Boost holding nicely until 4k+ RPM... then dropping

[free5ty1e]

What's up guys... been trying to get my Legacy up to roadworthiness for my cross-country trip, trying to button things up here so's I can get back to work on (among other things) the LegaCU prototype... and have discovered a little situation:

I've been playing with the location of that stock restrictor in my hybrid t-bird turbo / MBC / coffee muggified setup in order to get my boost proper. I've found that when the restrictor is in the line between the compressor and MBC, the boost spread between gears is lessened somewhat (but still significant, 3-5 psi difference). Now, since I hadn't had my crank pulley bolt tightened to specs before yesterday, I really haven't been letting my car rev above say 4, 4.5k RPM. With the bolt properly tightened, I decided to see what she could do. She doesn't like them high RPMs, because boost starts dropping (yes at WOT) and continues to drop until redline. In 1st, it hits 11 psi around 2.3k RPM or so, then begins to drop at 4k RPM, and by 5.5k RPM it's 10 psi. By 3rd gear, it hits 14 psi, begins to drop, and by 5.5k RPM it's 11 psi. 4th and 5th gear weren't fully tested like this due to lack of proper testing grounds, but they smack 15 psi pretty easily and begin to drop at the same place.

The restrictor doesn't do much when on the other side of the boost controller (between MBC and wastegate actuator). Reversing lines on the MBC results in wastegate boost so I know the controller's hooked up properly. I removed the restrictor entirely and the boost spread between gears increased a bit - and the boost still drops at high RPMs...

I made sure to keep my FMIC piping all above 2.5", except for the turbo outlet (about 2") and the BOV piping (about 2.25") as I had to have some crazy bends to get around the radiator and such, but I am relatively sure those bends don't restrict things much as they are made with pre-formed diesel radiator hose with an inside diameter of over 2.5"... I've really got to get some photos of this routing. But anyway, I've also got a 3" turbo-back exhaust with no cat.... it's mated up to the muffler that came with my car, which granted isn't 3" but it's not very restrictive at all and I doubt that's the problem. But damned if this situation doesn't sound like a restriction somewhere! I'm stumped.

Shouldn't a properly-rebuilt hybrid t-bird turbo with a 3" TBE and 2.5" FMIC be able to hold boost levels all the way to redline?

Hey - if it is a restriction, would my MAF sensor's reading seem to cap off at a certain value? Perhaps this is an excellent time to have MAF readings close at hand when driving.

Any suggestions?

[legacy92ej22t]

I'm not sure about the T-bird Hybrid but it is totally normal for the boost to drop off on a stock VF11. It will boost strong until around 4800 rpm and then fall back to 10-11 psi.

[free5ty1e]

Hmm... I was under the impression that without the restrictive stock exhaust, even the stock VF11 would hold boost until redline. Maybe it's just too small of a turbine, no matter what compressor's stuck on it.

The T-bird's compressor wheel is only like 2% taller than the stock VF11's compressor wheel. The housing is noticably different but the wheels had to be inspected closely next to each other to see the difference. I guess I need that 16G.

My next question, then - for those of you who regularly get quarter mile drag times for your Legacies (with the VF11 turbine still in), where do you shift? It would seem our powerband is not near redline...

And while I'm at it... do our stock injectors have enough capacity to support anything the MAF sensor can read out? That is to say, can I increase boost until my MAF voltage is near 5V and assume the injectors can keep up with anything that sensor can read? Or did Subaru not match up these two portions of our cars...? I'd like to turn the boost up now that I'm intercooled and such, but I don't want my first indication of maxing out the injectors to be the A/F ratio suddenly dropping off to the lean side...

[legacy92ej22t]

Having the bigger exhaust allows you to run boost levels above 12 psi, which is where you hit the wall on the oem exhaust, but the VF11 still will not hold boost in the upper rpm's.

I had to vote #3. My 16G holds all the way to redline.

Shifting- I usually tried to shift in the 5800-6200 range on the VF11. It does feel like the car is still pulling all the way to redline but I think it has been proven on the dyno that our heads don't flow very well past the 5800-6000 rpm range.

On the 16G I'm not totally sure yet. I only got to race the one day and the car was only really punching on 3 cylinders. I still think our heads will have a hard time producing much power after the 5800-6000 range even on the 16G but I can tell you that the car sure feels like it pulls very hard all the way to redline and boost doesn't drop.

In terms of the MAF. Our MAF hits the rail at about 12 psi, causing the injectors to go 100% IDC. I highly recommend that you install a Walboro 255 fuel pump if you're going to up your boost too.

[professor]

It is very rarely advantageous to shift before redline...the power may be lower at say 6500 rpm than 5800 due to breathing issues, but it doesn't drop off that far and its still better than the next gear for acceleration purposes

[biggreen96]

my boost falls off too... neeeeeeeed vf39

[BAC5.2]

The stock turbo does NOT hold past 4500 or so, nothing about 11psi.

The 16G, however, pulls to redline and thensome.

As a bonus... I make peak torque at the wheels with the 16g, only a few hundred RPM after peak engine hp is made bone stock .

16G represent!

