I must be missing something due to lack of hands-on experience...

Is there any reason to taper boost while approaching redline when given a turbo capable of sustaining peak boost to redline?

No not really. As long as you have enough fuel, and your EGTs are not getting that hot I can't see a reason to taper the boost.

my vf-10 won't hold boost till redline it self tappers from ~1 bar at 3000 down to ~.75 at 7k but its TINNNY

Okay, part 2 of my question is: What about the TD04-13G design is keeping it from boosting past ~13psi at redline? I have heard everyone state that the exhaust housing and/or the turbine itself limits flow, but I can't wrap my head around that really. Is it simply choked up and the air to produce only ~13psi is making it through quick enough? One would think if it can reach 18+psi with less air, say at 4500RPM if you chose to make it do that, then it should be able to reach that with more air due to the increased RPMs at 6500-7000.

I'd like to hear as detailed an explanation as someone can give about what is happening.

Thanks for any info.

The stocker only pushes about 350-360 cfm IIRC. The compressor wheel (maybe housing too but I'm not totally sure) is too small to physically push enough air out to create more than ~13-14 psi at redline. The engine is basically consuming air faster than the turbo can push it out. This is also why you can flow 18-19psi in the midrange, but not at redline.

The way I understand it is that at redline, our engines are obviously moving more air than at 4k. For the turbo to compress the intake manifold to 18 psi at 4k, it has to be spinning at say 70000 rpm. To hit 18 psi at redline, it'll have to spin at almost twice the speed to keep up the compression. At that speed, the tips of the compressor blades are passing the speed of sound, creating shockwaves that effectively lowers the efficiency of the turbo and at the same time increases the outlet temps a lot. This is just the compressor side of the turbo. The TD04 is also hampered by its small hotside (turbine+housing), which creates a whole lot of exhaust gas back pressure at higher rpms. This futher reduces the VE of our engines.

I've also learned from my experience that a small hotside matched to a larger cold side (compressor) typically will not produce stellar results. Best to get a balanced solution that increases the VE of your engine (not just a good turbo, but heads, cams, IC, etc).

I'm with mbiker97. I don't think anyone is intentionaly tapering boost.

Up to a point the turbo is compressing air that creates boost, beyond that point the turbo is compressing the same amount of air, the engine is gulping more air, but the turbo's compression contributes less percentage-wise to the total air consumed, so the boost pressure falls off.

You also have back pressure building up behind the turbo because you just can't push more air through that hole without more pressure. You are now wasting energy with back pressure as the engine produces more gas than the turbo can flow.

The VF39 spools very early, but for the capacity of the engine it is too small. On the exhaust side I think it dies around 350 CFM, but the engine is capable of ~450 CFM with the stock injectors.

I think most of us would like have a larger turbo with some sacrifice in spool up. How much larger you can go without running out of fuel from the stock injectors has been a debated subject. I believe the stock STi is fuel limited, but just barely before the turbo becomes the limiting factor. If you look at say all the PDX tuning dyno graphs that's what I see.

Thanks for the descriptions. That makes it very concrete to me now.

I've just read so many places where the statement is made that "it can't do it" or it "runs out of breath" but no real explanation of why it is flow-hindered.

Ask and you shall receive :)

You guys missed one major point... heat. At higher RPM's, as the turbo tries to make full boost and is spun much faster, it runs out of its effeciency range. This means that the air its pumping is much hotter. When that charge is cooled down by the IC, there is not enough air to make full boost and spinning the turbo any faster will just heat the air up rather than actually pumping a significant amount more in... it will still be puming a bit more air, but the temp difference will almost negate the extra air.

-- Ed

Something else to think about...
I've heard that if you push 18-19psi in the midrange, the TD04 will hold 15psi at redline. I have yet to figure out why or how. My guesses are built up heat, or a sort of inertial effect. No data either way yet though :)

-- Ed

You guys missed one major point... heat. At higher RPM's, as the turbo tries to make full boost and is spun much faster, it runs out of its effeciency range. This means that the air its pumping is much hotter. When that charge is cooled down by the IC, there is not enough air to make full boost and spinning the turbo any faster will just heat the air up rather than actually pumping a significant amount more in... it will still be puming a bit more air, but the temp difference will almost negate the extra air.

-- Ed


no, we didn't miss that point... from my post:


At that speed, the tips of the compressor blades are passing the speed of sound, creating shockwaves that effectively lowers the efficiency of the turbo and at the same time increases the outlet temps a lot.



Also, a TD04 can hold 18 psi at redline. Look at the fastest stock turbo results on the 1/4 mile. They're running 22 psi at the midrange falling off to 18 at redline and pulling 107 mph+ traps. Sure, the intake charge will be damn hot unless you've got a really good IC doing a fine job cooling it off - but it has been done..

Sorry, I missed that part of your post.

Now about the 18psi at redline.... I don't understand why running more boost in the midrange allowes the turbo to make more boost at redline... Sorry for the hijack

-- Ed

That poor poor little turbo.

