Guppy, there's some good info in that thread - and a lot of garbage. Additional material has come to light since that time.
Here’s my summary from our
Mustang Performance page that might be of interest (I've posted much the same here previously):
The Merlin engined P-51 went operational 1 December 1943 with the 354th Fighter Group based in Boxted UK. By D-Day, seven 8th Air Force Groups would join them as well as seven RAF Squadrons. In the MTO, the 31st and 52nd Fighter Groups exchanged their Spitfires for Mustangs in spring 1944. The P-51B that equipped these units was delivered with the Packard Merlin V-1650-3 until April 1944, when replacement aircraft began to be delivered with the V-1650-7 engine featuring maximum power peaking at lower altitudes. The P-51 operated on 100/130 grade fuel with War Emergency Power limited to 67" up to June 1944. On 29 March 1944 the Commanding General, Army Air Forces authorized the procurement of the necessary parts to modify all P-38, P-47 and P-51 airplanes in the United Kingdom for the use of Grade 150 fuel, subject to the relevant engines being cleared to use the fuel.
(1) (2) During March & April 1944 flight tests were conducted at Wright Field on the P-51B-15 airplane, AAF No. 43-24777, using 44-1 fuel, at the request of the Power Plant Laboratory, Engineering Division.
(3) These tests were made to determine the performance of the airplane at the higher powers allowable with 44-1 fuel as compared with the performance at powers allowable for standard aviation fuel. Parallel tests were conducted by Proving Ground Command at Eglin Field, Florida on P-51B airplanes, AAF Nos. 43-24755, 43-24757, and 43-24775. The Power Plant Laboratory concluded in a 19 April 1944 memorandum report that the "Packard built Rolls-Royce V-1650-7 engine will satisfactorily comply with a 75 In. Hg manifold pressure war emergency rating with Grade 44-1 fuel".
(4) As a result of the engine clearance and airplane trials the P-51-B airplane was cleared for operation at 75" Hg by late April.
(5) The modifications required to the P-51 to use the 150 grade fuel were: modify manifold pressure regulator, modify supercharger volute drain valve, install new type induction center manifold extension gland seals, use of Lodge RS5/5 or KLG RC5/3 spark plugs, installation of bulged exhaust stacks, and reset supercharger aneroid switch.
(6) By June 1944, final release on Project P.P.F. had been made approving 75" manifold pressure for the P-51 (both the 1650-3 and 1650-7 engines), as well as increased powers for the P-38 and P-47.
(7)Deliveries of Grade 100/150 aviation fuel to Eighth Air Force fighter airfields commenced in June 1944.
(8)(9)(10) This coincidentally occured about the same time as the introduction of the P-51D into service. Even though the USAAF had cleared the P-51 for 75" Hg., the Eighth Air Force chose 72" Hg as the P-51's War Emergency Rating.
(11)(12) Apparently there is more to the story, however, as Encounter Reports demonstrate that 75" Hg was used operationally.
(13)(14)By January 1945, fourteen of the Eighth Air Force's fifteen Fighter Groups were operating Mustangs, the sole holdout being the 56th FG in P-47's. Maintenance difficulties with spark plug fouling led to the decision to convert all fighter groups to 100/150 grade fuel reformulated with increased levels of ethylene dibromide (1.5T). Deliveries of PEP, as the new 100/150 blend was called, began to be issued to all fighter groups in February 1945. The use of PEP, however, cooroded the valve seats of the V-1650 at an unacceptable level. Consequently, the standard 100/150 (1T) grade fuel was reverted to by the end of March 1945.
(15)(16) The Eighth Air Force also had hoped to supply the 352nd and 361st Fighter Groups based on the continent with 100/150 grade fuel. This was deemed impractical from a logistical viewpoint, although admittedly such difficulties did not prevent the RAF's 2nd TAF from being supplied with 100/150 grade fuel.
(17)
