enginehistory.org

[jrussell]

Can anyone tell me why the Merlin is capable of more power for a racing engine than a Allison? All I have read indicates the Merlin can run higher manifold pressures, but what specific design difference ( or combination of them) gives it this ability? I used to be a crew chief on superchargeed race cars, so I understand how on a supercharged engine thermodynamic loading can become as important as mechanical design attributes. As an example, when Top Fuel Chevrolet reached about 2000hp, the mechanical strength of the block, and head castings, were the mechanical limit to increasing hp, whereas a Chrysler Hemi did not suffer this problem. As a result, we kept on boosting manifold pressure on the Hemi, and the Chev engines became extinct in Top Fuel. On other supercharged engine designs, the head gasket has been a very common problem. On the Porsche 935 engines, they finally had to electron beam weld the head to the barrels to eliminate head gasket failures. I always laugh about that one, because Harry Miller sidestepped that problem by a single piece block and head ass'y - in the 20's! So, as far as the Merlin vs the Allison, is it combustion chamber design as relating to detonation control, bridge strength of the cylinder head at the gasket surface, or what?

[jjuutinen]

Interesting question. The Merlin vs. the Allison affair is discussed in considerable length in Daniel Whitney´s "Vee´s for Victory" , published by Schiffer. However, I do have one observation. The 100 series Merlin was cleared for +20 boost on 100/130 fuel. This setting provides some 1830 hp. The highest power without ADI on 100/130 which a purely mechanically supercharged V-1710 was capable of was, if I remember correctly, about 1500 hp. So, in my opinion, there was definitely something wrong with the V-1710 design as it clearly reached detonation limit at considerably lower BMEP than the Merlin. Basically, tod o what the Merlin did on 100/130, the Allison seems to have required 115/145 fuel.

Jukka

[jwells]

I think one basic aspect that is being overlooked here is that the overall strength of the Merlin engine (ie the ones that are used in modern day racing) is much greater than that of the Allison.

If you have a look at the chart on Merlin development that is on p 10 of T.M. 5/2, the development lines according to basic construction show, what I call the "Super Kestrel" line, where the cylinder head and block are integral and these head/block units DO NOT sit on the crankcase. The liners support them and are, therefore, affected by any flexing of the crankcase. Thus, such engines are only semi-monoblock types. The Allison engine is of the SAME construction as the Super Kestrel motors and therefore of the same strength.

When the two-piece block Merlin was introduced the liners ceased to be a constructional member of the engine because the cylinder blocks bedded directly onto the crankcase to give a true monoblock form of aero-motor which is inherently stronger than the semi-monoblock form. The Curtiss D-12 had the same kind of construction.

Given that the Allison remained a semi-monoblock type engine throughout its history, it is not surprising that the degree to which it can be super-tuned is less than that of the later model Merlins.

JerryW.

[jjuutinen]

Interesting point though I am not entirely convinced that this factor has that great an influence!

[jrussell]

Very interesting observation. A similiar situation existed in drag racing in the late 70's. In the Top Fuel and Funny car classes, Big Block Chevrolets were very common, and were competitive. When the power output reached about 2000 hp, the Chevrolet block and heads could not stand the strain. The combustion chambers would blow in every run. the blocks would last two runs, before literaly cracking in half along the line of the main bearing bores. I saw several engines that did this, and the amazing thing was the crankshaft and rod assemblies were undamaged and used in the replacement block the next run! The late model Hemi's were physically stronger and became absolutely dominent.
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