Straight from the Army http://www.transportation.army.mil/ADSO/PS_Hot-Topics/Oct_2013_HMMWV%20detuned%20engines.pdf
And another reference with pics https://halfmastpsmag.wordpress.com/2013/10/23/know-the-difference-between-the-hmmwv-6-2l-and-6-5l-detuned-engines/
I have seen several on GP...