max boost with stock turbo pistons

[berniek]

Thanks for the reply. No, the pistons cannot be hypereutectic from the factory, but most certainly must be eutectic, probably at around 10%-11% Si. From the small amount of metallurgy I know, hypereutectic pistons tend to crack more easily than eutectic, although hypereutectic pistons are stronger. There was (is?) one fellow in Denmark running 30 PSI boost, but don't know any details aside from the stock ignition system not being high enough in voltage to break down the plug gap. Obviously, a casting is not as dense and strong as a forging, but is used in the stock turbo application with oil jet cooling. 17 PSI boost sounds high, but on an absolute pressure basis its only 53% more than the stock 6 PSI boost. But 30 PSI is a different story. If he runs hypereutectic pistons, the stock turbo cast eutectics must be OK for 17 PSI. At least that was my rationale at the time, but I've had lingering doubts about it.

Mixture under boost is about 12.5:1, known by a reworked thermal conductivity A/F ratio sensing cell following a coil of 1/4" cooling tubing run from the Lipp to the front. A small 12V solenoid (stainless) valve blocks it in to retain the last reading, which gas velocity effects through the cell require. Early-on I bought a Dawes 4 LED device which reads the last couple of millivolts at the rich end of the stock A/D ratio sensor. Its got a precision voltage reference and a couple of National voltage to LED drive chips inside. It has matched the thermal conductivity cell exactly. The thermal conductivity cell is not really visible unless the roving (NJ) emission police look under the horizontal sheet which is integrated with the hood latch. The LED readout and small meter for the TC cell are in the ash tray. Speaking of the police, the Raptor BOV is inside the right fender, and is recirculated to the bottom of the accordion hose.

Thanks for the assistance. I'll be happy to send a tip for any further information.


Berniek