Merlin and bus mash up

[Gob]

[MajGenl.Meade]

Bit a fun, that!

[RayThom]

... a "stock" Hennessey Venom GT does much better... 270.49 MPH

http://www.venomgt.com/

[Gob]

Prefer a Veyron...



The Super Sport version of the Veyron is recognised by Guinness World Records as the fastest street-legal production car in the world, with a top speed of 430.9 km/h (267.7 mph),[5] and the roadster Veyron Grand Sport Vitesse version is the fastest roadster in the world, reaching an averaged top speed of 408.84 km/h (254.04 mph)

[kmccune]

One Guy had a 65 Chevelle with an Allison V-12 under the body ,dont sell the Allisons short ,properly modified they seem to hold their own against the Merlins .
Someone said if a Merlin hit 4000 rpms ,it was trashed ,is that true ?

[datsunaholic]

A Merlin won't live long at 4000 RPM. They were only meant to turn around 3000 RPM, and that was limited time, War Emergency power.

Unlimited Hydroplane teams routinely ran them at 4000 RPM, though. They were pushing 2500 HP. Some of the Reno Air Racer planes pushed 4000 HP, though those engines were highly modified and one modification was to use the Allison V1710 rods (which are significantly stronger, especially the 100-series or later G6 rods). Merlin rods are 3-piece, fork and blade rods, Allison used fork and blade rods as well but only used 2-piece rods with a stronger I-beam profile.

Merlins are extremely complicated engines compared to the Allison. What Allison could do with 1 part, Rolls made do with 3. Part of the complexity is the 2-stage, 2-speed blowers used in the later engines, but that's not all of it.

I'm helping assemble a merlin blower now, actually. We took it apart a couple weeks ago, the case got painted last week so it's time to reassemble. Fortunately it wasn't missing any parts because that stuff is next to impossible to find, especially on a non-profit Museum budget.

[kmccune]

Thanks ,didnt the Merlin have a peculiar valve system ? never having been inside a Allison ,I assume they have standard poppet valves ?

[datsunaholic]

Both Allisons and Merlins use poppet valves. Both are 4-valve per cylinder, overhead cam engines with the cams driven by geartrains and shafts. The difference is in the rocker assemblies.

Merlin camshafts have a lobe for every valve with wiper rockers. The camshaft rides in a series of cam towers which also hold the rocker shafts on each side.

Allison camshafts have half that number of lobes, because each intake pair and each exhaust pair are driven by a Y-shaped rocker. It's also a roller rocker rather than a wiper type, so Allison camshafts rarely wear out (Merlin ones are only good for a couple hundred hours of operation). Allison cams ride in the head (on brass bearings) and the bearing "caps" serve as attachment points for the rocker pivots, so each cylinder's rocker set can be changed individually rather than having to pull the entire cam rack assembly like the Merlin. And yes, I've changed rockers on both types of engines (changing a rocker on an Allison takes under 10 minutes. Rolls, 1-2 hours, depending on which rocker it is).

Rolls Royce later designed roller rockers for the Merlin-derived Meteor engine, which was a de-tuned, naturally aspirated version of the Merlin used in British Tanks. However, some folks (like Roush Aviation) have come up with Aviation-Certified roller rocker/cams for Merlins. And as long as you aren't flying them, there are several Meteor parts that work in Merlins. Can't use the cams though because they turn the opposite way.

Another difference is Merlin valves are parallel to the piston crown (the combustion chamber is flat) whereas the Allison uses a Pent-roof head (valves are at slight angles) Between that and the fact that Allison valves are larger (much larger stems, slightly larger faces) made it possible to turbocharge an Allison whereas Merlins simply burn valves if you have any backpressure at all.


More than you needed to know, but I've been fascinated by these things ever since I got a chance to work on them. It's a very happy sound to hear one you built (out of original 70 year old used parts, no less) roar to life.

[kmccune]

Thank you ,used to hear that distinctive Allison V-12 sound at the tractor pulls ,the Allisons ,strangely enough ,didnt do all that well in the lighter classes (too much power ,hard to "hookup " ) they shined in the heavier classes ,however .
Nope ,never to much info ,thanks again .

[kmccune]

As an after thought ,what is a sleeve valve ?

[oldr_n_wsr]

More than you needed to know,

I don't know much beyond Chrysler Slant 6's, but I like reading your posts datsunaholic.

[Fafhrd]

The Merlin was designed for high altitudes; the Allison wasn't. This has something to do with the superchargers.

[Jarlaxle]

Actually, not the case...the Allison-powered P-38 would top 40,000'!

The correct engine for high altitude is the Pratt & Whitney Double Wasp radial.

[No Greater Fool]

The P-38's engines were turbocharged.

[Jarlaxle]

No kidding.

[Bicycle Bill]

Actually, not the case...the Allison-powered P-38 would top 40,000'!

The correct engine for high altitude is the Pratt & Whitney Double Wasp radial.

