The "basic" A/F gauge that everyone sells (cheap one) operates on a 0-1 volt signal off of the standard O2 sensor...very low resolution due to the small voltage variance. A wide band A/F gauge such as a Innovative or AEM needs a wide band O2 sensor which has a 0-5 volt signal that gives you much more resolution thus allowing you to accurately measure air/fuel ratios at any RPM and adjust fuel to achieve the desired A/F ratio at any given RPM to create a smooth even A/F ratio across the entire power band usually somewhere between 11-12. And yes you need to install a bung and a wide band O2 sensor in the exhaust somewhere between the Turbo and Cat...usually in the DP somewhere.
Probably just as important as a knock gauge if the car is not tuned properly...a very lean condition can be catastrophic as can high knock
for examlple...
I recently finished putting my sons car together...92 Eagle Talon AWD TSI .5mm over 6-bolt with lots of mods: fully ported EVO Big 16G Turbo w/max puck, ported EVO O2 housing, ported EVO Exhaust manifold, BIG Slowboy Racing FMIC, FIC 550 injectors, Walbro FP, 2nd Gen Maf, P&P heads, Crowler springs, 1mm oversized S/S valves, HKS Cams, Greddy Type-RS BOV, 2 1/2" DP and exhaust, Apexi SAFC II, Innovative LM-1 wide band, etc. Sarted it up and drove it around some...ran pretty good but needed some tuning...hooked up the LM-1 wide band and found out we were way lean 18-19 A/F. With out the wide band we may have had a melt down to the tune of about $6,000 in new parts and engine work
Car is now Dyno tuned with the Apexi SAFC II and at 18 psi put down 290 AWHP and 260 lb torque A/F is about 11.6-11.8 from 3500-7000 rpm :biggrin:
anyway back to the topic...my choice would be:
3-gauge pod (Boost/Vac, electric FP, and knock)
2-gauge pod (Boost/vac and knock)
-Scott