ScoobyNet.com - Subaru Enthusiast Forum - Changing dumpvalve causing AFR change?

This is not as straight forward as it sounds, so any thoughts or opinions are very welcome.

The car is MY99, VF34, 3inch exhaust, induction kit, 20psi boost, etc

I've been using a forge recirc with stiff spring and 3 shims for some time. Today I thought I would try it back with the original DV, mainly on John Banks advice to improve the MAF reversion issues. Now with the forge AFRs are 11.2-11.4:1. On goes the oem DV without problem, nothing else changed. First impressions, throttle transitions are much smoother, chatter gone, DV quieter, but also turbo much quieter (used to get lots of jet noises and fair amount of whine from turbo, particularly on part throttle boost). Give it some WOT and AFRs are now 10.7-10.9:1! I have never seen AFRs richer than 11:1 before (continuous wide band sensing). Ever.

First thought, I've got a boost leak, causing MAF over-read and overfuelling. Get home, check my install, everything is as it should be. Decide to adjust fuel pressure (reduced by 0.2bar static), and do some delta-logging. With the adjusted fuel pressure AFR now 11.1-11.2:1, so still a little richer than before.

Now it gets really confusing when you compare the logs (luckily I'd logged the car just before changing DV this morning). MAF voltages, hence airflow, is near identical, I've got boost held better above 5500rpm with OEM DV, ignition timing is similar, BUT IDCs are lower :confused: with oem DV, but AFRs are still richer (on logged simulated NB), this is despite a 0.2bar drop in fuel pressure.

What the hell has happened? The data suggests I've somehow improved combustion efficiency by changing the DV, resulting in more complete combustion for less fuel injected. Have I also improved VE?

Could the OEM DV be constantly recircing some charge, resulting in a lower PR across the compressor for any given boost, thereby allowing the turbo to spin slower and resulting in lower EGBP?

Simon

PS Before and after road dynos on same road, same direction, 1hr apart, possibly warmer on OEM run, show identical spool, but more torque with the OEM DV from 4000rpm to 7000rpm (only marginally, approx 5lbft at wheels)