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Block Party: Creating a V-6 By Declaring Two Cylinders Redundant

By: · April 12, 2016

2016 Jaguar F-type R coupe supercharged 5.0-liter V-8 engine

2016 Jaguar F-type R coupe supercharged 5.0-liter V-8 engine

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From the May 2016 issue

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Mankind’s ability to make things, sometimes out of practically nothing, is what distinguishes us from our knuckle-dragging ancestors. We can dig up ore, melt it, and ladle it into a mold, and out pops an engine block. Jaguar Land Rover’s AJ126 engine is a more economical form of human ingenuity; it’s a supercharged 3.0-liter V-6 made from the existing AJ133 5.0-liter V-8. Although this is not an original idea—see our sidebar, below, on Buick’s Fireball V-6—the JLR gambit deserves the Queen’s Medal for creativity.

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THE WHOLE TRUTH

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Jaguar Land Rover’s AJ126 isn’t just an eight with two dead holes, as its six cylinders have a smaller bore and stroke than the V-8. In their highest-output guises, the two engines scale thusly:

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V-6

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V-8

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BORE

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84.5 mm

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92.5 mm

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STROKE

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89.0 mm

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93.0 mm

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DISPLACEMENT

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2995 cc

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5000 cc

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POWER

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380 hp

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550 hp

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TORQUE

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339 lb-ft

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502 lb-ft

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HP/LITER

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127

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110

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The Hole Truth

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1987 Buick GNX

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PRECEDENTIAL ELECTION

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Buick created its Fireball V-6 in 1961 by pruning two cylinders from a troublesome aluminum V-8. The engine did a stint with Jeep, and then GM refined it with a balance shaft and even firing. In the turbocharged 1987 Buick GNX, it made 300 horsepower. As the 3800, it powered everything from Chevys to Cadillacs and sold more than 25 million copies. Race versions landed four Indy 500 poles. Its final assignment was in the 2009 Buick LaCrosse.

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Buick’s V-6 was a hack job, a V-8 with two cylinders lopped off. JLR took a more direct path to save on design, engineering, tooling, and machining costs. It plopped a pair of three-cylinder heads atop the eight-cylinder block. That’s daft from the bulk and mass perspectives, but the logic behind this twofer makes some sense. Sharing the exterior-block details enables one mounting system to serve both V-6 and V-8 engines. New partitions inside the die-cast aluminum V-6 block supplant the deleted cylinder bores.

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split crankpins

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The split crankpins are highlighted here. On a normal crank, those surfaces are neatly aligned.

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A 60-degree bank angle is generally preferred for V-6s to minimize noise and vibration, but the AJ126’s 90-degree design allows more space between cylinder heads for the supercharger and intercooler. To avoid odd firing intervals, JLR fitted a split-pin crankshaft. This splits adjoining connecting rods so that a cylinder fires every 120 degrees of crankshaft rotation, compared with the original Buick V-6’s 90-degree, then 150-degree syncopation.

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This V-6’s five main bearings, versus the usual four, are bad for mass and friction but good for NVH. To ensure that the AJ126 engine meets Jaguar Land Rover’s comportment standards, there’s a counter-rotating balance weight spinning at each end of the crankshaft.

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Trimming the V-8’s length wasn’t crucial because all JLR cars and SUVs have engine compartments long enough to harbor inline-fours. But the evil side of having two cylinders on permanent vacation is the weight of the longer-than-necessary block, crank, and oil pan. JLR won’t reveal exactly how much that is, but our educated guess is 50 pounds. That’s got to be frustrating to the body and chassis engineers who sweat every ounce.

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Customers have nothing to gripe about, though: This V-6 delivers exemplary power, poise, and performance. That said, the Jaguar F-type owner would be wise to take leave when the bar debate turns to engine design.

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Thanks to automotive-assessment firm A2Mac1 for providing crank photography and reference for our block illustrations.

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Filed under: Engine,News

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