High Performance Piston Options - Engine Builder Magazine

High Performance Piston Options

Ask the kid on the street what performance is and he’ll raise the hood of his Honda. Ask the Funny Car drag racer about performance and he’ll take you to the starting line to breathe the nitromethane. Ask the mother on vacation and she’ll tell you about driving to California from Indiana pulling a car BEHIND the motorhome through the Rockies in the western United States.

Ask about selecting the proper piston for each of these applications and you’ll probably receive just as many different answers.

While the basic operation of an internal combustion engine hasn’t changed significantly in more than 100 years, the uses (and misuses) of said engine certainly have – especially with regard to high performance applications. Piston manufacturers in particular say that while today’s performance choices are exciting, they also mean selecting a piston is a challenging task. Just as you can’t paint the performance picture with a broad brush, piston selection criteria have become so challenging – yet so important – that many engine builders are understandably confused by the options.

Bake The Pie Higher

Sport compact, Detroit Iron, Saturday night claimers, pure drag racing, classic and restoration – each of these is a piece of the performance pie and of course, there are many other slices out there. And based on recent activity, that pie may actually be getting bigger.

“Based on our experiences at the PRI show, the market for performance seems to be improving,” says John Levis, Wiseco Pistons. “In 2002, people just seemed to be window shopping. But this year, there was a ‘buzz’ in the convention center – people were actively looking at engine combinations they could build.”

Levis says his performance engine building customers have all told him the same thing: since the beginning of this year, things have been extremely busy.

Levis and others say much of the buzz is in the sport compact performance market.

“We saw very gradual growth in this arena up to about four years ago,” says Jeff Ginter, JE Pistons. “Then, it just went crazy. We started seeing the import racing associations and events which certainly helped push the excitement. But we also saw something else – the advent of import-specific magazines.”

These magazines – as well as the Internet, cable television and a variety of computer and video driving games – featuring brightly colored cars and beautiful models particularly appealed to the youth of our country, igniting a wave of automotive enthusiasm that has swept up more than 30 million people nationwide and accounts for more than $2.5 billion in annual sales.

“We had lost a whole generation of performance enthusiasts,” explains Moe Mills, Ross Racing Pistons. “Back in the mid-’60s, when I was a kid, I’d go to the drag races when I wasn’t working. A lot of us did – we were all the young guys hanging around asking questions of the older guys with their sprint cars and top fuel rails.”

It was during the 1970s, Mills recalls, that America’s youth “turned its back on performance. You’d go to the drag races and see those same guys from years before still hanging around asking questions…but we were all older. We started to worry that the business would soon be gone because we were all going to die and no one would be around to replace us.

“The sport compact performance market has breathed life back into the automotive industry,” Mills says. “There may be a lot of people who say they’re against those cars, but I think it’s a good thing. It gives this industry a good chance for continued growth.”

Honda is generally accepted as having the lion’s share of the sport compact performance market right now. The VTEC engine (like that built and auctioned by AERA’s Vanguard Group at last year’s AERA Expo) is sometimes called the small block Chevy of this generation. But some piston manufacturers aren’t so quick to crown the Honda king.

“When you look at the magazines, it’s Honda at the top. But actually, when you look at who’s doing the real work, it’s not the Honda owners who are ringing our – or the engine builders’ – cash registers,” says Mills.

He says the second most popular engine (and most popular import) his company sells pistons for is the Mitsubishi 4G63 2.0L engine found in the Eclipse and Talon. The reason? “That engine came from the factory with a turbo. Racers can keep upping the boost on it until they blow the stock cast pistons out of it. Mitsubishi may have less than one percent of the market but it’s still our most popular stock line of import pistons.”

Although Mills says the industry may characterize many Honda owners as guys who think performance is a set of bright purple spark plug wires and a big wing on the back, he acknowledges that there is some growth in the sale of pistons for these engines thanks to one phrase made infamous in The Fast and The Furious: “I need Nos! I need Nos.”

“The growth we’re seeing in the Honda market is because people have started bolting on nitrous oxide bottles. At that point, they can do some serious engine damage. They’ll need pistons, they’ll need to get the block bored out…good news for engine builders.”

