I am running a standalone engine management system on an Evo 8.

My understanding from talking to my tuner is that no matter what the settings - the stock turbo is not capable of holding 20psi to the redline. Typically it spikes to 20 / 21 psi and then fades to about 16 psi at redline.

So my question is - what causes the fade and what can I do to get it to hold that boost? Does the actuator needs upgrading? Is there an issue with the way the boost control solenoid is plumbed? Or is it just a design limitation of the turbo?

 

The truth is it will hold boost to redline. It's the stock solenoid that tapers boost, not the tubro running out of juice. I run 1.45 BAR to redline on the stock turbo with no problems. A cheap manual boost controller will cure the boost taper problem.

Did your tuner by any chance try to sell you a turbo that would hold boost until redline?

 

It cant flow enough air at Redline....As the consumption of air per period of time increases with an increase in revs, the turbo cant stay ahead of air consumption rate at max boost....Not without seriously overworking and heating up the air significantly at least.

 

ur being jerked around i hold 21 to red line. get a boost controller

 

Yes, I am an idiot... I thought you meant hold more than stock boost rather than stock boost...My bad, I thought everyone knew the stock turbo can easily hold stock boost to redline with a MBC.

 

And one other thing..my redline is 7800 RPM.

 

your boost willl be peak around 35k~45K and dies off even with MBC. The reason is simple. when rev is high, your pistons will suck out the air faster than the stock turbo can supply.

 

That's your rev limit. The stock turbo cannot hold 21psi to 7800rpm. It naturally tapers as you reach 7k rpm, and after 7k rpm it drops off significantly. The only way to get it to hold as much as 21psi by 7k rpm is to run much higher than 21psi and let it fall to 21. Either way, by 7800rpm, there is no way you're holding 21psi unless you force it to do so...it won't do it on its own with just a regular MBC. If you make it hold 21psi by 7800rpm, then it's just blowing hot air. The stock turbo is not efficient at that rpm.

I have to give this example a LOT, but Sean Glazar runs 11.5s at 119.5 on his stock 9.8-hotside turbo, and even with the wastegate completely open running max boost the whole way, his AEM EMS reads 17.3psi at the top of 4th (7600rpm).

 

Ok - so if I am peaking at 23 psi between 4 - 5k rpm then its reasonable to expect the stock turbo to fade to 18 - 19psi at 7krpm? (I dont want to go over 23 psi on straight pump).

And theres no point in trying to block the taper at this level because the turbo would be out of its efficiency range?

 

19-20psi your turbo will hold till redline. with boost controller

 

If you are running a 23psi spike, then with boost control, it should not taper below 20 by 7k. After 7k as you approach the rev limit, it should taper to 18-19, but not that low by 7k...that's a bit too low. Remember, we are also talking about no lower than 3rd gear. You can't set/measure/tune your boost in lower gears. 4th would be preferable, but you can't normally do 110mph runs all over the place...

 

Thank you that answers it in a nutshell!

 

actually it is the internal wastegate and actuator design there is too much leverage against the actuator spring cause of the long lever on it , pressure on the internal gate is atleast 60 psi against it and with the lever equals too much pressure against a long arm equals a wastegate opening, a mbc just helps u set where the actuator starts too bleed off pressure but cant control the wastegate totally, the forge unit allows u to insert higher pressure springs so it can hold better but it still cant control like an external gate

 

If Im peaking at 23psi is it worth upgrading the actuator on the stock turbo if you have gotten the most you can out of boost control and advancing timing?

 

boost controller and a aftermarket internal or external waste gate is what i have put together to hold boost to redline.