Boost Vs. Juice: Designing A Piston For Nitrous Oxide

March 13, 2018 / by Paul Huizenga

Nitrous gets a rap for being hard on parts, but with the right engineering, a nitrous engine can be a tough, long-lasting performer. Learn what it takes to create a piston optimized to survive and thrive on the bottle.

Thanksgiving dinner at the Power Adder family table is always awkward. No matter how much the three kids have accomplished over the past year, the conversation always turns to how Blower is a bit of a parasite, and Turbo keeps putting too much pressure on everyone. But the real black sheep is always Nitrous – too cheap, and always wrecking things when he gets a little greedy.

Nitrous gets a bad rep not because of its inherent flaws, but because it’s such an ‘easy’ path to power - or at least it seems. You’d never dream of throwing an 8-71 or a pair of 88mm turbos on an engine with stock internals, and yet the ease of slipping a little bit bigger jets into a nitrous kit often tempts even the most level-headed racer or performance enthusiast who should know better.

Nitrous oxide has a reputation for eating pistons, but with the right tune and the right parts, engines can live long, healthy lives on the bottle.

With the right parts and the right attitude though, an engine built for spray can live a long, healthy, and happy life, and the recipe starts with the part that sees the most abuse on the bottle – the piston. To find out the specifics of what a slug designed especially for use with nitrous oxide are, we sat down with Nick DiBlasi at Wiseco Pistons.

“A nitrous engine is a very volatile animal. There are extreme loads within an instant, and every part of the piston is designed with that in mind,” he begins. One important aspect of piston selection depends on whether it’s destined for a street/strip engine that will spend most of its life running naturally-aspirated, or if it’s slated for a race build that will see nitrous oxide applied more or less all the time.

Defining the Problem

“It all starts with completely understanding the customer’s needs,” DiBlasi continues. “While we have off-the-shelf pistons that are designed for nitrous, there are many customers who are going above and beyond what we call standard. For those customers, we build something custom, exactly to their needs.” That crossover point depends not only on horsepower, but the intended application as well.

Because of the additional heat and pressure generated by nitrous oxide, a typical piston designed for that application will move the ring package downward, away from the crown, to protect the top ring while still providing good sealing.

Per DiBlasi, “If the customer is requesting to put nitrous on a part that was never intended for nitrous, then we will have to custom make it. Most of the off-the-shelf parts are intended for naturally aspirated use, so we will have to convert them to a custom piston as it’s a unique situation. Also, if we have a shelf part designed for nitrous where the majority of the market uses a 200-300 shot, but someone wants to use 600 shot, then we will most likely move them to something custom. There is no exact formula, as every application and duty cycle is different. We have seen drift cars use nitrous a lot lately and will want to move those to custom pistons, as the duty cycle is much longer than of a drag racing engine of the same specs.”

Step one in laying out the specifications for a custom nitrous piston is ensuring that the customer isn’t in the market for the proverbial one-ended stick. DiBlasi says, “Based on the application, duty cycle, and other engine components, we will design our pistons to survive the conditions presented, but there are many cases where the customer is asking for something that simply defies physics. One of them that comes to mind is putting a 4-inch stroke in a Small Block Chevy with a 6-inch rod. This makes a compression height of 1.000” and simply does not give enough room for the ring lands to live.” 

Nitrous Piston Design Considerations

Once the impossible is discarded, what remains is to optimize the piston design for durability and power. Nitrous oxide applies cylinder pressures and temperatures very differently from a naturally aspirated or boosted engine, requiring a matching approach to things like where the ring package is located relative to the crown, the thickness of the crown material, and even the construction of the skirt and pin boss.

“In the design process we will adjust the ring thicknesses, materials, and lands to match the loads required,” says DiBlasi. “The crown thickness and taper angle of the top land will also be configured for the specific application. The proper pin thickness and length are designed to handle the demands of the engine while making sure the pin bosses have enough pin engagement material.”

Another characteristic of a nitrous piston design (right) is a thicker-than-usual crown to act as a heat sink, as well as special attention to the material thickness around areas like valve relief pockets. The piston on the left is a max-effort NA design with a much shorter top ring land.

The location of the top ring becomes a compromise; while modern factory engine designs tend to place the primary compression ring as close to the crown as possible to reduce crevice volume in order to help emissions, this is less than ideal in a nitrous engine. “The further the top ring is moved down, the more it is protected,” DiBlasi explains. But because a proper ring seal depends on the gas pressure applied to it, he adds, “Moving it too far down will cause a reduction of power and sealing ability.” 

One added design feature that’s common in high-performance naturally aspirated pistons is also useful for nitrous - per DiBlasi, “In racing applications, gas ports can do wonders for sealing. It’s not much different from boosted or high performance N/A. The only thing to watch out for is the premature wearing of rings due to the increased pressure that the face of the ring sees. Modern rings last a very long time, so it’s less of a factor with stainless and carbon steel nitride rings.”