[biggreen96]

i mean: neeeeeeed 16G

[free5ty1e]

Heh... OK at least I know I'm not alone on this, I was going nuts trying to figure out what I had done wrong. Now I know: I haven't picked up a 16G yet.

Thanks.

Matt - are you saying the stock VF-11 at 12psi smacks the upper MAF rail? Or the 16G at 12psi smacks the upper MAF rail? Because I've never had a problem running 14-15 lbs with the pig rich a/f reading that our cars like to maintain...

(OK - who said their VF-11 holds boost until redline on the poll...? Please 'splain.)

[legacy92ej22t]

Matt - are you saying the stock VF-11 at 12psi smacks the upper MAF rail? Or the 16G at 12psi smacks the upper MAF rail? Because I've never had a problem running 14-15 lbs with the pig rich a/f reading that our cars like to maintain...

Both. It's around 200 g/s at 12 psi. I'm not suprised that you're seeing rich readings. Anything over 12 psi and the ecu is running the car full rich, 100% IDC. You really don't want IDC #'s over 85% but I guess that's why we're all so excited about your ECU project.

If you're running a run of the mill a/f gauge I wouldn't take it as gospel either. It very well may be reading rich but at 15 psi I'd say it's a safe bet that you're leaning out a bit. Phil and I both were experiencing lean condition induced fuel cut at around 16 psi on the VF11 (as well as on the 16G of course), even though we were running FCD's. A Walboro 255 seems to cure this fuel cut condition.

From Phils recent dyno pulls it seems apparent that fueling at 15-16 psi on the 16G is definitely on the lower edge of "the safe side" though.

[legacy92ej22t]

It is very rarely advantageous to shift before redline...the power may be lower at say 6500 rpm than 5800 due to breathing issues, but it doesn't drop off that far and its still better than the next gear for acceleration purposes

Ya, I know exactly what you're saying and I agree with you in principle. I used to always shift at redline when I first started doing 1/4 miles. This past year at the Shootout I was playing with my shift points though (since I wasn't very competetive when running on 3 cylinders) and when I started trying to hit my shift point in 1st and 2nd at around 6000 rpm, instead of 6500, my times dropped by about .3-.4 . I would usually end up redlining 3rd because I would top out only a couple hundred feet from the line. My fastest time of the day I got into 4th about 800' before the line though because I got a great launch and shifted around 6000 rpm on 1st-2nd and 3rd.

About the power not dropping off that far after 5800 rpm I would have to disagree. On the dyno graphs I've seen for the EJ22t, it looked like the motor hit a brick wall at 5800. Power fell of sharply. I do think that was on a VF11 though and not a 16G. The 16G might be able to push through the heads weakness a little better then a VF11.

[BAC5.2]

I peaked torque at 3200 RPM. At redline of 6500, I was only making 140lb-ft at the wheels.

I made over 200lb-ft until 5500. So in that last 1,000 RPM, I dropped 60lb-ft at the wheels.

I also made more than 200whp until 6000, where I dropped 20whp to redline, ending at 180whp.

I can't tell if that's from running "unsafely rich" or if it's from the heads. I start to lean out (though still above stoich) to around 13:1 right before 5,000 RPM, and I hold 13:1 until redline.

[professor]

Wow that is a big drop in torque. I'm not as familiar with dealing with the quirks of turbo curves. Considering that 3-4 and 4-5 shifts are usually a ratio difference of only 20-25% it may indeed pay to shift a bit sooner, but not likely the 1-2 or 2-3 shift where the mechanical advantage change is 30-40%.

There are some way-cool drag race simulators out there. You feed in your hp and torque curves, and you can play with shift points, gear ratios, rear end ratios, etc.

Neat stuff if you are considering mods, to see where your dollars could be best spent to reduce your times, and to learn how to set up your car best. Particularly cool to stick in changes to the final drive ratio.

[Brat4by4]

mated up to the muffler that came with my car, which granted isn't 3" but it's not very restrictive at all and I doubt that's the problem.

What kind of muffler came with the car? If it is not a 3" straight through perforated core muffler, then it most certainly is a restriction to the system!

My exhaust has no point that goes below 3" from the turbo to the tailpipe, muffler is 4" I.D. I would hit fuel cut in 4th gear above 100mph in the winter before this one. stock turbo, stock intake.

[free5ty1e]

Matt - are you saying that the VF11 and the 16G both flow 200g/s at 12psi?! I do believe the compressor maps would disagree on that point... Or are you saying our MAF sensor can only read up to 200 g/s...?

Hmm... Hopefully I will have some free time to get back on my LegaCU project, I'd like to see exactly what my converted MAF reading is while driving. I definately plan to get the Walbro pump anyway as the first upgrade to the fuel system. I think I'll run VRG's scan tool just to see what kind of IDC's I'm running above 12psi. I never thought I'd be running above 80% with my current setup, as I've seen the car compensate for 20+ psi of superheated plasma on the occasion when my wastegate control line blew off and decided to be free. (of course I didn't hold this boost at all, as soon as I saw the gauge climbing that high I stopped and fixed the problem)

Brat - Well I have no idea what muffler it is, but it's stainless steel and looks to be about 2.5" on the opening. I don't think it was the actual stock muffler, the guy I bought the car from had it put on at one point or another. I had the exhaust shop flare the opening out so's the 3" pipe would sit in there without a problem, and it really doesn't look too restrictive. But I went ahead and searched around and found a 12" long 3" glasspack, I've got it sitting in my living room. I just really like the way the car sounds in its current configuration. Although... the glasspack would only let more of the boxer sound out now wouldn't it? Guess I should try putting that on at some point, huh...?

