I'm going to be totally honest here- I didn't read your entire post as carefully as I might have otherwise, as I find it painful on the eyes. However I'd like to address a couple of specific points.

First, let us visualize the path of air from the compressor to the engine. I will refer extensively to the following diagram, which I have borrowed from Bell's "Maximum Boost" and modified to suit our discussion.


Here, we see the entire pressurized intake system. I have marked three points of interest. "A" is a point after the compressor but before the intercooler, typically the little hose nipple that comes stock on most compressor housings and is the classical boost reference. "B" is a point after the intercooler but before the throttle body, which is where my boost controller is referenced to. "C" is a point after the throttle body, within the intake manifold itself.

Now, putting aside arguments of boost threshold and such, can we all agree that, all else being equal, a boost controller is going to try to achieve the pressure to which it is set, at the point in the system being measured? And we'll also agree that the boost gauge on the dash is always fed from a point after the throttle body, such as the brake booster hose, the cruise control hose, etc.

Good. Let us assume that we have a hypothetical perfect boost controller (and a perfect wastegate) and that the boost controller is set to 12 PSI.

We'll start with point A, since that's where most people are referencing their wastegate to. In this scenario, the boost controller is always going to try to achieve 12 PSI at the compressor outlet. At relatively low speeds, the flow through the intercooler is relatively little, and thus the pressure drop across the intercooler is relatively small. Assuming WOT then, the pressure in the system at points B and C will be fairly close to 12 PSI. As RPM increases, the magnitude of the pressure loss across the restrictive intercooler also increases. So while the boost controller is ensuring that 12 PSI exists at point A (before the restriction), the pressure at B and C will decrease by the amount of pressure drop across the intercooler. By redline, we may be down to 9 or 10 PSI at points B and C, despite the fact that there is still 12 PSI at point A.



Next, we move the boost controller's feed to point B, and re-run the test, still at WOT. As airflow increases and pressure drops across the intercooler, the boost controller works to maintain 12 PSI at point B, and so we see 12 PSI being reported on the boost gauge, assuming zero pressure drop across the throttle body. (Give me this one for the moment on faith, and I'll come back to it.) Now, since we're still seeing 12 PSI at points B and C, and yet pressure drop exists across the intercooler, this means that the pressure at point A is steadily increasing. Under the same assumptions as in the first test (that 2-3 PSI of drop occurs across the intercooler at WOT at redline,) the pressure at the compressor outlet will have risen to 14 or 15 PSI by this point. This, of course, is because the boost controller is ignoring point A, and looking only at point B. Some extra heat is going to be generated owing to the fact that the compressor is now working harder than before, but as a percentage of the total heat being generated, this is relatively inconsequential.



Ok, now we'll move the boost controller to point C, and we'll also change one other assumption- we're no longer going to be at WOT. We'll be going up a fictitious hill (or accelerating past a fictitious truck on the highway) and so you are modulating the throttle with your foot to achieve, say, 8 PSI. What's important here is that we're added a second restriction in the system- the throttle plate. And there's going to be pressure drop across this. If boost starts to creep up, you're going to close the throttle a little, increasing the pressure drop across the throttle, and holding MAP at 8 PSI. Because you are actively holding MAP below 12 PSI, the boost controller is never going to reach its activation point, and the wastegate is going to remain completely closed.

Now the problem is that at this load condition, you're generating more than enough exhaust gas to spin the turbo well beyond 8 PSI. If you were to measure the pressure in the system at points A or B, you might find that you've got 20 PSI. Or 30 PSI. Or 40? Who knows, really. A lot is going to depend on the size of the turbo, the exhaust system, etc. But it's going to be a hell of a lot more than 8 PSI, or 12 PSI, or even the 15 PSI we saw at the compressor outlet in scenario B. And this is going to generate a shitload of heat. And heat is the enemy. Granted, reducing the throttle opening is going to reduce the volume of gas available to spin the turbine, and there will be an equilibrium point somewhere, but remember that the whole reason turbuchargers work in the first place is that there is presumed to be a significant excess of exhaust gas available at all but the lowest load conditions- otherwise, we wouldn't need wastegates, and for that matter, the damn thing would never spool up.


"But" you ask, "won't this also be the case at part-throttle conditions in scenarios A and B?" And the answer, of course, is no it will not.

We'll step all the way back to scenario A. You're climbing the same hill, at the same part-throttle condition. You're making enough exhaust gas to spin the turbo up into orbit, however the boost controller is going to make sure that there is never more than 12 PSI in the pipes at point A. Doesn't matter if you are working the throttle to regulate MAP to 8 PSI, or 10, or 4. The pressure in the intake pipes will never exceed the boost controller's setpoint, because the boost controller is watching the intake pipes, not the manifold. Once the pressure in the pipes reaches 12 PSI, the wastegate will open to ensure that it does not exceed this. Thus, the compressor will never generate more heat than it would in a non-throttled run.

