This does of course depend on design, material, maximum combustion pressure and desired lifetime but in general the piston crown should have a thickness of at least 7-10% of the piston diameter.
You see the pistons are a set of original 320i BMW ones from an engine in my garage. I am turbocharging it, but i am not intending on buying fancy pistons. I am going to turbocharge the with a lot of stock components, albeit modified ones.
The pistons have just come back from a workshop where i have has 1mm skimmed from the top. I wont be running massive boost, but it will be forced! The point is i was wondering whether to have these ceramic coated, or just see how they do?
I tend to work like that, a bit of trial and error. I will take some measurements now!
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Depends on what you mean with ceramic coating. Is you mean a thermal barrier coating I recommend against these based on the results Cosworth got when they tested this. Real TBC's that actually works (not the ceramic powder/matrix coatings you can buy from several small manufacturers), similar to those found in jet engines are very fragile, they also have a different ratio of expansion from the substrate material (the aluminium alloy in the piston). Another problem is that you lose some cooling of the end gases during combustion, this means that there is an increased risk for detonations. I recommend that you use oil cooling of the pistons instead to reduce their temperatures.
There are also ceramics used to reduce wear. On a turbocharged engine I can recommend the use of for example hard anodized ring grooves (unless your piston has cast in steel groves). I also recommend the use of nitrided steel rings, conventional iron rings loose their tension too fast in a turbo engine and moly coated rings have also been showed to be unsuitable for turbo applications.
Piston diameter = 79.5mm
Piston height (top of crown to bottom of skirt) = 61mm
Piston height (inside measurement to bottom of skirt) = 55mm
55mm minus 61mm leaves a 6mm crown. With the diameter being 79.5mm, this means that if the crown were to be 10% of this, it would need to be 7.5mm thick, and the lower 7% would be 5.56mm. This means that my crown is within safe (ish) limits at 6mm, according to your post.
So, i should be able to use these pistons in my engine as they are, as long as i dont use too much boost. The ceramic coating i was on about was the thermal barrier as you rightly guessed, and i have heard from a few people that it may not be the best route. It was the cooling issue i was on about, and obviously the prevention of detonation.
Squirting oil onto the bottom of the pistons is a great way to do it, and i should look into this more.
does any of this make sense?
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Well, your piston hasn't the thickest of crowns but it seems to be enough. Sure, you can know exactly without actual tests or a more complicated analysis.
You should probably try to avoid too high combustion pressures. Boost pressure is of course one factor that affects the combustion pressures, two other important factors is compression ratio and ignition advance. If there is no detonation it's easy to fall for the desire to increase the ignition advance, this does not only increase torque but also the maximum combustion pressure, and taken too far it can destroy your pistons (or other components). Professional engine builders use sensors fitted in the cylinder to measure this pressure, a solution too costly for most, so be careful.
Oil cooled piston is very common on modern turbo and other high performance engines. Some even use an oil gallery in the piston improving cooling, this is found in many turbodiesels, in some WRC and most turbo F1 engines from the eighties. Oil spray cooling is a balance between the desire to have a cool piston and the increased losses caused by the oil (crankcase windage losses for example).
Johnathan me old fruit -- as stated by member of this forum -- oil sprays are the best way, my 1242 turbo lump has oil spray 2 per piston -- one per piston was stock but adding the turbo up's the running temp so i added another one per piston just incase, i have also gone down the 1mm off crown method as im strugglig to get the pistons i want.
I did not think many people had actually been as brasen as me with the lathe methods, but its nce to see people like you are, reasures me a bit. So i mean, i do know about oil sprays, but how have you done it?
have you got any pics, or kits or??
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like i say Mr 1 PSI the fire engine i have has oil sprays as stock -- all i did was to drill & tap the back of the stock sprays to add the second one to each cylinder -- will get some pick dont tomo for ya mate ;o)
Mine upon inspection has some jets which lubricate the shells internally, but apart from that, there is nothing. This would be fun to retro fit, or custom make!
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Mine upon inspection has some jets which lubricate the shells internally, but apart from that, there is nothing. This would be fun to retro fit, or custom make!
if you need any help (I KNOW NOTHING) LOL
no if you need help just ask -- im not saying i know everything but i can think on my feet --- as you know Mr 1 psi ;o)
Got a few things to do first, but at least i kow i can fit these pistons as they are, and as long as i keep them cool, i should be okay for 1.5 bar tops. Not as much as some, but adequate
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my pistons are only good for 1 bar (FACT) i know this as my friend in poland found out-- but 1bar + 1242 fire engine + T2 turbo + gt fuel pump = 175bhp into 500kg is a riot i recon
--- remember your RST piston with the valve in it LOL
Its adequate though Tim, your running a smaller engine. Its power to weight ratio will be more than enough. I will have to compensate with things like extra boost, although i plan to only run 1 bar most of the time.
here is the 'Palve'. Piston + Valve = Palve!
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