[legacy92ej22t]

Matt - are you saying that the VF11 and the 16G both flow 200g/s at 12psi?! I do believe the compressor maps would disagree on that point...

No, I'm not saying that but rereading my post it sounded that way. The 16G should be pushing significantly more CFM at that pressure.

Vikash and I were trouble shooting my FC problems (even though I was running a FCD) with the scan tool and during this we discovered that the ECU was throwing my injectors into 100% IDC at around 12 psi. This was with the 16G. So after some more poking around with the scan tool we saw that the MAF was also hitting the rail at around 12 psi. So we concluded that the ECU throws the injectors into 100% IDC when it sees the MAF hit the rail.

We double checked this on Phils car (also a 16G) and I was thinking that we also saw the same thing on the VF11, that the MAF hit the rail at around 12 psi but I'm not 100% sure. I am however sure of the numbers with the 16G. If Vikash pokes his head in here we can ask him for sure if we checked against the VF11, I think we did but....

Or are you saying our MAF sensor can only read up to 200 g/s...?

Yes, I am 100% saying that.

[vrg3]

With the stock turbo you'll have a hard time pushing enough air to max out the MAF sensor... maybe with a really good intercooler and exhaust... But, yeah, we only saw the MAF sensor hit its rail with the TD05-16G.

When playing with the scan tool keep in mind that when things get heated up the ECU sometimes stops reporting data because it's busy concentrating on its other jobs.

[legacy92ej22t]

Heh, I wonder why I was thinking we checked it on the VF11 too? Oh well.

I would be really interested to see what kind of MAF voltage and IDC numbers you guys are getting on the VF11 at say, 10-12-14 and 16 psi.

Hmm, that may be a thread starter.

[professor]

Isn't the flow rate THROUGH the engine solely determined by the pressure differential between the intake and exhaust, rpm, and the temperature of the charge ?

So if the air is the same temperature (a bad assumption), it wouldn't matter what turbo is pushing the 12 psi, the flow through the engine would be the same. The driving force for flow is the pressure differential alone, there isn't some secondary factor making it flow faster with a bigger turbo. You still have the same heads to go through.

The bigger turbo may excel but only because it can either:

deliver the 12 psi at a lower charge temperature, or

it can maintain the pressure at higher rpm, or

its efficiency is better so lower back presure is generated



The turbo maps tell you what the turbo can theoretically flow under some bench conditions, but if the process is throttled through the same engine, the flow would be very close to the same at the same rpm and boost pressure

[vrg3]

You're right, professor. For the reasons you give, you get much more airflow with the TD05-16G at 12 psi than with the VF11. The 16G compresses the air more efficiently which heats the charge less. The TD05 provides less restriction at the exhaust which improves volumetric efficiency. And the turbo's big enough to sustain high boost at high engine speed.

[free5ty1e]

Making more sense now. I'll see what the scan tool reports, and if I've got half a chance I'll see what my LegaCU MAF conversion reads.

I'm still wondering who says their stock turbo holds boost until redline on the poll. Please 'splain. Inquiring minds want to know.

Hey - with the wastegate control line off, and the vf11 pumping out 25 psi... think it'll still drop off to 11psi @ redline? hehe

[legacy92ej22t]

I'd be willing to bet that who ever voted that their turbo holds boost to redline is running stock boost, or at least under 11 psi of boost. If you were running 11 psi or under it would hold strong baby.

[Brat4by4]

I would hit fuel cut in 4th gear above 100mph in the winter before this one. stock turbo, stock intake.

Fuel cut is over 13 psi. But it was cold on the other hand... I have never cranked up my boost because I don't have the equipment installed to warrant that. Its all sitting in my room or the garage

And yes, at stock boost it holds the gauge pegged all the way to rev-limiter.

[free5ty1e]

(damn technicality... forgot to specify higher-than-stock boost levels)

Bastids

[dzx]

Matt - are you saying that the VF11 and the 16G both flow 200g/s at 12psi?! I do believe the compressor maps would disagree on that point...

No, I'm not saying that but rereading my post it sounded that way. The 16G should be pushing significantly more CFM at that pressure.

I'm pretty sure on the same engine at a lower rpm, the two turbos will flow the same amount of air. The point where you will see the difference will be at the higher rpm where the engine is consuming more air. The smaller turbo won't be able to flow enough air to keep the intake manifold pressurized so you would see a drop off in pressure. I don't really understand cfm ratings on turbo's because on different engines, they will flow different amounts of air.

To make it accurate at all, I would think they would need to measure the airflow on the engine in question or your application with the turbo at a higher rpm to show where one turbo would flow more air than another. If that makes any sense.