Boost sag? No, you don't need EBC.
05-18-2010, 03:32 PM
#1
Boost Pope
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Boost sag? No, you don't need EBC.
This topic has been coming up every now and then. Specifically, a lot of folks seem to be having boost curves that look like this, and perceive it as justification for the fact that their manual boost controller just isn't cutting it, and they need to go out and spend money on a fancy electronic boost controller.
So, does that look familiar? If so, save your money. Fixing this problem is going to cost you about $3.
First, let's take a look at why this is happening. Here's a simplified diagram of your turbo system, where we have the turbocharger itself, then the intercooler, and then the throttle body.
Point "A" in this system is where a lot of folks have their boost controller connected. It's that nipple that came from the factory on the side of your compressor housing, probably with a hose already attached between it and the wastegate actuator.
Well, that's just stupid.
What's happening here is that your boost controller is in fact maintaining a constant level of boost, however it's doing it in the wrong place. Specifically, it's maintaining a constant level of boost at the compressor, but that's not what your engine is actually seeing.
Confused?
Yes, behold the simple drinking straw. Solver of great mysteries.
Here's a quick experiment. Stick a drinking straw into your pie-hole, and blow through it. Not too hard, very gently in fact. Very, very gently. This ain't Hustler's mom we're dealing with.
Feel the resistance that the straw is offering? No? Of course not. At the rate at which you are blowing into it, the straw is not much of a restriction at all.
Now, blow harder. And now you start to feel the straw fighting you.
A funny thing happens when we try to flow a gas through a restrictive orifice. The more we try to flow through the restriction, the more restrictive it becomes. In practical terms, at low rates of flow, we get very little pressure loss across the restriction. As flow increases, so does the pressure loss. And it's not linear, either. The magnitude of pressure drop increases almost exponentially with flow rate.
Now, it may not look like one, but your intercooler is a drinking straw. A large, heavy, aluminum, multi-faceted drinking straw. Or, at least, it exhibits a lot of the same characteristics as one. So, back to the diagram:
Say that we have our boost controller set such that we see a peak of 12 PSI in the intake manifold. At 4,000 RPM, we reach that point. We are, incidentally, flowing about 130 CFM through the intercooler (our engine has a 100% VE at all speeds) and we're loosing about 1 PSI across the intercooler. So while the pressure at point B is 12 PSI, the pressure at point A (which is what the boost controller is seeing) is actually 13 PSI.
Now, we increase the speed to 7,000 RPM. At this point, we're moving about 220 CFM, and yet, what's this? The pressure drop across the intercooler has increased to 3 PSI! We didn't even double the flow, and yet we tripled the drop. (Well, I'm ignoring the fact that these are relative, rather than absolute pressure values, but you get the idea.) So now, even though the MBC is faithfully holding 13 PSI at the compressor, we're only seeing 10 PSI at the manifold.
The solution here should be pretty obvious by now. Move the boost controller from point A to point B.
By doing this, we are now telling the boost controller, in essence, "Hey, I want you to do whatever it takes to maintain a constant pressure at point B in the system, and to hell with what's going on over at the compressor." And it will comply. (Boost controllers are pretty simple-minded like that. They don't question orders.) Specifically, it now does not matter what the drop across the intercooler is, at least insofar as your actual manifold pressure is concerned. As drop across the IC increases, the boost controller will cause the compressor pressure to increase accordingly. So by the time you get to 7,000 RPM and are experiencing 3 PSI of drop across the IC, the pressure at point A will be up to 15 PSI, and you'll still be getting your 12 PSI at the manifold.
And here's everything you need to make it happen:
Yup. One 1/8" NPT hose-barb fitting. About $3 at your local ACE Hardware store. Drill ye' olde hole into the pipe which leads up into your throttle body, install this fitting into it, and plumb a hose from there to your MBC. Using all-silicone tube? (***.) Well, just drill a hole in the colid-side end tank of the IC itself. Anywhere is fine so long as it's after the IC core, and before the throttle body.
You'll probably have to turn the MBC down just a tad in order to achieve the same peak boost you had before, as it's no longer having to factor in even the smallest IC drop.
Vaya con Dios, friends. May your manifold pressure be stable.
So, does that look familiar? If so, save your money. Fixing this problem is going to cost you about $3.
First, let's take a look at why this is happening. Here's a simplified diagram of your turbo system, where we have the turbocharger itself, then the intercooler, and then the throttle body.
Point "A" in this system is where a lot of folks have their boost controller connected. It's that nipple that came from the factory on the side of your compressor housing, probably with a hose already attached between it and the wastegate actuator.
Well, that's just stupid.
What's happening here is that your boost controller is in fact maintaining a constant level of boost, however it's doing it in the wrong place. Specifically, it's maintaining a constant level of boost at the compressor, but that's not what your engine is actually seeing.
Confused?
Yes, behold the simple drinking straw. Solver of great mysteries.
Here's a quick experiment. Stick a drinking straw into your pie-hole, and blow through it. Not too hard, very gently in fact. Very, very gently. This ain't Hustler's mom we're dealing with.
Feel the resistance that the straw is offering? No? Of course not. At the rate at which you are blowing into it, the straw is not much of a restriction at all.
Now, blow harder. And now you start to feel the straw fighting you.
A funny thing happens when we try to flow a gas through a restrictive orifice. The more we try to flow through the restriction, the more restrictive it becomes. In practical terms, at low rates of flow, we get very little pressure loss across the restriction. As flow increases, so does the pressure loss. And it's not linear, either. The magnitude of pressure drop increases almost exponentially with flow rate.
