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Old 09-06-2008, 02:20 PM   #1
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Default Stock ECU only; is it safe to run the car below boost?

The mechanical components of my BEGi-S2 kit are installed, along with an Autometer vacuum-boost gauge and an Innovate LC-1 with G-5 air/fuel ratio gauge. I'm waiting on BEGi to complete testing and tweeking on their Zoom3, but I'd like to be able to drive the car so I can shake down all of the minor issues.

It's easy enough to keep the manifold pressure out of positive boost pressure, though the turbo really wants to put it there, by monitoring the vacuum-boost gauge and adjusting my right foot to keep manifold pressure at or below zero gauge pressure.

Here's a prelude to my question:

When accelerating through the gears at or near zero manifold gauge pressure my air/fuel ratio remains around 14.5:1. The shop manual ('95 Miata, page F-11) Engine Control Operating Chart indicates fuel injection should go "rich" under "acceleration" or "heavy load"; I'm not seeing that happening.

Now I don't know what Mazda means by "rich", but I'd guess it would be around 13:1 or so. I'm beginning to think that because the turbo's helping move the air (I can hear it spinning and the BOV opening when shifting) the throttle position sensor is not indicating a significant "acceleration" or "heavy load" condition to the ECU, and the ECU is being fooled into thinking the load is moderate when it isn't necessarily so.

Now I'm told that Zoom3, and other piggy-back systems don't even begin to operate until a positive manifold signal is read by the system. If so, and assuming my stock ECU is running properly, everything should be cool.

Here's my question:

Can I safely run at or below zero manifold gauge pressure or not?
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Old 09-06-2008, 02:27 PM   #2
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A boosted engine at 5in.hg is moving more air than a naturally aspirated engine at 5in.hg. This is why you are seeing the "lean" condition. It's not good to do, since you REALLY need to keep close tabs on the vacuum gauge, but yes, you can do it.
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Old 09-06-2008, 02:57 PM   #3
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A boosted engine at 5in.hg is moving more air than a naturally aspirated engine at 5in.hg. This is why you are seeing the "lean" condition. It's not good to do, since you REALLY need to keep close tabs on the vacuum gauge, but yes, you can do it.
Hey Savington, thanks for the answer.

But I'm a little bit confused by the boosted engine moving more air than a naturally aspirated engine at negative gauge pressures. The way I figure, the vacuum gauge is pretty dumb; it only understands vacuum and has no clue there's a turbo upstream. So, the motor's asking for air, the throttle plate between the motor and vacuum port is saying, "not now", and the vacuum gauge is reading the situation as a negative (below atmospheric) pressure. The air requirement is there (cylinders trying to fill) but the demand is not being met (negative pressure), turbo or no turbo. The pressurized air may be out there if the turbo's spooled up, but the throttle plate is only passing what it wants to let in, and that's not as much as the engine would like to have. So, at say -5 in.hg. or any other value below zero gauge pressure, how is a turbo motor moving more air than a NA motor?

Then there's that nagging question about how piggy-back systems are supposed to work. Are they also causing the motor to run lean under moderate to heavy loads and accelerations because they don't step in until they see positive pressure?

I'm not disagreeing, but I'm having a bit of trouble understanding the why's and wherefore's of your answer.
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Old 09-06-2008, 03:10 PM   #4
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A boosted engine at 5in.hg is moving more air than a naturally aspirated engine at 5in.hg.
That's true, however he's still using a MAF sensor, so the ECU is seeing an accurate measurement. This will eliminate improved VE as a variable.

Thing is, if it were me, I would n't trust myself to keep it out of boost. It's amazingly simple to find yourself making 5PSI without even realizing it.


Thucydides, your engine is not running lean. You have to really hammer on it to get the mixture down into the 12s and 13s, what you're describing sounds to me like normal operation. Go cruise at 95MPH on the highway for a moment and you'll see it.

Personally, I think you're an idiot for installing the turbo before the fuel and ignition management, and for not doing "before" measurements with your fancy new LC1 if you're so hung up about AFR off-boost, but that's just me. If you truly do keep it out of boost, you can drive the car without causing any harm.
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Old 09-06-2008, 03:21 PM   #5
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****, newbie making me think about my answers.

First, after rereading your post, I'd guess you didn't go above 4000rpm. Below 4k, the stock ECU runs in closed loop mode, which means the o2 sensors are helping the ECU keep the mixture as close to stoich as possible. Above 4k, it goes open loop, uses a set fuel map, and you'll see those 13:1 AFRs that you normally see at WOT up there.

Second, as far as the vacuum gauge goes, it measures a difference in pressure between the intake valves and the throttle body. With a turbo in the intake stream, there's much less restriction, and less restriction = more airflow. You may still see that 5in.hg difference, but the intake valves are using more air because the throttle body is "letting" more air past (air is being very lightly forced past). In reality, though, the 5in.hg NA would end up around 0psi with a turbo since the intake valves aren't nearly large enough to utilize all of the additional airflow.

edit: ******' MAF sensors screwing up my logic. Get your MS installed.
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Old 09-06-2008, 04:38 PM   #6
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Originally Posted by Savington View Post
First, after rereading your post, I'd guess you didn't go above 4000rpm. Below 4k, the stock ECU runs in closed loop mode, which means the o2 sensors are helping the ECU keep the mixture as close to stoich as possible. Above 4k, it goes open loop, uses a set fuel map, and you'll see those 13:1 AFRs that you normally see at WOT up there.
Actually, I have gone over 4k and that explains why I do sometimes see A/F ratio's in the 12's. I thought it was manifold vacuum controlled; apparently it's RPM controlled. That solves one mystery.