Isn't that the same engine that also powered the P-47 "Thunderbolt", the classic WWII ground-support fighter/bomber?
While the plane could reach angels 40 if push came to shove, that engine was also capable of giving "The Jug" a top speed of over 400 mph to allow it to get in, do its job, and then get out of Dodge before all hell could break loose.

One of my uncles was a crew chief on a P-47 in the ETO and he had nothing but good things to say about them and the way they could take a licking that would have KOed most other aircraft but still keep on flying.

-"BB"-

[datsunaholic]

Air-cooled radials vs liquid cooled V engines was always a fight. The radials made more power because they were much larger engines- the R2800 was 1100 cubic inches bigger than an Allison, but also weighed 1000 lbs more. Plus the huge frontal area made for a much less streamlined fuselage. Conversely, the air cooled engine eliminated the need for complex cooling systems and the added weight of coolant. That's why the Navy didn't use liquid cooled engines for carrier aircraft.


The Allison's poor high altitude performance was actually due to AAF requirements. The AAF wanted the engine used for low-altitude performance, expecting the turbo-compound version of the Allison to do the high-altitude stuff. Unfortunately it didn't work out that way, and the Rolls-Merlin turned out to be a relatively easy modification to the P-51, and the demands of war meant little resources were used to solve the Allison's high altitude problem when the Merlin was available. At the end of the war Allison had come up with an Auxiliary-Stage supercharger which looked to solve the problem, but the jet age had begun and the war ended before very many were built.

[kmccune]

Anybody read Smiths "Aircraft piston engines? (" anyway the title reads something like that) The last engine mentioned under development was the Gnome fairy H-12 turbocompounded engine (diesel I think ) it didnt make it ,the turbine came of age .

[Fafhrd]

The P-51 Mustang was considered a failure, initially, because of poor performance at altitude. It wasn't until someone put a Merlin into it that it became the best American fighter. Luckily, Merlins were being manufactured by Packard in the USA, and all production Mustangs were equipped with them.

I have read that parts for Packard Merlins were not interchangeable with parts for Rolls Royce Merlins. American Merlins were built to much more exacting standards.

I would have thought that the supercharger would have been easily changeable. I have no idea why the Allison wasn't considered a good engine for the Mustang--there had to have been a reason.

That big radial in the Jug was meant for Navy aircraft, and both F6F and F4U had them, making them easily better than most of the Japanese aircraft. I have no idea what either of those fighter aircraft could have done against German fighters.

I do remember reading that the British sent a squadron of Spitfires to the Far East; the Japanese chewed them up with great rapidity.

[datsunaholic]

Packard Merlins are compatible with equivalent British versions. They had to be- the Packard Merlin was originally built to SUPPLEMENT British production. That's why Packard Merlins still use those ridiculous "British Standard" nuts and bolts, requiring a whole different set of wrenches and the thread pitch is completely different from US SAE fasteners. A Dash-1 Packard Merlin (V1650-1) was based off the Merlin XX (or Merlin 28, depending on source). The 1650-7, the most common Packard variant, was based on the Merlin 66. Now, each variant was significantly different from the last, so not many parts interchanged. The V1710-1 was the only single-stage Merlin produced by Packard, the -3, -7, and -9 all used a 2-stage supercharger.

A (possibly an urban legend) has it that Rolls Royce came to the US looking for a suitable company to build Merlins. They went to Ford, but when Ford insisted on using production-line technology they went to Packard. Packard was chosen because they hand-built most of their cars. But after Packard produced a couple prototypes, they immediately modified the tooling for mass production. This ticked off Rolls Royce, who insisted that a mass-produced Merlin wouldn't meet the specifications Rolls insisted on by hand-finishing every part. They shipped the first run of engines back to Britain to be tested, and found that the tolerances were not only met, but were much closer to the blueprint than any British-built Merlin. So they let Packard continue, and eventually turned to mass production themselves.


As for the Allison supercharger, modifying the supercharger for high-altitude performance simply wasn't in the AAF's plan originally. The Allison's supercharger is integral with the accessory housing (the part on the Allison that drives everything not associated with the crankshaft and reduction gear). On a Merlin, the "Wheel Case" (which contains some of the accessories and the 2-speed blower clutches) is separate from the supercharger itself, so the supercharger could be designed independently. The Merlin XX (which the Packard Merlin was originally based on) always had the French-designed 2-speed clutch setup, whereas Allisons had the blower driven by a much simpler gear drive. Overall, the Merlin is about a foot LONGER than a Allison due to the immense size of the blower.

Allison had other ideas for higher altitude performance, including turbocharging (actually turbo-compounding), and then an auxiliary-stage supercharger driven by a shaft from the accessory housing that fed the main blower. The Aux-stage Allison produced nearly the same rated HP as a Merlin by war's end but it was a more complex setup and not particularly reliable, being detached from the main engine.