It isn’t only the import market that has seen a surge of enthusiasm lately. The domestic performance market has grown to include Fords, engines that, until recently, say suppliers, had limited aftermarket applications. New cylinder heads and crankshafts developed for the 5.0L Mustang have opened up performance possibilities to engine builders. And “Blue Oval” performance enthusiasts with less interest in building their own powerplants can now get Ford crate motors. “We have been supplying the pistons for Ford’s crate motor program,” says Chris Huff, Probe Industries. “They were experiencing some piston failures that made forged pistons look more attractive.”

To some enthusiasts, “performance” is real old-school. The restoration market is booming, says Steve Markley at Egge. Pistons for such classics as the Nailhead Buick, Rocket Olds, Hemis and 409s are available to bring new life to these old powerplants.

With all of the options, and with racers of all kinds trying to pull as much power from their engines as possible piston selection is critical. But piston manufacturers caution that knowing what’s right for your customers’ needs is not always an easy business.

Knowledge Base

“You almost need to start comparing some of the new sport compact engines to motorcycles engines,” says Russ Hayes, Federal-Mogul. “Today, several sport motorcycles develop astounding amounts of horsepower per liter. You’re seeing some naturally aspirated bike engines with 120-150+ hp per liter when measured at the rear wheel. Now, some of these import guys are winding their engines to 7,500-8,500 before shifting – and these short stroke engines can make similar power.”

Hayes says engine builders may find it difficult to choose components when their customers don’t really know what they need. “We find that the most guys on the strip understand what it takes to generate and transfer that power – the guys on the street may not.”

Some street enthusiasts think they know more than they actually do, but some of their confusion is understandable: is a forged piston always better than a cast piston? When is a hypereutectic piston the best choice? Was the piston designed as only a service replacement or specifically for performance? How does compression ratio, cam selection, and cylinder head choices impact your piston selection? Will the engine be supercharged, turbo-charged, or nitrous injected? If so, how much?

“One thing that is different from the last great car culture is that some of the participants don’t have the knowledge base that older machine shops and engine builders had back then,” says Wiseco’s Levis. “We have found ourselves doing more educating to the customers. ‘Replacement’ performance can be selected from a catalog. Upgrading the performance means things other than model year and bore size need to be considered.”

Where yesterday’s performance enthusiasts might have learned tricks and gotten ideas while hanging around the local speed shop, today’s speed junkies may be turning to another source – Internet chat rooms and discussion forums. Engine Builder readers share stories of potential customers who have created the “perfect engine,” with specs cobbled together from many different sources they found while “surfing the ‘net.” Individually, the components and specifications may be fine – collectively they are often laughable at best, dangerous at worst.

Discussion Groups

If you want to start a rousing debate, forget politics, religion or Super Bowl halftime wardrobe malfunctions: simply start talking abo

According to Mahle Motorsports, the Mahle Brothers revolutionized the piston industry when they created the first aluminum alloy piston in 1921. Because of their innovation, they were sought out by racers of the day.

Today, a variety of aluminum alloys are used to make pistons. The alloy used not only determines a piston’s strength and wear characteristics but also its thermal expansion characteristics. That aluminum is then cast (melted, poured into a mold and formed into a piston) or forged (formed into bar or slug stock and then machined into a piston). Most custom-made pistons for high performance uses are forged to a “near net shape,” usually machined using CNC equipment to the final result.

While it’s tempting to look at pistons from a “good, better, best” standpoint, some piston makers caution against doing that.

Forged pistons are recognized as the absolute strongest products available, designed for ultra-high performance applications, but hypereutectic pistons have an important place in many levels of performance engines.

“In addition to our forged products, we offer a variety of pistons designed to meet a customer’s needs and his wallet,” says KB Pistons’ Sulprizio. “Sometimes a claimer engine just needs a reasonable product at a reasonable price that does what it’s supposed to do.”

Federal-Mogul’s Hayes says that if you’re trying to use these high output small engines with substantial nitrous under the hood of a sport compact car, forged pistons are probably your best bet. Federal Mogul is developing a line of upper-end performance forgings that will also include import engines. However, he says don’t count out hypereutectic pistons when they’re appropriate. Years ago, when NASCAR ran open class engines, we tested some hypereutectic pistons in 750 hp engines running around 8,300 rpms,” Hayes recalls. “The pistons gave some definite advantages over some of the best forgings used at the time, including high heat strength, better endurance, no scuffing. We probably came away with 22-25 more hp due to the lighter weight and tighter bore clearances. We could have installed thinner ring packs, which would further reduce weight and friction. Overall this means less blow-by, more efficiency, and thus more horsepower.”

