Pete_89T2

iTrader: (4)

I'll preface this post by saying the experienced tuners out there probably already know this, but as a newbie guy trying to road tune his FD, this little trick can make the road tuning process much safer and perhaps even save your motor. Although the Link G4+ (and I would assume the newer G4X family) provides a very effective rotary-specific fuel & ignition cut function for over boost conditions (i.e., set your MAP or MGP threshold limit and if exceeded, the fuel/ignition cut comes into play), it's a bit rough on the engine when it does engage - you'll get some nasty backfires or other unpleasantness which generally isn't good for the engine or drive train.

So since I'm running a DBW throttle with my Link G4+, I learned that I can use a "Virtual AUX" output to manipulate which DBW throttle table is active as a function of MAP or MGP, aka - creating a throttle based boost limiter. Here's the theory behind it: Set up a virtual aux to monitor MAP, when MAP exceeds your set level (24.7 psi MAP or 10.2 psi MGP in my case), the virtual AUX output is used to toggle from your regular DBW map to a second/alternate DBW map. My normal DBW map is for the most part a simple 1:1 relationship between accelerator position and throttle position. But the alternate DBW map is set so the throttle position is 1/2 of the commanded accelerator position.

Here's a log screenshot of it in action. In this case I was in 5th gear holding a pretty steady throttle but gradually increasing it, expecting that it would cause some boost creep to occur beyond my spring pressure - boost control is still turned off, so it's strictly on mechanical springs. Typically my 7 psi springs (dual Turbosmart 40mm Gen IV external gates, recirculated into the DP) will hold a steady 8~9psi MGP boost on a sustained 3rd gear pull or a shorter duration 4th gear pull. The yellow cursor line is at the point where boost just exceeded my set threshold and MAP hit 24.8 psi. Just past that point, you can see where the AP% holds steady, but TP% rapidly trends down to about 1/2 of the AP%, and boost quickly drops off. Then as soon as the boost dropped below the threshold (about 9.5psi MGP), you can see the TP% and AP% tracking at 1:1 again, after it switched back to the primary DBW map.

Behind the wheel you'll feel an immediate reduction of power, almost like a misfire, but then it comes right back as you stay on the accelerator (as in the log snippet). So you get the safety of a boost limit without the nasty backfires or drive line shocks that a fuel/spark cut would result in.