Man, I forgot the joys of having a 16g and how great they hold boost at high rpm. The GT30 in my old VIII would hold insane boost at redline, NOT THE STOCK TURBO, BABY! Its just been too long to remember clearly.

We put the car on the dyno this weekend, which was probably stupid, because it has 1200 miles. I broke it in properly, but it was a little soft on the dyno, coming up about 20whp shy of a catless IX. I attribute this mostly to the fact the engine is green and I'm running a cat. I love the Mivec help post flash, and the midrange torque is incredible. The boost dropoff sucks wind, though.

In the past, I had an XEDE, and tuned with XEDE boost control. Sadly, when I sold my VIII, the XEDE was part of the deal, so I'm using a flash now. Pilling the stock line with the restricter removed from the BCS made me have too much of a spike. I pulled the bleed tube and it was a bit better, but still spiking more than I wanted it to. After the spike, it would really fall off.

No problem, I have a Forge MBC! I install that puppy, and hit close to 23psi in the middle for a second, then it tapers, recovers a bit, then tapers and totally pukes and dies to around 15.5-16psi at the ~7800rpm rev limiter we flashed.

Car has only the MBC, dump-back exhuast with cat, and flash. Once tuned in and logged, the knock counts were nonexistant (thanks Bruce!), but I really want that top end power that it should have.

In other opinions, do you guys lose that much boost or do you think its a new engine issue? Fueling control is rock solid, so there is no boost leak.

I guess I need to get a couple thousand more miles on her, run a test pipe, save those pennies for another XEDE, and try again.

 

why does the boost drop that much at high rpm's?

 

Internal small WG, smallish turbo, take your pick.
I'm just not really used to an MBC, but know other people have had success with them. I just want to sustain more boost like I could with XEDE's RPM controllable boost solenoid driver map.

 

Quote:

Originally Posted by Noize

Internal small WG, smallish turbo, take your pick.

A too small wastegate would cause boost creep and an overboost condition. Likewise a too small or tight turbine housing would also tend toward an overboost situation. Are you sure you installed your MBC correctly and eliminated all of the factory boost hose schemata, including the dreaded BCS and both restrictor pill orifices?

 

Get an RPM based boost controller, like E-01, Apex AVC-R or new HKS EVC 6. You can dial the taper out with that.

 

I'm nowhere near overboost.

Everything is installed correctly, this is my second Evo. Pills removed, triple vacuum line is completely out of the car from BCS to turbo and BCS is capped off.

Forge UNOS bottom nipple going to nipple at turbo, side nipple to WG actuator arm source.

I agree with Turbodawg that I need RPM dependent boost control like the XEDE offered, but there are some fast Evos with MBCs. My main question is do all these IX turbos all fall off this bad with MBCs?

 

i dont know man 15 16 is pretty low it should be in the 20's.

 

i have a 16g tme white rabbit turbo i hold 22 pounds to 7,700.and it starts at 24 pounds.

 

Most IV's I have seen fall off pretty badly. Even with a MBC, most will fall a minium of 3 psi from 6000 RPM to 7800 RPM. The backpressure in the turbine housing increase dramatically above 5800 RPM, IMO. That is what is the cause of the taper. The RPM based boost controllers allow you to counteract this increasing in turbine housing backpressure as the RPM's rise. In my mind, this is the only solution. IMO, people with MBC who claim no taper, are not watching close enough and/or not logging boost.

 

Quote:

Originally Posted by TURBODAWG

Get an RPM based boost controller, like E-01, Apex AVC-R or new HKS EVC 6. You can dial the taper out with that.

Yeah, there may be something in trying to sustain boost via tailoring the WG duty cycle with a good EBC. But, that would still beg the question of why you´re experiencing such exaggerated taper with your MBC.

 

I had a similiar problem with my MBC and I reseated the spring and put in the recommended lube and it was good to go.

Search for forge unos mbc and you should get some diagrams and other tips, good luck.

 

Up to a point, I agree with Turbodawg that excessive exhaust back pressure may be a contributing factor to your excessive boost taper. But, I am not convinced that an RPM triggered EBC is the sole possible remedy.

If excessive back pressure is indeed causing your taper, then, it would follow that an actual reduction in back pressure might mitigate the taper phenomenon as well. Yeah, the EBC may solve the problem of tapering boost, but it in no way does it reduce backpressure. It may turn out to be an effective band-aid, though.

BTW, TurboDawg, your car´s torque numbers are so impressive, so impressive, so i-m-p-r-e-s-s-i-v-e!

 

But its a well worn basic mod path that hundreds of others have already traveled. If there is any possible source of backpressure I could think of, it would be the cat pipe.

I just think I'm a piggyback guy and not a reflash guy at heart.

 

I'm more than willing to look at this as well.

 

my VIII had significant boost taper even with an MBC...it went away after I installed my turboback.