Forged Pistons v cast ones & stresses

Crank, rods, sealing, pistons, block, flywheel etc
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Guy Croft
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Forged Pistons v cast ones & stresses

Post by Guy Croft » Sat Jul 22, 2006 5:37 pm

A forged piston is made by stamping the material in a forging press. A cast piston is made form liquid alloy forced at high pressure into a precision steel mould. Forging alloys and casting alloys are not the same.

The forging process physically squeezes and deforms the material so as to produce grain flow patterns in the material that suit the stresses on the material in service whereas the grain structure in casting has no distinct pattern in any direction. Consequently the forging process gives a far stronger piston. This is important in race engines because the pin boss, ring lands and crown are potentially weak zones. Forging alloys have a much higher fatigue resistance as well, in other words they will withstand far more load 'cycles' at a given stress level.

If the accelerative loads around tdc are high enough the cast piston will break up, usually at the pin boss region, but sometimes, depending on the local stress, between the upper part of the pin boss and the crown. This can happen almost instantly from gross overload at too-high rpm or from fatigue - over a period of some hours use. You have to know the limits. I do.

Many cast pistons have cast-in steel struts for quiet running, again, this is an intrusion into the homogeneity of the material and a major weak point. To put it simply - forging gives strength in the right places. This reduces the 'k factor' - a compensation for stress concentrations that must be included in any engine stress evluation and which makes the piston weaker than simple load calculations would indicate

The Excel data (from actual parts) below shows that the RR58 forging alloy material is not only inherently stronger at working temperature but it can withstand far higher engine speeds without overload. Even at a low 6800 the cast item is well on the way to failure. And they would break up in this engine if I used them. My forged piston design is nowhere near failure even 7800 rev/min.

A forged piston can be lighter in some areas than a cast one needs to be to achieve the required strength and it is possible, because they are made in small batches, to design the strength 'where it is needed' and leave other less critical regions light. For example it is possible to arch the under-crown region to distribute the bending stresses evenly, and make the pin boss region stronger. A forged piston is only as good as the design and manufacture. I have seen a lot of forged pistons that are massively made, no attempt at lightweight design - way too heavy everwhere.

I do stress computations on many of my pistons, check the stress and fatigue data, match the characteristics against know failure modes and try and design them with minimum weight in mind.
Attachments
GC forged 8V TC pistons, medium and high compression dome.JPG
Two of many of my forged piston designs, by CP in California, left - medium high comp dome and right, high comp, would not use bigger on the 8v, and CR does depend on bore size.
GC forged 8V TC pistons, medium and high compression dome.JPG (104.9 KiB) Viewed 10303 times
GC 8V TC forged piston underhead milling.JPG
I do piston design on CAD (computer design) and specify the underhead thickness required in all regions, CP execute super-accurate computer milling to achieve this. Thus expensive hollow-dome forgings are not needed.
GC 8V TC forged piston underhead milling.JPG (115.61 KiB) Viewed 10294 times
GC 8V TC piston set 84.50mm bore.JPG
ALWAYS check availabilty of rings for your bore before ordering pistons and reboring, here CP race pistons with lightweight CP race rings. Note GC copyright 'offset valve reliefs' on 1 & 4 - huge VRs not needed on my pistons.
GC 8V TC piston set 84.50mm bore.JPG (151.6 KiB) Viewed 10288 times
HSR 2.3 pistons forged & cast.JPG
Comparison of weights between forged, left and cast. The forged item is far, far stronger and still much lighter.
HSR 2.3 pistons forged & cast.JPG (141.23 KiB) Viewed 10176 times
2.3 HSR piston stresses cast v forged.JPG
Piston stresses at 7800 on Vauxhall 2.3L engine, LM13 alloy cast item (left) and GC RR58 race piston (right).
2.3 HSR piston stresses cast v forged.JPG (185.24 KiB) Viewed 9638 times

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