Sabre IIB or not IIB on 150 fuel

[thedab]

I allways thought for the Sabre II engine to run at 11lb boost,it needed 150 fuel but then I came across this http://http://i.imgur.com/lUhGjmD.jpgwhich has the Sabre IIB runing at 11lb boost ,but just on 100/130 grade fuel,what going on here? is this right? or is this some post war thing,as so far i have not seen anything that says that the Sabre IIb even use 150 grade fuel,can someone shed some light on this?

thanks Ian

 

[tomo pauk]

Maybe it was the strength of the crankshaft in the Sabre II that was a limiting factor, not whether the detonation will appear when more than +9 lbs boost was used.
There was several ww2 engines that, when received strengthened crankshaft, were using increased boost and producing more power 'down low', like late BMW 801, V-1710s after Spring 1942, methinks also the single stage Merlins that went to 1600 HP for take off (on Lancasters, Mossies); that is with 130 grade fuel only, or thereabouts.

Link to the pic.

 

[thedab]

well it a odd one, I got this for the IIA http://http://www.wwiiaircraftperformance.org/tempest/sabrecurve.jpgit says 11lb boost with 150 fuel but with strengthened propeller shaft,but it don't help me in understand why that engine card says 11lb boost on 100/130 fuel,mind you there alot about the Saber i don't understand.

 

[tomo pauk]

Both the links to the pics are not working
It is indeed prop shaft, not the crankshaft, my bad. At any rate, my take is that engine needed a reinforced part to be installed to develop more power. Maybe the modified engine was later tested on 130 grade fuel and passed the test on +11 lbs boost?

 

[Neil Stirling]

The Sabre IIB was merely a modified IIA and this a strengthened Sabre II. The IIB modification could, providing the engine had the modified/strengthened propeller reduction gear assembly be carried out at squadron level and consisted of a data card, a new boost control cam and a new boost control capsule. This indicates that the Sabre IIA was already strong enough to use +11 lbs boost. http://www.wwiiaircraftperformance.org/tempest/sabrecurve.jpg

The IIB rating, apart from the flying bomb emergency was not officially cleared until January 1945.

Despite the initial use of 150 grade fuel, the Sabre proved it could run at +11 lbs boost when using 100/30 grade, and in fact at +15 lbs with the Sabre VA.

Neil.

 

[Shortround6]

It is very tough to make generalizations about what was required for specific engines to be up-rated for higher boost pressures. For instance the Merlin needed a stronger supercharger drive shaft to make 18lbs of boost. The older supercharger drive shafts (or quill shafts) weren't strong enough to handle the power required to drive the supercharger to get the higher pressure even if the crankshaft, crankcase and propeller shaft could stand up to the power. Sort of the same for the Allison. The drive gears for the supercharger wouldn't stand up to the load needed to run the 9.60 gear ratio without making the gears wider (thicker). Of course the Allison also used 4 different crankshafts and at at least two different crankcases over the years so it is a little difficult to pinpoint exactly which modification allowed for exactly how much of the allowable boost in WEP at any given time.