Inaccurate performance data for BOB fighters in COD comparing to RL data

[Skoshi Tiger]

Originally Posted by Seadog

Yet, in all the combat reports of the pilots pulling the plug, the aircraft responded with increased performance; there are no reports where the engine began to suffer detonation and/or failed...

Could that be that when the pilots 'pulled the plug' the boost was still at a level before detonation occurs.

One of the pilot stories (show in this thread) before did mention black smoke comming from the exhaust and the plane vibrating a lot. Sound like he was pushing it a bit too far. But none of the others mentioned it.

[TomcatViP]

Well, I can found dozens of 70+ smokers that did not get any Cancers and makes them testify about how safe was the cigarettes for them.

Would you then believe that Smoking is good for your health ?

[Seadog]

Originally Posted by Skoshi Tiger

Could that be that when the pilots 'pulled the plug' the boost was still at a level before detonation occurs.

One of the pilot stories (show in this thread) before did mention black smoke comming from the exhaust and the plane vibrating a lot. Sound like he was pushing it a bit too far. But none of the others mentioned it.

Prior to the 12lb mod/100 octane, if you pulled the boost override you would get 18lb boost up to about 5000ft, 16 lb boost to about 7500ft, 12 lb boost to about 11000ft and 7lb boost at ~17500ft. 7lb was the maximum permissible with 87 octane so the altitude range that it would be beneficial is very restricted; below that altitude power would be reduced because detonation would occur, engine would no longer run smoothly and the engine would quickly fail altogther. The only way that the boost override would be beneficial would be if it had a mod to restrict the additional boost to ~7lb, but of course this would only result in a very modest power increase, and this mod was never done.

AFAIK, the 109E was limited to 7lb boost as well (1.4 ATA) when using 87 octane.

[TomcatViP]

That's a poor argument.

The DB of the 109 had far more cylinder volume than the Merlin.

RR did ran the Boost horse simply because that was the way they had to go against the DB. Latter in the war they might also hve understood the huge advantage they had in therm materials of quality.

My old 2L Swedish SAAB engine had as much power than an average 5+L US V8 without supercharger .... But a 2+bar boost level.

I hope you will understand that way

The boost level in German plane is more linked to the lack of Nickel in their engine material. They had to build thicker internal surface and build bigger eng in order to run their eng at a lower temperature.

This has nothing to do with a comparison with the Merlin.

Interestingly, if we do compare the application of aviation engine in tanks, we might show that reciprocally, the bigger German eng where more reliable in that application. But honestly this is pure speculation.

[camber]

Originally Posted by Seadog

Prior to the 12lb mod/100 octane, if you pulled the boost override you would get 18lb boost up to about 5000ft, 16 lb boost to about 7500ft, 12 lb boost to about 11000ft and 7lb boost at ~17500ft. 7lb was the maximum permissible with 87 octane so the altitude range that it would be beneficial is very restricted; below that altitude power would be reduced because detonation would occur, engine would no longer run smoothly and the engine would quickly fail altogther. The only way that the boost override would be beneficial would be if it had a mod to restrict the additional boost to ~7lb, but of course this would only result in a very modest power increase, and this mod was never done.

AFAIK, the 109E was limited to 7lb boost as well (1.4 ATA) when using 87 octane.

Exactly right as I understand it Seadog. Although I would add one proviso...the boosts you mention are at maximum throttle. There would be nothing to stop a pilot pulling the boost override and adjusting his throttle handle (which is now directly linked to the actual throttle valve) to get a desired boost. So the pilot could possibly get some more performance, say by adjusting to +7psi where before he was stuck on +61/4 psi. Boost is no longer controlled, so if he dived to a lower altitude the boost will rise on it's own and possibly damage the engine. No wonder it was not an approved way of getting combat power on 87 octane...the gain was probably modest, the risk of misuse large. Whether or not it was used unofficially is open to conjecture.

Good point about the 109E, 1.45 ata is about +6.6psi boost. So I disagree Tomcat, it appears on 87 octane both the DB601 and Merlin were restricted by the maximum usable boost avoiding fuel predetonation..not by engine design paramaters or materials.

I am still intrigued by the engine test bed report apparently stating figures of 12500 feet, ~10.5psi boost and 1300bhp. But whether these are calculated figures, an engine tested with 100 octane, whether intake pressure was actually set to 12500 ft equivalent etc. does not appear to be available. Interesting to know what would happen if in a 87 octane Spit pre BCCO mod if you pulled the cutout and progressively raised boost above 6 1/4 psi. At what boost points would:
1) predetonation be detected
2) bhp start to decrease with increased boost (due to predetonation)
3) Significant loss of engine life occur
4) High risk of rapid engine failure.