On my Spitfire replica builders’ forum, a number of discussions have recently been had about choice of engines in full-scale replica Spitfires, ranging from suped-up car engines to proper aero engines – naturally the Merlin and the Allison as used in Bob DeFord’s aircraft (see pic).
The reliability of Merlins and Allisons has recently come up for discussion, so I ask the obvious: which was the more reliable engine?
You can understand why these guys are asking the same question – they’d rather be flying than sitting on the ground!!
By: ozplane - 9th June 2004 at 09:33
I gather the Barracuda was such a gormless aeroplane, even the Merlin 32 couldn’t make it a silk purse. It was an original pig’s ear.
By: dhfan - 8th June 2004 at 23:39
I remember reading, on here or in one of the mags, a pilot’s impression. He said the Allison was smooth, the Merlin sounded a lot rougher at low revs, and the Griffon sounded like it wanted to go and kill something.
By: TempestNut - 8th June 2004 at 22:45
A fairly superficial answer but I’m a civil engineer not an aero engineer so I stand to be corrected. The Allison, I understand, works well at low altitude (P-40 ground attack variants) and the Merlin works well at altitude (P-51 escort variants). At a guess this is probably tied up with the supercharging arrangements. Thus if your replicas are to be used for air-shows perhaps the Allison route is the correct one. I believe it was difficult to prise Ray Hanna out of his P-40 when he had the choice of the Breitling Fighters so that has to be some sort of recommendation for the Allison.
Lots of Merlins were configured to run at lower altitudes, and produced more power than the equivalent Allison. Supercharging is not just about high altitude. Fleet Air Arm Merlin 32s in the Barracuda produced max power 1600hp at around 5,000 feet. This made for a more efficient engine and allowed for increased range.
Yes the Allison is a cleaner engine and there are a number of features that are more up to date, than on the Merlin. RR incorporated many of these features in the Griffon. Any choice should be based on availability and service support.
By: ozplane - 8th June 2004 at 20:21
A fairly superficial answer but I’m a civil engineer not an aero engineer so I stand to be corrected. The Allison, I understand, works well at low altitude (P-40 ground attack variants) and the Merlin works well at altitude (P-51 escort variants). At a guess this is probably tied up with the supercharging arrangements. Thus if your replicas are to be used for air-shows perhaps the Allison route is the correct one. I believe it was difficult to prise Ray Hanna out of his P-40 when he had the choice of the Breitling Fighters so that has to be some sort of recommendation for the Allison.
By: Cleco - 8th June 2004 at 18:01
Allison config
Although I am know expert on Allisons, I understand the Allison made much more use of internal cavities for oil and cooling and thus makes for a slightly less cluttered installation (less external tubes running forward and aft around engine).
Keith
By: TempestNut - 8th June 2004 at 16:11
Daz It very much depends which Merlin or Allison you are referring to. The Merlin was developed extensively post war to make it last longer in civil use and these engines and or some of the components are much more reliable than the original wartime engines.
Mid war the Merlin underwent some major mechanical updates and again at the end of the war a further mechanically refined engine was introduced. Look at the model numbers and max boost rating of the engine and this will indicate how strong and reliable it could be made.
Original Merlin’s were restricted to 6lbs on 87 fuel and later 12lbs on 100 fuel. About 1300 hp
When the Merlin 45 and the XX were released 15lbs boost was available. About 1450 hp
Along came the 2 piece head, and Plus18 was available. This gave about 1600 to 1700hp
Some special engines and selected Merlin 24s (Lancaster B1 special) 25s (Mosquito FB6, NF13 NF19) and 66s (spitfire IX & XVI ) were available with 25Lbs in 44 About 2000 hp
The Merlin 100 series released in 44 had 25lbs available as std. These engines were the basis for some of the civil engines. The Merlin 24 was developed with some of the new parts in to the so called transport engine.
This is a very cursory summing up I thing you will find that most of the warbirds will have the newer style engines in them in the interests of reliability and to reduce costs. All Packard built Merlin’s had newer style heads apart from the very early engines that used a hybrid head.
The Allison engine had a similar history, and the later engines as fitted to the P82 are much converted, although small in numbers. Allison’s had some features that are used in racing Merlins, although these mods are not used in normal warbird engines.
In most respect the core engines are very similar, and produced the same power. The Merlin had a better supercharger and intercooler, and this contributed to better reliability. The Allison in the Lighting suffered from a poor installation and misadjusted fuel system especially in the early P38J’s leading to a very poor reputation for the engine that generally was not the engines fault.
So it’s a matter of doing some serious research to get the answer you need. By the way the engineering on both the Merlin and the Allison is light-years ahead of anything produced in the automotive world.
By: dhfan - 7th June 2004 at 23:06
The Merlin was a high performance engine by the standards of the day. The Allison was slightly larger in capacity and less powerful. The Yanks know a bit about big engines that last forever. Logic says to me that if there is a difference the Allison would be more reliable, but the full potential power of Merlins isn’t generally used nowadays.
Obviously not speaking from personal experience…
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June 7, 2004 at 6:18 pm