Why are Big Bore Engines Bad??

Spruce Goose engines- 8x radial, P&W R4360 @ 3000hp
Thanks,
Dan H

Probably hard to get the whole charge to fire in a reasonable time frame before the pressure and heat of the moving flame front causes the mixture to spontaneously ignite (detonation).

Oversquare bores are quite common in high performance motorcycle engines because the shorter stroke ( for the same displacement) allows for much higher revs and higher output. One fo the most extreme examples of this is the Yamaha R6, which will rev up to 16,000 rpm. Engineers can take this too far, however, as they did with the 1988 Suzuki GSXR 750, which had such radically oversquare dimensions that it left a lot of riders, and racers, complaining about the complete lack of torque at lower rpms, a complaint also leveled at the current R6.

Perhaps the epitome of the oversquare design was the Honda NR500, which siamesed two round pistons into one oval piston, to create a 4 cylinder motor with the valve area of a V-8 ( 4 cylinder limit on bikes racing GP's). This engine turned 22,000 rpm in race trim, but had a very narrow powerband from just under 20,000 rpm to redline. It was Hondas attempt to make a four stroke engine competitive with the contemporary two strokes which ruled the roost performance-wise. It was not a successful design, but displayed Honda's amazing engineering abilites and willingness to take the hard path and innovate.

Honda had test engines turning over 30,000 rpm back in the late 1960's to find the limits of combustion speeds in a cylinder.

I would think that the alloys available at the time (60+ years ago) would have been part of the size limiting. By the time you build all the engine parts beefy enough to take the stress of a 6" bore you've made it heavier then if you split the displacement between 2 cylinders. Todays higher strength alloys probably make the 6" rule of thumb absolete.

Got to remember at the time this engine was designed the Ford flathead was considered the cutting edge of technology and they were still trying to figure out how to build a jet engine that wouldn't melt.

A double, triple (were there any made) and quad engine has four crank throws, I don't remember how they are arranged. There are several sectioned late model radial engines in the SAC Museum in Nebraska.

Often the cylinders are not directly in a row but twisted so the rear cylinders get some fresher cooling air. Cooling these huge yet compact engines can be a pain in an aircraft and had to be even harder in the back of a tank.

Lubrication is by pressure with a dry sump, though when parked the lower cyinders can accumulate oil. That's why radials are always turned over a turn or two of the crankshaft (and multi bank engines were always geared to run the prop slower) before turning on fuel and ignition. Hydraulicing an engine from excess lubrication oil ruins the mission really fast, before it starts, practically.

Gerald J.

A couple of "zeros" must have wandered off from that 95,000 BTU figure!

As far as the 750 Gallons per minute across (assumed) 4 engines with enough volume to create a cooling cycle is a lot of agua on board. At 8.34 lbs/gal thats a lot of weight to be taking aloft. Aircraft are typically air cooled for the main reason of weight savings.

580 GPH X .70 = 406 GPH X 114,000 BTU per gallon = 46,284,000 BTU's to be "shed" through the exhaust and cooling systems, what ever the "split" would be!

Here's a photo of the engine:

Wikipedia has a article on the R-4360 "corncob" radial.

• Wikipedia R-4360 article

For the an equal displacement engine, one with a smaller bore/longer stroke will generate more torque at a lower rpm than one with a bigger bore/shorter stroke.

Engines can be undersquare, B < S, Square B = S or Oversquare B > S to make the power where needed.

In the case of the aforementioned aircraft engine, Props turn at relatively lower speeds than you'd first think. Wood props often can't survive 2500 rpm. Many planes had gear reduction boxes to slow the prop down but keep the engine at a higher level of torque.

The longer props on say a big bomber the rpm would have to be kept well below a point which would be approaching supersonic speeds at the far tips. Since the props were running lower speeds smaller bores and long strokes along with the many more power pulses of 28 cylinders made monstous torque at those lower revs and could turn those variable pitch props at high angles of attack for greater cruising speed and efficiency.

The radial designs were more about compactness and aerodynamics than anything else, Lord knows it made them ultra complcated to build and work on.

As for a substitute for displacement..... All my racing buddies have assured me for years that cubic inches will never outrun CUBIC DOLLARS.