23.11.2008, 05:30
Now that's the kind of posting I love. Simple to understand, illustrative and very authoritive.
I'm always confused by the old imperial versus continental metric boost ratings. I like mm of mercury, a far better system than trying to convert one to the other.
According to the above quote the Merlin achieved a manifold pressure of up to 2070mm of mercury.
Whilst the DB-605DC achieves up to 1504mm of mercury.
Soviet planes seem to get up to around 1150mm of mercury iirc.
However I am unsure as to how accurate the British "+25lbs" system of measuring is, according to engineers much less so than inches or millimetres of mercury. This rating is to say, that the equivalent of 40PSI total pressure is being forced into the engine. A top fueller sucking alcohol doesn't get that much. It's ridiculously high octane rating isn't enough to prevent the heads blowing right off. Its titanium pistons and one piece shotpeened billet steel crankshaft would desintegrate even if this didn't happen. It gets around 3000hp from 8 litres with about 28PSI and all that expensive hardware is just to cope with this much.
I'm not one to doubt the experts. I just...wonder.
Then there is the consideration of the regional manner in measuring fuel ratings. According again to engineering sources the German fuels were measured differently and very conservatively compared to the American/British system of rating octane. I've been instructed the 87 grade B4 fuel is approximately equivalent to US 130 grade Avgas, whilst 96 octane C2/3 is roughly equivalent to 150 grade. Certainly other factors are involved in fuel quality. But I should mention volatility is what octane describes, it actually goes down as octane goes up, that's how it stops pinging, that's what octane rating means. But different fuel qualities regardless of octane atomize differently and burn differently, so I should let that one pass since I think it is what the author meant by "volatility."
Again, an argument is not what I'm looking for at all. I noted this in passing (researching other stuff) so if I hunt down a link for you I'll let you know. I don't really care so much, but neither do I about challenging popular (mis)conception.
The higher output/capacity relationship of the Merlin engine was achieved by running at higher operating speeds than the Daimler, not amazing boost and fantastic fuel types...from a race engineering point of view leastways. But sure, from my armchair it was magical liquids and more boost than Kitt the Knightrider car :wink:
I could run a model using Engine Analyzer Pro (race engineering software) but I already know what it'll turn out. Horsepower is a product of fuel combustion, period. Superchargers must be sized for volumetric capacity. To get around 2000hp out of 27 litres @ 3000rpm I imagine you need something like 15 psi total supercharger pressure, and an appropriately sized casing (cfm rating).
But who am I to argue, a mere human.
I'm always confused by the old imperial versus continental metric boost ratings. I like mm of mercury, a far better system than trying to convert one to the other.
According to the above quote the Merlin achieved a manifold pressure of up to 2070mm of mercury.
Whilst the DB-605DC achieves up to 1504mm of mercury.
Soviet planes seem to get up to around 1150mm of mercury iirc.
However I am unsure as to how accurate the British "+25lbs" system of measuring is, according to engineers much less so than inches or millimetres of mercury. This rating is to say, that the equivalent of 40PSI total pressure is being forced into the engine. A top fueller sucking alcohol doesn't get that much. It's ridiculously high octane rating isn't enough to prevent the heads blowing right off. Its titanium pistons and one piece shotpeened billet steel crankshaft would desintegrate even if this didn't happen. It gets around 3000hp from 8 litres with about 28PSI and all that expensive hardware is just to cope with this much.
I'm not one to doubt the experts. I just...wonder.
Then there is the consideration of the regional manner in measuring fuel ratings. According again to engineering sources the German fuels were measured differently and very conservatively compared to the American/British system of rating octane. I've been instructed the 87 grade B4 fuel is approximately equivalent to US 130 grade Avgas, whilst 96 octane C2/3 is roughly equivalent to 150 grade. Certainly other factors are involved in fuel quality. But I should mention volatility is what octane describes, it actually goes down as octane goes up, that's how it stops pinging, that's what octane rating means. But different fuel qualities regardless of octane atomize differently and burn differently, so I should let that one pass since I think it is what the author meant by "volatility."
Again, an argument is not what I'm looking for at all. I noted this in passing (researching other stuff) so if I hunt down a link for you I'll let you know. I don't really care so much, but neither do I about challenging popular (mis)conception.
The higher output/capacity relationship of the Merlin engine was achieved by running at higher operating speeds than the Daimler, not amazing boost and fantastic fuel types...from a race engineering point of view leastways. But sure, from my armchair it was magical liquids and more boost than Kitt the Knightrider car :wink:
I could run a model using Engine Analyzer Pro (race engineering software) but I already know what it'll turn out. Horsepower is a product of fuel combustion, period. Superchargers must be sized for volumetric capacity. To get around 2000hp out of 27 litres @ 3000rpm I imagine you need something like 15 psi total supercharger pressure, and an appropriately sized casing (cfm rating).
But who am I to argue, a mere human.