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>>Top ring placement and piston ovality in competition engines

by

Titus Bloom

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itrous oxide systems substantially increase the race engine’s power output. Because of the higher partial pressure of oxygen injected into the fuel mixture it enables fuel to be burned at a faster rate.

For the most part, the use of nitrous oxide in drag cars has carried a winning promise. But nitrous can also prove to be ruinously expensive, reducing a previously powerful race engine to impotence in seconds. To realize its potential and avoid unwelcome drama, it’s best to investigate its winning formula at the outset.

So, where might it malfunction first? Usually pistons and rings are the components that tend to suffer most and, therefore, it’s advisable to speak to piston companies with broad experience in nitrous competition.

1: On naturally aspirated engines, the higher the top ring placement, the smaller the crevice volume between the piston and the cylinder wall, thus the quicker the reaction of the charge.

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2: On forced induction and particularly nitrous engines, the top ring groove is moved down to around 0.420in to 0.430in from the piston crown.

2 Robbie Giebas of Gibtec Pistons of Denver, Colorado says, “On forced induction and on nitrous applications, which encounter extreme shock loads, we move the top ring down from the piston crown to around 0.300 inches. In actual fact, the top ring could be moved down by as much as 0.450 inches, depending upon valve size and configuration, as well as

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the positioning of the valve pockets, the radial width of the top ring, the piston pin height, and the amount of nitrous power being added.

“Often it’s the intake valve pocket—which is always bigger than the exhaust pocket—that determines the position of the top ring.

Compact rings and therefore small ring grooves provide more potential for variation

april 2016 | RPM Magazine

in ring placement than larger ring grooves. For example, a naturally aspirated engine with a top ring of 0.6 mm (0.0236 inches) axial depth and 0.110 inches radial width, which requires a ring groove width of 0.115 inches, offers more pocket clearance than the top ring spec of a nitrous engine, which might measure 0.043 inches axial depth and 0.173 inches radial width.

For more than 20 years Robbie Giebas of Gibtec Pistons has been developing nitrous racing pistons and in 2003 founded one of the first billet production facilities.

3 “But on most small-block applications with a standard inline valve pattern and a power adder, lowering the top ring to around 0.300 inches protects it and the top land from potential detonation and other conditions that may occur in forced-induction applications.

“With the top ring positioned at a dimension of 0.260 inches down from the piston crown, the piston will accept a 250 to 260-hp shot of nitrous.

The big consideration with nitrous is air-fuel ratios. If they are inconsistent, no amount of top-ring-down placement will save the piston.

“On naturally aspirated engines a higher placement of the top ring is desirable. As a result, the crevice volume between the piston and the cylinder is smaller. The higher placement of the top ring also impels the induction gases to enter the chamber faster, as smaller crevice volumes provide quicker reactions to the initial pull. Super Stock racers, whose engine changes are closely regulated, always place the top ring as high as possible.”

When Giebas entered the competition piston business over two decades ago, top ring dimensions of naturally aspirated applications were likely to be 0.043 inches x 0.145 inches or 0.155 inches. Today, axial depths of 0.6 mm (0.0236 inches) x 0.110 inches or even 0.95 inches ring radial widths are commonplace. Generating less friction in the cylinder remains the endeavor.

3: Ovality, which refers to a piston’s out of roundness, is necessary in all pistons to allow for thermal expansion. The piston is narrower along the piston pin axis (the minor axis) than the thrust axis (the major axis) and the amount of ovality is determined by factors such as heat, cylinder pressures and side loadings.

PISTON OVALITY

Piston ovality is a subject not commonly discussed. By necessity, pistons are neither round nor do they have parallel sides. Instead, they have a larger diameter towards the bottom, usually between the base of the piston and the piston pin hole, and become progressively smaller toward the top of the piston. This design logic allows for greater expansion of the piston crown as this is the region closest to the heat source.

Ovality, which means out of roundness, is essential in all pistons to allow for thermal expansion. It also compensates for

4: A further form of ovality can be found in the middle of the piston skirts. A bearing area, it rides in the cylinder and is surrounded by a boundary of piston skirt that should not touch the cylinder wall to avoid friction.

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6 5 5: Aluminum billets are cut, heat treated, and machined. Those for big-block Chevy typically measure 4.750 inches, small-block Chevy and Ford 4.250 inches, Hemi 4.650 inches and 4.800 inches x 2.600 inches for Pro Stock and Pro Mod. deflection of the piston skirt caused by side loads. Pistons are manufactured with single, double, or triple ovality and the ovality is best measured by a precision roundness form measuring instrument.

Ovality in simple terms means the piston is narrower along the piston pin axis (the minor axis) than the thrust axis (the major axis). Because ovality is a function of heat (and therefore the growth the piston is likely to encounter) it is the amount of ovality assigned to

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6: For race engine builders, the advantage of the billet custom piston is its propensity to accommodate change: valve sizes, angles, compression ratios, and skirt profile . If a race team concludes their bores are too wet or too dry, they’ll intervene and change the number, size or shape of the drain holes.

a piston that’s the clever part. Ovality values differ, depending upon whether the engine is naturally aspirated, supercharged, turbocharged, or charged with nitrous oxide.

Obviously, heat, cylinder pressures and side loadings are much less in the naturally aspirated engine and hence less piston ovality is required.

Correct ovality usually manifests itself in the form of a bearing area. This is the area in the middle of the piston skirts that rides in the cylinder and it is surrounded by a

april 2016 | RPM Magazine

Gibtec’s Nick Plantus says their specialty lies in the rapid design and production of single sets of custom billet pistons.

“We have a formula for swiftness, it’s where we shine.”

boundary of piston skirt that should not touch the cylinder wall. If the boundary area does make contact it creates friction. A similar situation exists higher on the piston at the ring belt, which is the thicker area that accommodates the piston rings.

Carefully considering the myriad of shapes of today’s racing pistons is the challenge for Nick Plantus, Gibtec’s piston designer. Plantus, whose career in competition piston design stretches back to the 1970s, is often the one in charge of creating

7: Designer Nick Plantus will admit: “We produce a lot of conventional full-round piston designs, but unlike the inboard competition piston (pictured) it’s worth knowing that the full-round style contains unproductive weight without any additional stability.”

optimum ovality. “The science,” he says, “is constantly advancing—and if it didn’t there would be nothing more to learn.”

SOURCE Gibtec Pistons Denver, Colorado www.gibtecpistons.com 303.243.3340 RGiebas@GibtecPistons.com

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