you're really committed to moving at 64.8kph, downgrading to a lighter 'Mech will actually give you more payload tonnage. Amazingly enough, dropping 10 tons from a Banshee to Battlemaster allows you to mount 5 more tons of weapons and armor while keeping 64.8kph (4/6/0) mobility.

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**References:**

http://mwomercs.com/...dsheet-warrior/

http://www.ci-n.com/...imalweight.html

**Excel Spreadsheet at:**

https://drive.google...iew?usp=sharing

**Edited by Piousflea, 25 March 2016 - 01:08 AM.**