Same goes for scenario B. The pressure after the intercooler will never exceed 12 PSI, and the pressure before it will never exceed 12 PSI plus drop across the intercooler. And since the mass of air flowing through the intercooler at this part-throttle condition will be less than it was in WOT scenario B above, the pressure drop across it will be less and the pressure at the compressor outlet will be less.


Any argument here?



Yes, like the intercooler. Which is why taking the MBC reference from a point after the intercooler provides the same benefit of a manifold-sourced reference, with none of the disadvantages.



You are correct. However in this case, we are less concerned with what's going on inside the motor as we are with what's happening outside it- namely in the compressor.



كما يمكن للمرء أن يجادل بأن لأنه بعد انتهاء عملية التبريد إما يسكونسن جيم أو غير ذلك من كثافة استهلاك الاتهام والضغط أيضا موثوق يقاس في المدخول متعددة. أي شخص مطلع على النحو الأمثل للقوانين يعرف حجم الغاز هو البديل في ضوء الضغوط ودرجات الحرارة نسبيا منذ دينا ثابتة ، ثم لدينا حجم الضغط ودرجة الحرارة والعمل بسهولة تماما في الطريقة التي خفض درجة الحرارة أكثر كثافة والضغط النسبي. حيث عندما ننظر في منفذ من وضاغط الهواء في دورتها أرق وسخونة لدينا قراءات الضغط سيكون أقل بكثير من تبريد الاتهام. تولي ضاغط لديه القدرة على ملء كامل المدخول المسالك بسعر لمواصلة الضغط والحرارة. لو كان التوربينية الخروج من كفاءتها مجموعة الخاص بك ، ثم ضاغط على عجلة الغزل دورة في الدقيقة ببساطة نفس النتائج في الهواء واستهلاك أقل حركة وسرعة في نفس الكلية أو أقل مما كانت عليه عندما ضغط ش فيها في نطاق الكفاءة. هذا يولد حرارة أكثر مما تدفق وحتى عندما تبرد ش تواجه فقدان المعادلة من حيث الأداء

This is basically what I see when I look at the rest of the message.

 

woot i speack sans scrit.

One could also argue that since it is post the cooling process either IC or WI etc. The Density of the intake charge and its pressure would also bemore reliably measured in the intake manifold. As anyone familiar with the ideal gas laws knows volume is variant given pressure and relative temperature since we have a fixed volume then our pressure and temp work inversly and quite readily.

In the way that the lower the temperature the more density and relative pressure. Where as when we measure at the outlet of the compressor and the air is at its thinest and hottest our pressure readings will be much lower than that of the cooled charge.

Assuming the compressor has the capacity to fill the entire intake tract at a rate to keep the pressure and heat up. If your turbo was going out of its effeciency range then your compressor wheel spinning at maxx rpm would simply be recompressing the same air results in less movement and intake velocity overall and the same or less pressure than when u where in your effeciency range.
This ultimatly generates exponentialy more heat than flow and even when cooled u are facing a loosing equation in terms of performance.

 

I see what u are saying joe but im not buying it the setup i describe will give u more power ultimatly and eliminate part throttle lag going to full boost. This is good and since i run a cooling method that is volume and heat dependant it works idealy for me in all situations. I can retain maxx boost longer and garner it in sooner comming from a partial throttle situation IE your hill etc.
This is all about what goes in your motor and makes the most hp man it was never any different.

 

I understood the sentiment of the latter half of your previous post, I simply don't find it relevant.

Let's say that in addition to an intercooler, you are also using water injection. And that like me, you have the nozzle located at a point just prior to the throttle body. In such a scenario, there will likely be a difference in air temperature and thus density between points B and C of the diagram.

However at WOT, there will not be a difference in pressure.

The ideal gas laws deal with the behavior of a gas of uniform temperature in a closed space. If you raise the temperature, the pressure increases, and so on. However I believe you are treating the volumes of the intake manifold and the post-IC, pre-throttle pipe as two separate spaces, and this is not the case. In order for a pressure differential to exist between two spaces between which a gas is flowing, there must be a restriction separating these two spaces.

While it can be argued that a wide-open throttle body does present some manner of restriction, it is relatively insignificant at the levels with which we are dealing and the argument is largely academic. Since there is no meaningful restriction between point B and point C, the pressure at these two points will always be the same, regardless of any difference in air temperature and thus air density that exists between them.

If temperature alone were enough to cause a significant change in pressure along the length of an open and relatively unrestricted path, than your boost gauge would read incorrectly, since the air at the gauge side of the tubing is a hell of a lot cooler than the air at the manifold side, particularly if you've got the top up and the A/C on.


