No life (a.k.a. DattoMaster)  
Joined:
2008/10/10 22:02
From Melbourne Australia (and likely under the car)
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It's probably worth adding that not all cast pistons are alike. There's some genuinely brilliantly made cast ones, and then there are some that are a dodgy design (look around the oil ring, if it has small drilled holes or many tiny slots it's strongish - if it has long (1/4 the circumference or more) slots in it, don't attempt to run it at high rpm. You also have to look at the design around the gudgeon pin, some clearly offer far more support than others. The other place is crown thickness, thicker is better able to cope with heat, though it might go hand in hand with a slightly lower top ring placement on the piston. This might cost a hp or two, but the gains in life expectancy/resilience are imho worth it for anything short of a dedicated race setup where you plan to rebuild/freshen every season or something. The worst pistons of all have a 'split skirt' where there is a vertical slot cut into the piston skirt from the very bottom up to perhaps 5-10mm from the bottom of the oil ring. This lets the skirts flex a little and tends to run quiet in a std engine with very modest rpm limits. They are absolutely the worst thing out for a performance engine, and a sure bet they'll fail. I haven't seen any pistons for the a series that feature them, but I'm mentioning for the sake so all criteria can be checked out on a potential piston purchase.
It's also fair to say that the a-series bore and stroke aren't 'massive' by big US v8 standards, so the overall weight, and stresses at a particular rpm range are generally a lot lower than for a bigger engine, so relatively, you can get away without having an absolutely bulletproof piston.
FWIW, I've run ACL cast pistons in some fairly high rpm and high boost situations and I tend to really like em. They won't beat a forged for outright strength, but as mentioned, not many people run streeters hard enough to need a forged piston. The other things forgies are great for is boost. Even if you don't get detonation, the amount of air/fuel burning above the piston will heat it up, again, without detonation or pre-ignition, the heat can overwhelm the ability of the piston crown/skirts to conduct heat away and literally start to melt (which ironically htne leads to detonation/preignition, and finishes 'the job' destroying the piston). It's a phenomenon known as 'heat soak' and a forged piston will cope with a lot more heat energy (which isn't necessarily a higher peak temp, just much more air/fuel at hte same temp, more heat energy total to get rid of.)
Last tid-bit of info - obviously check piston to bore clearance, but very important - check the ring end gaps. On a highly stressed engine, the rings expand, and if the gap is too small, the ends of each end of the ring will expand and touch, and seize , and they they are the actual cause of a piston breaking apart. The ring gap is a very tiny (in total area, compared to the total area of the piston crown itself) thing, and stuff all compression/combustion pressure ever escapes. you could _literally_ run double the factory spec ring end gap and maybe lose 1% power over an 'ideal' ring gap on a moderately upgraded engine. People sometimes lose sleep thinking if the gap is too large, it'll bleed off and cost power, don't worry about it, 2-4thou extra gap over standard is no concern power wise, and a _hell_ of a lot cheaper than a ruined engine from a ring seizure and the avalance of damage it'll do before it's shut down.
Posted on: 2009/12/11 21:02
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