Now, it may not look like one, but your intercooler is a drinking straw. A large, heavy, aluminum, multi-faceted drinking straw. Or, at least, it exhibits a lot of the same characteristics as one. So, back to the diagram:
Say that we have our boost controller set such that we see a peak of 12 PSI in the intake manifold. At 4,000 RPM, we reach that point. We are, incidentally, flowing about 130 CFM through the intercooler (our engine has a 100% VE at all speeds) and we're loosing about 1 PSI across the intercooler. So while the pressure at point B is 12 PSI, the pressure at point A (which is what the boost controller is seeing) is actually 13 PSI.
Now, we increase the speed to 7,000 RPM. At this point, we're moving about 220 CFM, and yet, what's this? The pressure drop across the intercooler has increased to 3 PSI! We didn't even double the flow, and yet we tripled the drop. (Well, I'm ignoring the fact that these are relative, rather than absolute pressure values, but you get the idea.) So now, even though the MBC is faithfully holding 13 PSI at the compressor, we're only seeing 10 PSI at the manifold.
The solution here should be pretty obvious by now. Move the boost controller from point A to point B.
By doing this, we are now telling the boost controller, in essence, "Hey, I want you to do whatever it takes to maintain a constant pressure at point B in the system, and to hell with what's going on over at the compressor." And it will comply. (Boost controllers are pretty simple-minded like that. They don't question orders.) Specifically, it now does not matter what the drop across the intercooler is, at least insofar as your actual manifold pressure is concerned. As drop across the IC increases, the boost controller will cause the compressor pressure to increase accordingly. So by the time you get to 7,000 RPM and are experiencing 3 PSI of drop across the IC, the pressure at point A will be up to 15 PSI, and you'll still be getting your 12 PSI at the manifold.
And here's everything you need to make it happen:
Yup. One 1/8" NPT hose-barb fitting. About $3 at your local ACE Hardware store. Drill ye' olde hole into the pipe which leads up into your throttle body, install this fitting into it, and plumb a hose from there to your MBC. Using all-silicone tube? (***.) Well, just drill a hole in the colid-side end tank of the IC itself. Anywhere is fine so long as it's after the IC core, and before the throttle body.
You'll probably have to turn the MBC down just a tad in order to achieve the same peak boost you had before, as it's no longer having to factor in even the smallest IC drop.
Vaya con Dios, friends. May your manifold pressure be stable.
Last edited by Joe Perez; 09-04-2012 at 05:18 PM. Reason: Changed images to local embed
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05-18-2010, 03:47 PM
#6
I swear you could make literally thousands of dollars as a professional technical writer.
Very good. I knew this and still found it an entertaining read. Of course, even though I know this, i've still got my signal coming off of the compressor housing. This will change once my engine goes back in though.
Very good. I knew this and still found it an entertaining read. Of course, even though I know this, i've still got my signal coming off of the compressor housing. This will change once my engine goes back in though.
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05-18-2010, 03:52 PM
#10
Great write up again Joe.
https://www.miataturbo.net/showthrea...t=32479&page=2 -Post #21. Read about reference point "C".
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05-18-2010, 04:03 PM
#15
Boost Pope
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Or at least, a half-truth.
The real trouble with taking the boost reference from point C doesn't come into focus until we consider what happens when you are operating in boost, but at part-throttle.
So, let's say you're climbing a hill in 4th, and you're getting into the throttle enough to hold a constant speed but not accelerate. You're seeing maybe 6 PSI at the manifold. Because the boost controller is referenced to point B, the turbo is in fact probably making 12 PSI at that point. Ok, so it's making a little bit of excess heat, and our efficiency is off a few tenths, but no harm done.
But what if we had the boost controller referenced to point C? Well, the fact that you are using the throttle to modulate boost means that the manifold pressure is never actually going to reach the setpoint, however the boost controller isn't smart enough to realize this. (As I said before, they're loyal, but very simple-minded. Kind of like people from Minnesota.)
So now we're in a state where the boost controller is trying like mad to get the manifold up to 12 PSI, and it's fighting against a throttle plate that's not going to let it happen. Thus, the effective boost target (as seen from points A and B) becomes infinite!
That's right, anthropomorphic chibi-Dan. Well, maybe not infinite per-se, but the wastegate is definitely not going to open voluntarily. The turbo is just going to fight as hard as it bloody well can to achieve a target it'll never reach, making all kinds of heat in the process and sucking down your VE worse than a brain slug. Is this actually going to break anything? Probably not immediately. But you are putting a lot of undue stress on things without getting much, if anything, in return.
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05-18-2010, 04:16 PM
#17
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Join Date: Jan 2010
Location: Todmorden UK
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So let me see if i have this right.
1. Feed to boost gauge should be after the TB.
2. Feed to boost controller then to wastegate after IC but before TB.
3. Best location for blow off valve feed? Before or after TB?
4. Feed to MAP sensor after TB.
Thanks
1. Feed to boost gauge should be after the TB.
2. Feed to boost controller then to wastegate after IC but before TB.
3. Best location for blow off valve feed? Before or after TB?
4. Feed to MAP sensor after TB.
Thanks
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05-18-2010, 04:17 PM
#18
Uh, doesn't your line go to the MBC? If your MBC is not a bleed valve, the manifold tap wouldn't cause a problem with your WG. The issue is more with your turbo spooling when you lift since it would close your WG completely and it will also run your turbo hard when not WOT. Not only do you have the IC restricting, but also the TB. You only want your boost target right before the TB for that reason.
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