Sulprizio, too, explains that, when appropriate, hypereutectic pistons can be a fine choice. “We have a customer who has been a regular IMCA champion – he has run hypers forever. Properly managed, an engine may have several options.”

Getting To Know You

So if there are so many options, how do you choose the right one for your particular customer? Even if you don’t have decades of experience, piston manufacturers say technical support is as close as a phone call or a Web site.

“Our customers’ customers tend to want to overcam and ‘over-compression’ the engine it,” says Mills from Ross. “If an engine builder gives what the customer thinks he wants but it ends up blowing up, it’s going to come back on the builder. The resources are out there: you just need to take advantage of them. All of the parts manufacturers are there to help.”

The quality of the parts is higher now than it’s ever been, meaning engines can do more than they ever did before, says JE’s Ginter. “Customers are seeing this in better consistency and repeatability. There’s been staggering growth in technology.”

Yet, says Federal-Mogul’s Hayes, despite these advances, proper attention to detail is more important than ever. “We still hear from people who think they should use the biggest camshaft they can or go for the highest compression, but fail to complement the total engine package as needed. If you’re not doing the combination correctly, you won’t end up with more power – you’ll end up with a time bomb.”If you have doubts about how the components in your next performance engine should work together, including the pistons, contact the experts. Your supplier can discuss things you may never have considered, answer questions you never asked and give you details you might not have known you needed.

The alloy from which a piston is made not only determines its strength and wear characteristics, but also its thermal expansion characteristics. Hotter engines require more stable alloys to maintain close tolerances without scuffing.

Many pistons used to be made from “hypoeutectic” aluminum alloys like SAE 332 which contains 8-1/2 to 10-1/2 percent silicone. Today we see more “eutectic” alloy pistons which have 11 to 12 percent silicone, and “hypereutectic” alloys that have 12-1/2 to over 16 percent silicone.

Silicone improves high heat strength and reduces the coefficient of expansion so tighter tolerances can be held as temperatures change. Hypereutectic pistons have a coefficient of thermal expansion that is about 15 percent less than that for standard F-132 alloy pistons. Because of this, the pistons can be installed with a much tighter fit – up to .0005? less clearance may be needed depending on the application.

Hypereutectic alloys are also slightly lighter (about 2 percent) than standard alloys. But the castings are often made thinner because the alloy is stronger, resulting in a net reduction of up to 10 percent in the piston’s total weight.
Hypereutectic alloys are more difficult to cast because the silicon must be kept evenly dispersed throughout the aluminum as the metal cools. Particle size must also be carefully controlled so the piston does not become brittle or develop hard spots making it difficult to machine. Some pistons also receive a special heat treatment to further modify and improve the grain structure for added strength and durability. A “T-6” heat treatment, which is often used on performance pistons, increases strength up to 30 percent.

Machining hypereutectic pistons is also more difficult because of the harder alloy. Consequently, hypereutectic pistons typically cost several dollars more than standard alloy pistons. That’s why most OEMs (except Ford) have gone back to eutectic alloy pistons in their late model engines. High copper eutectic alloys offer most of the advantages of hypereutectic alloys without as much cost.

Forged pistons will generally come from one of two aluminum alloys: SAE 4032 and 2618. A 4032 alloy is high silicon and usually used in naturally aspirated engines, while a 2618 alloy is a low silicon material designed for the abuse of marine, supercharged and nitrous applications. A 2618 alloy is about 3 percent heavier and doesn’t have as good lubricity qualities as the 4032 because of the lower silicon content. A 2618 alloy will also expand more than 4032 because of the denser material. Because of this difference in expansion, the manufacturer will design the necessary clearances into the piston; the engine builder will not normally need to change recommended running clearances.

The 4032 composition will be what you find in virtually all circle track-type applications as well as street applications in which a forged piston is desired. The 4032 can be made as strong as 2618; it would just need to be thicker. The melting point of these alloys is about the same, so that wouldn’t be a consideration.

The most important factor in piston choice is application. What do you need the piston to do? Street and drag race applications are easy on pistons, circle track and road race applications are somewhat tougher, while marine, supercharged and nitrous engine applications will produce the toughest environments.

A related factor to consider is how often do you want to inspect and/or change the pistons? Do you want the pistons to last a full season or even several seasons? Or do you want the lightest pistons possible and plan to change them after every few races?

Jim Walbolt and Larry Carley

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