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New-ish 911SC Targa Owner
 
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any benefit to filing sharp points on pistons down?

An old wizard dressed like a human once told me that he never built an engine without taking a file to the sharp pointy areas on the pistons to prevent them from becoming little glow rods and causing preignition.

I've only rebuilt one other motor before and It had flat top pistons so it was moot point. On a 911 piston with sharp edges around the valve reliefs, would there be any safety against preignition if one was to smooth those sharp points down some?
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Old 10-18-2016, 07:21 AM
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If there was a foreign object in the cylinder running and hitting the piston putting nicks in the face, that's the kind of high points that would glow causing pre ignition.
Bruce
Old 10-18-2016, 07:37 AM
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Yes but difficult to measure... more so on a turbo car in my opinion. Tape the sides and bottom so you don't get dust everywhere. I would work only by hand to avoid mistakes that are more likely with a rotating tool. Work each piston the same with the goal of smoothing rather than removing material, and use a scale to keep weights between all pistons the same. This is good prep in advance of applying a ceramic coating to the piston crown as well.
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Old 10-18-2016, 08:26 AM
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The benefit is more about performance than an actual "hot spot"
We see 5-7 hp by simply rounding the sharp edges of some after market pistons.



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Old 10-18-2016, 10:51 AM
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Henry - how much does rounding as shown reduce the nominal CR stated by the piston maker?
Old 10-24-2016, 06:57 PM
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Quote:
Originally Posted by Walt Fricke View Post
Henry - how much does rounding as shown reduce the nominal CR stated by the piston maker?
Our experience is .05 to .15 points of compression. Adding a ceramic coating will replace most of the dome volume removed.
Interesting how you use the term "stated by the manufacturer".
We have found that "stated" compression ratios are rarely close especially with early Mahle.
Our experience has shown Mahle over-estimates CR making accurate performance predictions hit or miss.
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Last edited by Henry Schmidt; 10-24-2016 at 08:11 PM..
Old 10-24-2016, 08:07 PM
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Thanks, Henry. The phrasing was not by accident, of course.
Old 10-24-2016, 09:54 PM
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