I was staring at my boost gauge last night, and my boost doesn't hold constant till redline. It tapers just like as if it was stock, although it still makes more boost than stock.

Then, i took a look at my dyno chart and i guess i should have expected the boost taper. . . But is this normal?

Would the car make more power if boost held at a constant 19-20 psi till redline? Or is there a reason for maintaining this boost taper?

 

It is normal.

 

How much of a taper is normal? My car will peak at 21 then maybe bleed down to 20psi at redline. Sometimes it seems like it does not drop at all. Stock it would drop to 16 from19psi at redline which is much more drastic than now. Currently running the 93 octane stage 0+ .

 

Well. . . what gear are you in when you notice that it doesn't taper?

B/c mine doesn't taper at all in 3rd , 4th, and 5th, consistent with what the people above were saying about the amount of load on the car affecting the boost.

 

Just about all the XEDE maps have the boost control solenoid maxed out at 100% at higher engine speeds. So any taper in boost is caused passively by turbo flow limitations. From what I've seen, it's rare to find a free-flowing EVO that can hold more than 17psi at 7000rpm.

my 2c,
shiv

 

Boost taper is a function of the factory ECU. This is clearly apparent with wastegate dutycycles set at 100% for the high RPM/Loads. Removing the boost pill in the hose leading to the wastegate solenoid increased boost for both peak and final boost pressure once it's tapered down, retaining the factory tuned characteristics. In order to increase boost pressure or maintain a steady state pressure of 20-23lbs. through redline an additional pill must be installed in the hose from the compressor outlet to the T fitting leading to the wastegate and solenoid. This additional pill will restrict the boost signal to the wastegate and solenoid thus increasing boost pressure. The additional pressure can now be managed with the Xede by lowering the dutycycle, so it's possible to have steady boost of say 21lbs. with no taper to redline if one choses with proper tuning of course. The question is now will the factory turbo exede it's efficiency with the increased boost at higher RPMs?

 

Well, now that I have a 3 bar MAP sensor and I am able to log boost with my Innovate LM-1 I can clearly see the boost taper even with the Xede. There is as much as 3 psi of boost taper between peak boost and 7000 rpm. Here are the results from the logs:

21.50 psi @ 3500 tapering to 18.18 @ 7080
21.72 psi @ 3700 tapering to 17.69 @ 7010
21.45 psi @ 3860 tapering to 18.36 @ 7020
20.60 psi @ 3740 tapering to 17.82 @ 7060 this was done at 2 pm and car driven for a while, hence the lower peak boost
21.23 psi @ 3480 tapering to 17.78 @ 7020

I used to rely on the Defi D gauge and it told me that I was boosting to 23 psi. So I went ahead and backed off the Xede boost table by about 5% since I thought that I was boosting too high on 91 octane gas. Turns out the Defi D gauge is off the mark by about 1.5 psi. IMO, all boost gauges are off the mark. The best way to get your boost is to log it.

 

the taper is due mostly to the limitations of the stock turbo

 

Yep.. most japanese electronic boost gauges i've seen read too high. Sometimes by as much as 3psi under high boost conditions. This is why I base all my boost claims (taper, peak boost, etc,.) from boost logs from the dyno

Shiv

 

There are a ton of discussions going on about this right now. Basically boost isn't as important as airflow. If you are flowing 42LBS of air and only maintaining 17PSI at redline that is just as good if not better than flowing 42LBS of air at 20PSI. This means that more boost doesn't always mean more airflow.

The stock turbo is somewhat limited at 42/43LBS of air. If you attempt to run higher boost all you are doing is turning your turbo into a mini flame thrower and not getting any more performance. Unless you can actually use the higher boost with race gas or alchy you may actually be hurting your performance more than helping it.