Allison engines vs Merlins for racing

[MIKE NIXON]

I have been asked why no Allisons in gold racing.
First off the Allison is a great engine and used about 20% less parts to build. The cam & roller rocker arms along with the valves have almost no problems. The late con-rods are in all the top Merlins.
That said, the Allisons had the usual changes as they went from C's, E's, F's and then G models. They also had aux stage superchargers for high alt.
The pro/cons of the Allison vs the Merlin:
Starting at the front, the A has a .500 gear ratio, the M somes with .479 and .420 ratios.
The bigger number turns the propeller faster which hurts A/C performance with tips reaching Mach +.
The heads in the A are great with hemi-chambers. The valve stems are larger than the M which hurts. Ports are smaller on the Allisons.
The late con-rods (G) with new bolts are the top choice.
The induction manifolds are better with the Merlins.
Merlins have a 12" fan flowing into a 10" fan for the supercharger. The Allison has a 10" fan.
The fans are propellers for the most part and spinning faster increases heat and loss of power so gear changes can hurt.
The Merlin has intercooling and after-cooling of the air/fuel charge to keep temps down.
Some later Allison's had aux superchargers with aftercoolers, but the drives and all added to the weight and size needed.
There are some other variations that can help the Allison perform better but they are rare.
MN

[Airrace7]

Re: Allison engines vs Merlins for racing

I have been asked why no Allisons in gold racing.
First off the Allison is a great engine and used about 20% less parts to build. The cam & roller rocker arms along with the valves have almost no problems. The late con-rods are in all the top Merlins.
That said, the Allisons had the usual changes as they went from C's, E's, F's and then G models. They also had aux stage superchargers for high alt.
The pro/cons of the Allison vs the Merlin:
Starting at the front, the A has a .500 gear ratio, the M somes with .479 and .420 ratios.
The bigger number turns the propeller faster which hurts A/C performance with tips reaching Mach +.
The heads in the A are great with hemi-chambers. The valve stems are larger than the M which hurts. Ports are smaller on the Allisons.
The late con-rods (G) with new bolts are the top choice.
The induction manifolds are better with the Merlins.
Merlins have a 12" fan flowing into a 10" fan for the supercharger. The Allison has a 10" fan.
The fans are propellers for the most part and spinning faster increases heat and loss of power so gear changes can hurt.
The Merlin has intercooling and after-cooling of the air/fuel charge to keep temps down.
Some later Allison's had aux superchargers with aftercoolers, but the drives and all added to the weight and size needed.
There are some other variations that can help the Allison perform better but they are rare.
MN

Great Information, Thanks for letting us all know.

[FlyKidChris]

Re: Allison engines vs Merlins for racing

I have been asked why no Allisons in gold racing.
First off the Allison is a great engine and used about 20% less parts to build. The cam & roller rocker arms along with the valves have almost no problems. The late con-rods are in all the top Merlins.
That said, the Allisons had the usual changes as they went from C's, E's, F's and then G models. They also had aux stage superchargers for high alt.
The pro/cons of the Allison vs the Merlin:
Starting at the front, the A has a .500 gear ratio, the M somes with .479 and .420 ratios.
The bigger number turns the propeller faster which hurts A/C performance with tips reaching Mach +.
The heads in the A are great with hemi-chambers. The valve stems are larger than the M which hurts. Ports are smaller on the Allisons.
The late con-rods (G) with new bolts are the top choice.
The induction manifolds are better with the Merlins.
Merlins have a 12" fan flowing into a 10" fan for the supercharger. The Allison has a 10" fan.
The fans are propellers for the most part and spinning faster increases heat and loss of power so gear changes can hurt.
The Merlin has intercooling and after-cooling of the air/fuel charge to keep temps down.
Some later Allison's had aux superchargers with aftercoolers, but the drives and all added to the weight and size needed.
There are some other variations that can help the Allison perform better but they are rare.
MN

Very cool and thank you for the summary.

[Curt_B]

Re: Allison engines vs Merlins for racing

I have been asked why no Allisons in gold racing.

MN

Can skinny stem valves be substituted. A buddy left this in my email a while ago. Any of it make sense since I'm not an engine guy, or is it too heavy;

"Allison powered Unlimited employing all the technologies embodied in the E27 (turbocompound) version of the V-1710 shown but possibly integrating PRT components from the Wright TC18 turbocompound engine (plentiful) and intercooling features (from Merlin maybe) that most racers remove and discard?
Also employ Leeward’s Total Loss Boiler approach and only take Ram Induction air aboard at race power.
Race Fuel, Boost, Water and PRT combined could approach 4000 HP on an ultra-lean airframe."

Of course the 0.5 reduction would out rule it all at current speeds. Can the Allison gear ratio be changed, or is it package limited.

Curt

[440_Magnum]

Re: Allison engines vs Merlins for racing

That pretty well lines up with he old summary I've always heard, which is "it really all comes down to the blower." Merlin for the win when it comes to the blower- Stanley "Doc" Hooker (who later went on to renown in jet engine design with Rolls, then Bristol, then Rolls again) was involved in the evolution of the Merlin blower if I recall right. The man was a genius when it came to turbines, and a supercharger is a gear-driven turbocompressor. It was way ahead of the little single-stage blower on the Allison.