Don't mind the hijack. I got my answer :)

np :)

warning been partying tonight...
Boost pressure is based on air restriction (load). The faster the engine is spinning, the more air it breathes and usually the turbo has to spin faster to keep the boost pressure at a constant. If the turbo falls far enough off its efficient operating speed and begins to create super heated air, the boost pressure tends to decrease. There are exceptions to this though, if a very large turbo is hooked up to an ej20, it may never begin to reach the falling side of the turbo's efficiency "curve" and could happily keep building boost till redline.

did that make any sense? lol :banana:

It made sense, but it didn't answer my question at all. I understand that a larger turbo will hold more boost at redline without a problem. The question is why would running 20psi on a TD04 in the midrange, allow the same little TD04 to make 18psi at redline while it won't come close to 18psi at redline otherwise?

-- Ed

i'm with you Vaus. i dont seem to understand it either. if pushing the stock turbo to 22 in mid range, why can it fall to 18. why couldnt you just tune for 18 midrange all the way to red line? obviously we are talking hot air and you'd get more power from a larger turbo at the same pressure.

also these fastest stock turbo times, are the turbos ported or 100% stock?

I'm pretty sure they're stock. A ported turbo will mainly help with spoolup, not topend.

It seems that the more boost you run in the midrange, the more boost the TD04 will hold at redline. I just don't understand why?

-- Ed

Hmmm, there are a lot of possible reasons to this.

We're still dealing with a turbo that has a wastegate / boost controller. Setting the boost controller so that it runs 18 psi at midrange and at redline is easily done - all a boost controller does is leak boost away from the wastegate diaphram so that the wastegate only sees x amount of boost - say 6 psi (8 psi is when the stock wastegate opens). But if you'd remove your wastegate hose, it would never open (unless the exhaust gas back pressure forces it open - a common occurance on sti's with catless turbobacks and the stock vf39) and you'd probably see more than 22 psi at the midrange but then would see it gradually fall off towards redline (less load).

Using any electronic boost controller, you could set it up so that you see 13 psi at midrange and 18 at redline if you really wanted to - it be kind of pointless though since the TD04 really shines in the midrange. It doesn't have to make more boost in the midrange in order to make more boost at redline.

Thanks guys...this thread just answered a question I had after going up to a Cobb Stage 2 w/ DP,UP, TBE.
I couldn't figure out why I'd peak at 16psi@5K(+-) and then drop down to 13psi by 6K.
Makes sense to me ;)

Hmmm, there are a lot of possible reasons to this.

We're still dealing with a turbo that has a wastegate / boost controller. Setting the boost controller so that it runs 18 psi at midrange and at redline is easily done - all a boost controller does is leak boost away from the wastegate diaphram so that the wastegate only sees x amount of boost - say 6 psi (8 psi is when the stock wastegate opens). But if you'd remove your wastegate hose, it would never open (unless the exhaust gas back pressure forces it open - a common occurance on sti's with catless turbobacks and the stock vf39) and you'd probably see more than 22 psi at the midrange but then would see it gradually fall off towards redline (less load).

Using any electronic boost controller, you could set it up so that you see 13 psi at midrange and 18 at redline if you really wanted to - it be kind of pointless though since the TD04 really shines in the midrange. It doesn't have to make more boost in the midrange in order to make more boost at redline.


Nope, I'm talking about a ball and spring MBC as boost control. If you set it for 18psi in the midrange, the MBC will remain closed, not letting any boost through the wastegate signal line, as long as boost is bellow 18psi. This is the case in the midrange as well as redline. Yet, the turbo makes more boost in the midrange which suggests that it cannot make any more than it does at redline. If you increase midrange boost, however, the turbo is able to hold more at redline... all with a ball and spring type boost controller.

Thinking about this more, I have another guess of why this is happening. Remember that the boost controller is acting on the boost at the turbo's outlet which is pre-intercooler. Maybe at higher RPM's the pressure drop through the IC increases, and therefore results in a lower manifold boost while the pre-IC boost may be maintained at the same level as it was in the midrange. Just another guess to throw out there.

-- Ed

Good thought, but I just can't buy 4-5psi of pressure drop because of 2000RPMs.

well we're talking about a pressure drop difference of about 2-3psi as you approach redline... I think that might be a bit more believable, but who knows... I guess the only way to prove/disprove this is with two boost gauges.

-- Ed

Hmmm I'm not too sure why it would react this way with a MBC. I've worked with the AVC-R and the stock EBC + MBC setup as a leaker. I'm going to have to think about this one. :)

The stock engine management tapers boost at redline beyond the natural taper of the TD04. Cobb and Vishnu have made a fortune based on this.

The stock engine management tapers boost at redline beyond the natural taper of the TD04. Cobb and Vishnu have made a fortune based on this.

yes... this is old news. Both cobb and vishnu claim more power by tapering beyond the natural taper... but I have yet to see any proof of this.

-- Ed