Now, I do accept your argument that placing the MBC reference at point C rather than point B will improve throttle response somewhat in the part-throttle scenario discussed earlier- the "going up a hill" experiment. But this is because, as I stated earlier, the closed wastegate is going to ensure that you've got 20 or 30 PSI sitting in the pipe just waiting to flood into the manifold as soon as you floor the pedal. Unfortunately, this performance is not without cost- the whole time that the turbo was packing in the air, waiting for you to finally floor it, it was generating great gobs of heat, thoroughly heat-soaking your intercooler in the process. This wasn't so much an issue before, as the act of the intake air de-compressing as it passed over the throttle plate would have returned its temperature to a more reasonable level, but once you're at WOT, that benny is out the window and the interheater is in full effect. Where's the density advantage now?

Cliff's Notes: EBC reference taken after the throttle does give you the best throttle resonse on transition from partial boost to full boost, but at the expense of considerable heat production, increased exhaust backpressure, and decreased VE during part-throttle conditions. This is an unacceptable compromise in a street-driven car.

 

How much of a pressure difference is there at part throttle on each side of the throttle plate?

 

That depends on the throttle angle and the amount of boost in your charge pipes. Anywhere between 0 and 30psi, conceivably.

 

Looks like I'll be hooking up the magnehelic gauge or a secondary boost guage.

 

Yay! Empirical data!

I wonder if I could disable barometric compensation yet still datalog the second MAP sensor on my MS...

 

I would imagine one would need Hulk grade T-bolt clamps in order to even be able to run with the wastegate sourcing signal from the manifold.. 30 psi is not something I want to subject my pre-throttle body components to just yet.

I am curious to know whether all of the talk of the greddy wastegate being a feeble and weak part is actually based on a misunderstanding and in fact the problem is that the wategate's vac source is not optimal... given that a cheap and reliable MBC is still necessary to go over 6 psi.

-Ryan

 

Sorry I didn't answer these questions earlier- got wrapped up in thermodynamics.

In the picture above, the factory cruise control port is being used to operate the bypass valve, which is not visible (obscured behind the upper radiator hose) however it is just to the left of the crankshaft pulley, mounted midway up the coldside charge pipe that feeds into the throttle assembly.

The vacuum source for my boost control apparatus, as I said earlier, is a brass hose barb fitting mounted to the same charge pipe, just above the bypass valve. I drilled and tapped a 1/8" NPT hole in the pipe, and inserted a 1/8" brass hose barb fitting from the plumbing section of the hardware store. The blue tube that crosses the radiator fan is attached to this.

The vacuum source for my boost reading (both my in-dash boost/vac gauge, my MS's MAP sensor, and the MAP sensor for my WI controller) are taken from the large hose that feeds the brake booster. If you look just above the brake master cylinder reservoir (the plastic cup with the black cap) you will see a brass Tee fitting. (Yes, a lot of the stuff under my hood came from ACE Hardware.) That tee is located just before the check valve that's built into that hose. There's a thin semi-rigid plastic tube coming off of it (not visible) which passes through the firewall at the grommet for the cruise control, and is then split to feed the gauge and the MAP sensors. Yes, everything is fed off of one skinny plastic tube, of the sort that comes with vacuum gauges.


I threw away the original Greddy can after I installed the Garret one, so I'm unable to go back and make a comparison. All I can say for sure is that the helper spring on the stock Greddy can was not effective at combating the boost dropoff, however the Garret can was no better in this regard, given the same vacuum source at the compressor outlet.

 

I dunno, I've got a feeling, going up a hill with just maintenance throttle, it's not going to be 30psi difference. If that was the case you'd probably be able to hear a hiss like when a BOV opens every time you cracked the throttle, and there would be insta spike on boost guage.

But that's my hypothesis. I'll get around to testing probably in a few weeks (I know lame).

 

A few grammatical errors and misused words, but otherwise good.

 

i would love to see imperical data. joe the ideal gas law realy only limits to a specific quantity of gas in most examples it is a mole.
This being said all variables that can change are subject to the unchanged and thus move inversely. For simplicity i assumed we had a constant or near constant mass of gass for the entire experiment.

 

Well, I would consider this a productive thread. Why? Here's why:



I gained roughly 2.3 psi on the top end by applying the fix Joe clued me in on. Or in other words, I don't have to crank the MBC up to spike to 13 psi to get 10 at redline.

 

Wow, amazing info. You take any pics?

 

try it from a vac source and you will see even better performance man.

 

Win.

 

I did now:



Left to Right..
Pic 1: Barbed fitting drilled/tapped and JB welded into coldside intake pipe just before the throttle body (the one that is higher up).
Pic 2: Vac line runs from barb under the coldside pipe up to front of valve cover
Pic 3: Line runs along front of valve cover to MBC and then out of MBC to wastegate.

-Ryan

 

Maybe the title of the thread should be changed to make it easier for noobs to find this great info.

I suppose I should re-do mine now, having to spike to 13lbs to hold 10-11 is annoying.

 

I re-did mine yesterday, and it doesn't drop off but I still get a spike at the beginning. It overshoots my target by a PSI or two and then settles (rock solid) at 13psi.