Not sure what exhaust it is. Looks like a custom job. I'll take a look around this wekeend and report back but I'm fairly confident its a hole soemwhere in the exhaust near the O2 sensor. I've never had a hole int he exuast so I didn't know if it makes a whistle sound as it sounds a lot liek an intake leak. I read on Suby forums though that when they get holes in the up pipe to the turbo it makes a whistle sound liek the turbo spooling.

Look for black soot to locate leak.
 
I wouldn't have thought a leak small enough to create a whistle would effect the mixtures to such an extent?
 
Cheers

Yeah I'll be looking for the carbon stains if its leaving any. Otherwise I'll try a few other tricks to locate it. Worste case it will go on a smoke machine. O2 sensors are very sensitive so if the leak is anywhere near it will detect extra oxegon. It will keep adding more fuel till it doesn't detect any excess but this will never happen, thus runs very rich. Anyway finding this leak will not be fun. Both exhuast brackets have snapped after the last race so need it rewelded. If those broke i'm not suprised there could be a crack somewhere else.

As suggested prior, have you tried disconnecting the O2 sensor?
 
If there is a small leak, the sensor will not detect extra oxygen, you will have positive pressure in your manifold so no outside air will be sucked in.
 
Yes O2 sensor only starts generating a signal once its warm, on my previous gen2, it started to work after about a km of gentle diving following a cold start.  The gen3 has a built in heater so should start to work sooner.
 
Another way of finding leaks (including vacuum) is to use a stethoscope with the tube open at the end.  You can easily hear any wind, its very sensitive.
 
Cheers

I'm fairly confident in the exhaust leak so i'll sort it out thanks anway. Smoke testers make for the easiest diagnostic for vacume and exhaust leaks. Just don't want to have to take it to a shop with a smoke machine if I don't have to.
On a modified turbo you have a lot of noises going on. Also exhausts will drawn in air on the pulse, hence why a hole will cause problems especially near the the sensor. When tuning EFI systems on previous race cars even we wouldn't use the O2 sesors you can hook into the back of the exhaust as we found them inacurate. Instead would use a drilled bung much like a factory O2 sensor bung to take the readings and that wasn't just for idle, thats for full dyno tuning runs. Thanks anyway, I'm 90% sure I'll be able to find a leak and get the car ready for its next track event.
 
 

All sorted. Looks like from pressure testing the intercooler there was some water sitting in there. After driving around enough its blown it out. Everything is working fine again but there is a tiny exhaust leak somewhere, it looks like the gasket on the dump pipe. Really hard to tell because I can't manage to see it, too much stuff in the way, but I was getting water droppets dropping out from somewhere there. It's definately water and not coolant so must be a slight leak somewhere in the exhaust. When i get the exhaust brackets welded I'll get them to take a look as it will be easier on a hoist.

Just an update, found the hole in the exhaust, it was on the exhaust just after the flange to the dump pipe. Hopefully it can be welded rather then having a new mid section of exhaust pipe made up......