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broken pistons along valve relief

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BadIdea

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Discussion starter · #1 ·
#2 ·
It's not uncommon. The pistons are Forged and Forged isn't as forgiving as cast or hypereutectic. They can break at the first sign of detonation.

Looks like you where running pretty rich, too rich almost. But the corners that broke where much leaner than the rest of the cylinder, meaning it was the hottest point. Odds are the heat weakened that spot or contributed to a detonation situation.
 
#3 ·
FociSHO, I love that you spelled "hypereutectic" correctly. For some reason, that made me happy to read. Strikes me as interesting that the break happened in the exact same spot on both pistons. Maybe I'm not that smart, but what are the odds that your engine would develop hot spots in the same place in two adjoining cylinders?
 
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#5 ·
Well, is there any indication of valve damage on the 2 & 3 cylinder valves? I would think that if the pistons look like that and it was a valve issue, there would be some VERY noticeable damage to the valves.
 
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#6 ·
The SVT pistons are *not* forged. Only the rods are. The pistons are cast hypereutectic.

The damage resembles damage that John sustained in his motor. Looks a lot like heat. If a valve hit the piston, there normally would be little nicks and cuts all over the place...not to mention you would have bent/broken valves making lots of noise.

FordFoci: where did you hear that forged is unforgiving? Forged rods/pistons are very strong and can take waaay more detonation than cast. Really really good example is the Honda S2000. 11:1 compression and yet can take 600whp/400 ft-lbs for thousands and thousands of miles if tuned right. That's what forged internals can do.
 
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#7 ·
bmylez said:
The SVT pistons are *not* forged. Only the rods are. The pistons are cast hypereutectic.

The damage resembles damage that John sustained in his motor. Looks a lot like heat. If a valve hit the piston, there normally would be little nicks and cuts all over the place...not to mention you would have bent/broken valves making lots of noise.

FordFoci: where did you hear that forged is unforgiving? Forged rods/pistons are very strong and can take waaay more detonation than cast. Really really good example is the Honda S2000. 11:1 compression and yet can take 600whp/400 ft-lbs for thousands and thousands of miles if tuned right. That's what forged internals can do.
Click to expand...


I know the stock zetec pistons are Autothermic, designated by the "V" in the part #. I've never come across the actual part # for the SVT/ST170 piston. THought I'd read years back that they where Forged though. Reading online now is still a crapshoot.
I sold my set and I can't recall what the # on the bottom of them was. A "P" designates it as a Forged piston.

... and if I'd listened to myself I'd know better :screwy:
http://forums.focaljet.com/svt-headquarters/631807-original-ford-svt-press-release.html



But my comment on the strength was in lieu of heat stress. Forged can handle higher impact stress but can not handle higher heat stress. Detonation is both. If the spot was lean for a long time then subject to higher than normal pressure it could easily break.
And that's what I see in those pistons.

Tuned perfect, yes Forged will always be stronger. But there is no such thing as a perfect tune.
 
#8 ·
Ok...but that doesn't prove your point at all, then. You said a forged piston is more prone to heat damage (which I'm not sure if that's true...) but this isn't a forged piston so...
 
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#10 ·
Who Built the engine? its almost like the tolerances were to tight for heat expansion. but i agree with the other guys wrong fuel mixture combined with detonation u can c the spots on top of the pistons
 
#11 ·
The piston and rod assembly used in the SVT was made by Mahle and as mentioned the rods are forged steel (conventional machined and not the new fracture spit designs) and the pistons were a cast, hypereutectic aluminum. This was chosen as the design and materials used allow for a lighter piston that in turn reduces mass of the rotating assembly and was deemed important for the performance and higher rev range.


Casting refers to the process where the piston is made by pouring the molten alloy into a mold with machining afterwards compared to a forged piston which is completely machined from a billet "slug". Eutectic or hypereutectic are references to the specific aluminum alloy used and signify higher silicon content. Forged pistons are generally made from a non-eutectic (i.e. low silicon content) alloy such as 2618 or 4032.

Characteristically as you increase the silicon content the piston becomes stronger and therefore can be produced with less material which yields a weight advantage with hypereutectic pistons having even higher silicon content than eutectic pistons. The higher silicon content in the alloy also causes them to expand and contract less with temperature changes which make them better for engines machined and requiring very tight piston bore tolerances.

It's important to define stronger though in that we're talking about at the molecular level and the bond between the elements. In practical applications this has a down side of also making a eutectic alloy more brittle. When combined with heat and high peak combustion pressures this makes the eutectic-based pistons more susceptible to damage from detonation. In comparison most of the forged pistons use a much lower silicon content alloy and while technically softer and requiring looser tolerances (which can result in piston slap noise until the engine warms up if a super-low silicon content alloy such as 2618 is used), they are also able to withstand detonation events better as they are less brittle and won't crack or break up as easily as a common cast eutectic/hypereutectic piston.

This is primarily due to the material used in each piston type and not the difference between the casting and forging process. You could theoretically have a hypereutectic, forged piston, but when mass is a concern such as in a high performance and high-revving naturally aspirated engine (where that type of piston is normally specified), the casting process will generally yield a piston with less weight and be much less expensive to produce. A side note is that Mahle has also made a low-silicon content, "semi-forged" piston for turbocharged applications that starts off with a piston slug (solid cylindrical piece of raw material) that is somewhat cast into the general piston shape which requires less machining than a fully forged piston.


As for the pictures you posted I could only wager a guess in that there might have been high temps combined with detonation due to a fueling or timing issue or possibly a manufacturing defect with a batch of pistons. The ring in the SVT's pistons are placed up fairly high in order to minimize the amount of space above the rings in which combustion gasses and un-burnt fuel can collect. This is better for emissions but reduces the strength of the ring land area above the top compression ring because there is less material (Mahle and other manufacturers sometimes apply an anodizing process to that area of the piston to increase strength due to that). Combined with loss of material from the machined valve reliefs, that would make the areas that broke one of the weaker spots on the SVT piston's surface.

It's difficult to tell without looking and performing an inspection in person and knowing the history and setup of the car but if it was assembled and machined correctly and the surface on the face of the piston are showing signs of detonation and the spark plugs were as well (and even more so if they were also showing signs of too much heat in the combustion chamber) then I'd be looking into what might have caused that (too lean, running too much timing, inoperative knock sensor, etc.).
 
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#12 · (Edited)
A piston isn't part of the rotating mass of an engine...it's reciprical. But I'm sure that's just a typo on your part :) (although, when it comes to dynamically balancing an engine, the weight of the pistons/rings comes into play.)

A lighter piston would reduce the inertia effect on the rotating parts, i.e. crank and rods, but all told, it's true, it is also an important factor in high RPM engines...

I think cast pistons were chosen over forged, likely because forged pistons expand more under heat (so you need a larger piston/cylinder clearance when cold), and on cold startup, you have a noisey engine with possible blow-by...until it comes all the way up to operating temp. Both of which isn't desireable on a production, EPA certified, factory produced car....


....also, your pistons could of cracked there because of the highly concentrated stress of the 90 degree angle at the valve cut out lip...
 
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BadIdea
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