A bored-out Merlin or V-1710?

Daimler Benz increased the bore of the DB-601 in order to, along with other changes, develop the DB-605. Despite the initial problems, that were perhaps more due to a too much tight installations and insufficient oil cooling than with engine itself, the new engine offered more power at all altitudes than the current DB-601E, when on same power regime.
Wonder whether the increase of the bore volume of the two most important W. Allied V-12s gave any appreciable performance boost? The volume was increased by some 5% for the DB-605 - that kind of increase would meant the 28.5-29 L Merlin or V-1710.

I think you might get an increase, but am not really sure off the top of my head how much the bore could be increased without making the space between the cylinders too tight. I have not measured the thicknesses nor the distances between crankshaft throws. If other changes had been made along with the bore increase, there might be some gains.

You could increase the spacing between the cylinders and decrease the stroke a bit ... and get a slightly longer engine. Development would have to be done, at least the design stage, to see if there would be improvement or a reduction in performance. You might get a 10% - 15% improvement if done correctly and the question would be whether the expense would be worth it for a small increase in power. I can't say.

The Bf 109 seemed to gain with the introduction of the DB 605, but also got heavier. Since the Germans didn't develop the BF 109 with a DB 601 any farther than the Bf 109F series, we are left with conjecture on whether or not it could have kept pace with the Bf 109G and later models with the DB 605. I think it could have done so or have been made to do so with some effort, but they didn't pursue it, so it is a "what-if."

It is possible that with sufficient attention to weight reduction instead of engine displacement increase, similar performance gains might also have been made in Allied aircraft. Again, this is a "what-if."

So the potential was there and the only question would be whether the slight gains would be worth the expense of development. I would not care to hazard a guess on that score but, if in a position to make the decision, would have pursued weight reduction, finish improvement for all fighters. I'd ask the engine manufactures to pursue incremental engine improvement and the propeller manufacturers to pursue better propellers.

The decision that a first-line fighter was falling behind the competition would mean either improvement of that fighter to some specification or perhaps development of a new aircraft once the pinnacle of a particular design had been reached. I am not sure the end of any particular deign had been reached, even the P-40. A completely new aircraft could have been created with the P-40 if a 2-stage Merlin had been installed ... but they never DID that. The P-39 probably reached the limits and was discontinued well after it should have been, but the P-63 was not developed as IT could have been.

The P-51 was still being improved when the jet age overtook it, as it did ALL the piston fighters. But in the 1942 - 1945 timeframe there were probably airframe improvements that could have made about the same difference as a small increase in power.

Was the oversized Merlin not the Griffon? Using Merlin technology on a Buzzard base (yes I know I am oversimplifying.)

yulzari said:

Was the oversized Merlin not the Griffon? Using Merlin technology on a Buzzard base (yes I know I am oversimplifying.)

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Kinda, sortof. There was no real need to bore out the engine because the Merlin was able to keep pace with the need for increased power by increasing the efficiency of the supercharger and reinforcing and improving the internal structure, which allowed for increased boost pressures - the Merlin 61, for instance, could call on +12 lbs boost while the improved 63 could get +18 lbs and the Merlin 66, with 150 Octane fuel, could generate 2,000 hp at +25 lbs. Boring out the engine would have probably led to more complications for too little gain compared with using the Griffon.

Without substantial technical understanding and without knowing the specific construction details, it is difficult to say what could be done to increase the bore of a given liquid cooled engine without requiring extensive redesign. Certainly, it was common in the 1950s and 60s for the US automakers to introduce a basic engine design and increase the bore and/or stroke in subsequent versions. However, these engines would have been designed to provide the margins needed to allow such changes. In general, one would expect that some effort would have gone into optimizing the initial design of a cast block inline aircraft engine to maximize the bore for a given engine length - one does not want to make the engine larger or heavier than it has to be for aircraft use. I can't think of very many liquid cooled aircraft engines where there were versions with different engine displacements that retained significant commonality of either components or external dimensions. The DB 605, Jumo 222 and Klimov M-105 are about the only ones that come to mind. It may be that the design of the DB 601 was a bit conservative and that DB was able to modify the basic design to slightly increase the bore without changing other dimensions. The repeated modifications of the Jumo 222 seem to have been of a major nature and contributed to delays that made it irrelevant. Interestingly, Klimov actually reduced the cylinder bore of the M-105 engine by 2 mm in comparison with the earlier M-100 series designs. Presumably, this was to increase the strength of the cylinder block of what was a somewhat lightly constructed engine.

