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.

https://www.miataturbo.net/attachmen...1&d=1346793381

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.

https://www.miataturbo.net/attachmen...1&d=1346793381

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?

https://www.miataturbo.net/attachmen...1&d=1346793381

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:

https://www.miataturbo.net/attachmen...1&d=1346793381

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.

https://www.miataturbo.net/attachmen...1&d=1346793381


And here's everything you need to make it happen:

https://www.miataturbo.net/attachmen...1&d=1346793381

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? (fag.) 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.

*Standing Ovation*

BRILLIANT and funny:giggle:

Needs to be stickied asap

Am I missing part of this or is there a missing suggestion of also just using ports on the Intake Manifold. Lots of stuff to T into or just steal entirely.

I'm glad you explained it to everyone, alot of people don't know this. I had this issue about a year ago and it's been working flawless since then

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.

You cant take the signal off the intake manifold because of vacuum destroying your wastegate diaphragm. Isn't this a conclusion someone came to before?

Feels sticky

It has to be before TB... Oh and I got a little boost spike but at least it doesn't sag towards red line

Great write up again Joe.

https://www.miataturbo.net/showthrea...t=32479&page=2 -Post #21. Read about reference point "C".

When will this be available on video?

Huh? I've run many cars off the intake manifold with 0 issues :p

In for another thermostat testing video. But with mbc's this time:D

I'm just reaching this point in my build, awesome, saves me a tonne of research.

Thanks alot!!

Well, I think that may be a myth.

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!

http://img38.imagefra.me/img/img38/6...bm_b90cf1c.gif

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.

Best writeup ever

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

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.

dammit, Joe beat me

1. If that's where you want to measure boost. Would be the best place to read what's going into the engine.
2. Yes.
3. After TB and as close to the TB as possible.
4. Yes, best location being at the back of the manifold.

Put the boost controller at C and be my hero >:)

And yeah, this is a very concise explanation that should help lots of noobs.

I love simple solutions. Why the hell did it take this long for someone to figure this out! (or steal it from someone else who figured it out ;-) )

To clarify a tad, I've re-drawn the two charts above to show what's happening both at the compressor outlet (blue line) and after the intercooler (red line), just in case anybody is having trouble visualizing it.


In the first case, we are taking our reference from point A. So even though it appears that boost is dropping off (as we perceive it from the perspective of the manifold) the MBC is in fact doing its job.

https://www.miataturbo.net/attachmen...1&d=1346793540


Having moved the MBC pickup to point B, we merely change which point in the system is stable.

https://www.miataturbo.net/attachmen...1&d=1346793540


In both cases, there is a point in the system at which equilibrium is reached. But in the first case, that point is one that we don't particularly care about.

Trouble is, then I'd need to start paying for all the artwork that I presently am able to abscond with.

We are in preliminary negations with DreamWorks, I'm just not real happy about their insistence on Eugene Levy as the voice of the intercooler.

This is old, old knowledge, and just basic fluid dynamics, really. Seems like it's just one of those things that isn't obvious until somebody points it out. Hell, even I needed reminding (by Abe) a few years ago.

It's a full truth. See Mazdaspeed Protege. Also a bad idea for the other reasons you mention.

Indeed. Kinda surprised to see it actually. I thought this was common knowledge by now?

That's the problem with old knowledge like this, everyone assumes everyone else already knows it. I haven't seen this topic brought up here in a long time but there have been a number of people with droop complaints.

Now if there was a simple and cheap a solution for over-boosting we'd really have something!

Me too, but I thoroughly enjoyed the write-up and the visual aids. Needs more special effects and sparkly things to keep the attention of the idiots from CR and other noobs.

Sorry for my stupidity, don't mean to muck up a good thread... but what kind of stress (if any) does this put on the turbo? Especially for the guys running 18-20psi. I can imagine that the difference in pressure between the TB and the compressor is pretty large at that amount of boost. Wouldn't the cfm required to keep some peoples max boost push the turbo pretty far out of it's efficiency range?

That depends on the efficiency of the individual turbo, doesn't it? What it can flow is what it can flow. In which case, I'd guess that Point A is a better reference to what the turbo is actually doing. There's your efficiency reference. Intake temps are probably a better way to observe what the turbo is doin, as inefficient turbos start making lots of heat.

FRT, If you have a large enough intercooler you won't experience as much pressure drop across it at higher flow rates and your turbo won't be as likely to be out of its efficiency range. One of the problems I see is that people often use intercoolers that are sized for little peashooter turbos and 5 to 8psi factory turbo setups instead of the flow rates they actually aspire to run. One should aim for both great IATs and low pressure drop by going large if you really are trying to maximize your efforts. Ideally the air coming out of your intercooler should be as close to ambient temperature as possible. It is hard to do that with a tiny intercooler.

+1 for a video release.
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I really think Joe is thinking of a way to direct his next video...."hmmm, what would be the appropriate beer for this......"

