Inspecting Pistons for DET.

Hi,

I am in the process of rebuilding my street car, This is the car where I first started tinkering with tuning. Over time I have been through a few different ECU's. Ran all sorts of different tunes.

As this was my "learning" platform, I was expecting to be a fair amount of damage. I am no pro when it comes to inspecting internals.

But....Any ideas on why so much carbon build up on these pistons? and also can anyone with a tuned eye see much signs of DET.

The pistons are Mahle 4032 alloy in a Subaru EJ20. The engine has seen rich, lean, too much/too little timing. But over all ran 11:1 AFR and around 280kw at 2bar.

Any observations would be great.

i'm not a guru on engine analysis, so take my information with a grain of salt.

serious detonation will typically reveal itself on the outer surface of the piston. material may be depleted leaving a rough texture on the surface of the piston after detonating.

there's visible signs of carbon on both indicating most likely a richer mixture. on the right piston there appears to be some buildup in the center, wouldn't be able to tell exactly what it is from the picture.

Carbon deposits can occur in two ways, richer than stoich mixtures, and excess oil entering the combustion chamber. If the car was ran at 11:1 afr on average, this would be the probable cause.

Thanks for the reply Mitch.

Before stripping the engine there was some slight white smoke. AFR was 11:1 at 2 Bar of boost. Was 14-15 under light load.

Was expecting to see a lot of damage, But clearly these pistons aren't too bad. Or my initial tuning was too...far from the ball park!

Light detonation will usually show up as a light sand blasted appearance to the edge of the piston crown. Contrary to what would seem to make sense, this most often reveals itself first on the intake side of the piston crown rather than the exhaust. If the detonation is light you may struggle to see it and I'd start by cleaning the piston crown with scotchbrite to remove the carbon. More severe detonation will be obvious (an there doesn't appear to be any) as it will erode the surface of the piston in the same area. You also may find that the top ring land has been crushed slightly and this will present itself with the top ring being 'sticky' in the groove. Also check the side of the piston near the top ring land. If your piston has 'anti detonation' grooves or contact reduction grooves as they are also referred to, these can easily become damaged by knock, showing pitting.

The carbon build up probably isn't anything I wouldn't expect to be honest. One of the causes of carbon build up is oil contamination of the combustion charge and a forged EJ engine can tend to be worse than most in this respect because the engine is horizontally opposed, particularly if you're using a large piston to bore clearance and large ring end gaps. Obviously a rich mixture will affect things too, however I wouldn't necessarily say that 11:1 under WOT is excessively rich.

Thanks Andre, Clearly the HPA courses are working. I went bull at a gate with this engine and its not stuffed!!

Will give it all a clean up and have a look.

If you can't break an EJ20 then you're not doing to badly. In my experience even with forged pistons they are quite a fragile engine.