Increasing the bore increases the swept volume and can give more power on a conventionally aspirated engine but generally bigger pistons reduce the revs. As stated the Merlin used more boost and higher octane fuel to boost power from circa 850 to circa 2000 that cant be done with a big bore kit. The Germans were restricted with their fuel choices so increased the bore as an alternative.

yulzari said:

Was the oversized Merlin not the Griffon? Using Merlin technology on a Buzzard base (yes I know I am oversimplifying.)

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I think the point here is that the Griffon was substantially new, even if benefiting from knowledge gained from the Merlin development, and not simply a bored and stroked Merlin. Very few of the non-trivial parts would have been shared. If you are going to the trouble of developing a larger engine, you want to aim for a worthwhile increase in power.

High performance piston engines are not easy to scale in size for various reasons:

- thermal issues such as cooling the valves and pistons become more severe.
- mechanical design becomes more difficult since the loads on critical components increase. Also, the resonant frequencies will change, and what was previously an unimportant resonance that would not show up under practical conditions may become problematic.
- combustion issues limit cylinder bores to about 160 mm for fast running aircraft engines.

There are probably some points I've missed here. Nevertheless, it is interesting to consider several practical cases. Wright slightly increased the R-1750 cylinder size to make the very successful R-1820. Pratt and Whitney tried the same thing with the R-1690 to produce the R-1860, but the larger engine turned out to be unsatisfactory and was soon dropped. The Mitsubishi Kasei was a bored out Shinten which in turn was a long stroke version of the Kinsei (though I'm sure there were many other changes along the way). The initial versions of the 14 cylinder Shvetsov engines had a 174 mm stroke, but this was reduced to 155 mm in the very successful ASh-82. In general, it seems that the air cooled engines offered more possibilities for modifying cylinder dimensions, at least in terms of the physical layout issues, than the liquid cooled engines.

Junkers seems to have been successful in directly scaling the overall design of the Jumo 210 up to produce the Jumo 211. On the other hand, DB scaled the DB 601E up to produce the DB 603, and ran into various unanticipated difficulties. Interestingly, the DB 603 and Mikulin AM series seem to have had the largest cylinder dimensions of any high speed liquid cooled engines.

Looking a bit at the power curves, looks like the increased revs of the DB 605A were even more contributing to the increase in power at all altitudes? The DB 601E was doing 2700 rpm when in Notleistung and 2500 when in Steig Kampfleistung, against 2800 and 2600 rpm for the 605A.
When the V-1710 was running at 3200 rpm, it was a far more potent machine than when making 3000 rpm. It needed changes in crankshaft and piston rings to do that reliably, though; a far smaller effort than trying to bore it out.

All Allisons were designed for 4,000 rpm from the outset. They were run at 3,000 rpm due to other considerations including the propellers and the attached components. But the crankshafts were good to go at 4,000. They went from 6 counterweights to 12 ti reduce harmonics, not to increase the rpm. When they DID increase it to 3,200, the new Allisons coming off the lines were using the 12 counterweight cranks, but the early cranks could do it, too, if the props and auxiliary devices could handle it. They would, of course, vibrate a bit more, but they could do it.

All piston engines are air pumps. Put more air through them and they make more power. You can do it with bore, but an rpm increase usually accomplishes a lot more a lot sooner. You can also do it with a charge pressure increase (boost). A combination usually makes for the most dramatic power increases.

As an interesting aside, Joe Yancey builds Allison for all applications and he has maybe 15 or so in Europe on tractors for the tractor pull crowd. Some of those guys are turning them 4,600 rpm! And they are doing it reliably.

GregP said:

All Allisons were designed for 4,000 rpm from the outset. ...

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What page in the 'Vee's for victory' covers that?

They were run at 3,000 rpm due to other considerations including the propellers and the attached components.

