Tech Speak: Forged or Cast?  Piston Basics!

 

Not too many car owners spend a lot of time thinking about the pistons in their engines.  I can't say I blame them particularly either, as most of the time there is little reason to give them a second thought.  They do their job without a lot of fuss, compressing the air/fuel charge and then thrusting down the bore to generate torque and power.  There comes a time, though, when replacing/changing the pistons becomes a necessity, when either an engine rebuild, or the search for a bucket load of extra horsepower (from an existing combination) is required.

 

At this stage there are a lot of decisions to be made.  Do you stick with the standard cast items?  Do you invest in forged pistons?  Do you use the old pistons out of the engine?  The fact of the matter is that different engine builders have their own individual theories on piston application and at what horsepower levels different piston constructions are required.  ACL - Automotive Components Limited - are without a doubt the largest manufacture of pistons in Australia, making the factory items for all of our favourite Commodore PFI engines.  As well as making the stockers, ACL also manufacture a range of performance pistons, known as the ACL Racer Series, which offer different material construction, different design features and different horsepower load handling to suit numerous applications.  it is encouraging to see that such a huge parts manufacture has identified the need to supply after market performance pistons to help us in out quest to extract the maximum power/reliability from our favourite engines.

 

This is a factory 

ACL 5.0 PFI Holden Piston

This is a ACL Race Series

5.7 high compression piston

 

CAST AND FORGED PISTONS


The majority of original equipment and aftermarket pistons are manufactured through casting.  The technical description is 'gravity die casting'.  However for the sake of simplicity, a cast piston is manufactured by pouring molten aluminium/silicon alloy into a mold.  Forged pistons differ fundamentally in manufacturing and inherent character.  As opposed to casting, the forging process basically takes a lump of billet alloy and stamps the shape of the piston from a die.  Of course, both manufacturing procedures are a lot more complex and intensive that this simple analysis, but you get the broad picture.

 

Casting and forging results in two different types of piston.  A die for forged piston must be designed so it can easily be removed and, as a result, the forged blank (or unfinished piston) has a relative simple shape.  Casting can achieve a more complex blank and, therefore, facilitate lightweight construction.  Also, due to relative manufacturing procedures, forged pistons tend to be more expensive than cast items and forged 4.9 and 5.0 litre Holden V8 pistons have been traditionally scarce, in terms of availability.  Although 5.7 litre Holden Chev units ca be sourced as off-the-shelf product, you may need to have suitable 304/308 forged pistons custom made, with a tidy price to match.

 

CAST PISTON MATERIALS


No matter which piston you use, and no matter which engine a piston goes into they are all made from a combination of both aluminium and silicon.  It is the amount of silicon though, which determines the pistons overall strength verses wear resistance properties.  Silicon also controls the rate of expansion of the piston as the material becomes hotter (the less expansion then better!).  Silicon content also markedly affects actual material hardness.  More Silicon makes the piston much easier to machine-in the manufacturing phase.  There are traces of many other metals in cast pistons, including copper, nickel, manganese and magnesium, all of these adding somewhat to the overall behaviour and strength of the piston.

 

Coming straight from the casting, this 

blank piston needs a lot of machine-work 

before it can be installed into an engine.

 

HYPEREUTECTIC AND EUTECTIC


Pretty much the catch words in cast pistons towards the end of last decade, Hypoeutectic, Eutectic and Hypereutectic are metallurgical terms which describe little more than the amount of silicon present in the piston material and the way in which it is structured (uniformly) in the piston itself.  Hypoeutectic describes a molten mixture of alloy which contains a low quantity (up to around 10 per cent) of silicon to aluminium ration where the silicon can completely dissolve.  Manufactures don't use Hypoeutectic alloy much for cast piston construction, so this is the last we'll mention of it.