Handling the Heat

Besides providing a compression seal, the top rings also play a critical role in transferring heat from the body of the piston to the cylinder wall, and with the added energy in a nitrous application, this becomes even more important. Both ring material and the ring gap need to be specified with spray in mind.

According to DiBlasi, “We have various ring materials that are popular. If we are using a 1/16 ring, we have our hardened nitrous series of rings that are available for any shelf or custom piston. We suggest a hardened nitrous series ring, carbon steel nitride ring, or a stainless steel ring for best longevity.”

For durability, the design of the pin bores and supporting piston structure is critical. Higher cylinder pressures require additional reinforcement, and the entire piston needs higher rigidity to keep it cylindrical and help the rings maintain a good seal.

While the tendency in recent years for both racing and factory engines has been to run tighter end gaps for better efficiency, special care must be taken for nitrous powerplants. DiBlasi explains, “Each application and duty cycle will determine the ring end gaps. We include a guideline of instructions that walk our customers and engine builders through some math based on their bore, application, duty cycle, and a few other variables. The principle is the more heat, the more end gap you want to start with, so when everything is at its full operating temperature, nothing binds. When the rings end up running into each other, they lock in the cylinder bore and start yanking the top land from the rest of the piston. It results in over-heating the piston, detonation, and total piston failure very quickly. Incorrect ring end gap is probably in the top five reasons why pistons fail.”

Another recent trend in piston design has been to move the oil ring down to the point where the groove actually intersects the pin bore. This allows for shorter overall piston height, more room to put the ring package farther down from the crown, or a combination of the two. But is it a good idea for a piston designed for nitrous duty? DiBlasi says yes; “We have great success using oil rail supports to allow the pin to go into the oil ring. Providing the ring lands, crown thickness, and ring material is designed for the application, there should be no difference to the performance if using a rail support or not.”

Short piston heights combined with a lowered ring package can move the oil ring groove into the pin bore area, but with a proper support ring, this isn't a drawback.

Additional Support

The additional heat and higher cylinder pressure generated by nitrous applications also drive the shape of the piston in other ways, with DiBlasi noting, “Crown thickness, pin length, and pin thickness are all critical to nitrous engines.” The design of the crown must take into account how heat will transfer, especially in regions like the valve pockets where hard edges and thin spots can create potential detonation sources. Overall, the trade off between low piston weight and having a sufficient ‘heat sink’ favors the latter factor in a nitrous application.

“Additionally, we want to make sure that the pin is supported as much as possible by widening the pin boss towers,” DiBlasi adds. “This creates the least amount of unsupported area from the pin towers to the rod, limiting flexing.” Obviously, keeping the piston as cylindrical as possible helps the ring seal, as well as reducing friction and wear and promoting heat transfer from the piston to the cylinder wall. 

There are some finishing touches that Wiseco can apply as well, improving the performance, detonation resistance, and service life of a nitrous engine’s pistons. Starting at the top, the right treatment can give piston tops an additional layer of defense against the increased combustion heat load. “We suggest our ceramic piston crown coating for any forced induction or nitrous applications,” DiBlasi recommends. “This will help deflect the heat off the crown of the piston and give it longer life. Aluminum can only go through so many heat cycles, as they lower its hardness level. You can fight this by removing the heat that anneals the pistons.”

The right high-tech coating can help reject heat from the piston crown and improve both performance and longevity. Today's coating processes like Wiseco's ArmorPlating are engineered specifically for extreme environments.

A Choice of Coatings

Wiseco’s proprietary ArmorPlating process, which protects the crown, ring grooves, and pin bores, actually becomes harder when exposed to heat. It’s a metal-based auto catalytic plating that has the precision to accurately cover features as small as gas ports without impeding their function, and it will actually help prevent carbon fouling of the piston top, which eliminates a potential source of detonation, regardless of what kind of engine it’s being used in.

In addition to the tops, there’s no reason to miss out on the performance benefits of a coated skirt on a nitrous application either. DiBlasi adds, “Skirt coatings also help with friction reduction and controlling piston-to-wall clearance.” It’s an inexpensive ‘premium’ feature that’s become common on shelf stock parts, not just custom slugs.

While nitrous has the reputation of being hungry for pistons, we’ve learned that by specifying the right parts that are designed for the unique thermal and physical stresses spray imposes, it’s not impossible to put together a durable, powerful, nitrous combination. Just like any other power adder (or even a highly-strung naturally aspirated engine), the secret lies in knowing the limits of the components you’re using, and upgrading where necessary to handle the extra stress.

Topics: Tech, ENGINE TECH, PISTONS 101, featured

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Written by Paul Huizenga

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