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Neither of which was direct-driven, but was driven via reduction gearing. Ie. they don't 'care' whether the crankshaft does 2000 rpm, 3000 or 4000.
The props and attached components of the Napier Sabre have had no problems with crankshaft turning 3750 rpm.

As an interesting aside, Joe Yancey builds Allison for all applications and he has maybe 15 or so in Europe on tractors for the tractor pull crowd. Some of those guys are turning them 4,600 rpm! And they are doing it reliably.

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Think this is the 1st time that V-1710s doing tractor pulling were mentioned in this forum.

Engines are a black art, I remember Honda increased the RPM and reliability of their CB900 by reducing the size of the big end journal. There are a lot of complex calculations and compromises and very few free lunches.

Vee's for victory is not the only source or even a primary source. The 4,000 rpm design comes from former Allison employees who are friends of Joe Yancey. Joe is in his is 60's and has had friends who were there when it was happening, especially in the 1942 - 1945 timeframe.

He has many of both the 6 and 12 counterweight cranks and they have similar static balance, confirming what was said by former employees. We even have first hand recent knowledge of an Allison in the bronze race at Reno in which it was turned at 3,600 rpm (crankshaft rpm) for one lap. That would be 1,800 rpm for the prop.

Since you know I worked on Allisons, Tomo, what point are you trying to make about it being geared? After you take one apart or put one together, the fact that it is geared is pretty obvious, wouldn't you think? The props were designed for certain speeds with certain safety limits that were necessary for survival in high speed dives, so you CAN turn them that fast but, if you do, you certainly don't want to initiate a dive when turning a Curtiss Electric 3-blade at 1,700 or 1,800 rpm. It can stretch the blades and maybe render it unairworthy. Turn it fdast enough and you can shed a blade. But if you are flying straight and level (which they do at Reno), you can turn it up a bit safely ... providing it is balanced to start with.

I have posted the fact Joe has Allisons in Europe on the tractor-pull circuit before. One guy ran a tractor that had either two or three Allisons on it depending on what class he wanted to run. He usually drove it with 3 engines and his son usually drove it with 2 engines. Unfortunately he passed away a few years back when the sled weight came off on top of him at the end of a pull. I believe his son still has the tractor.

Joe even has one European who is running his Allison with two turbochargers and fuel injection. They seem to survive just fine at high rpm ... up to 4,600 and more. Of course, they aren't flying mission of several hours either. Each run is short.

So I'm not making any claims about what might have been possible in WWII since reliability was vital, not absolute maximum power for a short duration. The only places in flying where THAT was important were events such as the Schneider Cup races or other races early in the history of the airplane. Today in races like the Red Bull series, power is important, but they aren't stressing the engines to destruction. Heck, you don't see THAT many engine failure at Reno and they are really getting some power out there in the front runners.

And if you check, the two front running Mustangs (Strega and Voodoo) are both running Allison rods in the engine they call a Merlin. Bet you won't find Merlin pistons in them either.

GregP said:

Vee's for victory is not the only source or even a primary source. The 4,000 rpm design comes from former Allison employees who are friends of Joe Yancey. Joe is in his is 60's and has had friends who were there when it was happening, especially in the 1942 - 1945 timeframe.

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Unless someone post the document that proves that V-1710 was designed for 4000 RPM in 1930s (when it was designed), I'll maintain that it was not the case. What the V-1710 was capable for after 30 or 50 years after it was conceived has no bearing on what it was capable during 1st 10 years. And that was 3400 rpm for post war one-offs. The 1st V-1710-33s were returned to the factory for 'modernization', or were restricted to 2770 rpm. It counted what it was been able to give - 4000 rpm was not when it mattered.
I have respect for Mr. Yancey and people from Allison. With that said, a person working at Alison in 1943 had how much of insight whether the V-1710 was 'designed' for something in, say, 1933?

He has many of both the 6 and 12 counterweight cranks and they have similar static balance, confirming what was said by former employees. We even have first hand recent knowledge of an Allison in the bronze race at Reno in which it was turned at 3,600 rpm (crankshaft rpm) for one lap. That would be 1,800 rpm for the prop.