Fantastic writing as usual

I think this video should have a snuff film kinda vibe. Its time to break out the gimp mask Joe

There's a log of mine with the source right before the TB
2-3kpa difference all the way though Attachment 197465

Does this same theory apply to external wastegates as well as internal? If i'm not having such problem of boost falling off before the redline should I change? I simply have my MBC spliced in the middle of the boost signal from the compressor to the bottom wastegate nipple.

<Noob question>

"Using all-silicone tube? (fag.) 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."

Why a different place for silicone? Should one use Vac line another type instead?

</Noob Question>

He is referring to people that have the majority of their intercooler piping being made up of silicone pieces instead of hard pipes. In that case, you cant essentially drill and tap silicone.

Thanks for clearing that up, im a retard, i was thinking of the actual signal line :).

Excellent write-up, love the pics too. I would have gone with more of a Barney and friends theme though. Since this site is all about sharing the Love.:bowrofl:

Isnt a side effect of this location a possible boost spike/overboost? By the time location B sees 12 psi, there may be 16psi at point A on its way allready. (making up numbers here)... That was my rationalization for going with the compressor outlet location... I wanted to play it safe on the street so I opted for a slight performance loss in favor of more safety and simplicity.

I agree location B is probably best, just thought I'd mention this^

It makes all the sense in the world and excellent write up...!!

However boost spikes are really a possibility... Also the size of the IC is critical... it would be like the size of the straw, get a 2" hose and blow as hard as you can, and you will not feel resistance... Unless you are really good at blowing hahahahaha

I recently installed mine at the turbo nipple, and have seen steady boost, but I admit not going all the way to redline yet... Will do a run, as soon as I can, and will share!

miatex

The reason the straw was use is because we have to downsize our example since our mouths, even though we're all Miata driving gays, are not as powerful as even the smallest of turbos.

My only issue with this idea is that I use the amount of boost drop off as an indicator as to how much my bolts have stretched, and therefore how much exhaust is leaking out between my turbo/manifold. The flanges need to be resurfaced, so it always sounds like there's a bit of a leak.

I understand, yet the I dea I was trying to portray was the fact that the IC flow capacity will play a major factor... High flow IC, and you may not notice such droop... tiny IC and the setup is absolutely necessary...

I will see how my setup resist the current 13psi target... thanks for the clarification tho.

miatex

I don't see any evidence of my boost dropping at high rpm's. Admittedly, I tuned the gain on my Profec to maintain 12 psi to redline. In this scenario, is there any benefit to moving the signal source on the cold side of the IC?

C

If point C is such a bad thing, why does the MSM take its source at point C?
I believe the folks from FM also advice to use point C.

Fantastic write up Joe, my crusty ebay MBC actually recomends making a point b.

The article makes a lot of sense to me - but I'm a musician, what the heck do I know? I wonder what comments Corky might have since his systems (and mine) seem to use point A?

What happens when your wastegate and MBC does this:

http://www.boostedmiata.com/gallery2...work_boost.jpg

You make moar power?

well played sir.

^Very true this was the setup that came on my old 323 gtx engine that was from '93. How is it that Mazda went to poorer designed systems on the newer cars? Less rallying and more Ford influence?

Def an excellent post by Joe, as usual, but I feel like it should also address boost/power loss do to inadequate spring pressure on the Internal/External wastegate. From my exp and reading its generally not recommended to run boost pressures more than 1.5x your WG spring, regardless of boost control method. It will not cause your boost to drop off as much as it will cause you to spool much slower than you could if you have a more adequate spring for the boost you are running.

Further, I would say that 1.5x might even be too much when running an IWG with a small/crappy internal diaphragm. Say you are running 15psi. I would put my money down that you would see an increase in spool running a 15psi IWG vs a 10psi IWG. EWG can get away with more because they have much larger diaphragms and often run dual springs.

Great information.

Any pictures of how to actually run the lines for this and labeled components? Seeing it would really help me, at least.

Excellent exposition, Joe ;)

Thanx a lot.
Ben.

The MSM, takes it from point B. It has a port on the side of the tube that attaches to the throttle body, from that port it goes to the boost control solenoid on the valve cover and onto then to the WG.

I have always used the port on the backside of my IM. It is the 1/4" port that the automatic transmission refrences the vacuum. That port is perfect. No boost spike. Smooth to 12# all day long no matter the rpm on my 1.8

that port is turbulent as hell when your MAP sensor is sourced off it.

Yes it would be rock steady, but what is your turbo doing at engine loaded part-throttle cruise (uphill at 70mph on the highway)? To make 12psi in the manifold behind a partially closed throttle plate you might be making 30+psi at the compressor and huge amounts of excess heat for your intercooler to fight with. Plus you are restricting the airflow by forcing the turbine to fight the excess pressure that the compressor is seeing. I bet the turbine gets pretty hot at a cruise compared to one referenced at "point B". You should try switching it for a tankful and see if your mileage improves.