fivebob wrote:RedMist wrote:My bet is you are breaking the ring lands on cylinders 2 and 3. I wonder if its more of a block design issue and two and three are hotter than one and four, causing detonation or micro seazures.
I doubt that it is a block issue, though #2 & #3 are more common failures, with #2 being more common in Gen III+ and #3 in Gen 2 engines, I've seen all pistons fail.
Both my engines had severe failure of #2 between the second ring and the oil control ring, with minor failures in #3 between first and second rings. However in farmenrdaves engine #1 piston failed, #3 and #4 had broken lands between below the top ring. In all cases the crack originated on the left side of the piston (when looking at from the intake side).
In the case of both my engine failures, one MR2 & one Caldina, occured under the same circumstances, long uphill road on a hot day and too much boost for the stock ignition map. When the Caldna failed I noticed that the oil temp got very high (125C+ when it normally runs about 95C) and I know that oil cooling plays a big part in controlling detonation in the 3S-GTE.
When all said and done, engines detonate, it's just that with the stock pistons this causes ring land failure. As nothing can be done about the brittle nature of the stock piston then, as I see it, the only option is to reduce the chance of detonation, and remove any stress risers from the piston.
For my own peace of mind, along with using water injection, as I recondition the engines I am going to be replacing the pistons with Mahle forged items, as these are made out of 4032 alloy this allows for closer bore/piston clearances and as such have none of the forged piston slap on start. Only issue is that they seem to only available in 0.5mm oversize which may run into that other 3S-GTE probolem of thin walled blocks
Fivebob, thanks for all your reply!
It seems that detonation is an issue on 3SGTE's at relatively low boost levels. My experience with 4G63 (VR4-Evo) engines is that the factory pistons can handle a lot more punishment (like 22 pounds boost for 2 rally seasons). Have also seen a NA 4g63 running 16 psi for a season. Anyway running 11.5 AFR should help, but it is a shame to waste fuel simply to keep the pistons cool! I also find it hard to believe the greatest power/torque is achieved at 11.5 AFR but cannot put it on a dyno to prove it. In terms of oil cooling I use cold water from the jetunit so it is far better than a car situation.
Will have to investigate water injection I read in the "3SGTE power primer" to use an orrifice of 0.7-0.9mm but no mention of what pressure is required. I have plenty of pressurised water available.
Put an engine together in the weekend with gen 1 8.5:1 pistons and mildly chanfered the inlet valve pockets, will see how it goes later in the week!