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In case the prop gear is of 2.36:1 ratio (used eg. on the P-82F), it would not be 1800 for the prop
The engines with 12 cw were allowed for 3200 rpm on take off by manufacturer. For my money, I'd rather believe the manufacturer. Former employees might not be aware of all of the intricaties of engine's design, either. And there is a small chance that someone might actually remember not so accurately the things old 70 odd years.

Since you know I worked on Allisons, Tomo, what point are you trying to make about it being geared? After you take one apart or put one together, the fact that it is geared is pretty obvious, wouldn't you think? The props were designed for certain speeds with certain safety limits that were necessary for survival in high speed dives, so you CAN turn them that fast but, if you do, you certainly don't want to initiate a dive when turning a Curtiss Electric 3-blade at 1,700 or 1,800 rpm. It can stretch the blades and maybe render it unairworthy. Turn it fdast enough and you can shed a blade. But if you are flying straight and level (which they do at Reno), you can turn it up a bit safely ... providing it is balanced to start with.

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You were making a point about the V-1710 being good for 4000 rpm right from the box, but other things were holding it down. Prop doesn't care how big the RPM is at the crankshaft, since, in the most of the cases, the reduction gear is there to reduce the crankshaft RPM down to the RPM appropriate for the prop. Same goes for engine accessories. If indeed the, say, V-1710-33, was capable for 4000 RPM, it would have the, say, 3:1 reduction gear attached on it so the prop tips don't broke speed barrier. As it was the case for eg. Napier engines, that turned (not just designed for, really turned) 3750-4100 rpm.

I have posted the fact Joe has Allisons in Europe on the tractor-pull circuit before. One guy ran a tractor that had either two or three Allisons on it depending on what class he wanted to run. He usually drove it with 3 engines and his son usually drove it with 2 engines. Unfortunately he passed away a few years back when the sled weight came off on top of him at the end of a pull. I believe his son still has the tractor.

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I know that you've posted about tractor pulling before. Since we all know that, I was just pulling your leg

Well Tomo,

Joe has the most genuine Allison documentation of anyone I personally know. He has the only genuine Allison crankshaft gage as far as he knows, and that is why his bearings and engines run so well ... they are done to Allison factory specs. You have the nominal and first undersize all on one crankshaft gage. Simple and easy. You install it, torque the rod ends and main caps, and lower crankcase, and you must be able to turn it by hand. If not, the crankshaft will overheat and seize at some point. I believe the nominal clearance is about 2.5 thousandth's of an inch.

So, if you want to see his genuine documentation, you can do so at Joe's shop in Rialto, CA, US with an appointment. The actual business name is Yancey Enterprises and you can also view the 120+ Allisons ready for overhaul and sale at his shop. He can even sell you a V-3420 if you want one and an auxiliary supercharger if you want that, too. Joe doesn't have much time for tourists but if you are interested in an Allison, he has the time and will take it ... that is his business.

So you can believe whatever you want, but NOBODY designed an engine for the speed at which it ran or it would blow up in a dive. EVERYONE had a max rpm multiplier at which the engine was safe. Allison used a 1/3 safety margin. The only engine I know of that has almost zero safety margin is a Bristol Centaurus. If you turn it as fast as 150 more rpm over redline, it will blow up. They handled the safety margin for that engine with extra propeller pitch adjustment beyond the normal range. So say the only 3 owners of them I know and, so far, all three have had their cylinders done by Joe. In fact, Joe is doing the Centaurus cylinders for the British Crown's Sea Furies (don't know which ones but there are more cylinders than for one aircraft), too, as well as a few Centaurus owners in Australia and New Zealand.

This is not intended to start anything with you, Tomo ... I have no animosity. But former employees and genuine Allison design documents and manuals are primary sources to me and I had access to them for awhile. Not so much anymore, though Joe and I are friends and we see each other from time to time. I stop by the shop every few months to see how things are going. Actually, they are set to move by the end of June and they will probably move closer to me than they are now.

Cheers.

We may have a confusion of terms here.

The Allison may well have been "designed" to turn 4000 rpm for a few seconds in a dive at part throttle. Which is a far cry from running for several minutes at 4000rpm with a wide open throttle. (boost pressure would be???)

