Now this is a fun question. You could ask it two different ways.
What is the biggest engine in the Harley-Davidson range or, how big can I make a Harley-Davidson engine?
For starters, the introduction of the all-new Milwaukee-Eight in 2017 spelled a whole new chapter for Harley-Davidson engines. As you know, the 8-valve V-Twin is in all Softail and Touring bikes in capacities of 107, 114 and 117 cubic inches. (1750, 1870 and 1920cc)
Even though the weight is the same as the earlier Twin-Cam engines, the Milwaukee-Eight increases acceleration by 10 per cent (0-60 for the 107 vs the 103) and 8 per cent (0-60 for the 114 vs. the 110)
So the answer to the first part of the question is: 117 cubic inches or 1920cc.
Then the fun starts. Now, if we use parts off-the-shelf and don’t machine anything, we can go to 128 cubic inches (2100cc) like with our Dandenong FXDR shop build [https://www.harleyheaven.com.au/blog/2303-2/] where we used Screamin’ Eagle adjustable pushrods with an S&S 128 barrel and piston combination.
Now, if we want to get meticulous, the S&S folks also produce a “Long Block” conversion for the older engines which will create a monstrous 143 cubic inch (2350cc) fire breather with double the power of the original motor. On the drag strip, an expert rider can run a sub-10 with a trap speed of 135mph. Not for the faint-hearted.