DaveB.inVa
Airman 1st Class
- 225
- Dec 20, 2004
The difference is in the induction sytem. Just because the P-51 had such a huge jump in performance was NOT because a Merlin simply was stuffed in there in place of an Allison.
Heres something I wrote a while back, its a bit long but it does explain the P-51 Allison/Merlin performance gap!!
"A lot has been written about the Merlin and the Allison, namely that the Merlin was a much more powerful engine. When in fact it was not! The Allison and the Merlin were both closely matched in displacement and at sea level and up to about 10000' they both had very similar power levels!!
Up higher than 10000' you MUST consider the induction system. Most Allison's like the ones in the P-39, P-40 and early P-51's had just a single stage supercharger. Most Merlin's like used in later P-51's, Spitfires and Lancasters had a two speed two stage supercharger. This makes a world of difference!!!
Once the Merlin got above a certain altitude the supercharger on the Merlin would shift into high blower. This greatly increases power at altitude... not because its a Merlin... but because you have more air!
The P-38 was an exception to the Allison's mentioned above. The reason was because it has a turbocharger feeding the Allison in addition to its own engine mounted single stage supercharger. This is a more flexible system than the two speed two stage setup could be. You can regulate the amount of turbo boost much more easily than you can with a two speed two stage supercharger. Using a turbo also provides a seamless level of power from sealevel all the way up to rated altitude.
This was not so with the two speed two stage supercharger used with the Merlin. As the aircraft got to a certain altitude the low blower speed would begin to lose power. Sometime at a little higher altitiude the supercharger would shift into high blower. This was a fairly violent operation, but because it did increase the speed of the blower you will make more power because your now getting more air.
Later Allisons used a two speed two stage supercharger and these could definatley run with a two speed two stage Merlin at high altitudes!
You also have to compare apples to apples here. There were different versions of each engine. For instance most Allisons had a single stage supercharger that in the case of the P-38 was supplemented by a turbocharger. While the P-51 and most Merlins used a two speed two stage supercharger.
However there were single stage Merlins out there too for instance the P-40F had a single stage two speed Merlin producing 1300hp takeoff while the Allison engined P-40K produced at the exact same time made 1325hp takeoff.
Thats the closest comparison between the two I know of. The induction was similar and for takeoff both were making close to the same numbers.
There were later versions of the Allison equipped with two speed two stage superchargers that made 1700hp at altitude, these were used in later versions of the P-82 Twin Mustang as well as the XP-51J . In contrast the Merlins with a two speed two stage supercharger used in the P-51B/C produced 1450hp and the P-51D made 1695hp at altitude.
The Germans used a different system than the Allies that was slightly superior to the two speed two stage supercharger but not as great as using a turbocharger. On DB 601s and 605s they used a single stage vairable speed supercharger. Speed was varied by a viscous coupling. So at low altitudes they could have a high slip and as altitude increased slip would be reduced to produce higher supercharger speeds for more power. This was a seamless operation for them as well!
So Ill say it again. It was NOT the Merlin in its self that was so great. It WAS the two speed two stage supercharger that made the Merlin so great.
This is a fact, if you can give ANY engine the amount of air it requires to make its rated power and can cool it effectively you can make its rated power at any realistic altitude!
The reason the Allison wasnt able to make its rated power at 30000' was because it couldnt get enough air. The Merlin could get enough air!
If the Allison could get the same amount of air as the Merlin at 30000' then Im sure today that the Merlin would seem much less mystical... it would just be another engine."
Heres something I wrote a while back, its a bit long but it does explain the P-51 Allison/Merlin performance gap!!
"A lot has been written about the Merlin and the Allison, namely that the Merlin was a much more powerful engine. When in fact it was not! The Allison and the Merlin were both closely matched in displacement and at sea level and up to about 10000' they both had very similar power levels!!
Up higher than 10000' you MUST consider the induction system. Most Allison's like the ones in the P-39, P-40 and early P-51's had just a single stage supercharger. Most Merlin's like used in later P-51's, Spitfires and Lancasters had a two speed two stage supercharger. This makes a world of difference!!!
Once the Merlin got above a certain altitude the supercharger on the Merlin would shift into high blower. This greatly increases power at altitude... not because its a Merlin... but because you have more air!
The P-38 was an exception to the Allison's mentioned above. The reason was because it has a turbocharger feeding the Allison in addition to its own engine mounted single stage supercharger. This is a more flexible system than the two speed two stage setup could be. You can regulate the amount of turbo boost much more easily than you can with a two speed two stage supercharger. Using a turbo also provides a seamless level of power from sealevel all the way up to rated altitude.
This was not so with the two speed two stage supercharger used with the Merlin. As the aircraft got to a certain altitude the low blower speed would begin to lose power. Sometime at a little higher altitiude the supercharger would shift into high blower. This was a fairly violent operation, but because it did increase the speed of the blower you will make more power because your now getting more air.
Later Allisons used a two speed two stage supercharger and these could definatley run with a two speed two stage Merlin at high altitudes!
You also have to compare apples to apples here. There were different versions of each engine. For instance most Allisons had a single stage supercharger that in the case of the P-38 was supplemented by a turbocharger. While the P-51 and most Merlins used a two speed two stage supercharger.
However there were single stage Merlins out there too for instance the P-40F had a single stage two speed Merlin producing 1300hp takeoff while the Allison engined P-40K produced at the exact same time made 1325hp takeoff.
Thats the closest comparison between the two I know of. The induction was similar and for takeoff both were making close to the same numbers.
There were later versions of the Allison equipped with two speed two stage superchargers that made 1700hp at altitude, these were used in later versions of the P-82 Twin Mustang as well as the XP-51J . In contrast the Merlins with a two speed two stage supercharger used in the P-51B/C produced 1450hp and the P-51D made 1695hp at altitude.
The Germans used a different system than the Allies that was slightly superior to the two speed two stage supercharger but not as great as using a turbocharger. On DB 601s and 605s they used a single stage vairable speed supercharger. Speed was varied by a viscous coupling. So at low altitudes they could have a high slip and as altitude increased slip would be reduced to produce higher supercharger speeds for more power. This was a seamless operation for them as well!
So Ill say it again. It was NOT the Merlin in its self that was so great. It WAS the two speed two stage supercharger that made the Merlin so great.
This is a fact, if you can give ANY engine the amount of air it requires to make its rated power and can cool it effectively you can make its rated power at any realistic altitude!
The reason the Allison wasnt able to make its rated power at 30000' was because it couldnt get enough air. The Merlin could get enough air!
If the Allison could get the same amount of air as the Merlin at 30000' then Im sure today that the Merlin would seem much less mystical... it would just be another engine."
so it is in his hands
