Over the years I've done multiple boost gauge installs on my A4 and S4--three to be exact. (A4, S4, S4) My goal in each case was clean integration with function. In each case I was reasonably successful but, the O.CT Tuning boost gauge offered by Stratmosphere, may have the best integrated boost gauge I've seen to date for the B5 S4 chassis. 

The O.CT gauge replaces the factory tachometer gauge face with a custom face that retains all the stock functionality (warning lights and such) along with adding a half-circle gauge that displays up to 1.5 bar boost pressure via a 20 segment LED bar graph  living just inside of the tachometer numbers. The bar graph begins at 1400 rpm and wraps around the tach to 6400 rpm. Very neat and tidy, very factory looking, very cool. The best thing about this gauge is if you glance at the tach you can instantly see the boost pressure since it's right where you're looking. Very nice indeed. 

The gauge face is a match for the factory face so you retain factory appearance for the instrument cluster. Unfortunately, the S4 logo on the tach is lost with this upgrade--replaced by O.CT's logo. Generally speaking I prefer analog gauges but I'll make an exception in this case; this is a well done gauge.

The kit also includes a gauge face for the speedometer that will display up to 180 mph instead of the stock 160 mph display. I opted not to install the 180 mph gauge face since I have no hopes of ever going that fast. Of course, it could come in handy for someone with upgraded turbos. 

The kit includes two gauge faces, the necessary cables, wire connectors, tools to aid in opening the cluster and a circuit board that runs the gauge. 
 

The install took about 3.5 hours to complete and was a very straightforward aided by  well written full color installation instructions. I didn't encounter any fitment problems. The most  unnerving part of the install is taking the instrument cluster apart (an $800 item). The provided tools made opening the housing a snap. You simply lift the latches on the housing, (6 of them) slide in the provided tools, remove two #10 torx screws and unlatch two catches and presto... open instrument cluster with out any prying with screwdrivers or the risk of breaking off the tabs while trying to unlatch six pesky tabs simultaneously. Nice touch O.CT.

Careful attention to the removal of the gauge pointers and their location is a must for the gauges remain accurate. You must note the location of the pointers at rest and be sure to put them back to the same location when putting it all back together--the instructions clearly point out this information. My pointer was stubborn to come off but otherwise, not much of a hassle. Work carefully so you don't damage anything.

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Unique to this boost gauge (thus far) is that the O.CT gauge does not require tapping into any boost or vacuum lines on the engine. It receives the boost pressure information straight from the engine computer--no chance of leaking, kinked or damaged hoses with this one!

The installation requires connecting three wires into the engine harness via wire loc connectors (no soldering or cutting required); two for power and ground and the third for the boost pressure signal from the computer. The needed wires live in the wiring harness near the ECU which means the harness "tape" must be unwrapped to gain access to the wiring. I don't know what Audi uses, some kind of sticky goo impregnated fabric, but it sure is nasty stuff. You'll need some hand cleaner to wash off the goo when you're done--plain old soap and water won't touch this stuff. Of course, the benefit of the goo is when you wrap it all back up it sticks to itself instantly.

Installation and hookup complete, the gauge must be calibrated. Calibration is a simple procedure via a pot on the circuit board before installing the cluster back into the dashboard.

In operation, the gauge provides information to the driver with a quick glance. The bar graph is large enough (and well placed on the instrument panel) that it's easy to read and doesn't require you to study the gauge to figure out what it's displaying. Audi engineers--wake up and take notice!

The O.CT boost gauge is an impressive package--for a factory look nothing out there compares. The gauge is available from Stratmosphere, priced at $399. It is a bit higher priced than regular analog gauges but none of them offer the level of integration and clean factory look this gauge has to offer. For the price difference the O.CT gauge is a choice that should not be overlooked if you're in the market for a well done gauge.