Differences between Source Engine branches

This picture explains most of the differences between "Old Engine" (Source Engine 2006) and "New Engine" (Source Engine 2009). S. has made a YouTube video that shows even more differences.

As you might have seen or heard, there are multiple branches of the Source Engine. Some of Valve's games have had builds released on more than one iteration of the engine (e.g. Half-Life 2 and Episode One). The engine branches are very different from each other, so it's good to know what the advantages are of speedrunning on a certain build.

Half-Life 2, together with Lost Coast originally ran on Source Engine 2004 (known better as Old Engine along with SE 2006). In 2006, Episode One was released, bringing the introduction of Source Engine 2006. Although HL2 and Lost Coast were updated, only engine bug fixes (such as save deletion) applied to them compared to later engine updates, where changes to client and server functions were applied to all games using them.
In 2007, the release of Episode Two and Portal saw an updated version of that engine, Source Engine 2007 (widely known as OrangeBox Engine and New Engine along with Source Engine 2009), which had several glitches patched as well as introducing new glitches. At the time, Half-Life 2 and Episode One still ran on Old Engine (although due to the way Episode Two files were managed it was sometimes possible to run Episode One on 2007).
In 2010, Valve updated all of their games (HL2, the episodes and Portal) to run on Source Engine 2009, which while similar for the most part to Source Engine 2007 had more differences still. This branch of the Source Engine is used in the popular speedrunning mod, the Ghosting mod, which most New Engine HL2 runners use.
Finally, Valve released Portal 2 in 2011, which ran on its own branch of the Source Engine simply referred to as the Portal 2 Engine, which patched out the vast majority of glitches. Valve has no plans to move the other games to Portal 2's engine.
Valve has since moved the 2009 engine games to Source Engine 2013, better known as Steampipe. The Steampipe version is the current one on Steam. It's functionally identical to Source Engine 2009, so most glitches still work on it (for unknown reasons edge glitch and save glitch are either very difficult or impossible to perform). However, it's not used in the speedrunning scene due to demo incompatibility and other technical issues that Steampipe brought.

Branch	Commonly ran builds	Nickname
Source Engine 2006	2187, 2707 and 4044	Old Engine
Source Engine 2007	4104	OrangeBox Engine
Source Engine 2009	5135	New Engine
Source Engine 2011	(current builds)	Portal 2 Engine
Source Engine 2013	(current builds)	Steampipe

Differences

	Source Engine 2004	Source Engine 2006	Source Engine 2007	Source Engine 2009	Portal 2 Engine
General differences
Movement method	Forward Bunnyhopping Consistent, gradually rising speed	Forward Bunnyhopping Consistent, gradually rising speed	Accelerated Back Hopping Faster speed, but also more difficult to control	Accelerated Back Hopping Faster speed, but also more difficult to control	Portal 2 Bunnyhopping Slow speed acceleration, no air control past 300 ups
Collisions with gibs/small props	Yes, can be used for Flying	Yes (No in Episode One), can be used for Wallclimbing	No	No	No
Movable OoB (Out of Bounds)	Yes	Yes	Yes	Limited (Can move around freely close to Portals, while load glitched or when in a vehicle)	Yes, but cannot see OoB in maps compiled with Portal 2 map compilers without cheat cvars
Tricks and glitches
Flying and Wallclimbing	Flying: Very easy, possible with just about any prop and gib, capable of advanced techniques. Wallclimbing: See above	Flying: Very easy, possible with just about any prop and gib, capable of advanced techniques. Limited in Episode One (only possible with certain props) Wallclimbing: See above	Flying: Limited, only possible with certain props. No advanced techniques. Wallclimbing: Easy, possible with any collideable prop	Flying: Limited, only possible with certain props. No advanced techniques. Wallclimbing: Easy, not possible with small props due to no collision	Flying: Very limited, can only be used with certain props for slow descent or jumping small gaps. Wallclimbing: Limited, can only be used to climb large heights with select props and save-loading
NPC Teleportation	Very easy	Very easy	Difficult	Difficult	Unknown to be possible (no NPCs of such type in Portal 2)
Schrodinger's Crouch	Impossible	Impossible	Impossible	Possible	Impossible
Crouch-Clipping	Moderately easy	Moderately easy	Very hard	Very hard	Unknown (no-one has attempted or succeeded in performing the glitch)
Crouch Save Glitch (JClip)	Possible	Possible (impossible in Episode One)	Impossible	Impossible	Impossible
Pressing buttons through walls	Possible	Possible	Impossible	Impossible, but can be done by utilizing Schrodinger's Crouch	Impossible
Stucklaunching	Possible	Possible	Possible	Possible	Impossible (no boost upon jumping with Portal 2 movement)
Clipping through displacements	Possible, game pushes you through	Possible, game pushes you through	Semi-possible. You get stuck, but can get out by Save/Load buffering. Not doable in Single-Segment speedruns.	Semi-possible. You get stuck, but can get out by Save/Load buffering. Not doable in Single-Segment speedruns.	Semi-possible. You get stuck, but can get out by Save/Load buffering (must be crouched to work). Not doable in Single-Segment speedruns.
Save/Load Clipping direction	Upwards; can't be changed with Damage Boosting	Upwards; can't be changed with Damage Boosting	Downwards; can be changed with Damage Boosting and Reportaling	Downwards; can be changed with Damage Boosting and Reportaling	Downwards; can be changed with Damage Boosting and Reportaling
Grabbing items through walls	Possible	Possible	Impossible	Impossible	Impossible
Super Physics Deform	Impossible, however, trying to SPD an object will allow you to Stuckjump from it.	Impossible, however, trying to SPD an object will allow you to Stuckjump from it.	Possible	Possible	Impossible, however, trying to SPD an object will allow you to Stuckjump from it
Pistol Charging	Possible	Possible	Impossible	Impossible	Impossible
Save Deletion	Impossible	Possible	Possible	Possible	Possible
Triggerdelay and Save warping	Possible	Possible	Impossible	Impossible	Impossible

See Also

Game versions