Selecting new engine components for a rebuild can be a daunting task, with an endless number of brands and options to choose from. The parts you will need are dependent on a number of variables, which means there really is no one-size-fits-all answer for you. Over the years I have seen countless customers who have ended up with an engine combination that is either complete overkill for their application or, alternatively, is just not up to the required task. While in the first instance the overkill engine is only likely to leave a hole in your wallet, a combination that isn’t up to the task could just as easily leave a hole in your block.
This month we will take an in-depth look at pistons. We’ll discuss the pros and cons of cast versus forged pistons, and separate some of the facts from the fiction so you can make an informed decision about which pistons to select for your next build.
Cast pistons still remain the components of choice for production engine manufacturers. There are a number of advantages to the cast piston, however, it should be no surprise that their low cost is the reason they are favoured. Cast pistons also exhibit a very low rate of thermal expansion, which means they don’t expand much as the engine heats up. This allows the clearance between the piston and bore to be set very tight, which results in a very quiet engine.
Cast pistons are manufactured by pouring molten aluminium into a mould and allowing it to cool. This produces the approximate shape of the piston, after which several machining processes are performed to finalise the part ready for installation. The drawback with the casting process is that the resulting piston has a very low density of aluminium, and there’s no control over the grain structure in the finished part. This results in a piston which is very brittle and relatively weak. In particular, a cast piston cannot cope well with even light levels of detonation, the result of which will often be cracked ring lands and a loss of compression.
It would be unfair to write off cast pistons completely, as they have seen some huge improvements over the years, and some of the current offerings are genuinely excellent products. While they are not suited for a highly stressed nitrous or blown engine, a cast piston can provide a very low cost solution to a mild performance engine build.
Forged pistons have been the choice of race engine builders for several decades, and in more recent times the technology of these pistons has improved and the cost has reduced dramatically, making them a serious option even for our street engines.
A forged piston is manufactured by forcing a die into a hot ingot of aluminium under immense pressure (2000 tonnes). The heat and pressure force the aluminium into the shape of the piston, and results in two important advantages: a forged piston has a higher density of aluminium when compared to a cast piston, and it also exhibits a uniform grain structure.
It is these aspects of the forged piston that increase its strength.
One of the drawbacks with forged pistons is they have a higher rate of thermal expansion than a cast piston. This means they need to be installed with more clearance between the piston and bore than an identical piston manufactured by the casting process. The end result of this is that a forged piston can be noisy, or ‘rattle’ in the bore when it is cold.
During this warm-up period, the piston-to-bore clearance is excessive, and that can result in increased blow-by of combustion gases past the piston rings, as well as increased emissions.
Due to the reduced support of the piston, a forged piston will also tend to wear more during cold operation than a cast piston.
Since nobody likes noisy pistons, aftermarket piston manufacturers have put a lot of energy into developing forgings that exhibit all the advantages of a cast piston while retaining the strength of a forging. Manufacturers found that adding silicon to the aluminium alloy prior to forging helps control the rate of thermal expansion and allowed the pistons to be fitted with tighter clearances.
The addition of silicon to the alloy helps in two ways. First, it has a lower rate of thermal expansion than aluminium; second it is harder, which tends to help reduce wear of the relatively soft aluminium against the bore wall.
There are no free lunches though, and the downside of adding silicon is that the piston becomes more brittle and some of its strength advantage is lost, so it’s a delicate balancing act deciding how much silicon to add.
While forged pistons can be constructed from a number of materials, development has seen two different aluminium alloys emerge as the preferred materials for manufacture. Both alloys have advantages and disadvantages, and they’re suited to different purposes.
4032 Aluminium Alloy
The 4032 aluminium alloy contains a silicon content of around 11 to 12 percent, and exhibits similar thermal expansion properties to a cast piston. This allows the clearance between the piston and bore to be set quite tight, and results in a very quiet motor when cold. The high silicon content also results in a very hard piston that is resistant to scuffing and wear on the skirts.
A 4032 forged piston will give a significant improvement in strength and resistance to knock when compared to a cast piston. However, the high silicon content ultimately produces a piston that is more brittle and fragile than a piston forged from 2618 alloy, and hence it’s not as strong.
These 4032 pistons can be a perfect option for a performance street motor, which is likely to see a lot of use without the need for frequent rebuilds.
In a properly assembled and tuned engine, a 4032 piston is still capable of handling power adders such as blowers, turbos or nitrous, and it can provide great reliability.
2618 Aluminium Alloy
The 2618 aluminium alloy contains silicon at less than two percent. The low silicon content requires additional cold clearance between the piston and the bore when compared to a 4032 forging, but advances in both material and piston design mean these forgings can still offer quiet cold start performance.
The advantage with the 2618 alloy is the additional strength that is provided by the lower silicon content. These pistons are the best choice for very high rpm, high compression, naturally aspirated engines, as well as the likes of 5200 to 5900kW Top Fuel drag engines.
These 2618 pistons are the perfect choice for a serious street engine, as well as all-out competition engines. Their strength makes them the perfect option for applications subjected to very high specific power levels, and they are able to withstand high levels of cylinder pressure with ease. A 2618 forging will even be able to withstand occasional detonation without cracking.
Which To Choose?
Unfortunately there are no solid answers as to which piston is best for any particular application, and your choice should be made in consultation with your engine builder. It should be apparent that in some instances more than one type of piston may suit your requirements, and hopefully now you will be able to make a more informed decision about the advantages and disadvantages of each option.
Remember, though, that a forged piston is not a fix for a poorly tuned engine, and the strongest piston in the world will not last long in an engine that is suffering from severe detonation or a lean mixture.
Words: Andre Simon
This article is from NZV8 issue 63. Click here to check it out.