Quick question, is there any way to keep the boost from dropping so much on the stock turbo?
Been working on tuning and it seems like the higher boost level I go the more it drops in the upper rpm's. Just want to know if this is normal and or tricks to not make it so dramatic.

I’m using AEM tru boost controller.
Tephra v7 on sd. e85 I can set it at 25-26 psi at 2900-3k but tapers down to 19.5 by 7k

Is this just because the stock turbo poops out by that rpm range? I can post a log after work in need be.
Thanks.

 

Pretty much the nature of the beast some have had luck with Ecu controlled boost but you're still gonna have taper.

I absolutely hated it with the 3 stock frame turbos my car over the years got so sick of it bough a T3 set up and would never go back

 

I had the same issue..you're never going to get rid of taper on the 8/9 stock frame turbos...however, I helped a lot of my taper by going with the Hallman Pro RX MBC, and the Forge 25# WGA...I used to spike 26 and fall to 18-19 at redline...now I spike 28 and hold ~21 to redline.

 

Ok thanks. Just was not sure if it was uncommon. I’m going to max out my stock turbo before going bigger. At what psi do these turbos become inefficient? I know every setup differs but just for a basic rule of thumb?

 

looks to be the same 7-8 psi drop befor and after you got the mbc and wga..

 

No, I'm able to hold 2-3 more psi now with the Pro RX and 25# WGA..on the old setup, even when I spiked 27-28, it would still dip down to ~18...now I've never seen it go under 20, even over 7k.

People run stock turbos all the way up to 30-32psi...this is the max efficiency on e85, with supporting mods of course. On pump gas, I believe around 25-26 is what people usually stop at.

 

Why not try running the actuator straight off the compressor discharge nipple for comparative purposes? This will help rule out your EBC as a possible cause of your taper issue. Just increase boost up to your peak boost limit by adjusting the length of the actuator rod shorter and shorter. See if doing it this way will yield a more stable boost slope and less taper. However, I have a feeling that more than likely, yours is an EBC issue.

The VIII turbo shouldn't be tapering that much if the actuator spring pressure is set up properly. In fact the IX turbo is more prone to taper than the older VIII turbo was. This is just my .02, but I don't like the AEM Tru-boost myself. I prefer any MBC to the Tru-boost.

 

My BOV was leaking that's why mine was tapering that much. Swapped it out and taper is almost non existent.

 

ive actually noticed more of a drop and taper by doing what you suggested the op to do. i doubt it is his ebc unless its just a slight difference in our setups.

with my forge wastegate preloaded to stock 18-19 psi and the boost controller controlling boost to 26psi the taper seems to be less dramatic and holds around 21 at redline. with the preload on my wg actuator set to 25-26psi and the hks evc off the it seems to spike at 26 and fall to around 18 at around 5000 and hold till redline. i am running an o2 dump and a 10.5 hotside though so that prob makes a difference however small it is?

 

I'm also noticing this,but in my case the boost doesn't drop.It just feels like the car doesn't pull anymore at high rpm's.I'm running a stock IX turbo with perrin ebc and the full HKS Intake and exhaust/filter set up.I was wondering if there's a boost leak somewhere,the previous owner told me there was and after that i doubled up on the lower ic pipe clamp.
I'm a new Evo owner.My boost is set at 21 psi,but i have to admit it only feels like this in low boost.I've got a high boost map as well,and the high boost map has better timing.

Maybe i'm just full of $%#@!Or getting used to the power.

 

The Forge 18 PSI actuator has an inherent spike that usually manifests itself in the 19-21 PSI range. So, in a way it does not have a perfectly linear resolution. It is a good actuator but works best preloaded like you have it at 18 PSI or less. I have preloaded mine up over 21 PSI and it worked well. At any rate once you compensate for its built in spike then it works fine.

For the Forge unit to work really well though it is best to trim 3/8" off the threaded tip of the actuator rod. If you don't trim it then it doesn't really react that well in the upper preload levels like where you are complaining about with your Forge WGA. I take it that you haven't taken a hacksaw to yours yet. It will make a night and day different

Also, I would think that probably your turbine housing may not be adequately ported especially in the turbine exit area. So, the turbine's exducer is shrouded and this probably is contributing to your taper. But, first try cutting the length of the actuator rod.

At any rate the OP I think has a stock MHI actuator, which is somewhat different to your Forge unit. It has a weaker internal spring and it can benefit from more preload. There is more of a tendency for the flapper valve to lift off its seat early due to the weaker spring and this can contribute to boost taper at levels over 22-24 PSI.

 

Ok good info, and yes I’m using the stock wg actuator. I’m trying to get up to 30 psi. I’m having trouble getting over 27 with my Tru Boost controller. What do others have the spring crack pressure set up as. I think this has to do with it after re readings the manual.