924Board.org

[gegge]

I would like some input from you that have had problems with OEM pistons. What is the weakness: piston rings, crown or bowl? How do you spot a lean mixture? How do you spot extreme heat? How do you spot detonation?

The reason for asking is that a have got a set of used pistons and the bowl is looking very good and so does the skirts. The problem is just above the 1:st ring, the metal has eroded and the question are why and just how bad is it? Most likely has this being going on for some time.

If I would opt for aftermarket pistons, why should they be better?
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[ic932]

Gegge, As it was me that sold you these pistons ( I presume), I would like to know too. As we know, broken top compression rings are very common, I've seen plenty of broken ring lands too.

[Raceboy]

Stock pistons are cast items and they are not stronger than NA pistons. Actually a bit weaker because of the thinner rings which break very often.

And if detonation occurs, it doesn't matter which piston you have, NONE of them can withstand detonation for prolonged periods of time.

Signs of "erosion" are actually a sign of detonation and this piston is a good ashtray.

Best way to upgrade pistons is to make custom pistons using 931 piston design but with NA rings. Wiseco does custom pistons at a reasonable price.

Cheapest way is to obtain VW beetl flat-top pistons. This results in not the most effective quench area usage, but it has been proven to work.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[gegge]

I would expect evidence of detonation on the piston top, not on the sides? Maybe on the sqwish area, but the volume between piston sides and cylinder wall is close to nothing.

Is the material just weak or is the cylinder pressure/temperature extremly high? There is a problem but different solutions

IC932: My mechanic just told me to bin the pistons

The flat-tops are on my list when building a new engine. They will maybe withstand detonation better due to forged material, but the problem is engine management.
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[ic932]

IC932: My mechanic just told me to bin the pistons ... OUCHHH! any pics of the damage? I've got another set and DO NOT want to sell a set of ashtrays. What am I looking/measuring Gegge?

[TurboJak]

Had same damage on my pistons, some pics:




Maybe the engine had been running very hot at some point...

Changed to flattops and engine is standing ready to drop into car again


_________________
______________________________
'76 924 + '82 924T engine + GT look

[Raceboy]

Detonation pretty much always eats up ring lands and starts from the edge of the piston. On the pics there's clear signs of detonation.

It really doesn't make a difference if combustion temps are just hot or detonation occurs, the latter just can brake pistons and rods into pieces but mostly just melts pistons.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[gegge]

Pictures of my pistons after cleaning. Is it acceptable and usable or not?


_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[TurboJak]

Looks much better than mine did, but not sure if you should use them. My pistons had actually squeezed down on the second ring so second ring was broken on all pistons and very difficult to remove from pistons. So if you use these be sure that the rings fit perfect.

Could this damage be caused by a faulty wastegate/high boost? Did test my wastegate and it is leaking some through the wastegate valveguide...
_________________
______________________________
'76 924 + '82 924T engine + GT look

[Raceboy]

TurboJak, there's pretty much always enough vacuum/boost to operate wastegate and all of them leak some air through the stem.

Higher boost can cause detonation but not as likely as having a problem with the fuel system or ignition distributor set to too muc advance.
_________________
'83 924 2.6 16v Turbo, 470hp
'67 911 2.4S hotrod
'90 944 S2 Cabriolet
'78 924 Carrera GT replica
'84 928 S, sold
'91 944 S2, sold
'82 924S/931 "Gulf", sold
'84 924, turbocharged, sold.
http://www.facebook.com/vemsporsche

[ic932]

Gegge, I've got some pics I must post but a brief discription might help. They are of oversize S1 pistons. They look like new but have fractured piston lands (detonation). There are no signs of the corrosion that you have. Just plain simple catasrophic damage. It makes me wonder if what you see on your pistons is natural wear and tear? Given that your pistons would of most likely seen extreme heat cycles and moments of detonation in their lives...(Its a 931...very early turbo technology and little understanding from the owners back in the day lol), could they be "normal used pistons"? For example if your motor was down on compression and needed rings would this kind of "wear an tear" go un-noticed?

I have a std used piston set from a healthy motor, I will clean them up and report back with pics....my bet is that they will show similar features. If they don't then we can come to some sort of "exchange policy"

[ic932]

gegge?

[gegge]

Yes..?
_________________
Carl Fredrik Torkildsen

924 turbo -81 Carrera GT RESTOMOD
924 turbo -80 Dolomite De Luxe
924 -85 DP kit, BBS RS, M030 and tuned engine
924s -86 Black on black turbo with Fuchs

[ic932]

Did you make a decision about those pistons?

[gegge]