Tractor pulling can use whatever they think the engine will stand and if they are wrong????? They can walk back to the pits and tow the thing off the track.

Tractor pulling may also use modified supercharger or gears or intake systems compared to WW II aircraft engines. They may be using different fuels.

OK, I can rephrase. The crankshaft, rods, pistons, valves, valve springs and other rotating parts were designed to withstand a 4,000 rpm rotation speed safely. They were NOT designed to run 150 inches of MAP at 4,000 rpm for long periods.

As I stated above, NOBODY except Bristol failed to use a safety factor, knowing the engines would be in very high speed dives at some point in service.

I recall reading in Vees that there was a design limit of 4000rpm - but that was for a dive.

Greg, the point Tomo is trying to make is that if the prop has an rpm limit the engine's gearing would be modified to suit. Some V-1710s had 2:1 reduction gearing for 3000rpm, so if they could do 4000rpm they would have changed it to 2.67:1. The prop would not have limited the engine's rpm.

There were seven propeller gear reduction ratios for production Allisons. First, there were 77 dash numbers. 42 of them had a reduction ratio of 2.01 : 1 (rounded to 2 : 1).

Five had a ratio of 1.8; eight had a ratio of 2.23 : 1; one had a ratio of 2.26 : 1; sixteen had a ratio of 2.36 : 1; one had a ratio of 2.48 : 1, and four had a ratio of 2.772 : 1. If they had needed more, it wasn't all that hard ... cut new gears.

Why tell me the obvious? It's geared? I built and disassembled them. The prop rpm was limited? I stated that (and know the limits). I am up to speed on Allisons more than 99+% of people on the earth. It was all an accident of circumstance, not anything special about me. Which means almost nothing since they are in very limited use. It is a niche market at best. The owners are not very numerous.

But if I post some info on the Allison, it comes from primary sources / personal experience and is not speculation. I don't reference Vees for Victory very often ... only sometimes when it is convenient. We saw some really unusual things out at Joe's while I was there, including making an Allison powered dragster from the 1950's run again (The Green Monster). Believe me, stock is best. Modified Allisons are usually not better ... unless the modifier KNOWS what he or she is doing. Believe it or not, they aren't all that complicated but they DO require extreme care in preparation and assembly and ... most importantly, operation. Run it right and it will last a long time. Fail to do that and you are a tset operator with a test engine..

Merlins are no different in that respect. I don't know anyone in the USA who can build you a stock Merlin today. The parts aren't there. But you CAN make almost any Merlin run. Most of the V-1650-3, 5, 9's running today are running transport heads and a mix of parts. That isn't stock ... but works. Can't speak for Europe, but the Merlins in the USA are in need of spare parts today ... STOCK spare parts.

GregP said:

There were seven propeller gear reduction ratios for production Allisons. First, there were 77 dash numbers. 42 of them had a reduction ratio of 2.01 : 1 (rounded to 2 : 1).

Five had a ratio of 1.8; eight had a ratio of 2.23 : 1; one had a ratio of 2.26 : 1; sixteen had a ratio of 2.36 : 1; one had a ratio of 2.48 : 1, and four had a ratio of 2.772 : 1. If they had needed more, it wasn't all that hard ... cut new gears.

Why tell me the obvious? It's geared? I built and disassembled them. The prop rpm was limited? I stated that (and know the limits).

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It goes back to the original statement:

GregP said:

All Allisons were designed for 4,000 rpm from the outset. They were run at 3,000 rpm due to other considerations including the propellers and the attached components. But the crankshafts were good to go at 4,000.

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You are saying, and have said, that the Allison was designed for 4000rpm and that it was the propellor, and other factors, which forced it to be limited to 3000rpm. The reality is that had the base engine been capable of 4000rpm the propellor would not care because the reduction gear would be adjusted to suit.

Other factors, which I take for other components, may have also been limiting factors on engine speed. Which would mean that they were

• geared incorrectly
• not designed to match the crankshaft performance

Or that the crankshaft was overdesigned.

Also, didn't the early Allisons struggle at 2600rpm?