 

The most common piston found in production car engines, is constructed from a Eutectic alloy.  The term Eutectic mean that the piston allow contains around 12.5% silicon.  This is just about the point of total dissolved silicon saturation.  With older piston designs which have conventional ring lands, there is little need to have any more than this simple, reliable material which has served the industry so well for so long.  In fact all the worldwide passenger car engine manufactures we know of still use the 12.5 per cent alloy.

 

Hypereutectic alloy is pretty damn similar, but has a much higher degree of silicon in its makeup, something around the 16-18 per cent mark.  What this actually achieves in the piston-manufacturing process is a high degree of free (undissolved) silicon in the end piston.  The silicon/aluminium ration affect the metal's character.  The higher silicon content in the Hypereutectic alloy lends itself to improved scuff resistance and, importantly, a relatively low expansion rate.

 

Early in the peace it was common for US forged-piston materials to contain only 7 per cent silicon in their make-up.  This led to high temperature expansion rates, making engines sound 'rattly' when started cold.  The Germans came to the rescue here with new materials for forged pistons which did away with the undesirable expansion rates, by containing a much more acceptable 14 per cent silicon content.  As some top-level German cars have forged pistons standard (BMW V8, V12 ect) it was a requirement that these engines remained smooth and quiet, even when cold - hence the development of better materials.  Now, a lot of the aftermarket forged-piston manufactures have caught on and are offering the 'quieter' high silicon content alloy.

 

With Hypereutectic pistons, the primary reason for having all of this free silicon is to reduce piston ring groove wear.  This allows piston designers to move the top compression ring much farther up the side of the piston (where combustion temperatures are much hotter), and run much smaller, thinner piston ring lands (the metal section separating the ring grooves).  The reason piston designers want to do this is that it allows a lighter piston to be produced, and also has dramatic results in changing engine emission characteristics.  As emission laws become tougher, it will be commonplace to find true Hypereutectic pistons in road engines.  It must be added though, that Hypereutectic pistons are not automatically stronger than conventional Eutectics.  Their main advantages being reduced chance of ring/grove welding and reduced piston ring grove wear.  If higher piston strength is needed, then generally, a piston manufacturer will ass more copper and nickel to the alloy to gain extra high temperature strength.

 

All metallurgy aside, it is not so much what material your pistons are made from but their physical design that is will determine ultimate durability and whether they are going to break/seize.

 

The crown shape varies in 

some piston designs for 

compression, combustion 

and valve clearance reasons.

 

CAST VERSES FORGED


This is an age-old problem for engine designers.  At what sort of power level is it necessary to change from a conventional cast Eutectic/Hypereutectic piston to a forged item?  According to ACL's chief piston engineer, the only real disadvantage of a cast piston (in high output situations) is in the case of a piston failure, a cast items is more likely to shatter and damage the engine, as a whole, more than a forged piston.

 

A big advantage with forged pistons is they generally result in a more ductile material, with the effect being the piston can take a higher level of detonation before failing.  As far as I am concerned, this is not such a huge bonus as you engine should be tuned not to detonate in the first place.  In extremely high rpm/high horsepower applications, the great strength of the forged piston can add reliability, with ACL recommending they be used once power levels rise past about 80hp per litre of engine capacity.  This gives us around 450hp in a 5-litre Commodore before even thinking of changing to a forged piston.

 

Flat-top pistons are named 

thus for one very significant reason

 

CONCLUSION


All of ACL's performance orientated Race Series V8 pistons include some tricky design technology such as integrated oil drain back slots and a pressure balancer groove in the top ring land to help reduce ring land breakage, in the case of detonation.  In the instance if building a mildly supercharged 5 litre V8, or pushing for 450hp out of a normally aspirated engine, the is probable little advantage in investing large sums of money in forged pistons, when a correctly-tuned engine with either standard Eutectic items or modern design Hypereutectic (ACL Race Series or similar) pistons will give the same power output and reliability levels.

 

Thankfully, with new piston

design technologies like this

 Racing piston, cracking piston

skirts should no longer be a problem.