I'd read about the Allison advantages in mechanical strength (rods, primarily) and combustion chamber. The drawback of the thicker valve stems and smaller ports wasn't one I'd heard about- could that be overcome by "porting" to borrow an automotive term? Or is there not enough meat in the heads?

So I'm interested in your opinion here: Allison engines had their best successes when a turbocharger was added to act as a first-stage for that single-stage blower. What are your thoughts on the applicability of turbochargers for air racing, specifically on the WWII era unlimited V12s as opposed to the more modern Sport-class engines. "Back in the day," the aircraft manufacturers primarily used turbos for altitude compensation, not so much for drastic power increase at low density altitudes. For racing, they'd have to be providing a lot of boost at Reno altitudes and would need a lot of after-cooling, either via ADI or heat exchanger- a very different scenario and the ancient first-generation GE turbos that they originally had need not apply for the job. And of course there's the question of how the REST of the engine would respond to operating with the high exhaust back-pressure of a turbocharger generating a high level of boost at low density altitude. I understand that its NOT a simple question of tacking a turbo on and flying to a win. If it was, it'd have been done.

[1:1 Scale]

Re: Allison engines vs Merlins for racing

I'd be interested in what kinds of gains could be had simply by shoving a modern compressor design into the current superchargers...

[omalley1537]

Re: Allison engines vs Merlins for racing

I would welcome being corrected if I am wrong, but isn't the limiting factor now more about prop and aerodynamics than horesepower?
Seems like I read somewhere that at the current hp levels the prop isn't taking full advantage of the available power they already have. Again, feel free to tell me I am wrong. I am no expert and I didn't stay at a holiday inn express last night...

[Idaho_cowpony]

Re: Allison engines vs Merlins for racing

Interesting thread Mike. Thank-you.

Makes me curious;
If you had the various options at your disposal, and the $$ to make it happen, just what would you do with an Allison (any series) engine, to mount, in say a RO Yak?

- Joseph

[Big_Jim]

Re: Allison engines vs Merlins for racing

Jack Sandberg experimented with a pair of Garret turbochargers on the Allison in Tipsy Miss in 1974, but they never ran in competition.

A few years back I was working with Frank Taylor after he had bought Dago Red from the Costo's. He was open to all sorts of suggestions, including experimenting with a turbocharged Allison--to the point where I pointed him in the direction of Ed Cooper Jr., who built and ran possibly the strongest turbo Allisons ever run in the Unlimited Hydroplanes. I know Frank did a lot of feasibility discussion with Ed about getting a few for Dago.

At the same time I was talking to Kerch about his thoughts on running a turbo Allison in a Mustang since he had worked on J.R.'s P-63. He said that it certainly had possibilities, but thought that he figured you'd have to run at least three turbochargers PER BANK in order to get comparable horsepower of an Allison rod Merlin. And in the voice that only Kerch can muster he said "Where would you put it????"

[IcePaq]

Re: Allison engines vs Merlins for racing

There are cars out there making 3000hp on a single turbocharger or two smaller ones.

[440_Magnum]

Re: Allison engines vs Merlins for racing

There are cars out there making 3000hp on a single turbocharger or two smaller ones.

There are locomotives making 6000 horsepower on a single turbo. So what does that prove? (and few if any cars really produce 3000 horsepower for more than 3 seconds at a time.)

An unlimited powerplant system has to be able to make the needed power in a limited size/weight package, across the needed RPM range, with suitable mass flow, at the Reno density altitude with range for variation due to weather, tolerant of several positive and some negative G, lateral G, high vibration, with reliability to last the race week and protect the pilot, and a litany of other constraints. Its not easy, or it would have been done already. Automotive tech applied to air racing has had very limited success in the sport class (and "unlimited" failure - pun intended) because of the radically different operating environment than what it was created and evolved for over the decades.

[John]

Re: Allison engines vs Merlins for racing

As noted on another thread, Bruce Lockwood wrote an excellent article in AirClassics about the "Tube" Merlin; and he also mentions the amount of thrust produced by the exhaust of that type of engine setup. So, my question is: would not a turbo-ed set up loose much of this exhaust thrust?

[grampi]

Re: Allison engines vs Merlins for racing

I would welcome being corrected if I am wrong, but isn't the limiting factor now more about prop and aerodynamics than horesepower?
Seems like I read somewhere that at the current hp levels the prop isn't taking full advantage of the available power they already have. Again, feel free to tell me I am wrong. I am no expert and I didn't stay at a holiday inn express last night...

Agree...I would think they would want to go to a larger diameter prop, wider blades, or a combination of both to be able to take full advantage of these souped up engines...

[Samuel]

Re: Allison engines vs Merlins for racing

Pretty interesting discussion going on here: http://www.ww2aircraft.net/forum/avi...nts-34680.html

[boomtown]

Re: Allison engines vs Merlins for racing

Agree...I would think they would want to go to a larger diameter prop, wider blades, or a combination of both to be able to take full advantage of these souped up engines...

Larger Diameter = greater prop tip speeds = not good