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Originally Posted by Savington View Post
Second, as far as the vacuum gauge goes, it measures a difference in pressure between the intake valves and the throttle body. With a turbo in the intake stream, there's much less restriction, and less restriction = more airflow. You may still see that 5in.hg difference, but the intake valves are using more air because the throttle body is "letting" more air past (air is being very lightly forced past). In reality, though, the 5in.hg NA would end up around 0psi with a turbo since the intake valves aren't nearly large enough to utilize all of the additional airflow.
****, I thought I had it but now it's gone. What I was thinking was that the vacuum gauge is, in reality, a pressure differential gauge. In a nominally pressurized environment the differential may still be -5 in.Hg., but the net mass of fuel/air into the engine is greater. The problem with that theory is that the vacuum gauge is a pressure differential gauge but the reference is still atmospheric pressure. If the gauge lived entirely within the intake manifold the previous statement would hold true, but that's not how the vacuum gauge measures pressure changes because it lives beneath the radio in my wife's car, and unless she's in there pressure's atmospheric.

Let's look at the reduced restriction cause. Let's say the turbo was pushing 100 psi outside of the throttle plate. And let's say I wanted to drive at -5 in.Hg. manifold pressure at whatever speed the car achieves at that manifold pressure. To do so I would have to drive with the throttle plate largely closed. Let's now say that the turbo pushes 1 psi, but that I still want to dive at -5 in.Hg. To do so, because the turbo is hardly pushing at all, I'll need drive with the throttle plate largely open. All's the engine and the vacuum gauge understands is -5 in.Hg. I'm thinking the engine and vacuum gauge don't give a rat's *** what's happening upstream of the throttle plate, and only care what's happening within the intake manifold downstream of the throttle plate. Whether there's no psi's or 10,000,000 psi out there the engine cares not at all, so long as the pressure within the manifold is -5 in.Hg. In other words, the system is entirely throttle plate controlled.

Here's one more way of looking at it. If I understand a non-adjustable pressure regulator correctly (and there's a damned good chance I don't), within it's operating parameters it will deliver a downstream pressure independent of upstream pressure. The intake manifold is a pressure regulator; the device doing the pressure regulating is the throttle plate and my foot. The downstream pressure is still independent of upstream pressure and can be measured with a vacuum gauge.

I'm still not saying you're wrong, but as long as I can come up with reasonably counter points I'm in a quandary. And that piggy-back thing is still out there like a big matza ball. Let's grant you're 100% correct. Wouldn't turbo Miatas with piggy-back systems that only operate at positive manifold pressures be melting down by the hundreds for all of the reason's you've stated?

Anyway, thank's Savington. Saturday afternoon's a terrible time to be busting braincells over Bernoulli's Principles.

Last edited by Thucydides; 09-06-2008 at 06:59 PM.
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Old 09-06-2008, 04:52 PM   #7
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Personally, I think you're an idiot for installing the turbo before the fuel and ignition management, and for not doing "before" measurements with your fancy new LC1 if you're so hung up about AFR off-boost, but that's just me.
Joe, I appreciate your concern however badly expressed. And truely, if you knew me, you could say much worse. But trust me when I tell you that's not how I planned things.

But let's be fair Joe. How long can a man stare at an intercooler, BOV, stainless steel return lines, t-bolt clamps, stainless steel divorced downpipe exhaust manifold, ceramic coated exhaust and turbocharger, and all of those other tasty bits in their living room without hanging them on their car? I'm not Hustler, for god's sake!

Sure, I could have "waited" until every last bit showed up as planned, but I'll admit I'm weak and succumbed to the Siren's song that the management system was on it's way (no reference to Stephenie, despite appearances, unless she's Greek and has three heads).

But thanks Joe, and until I better understand what my ECU and fuel injectors are doing I'll stay out of the car.....as much as possible.....most of the time....and never above -10 in.Hg.
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Old 09-06-2008, 11:24 PM   #8
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Just wondering... why did you pick Zoom3 over MS or other ECU?
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Old 09-06-2008, 11:39 PM   #9
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Well, since nobody seems to have anything to add at the moment I'll share what I've learned about piggy-back systems:

From BEGi's website, and about the Exede:

"Installing a piggyback auxiliary computer between the factory ECU and its sensors and actuators provides bone-stock engine management during no-boost engine operations.

.... the piggy uses the enormous programming experience of the OEM computer for all off-boost functions."

From Flyin' Miata's website:

"Fuel management is handled by our own Voodoo Box, which uses the stock injectors to deliver more fuel. Because it only modifies the signal under boost, the car acts completely stock at idle and cruise."

From Brainacks, “Things you need to think about before turbocharging your Miata”:

“The MSD unit retards the timing 3 per pound of boost pressure with no more that 6 of retard being allowed. When no boost pressure is present, the ignition runs at the factory timing.”

So, turbo or no turbo, if you're running a piggy-back system and the information provided by Begi, Flyin' Miata, and Braineack is correct, if you're not making manifold pressure above atmospheric your running stock ECU injector duration and ignition timing. Presumably this doesn't do any significant harm.

That being the case I have to agree completely with Joe when he says it's amazingly simple to find yourself making 5PSI without even realizing it. It's also ridiculously easy and tempting to make boost for an instant to see how quickly the turbo spools, and in that instant bad things can happen. Maybe not catastrophic things, but bad things. Damage can be cumulative. And I don't think it's a very good idea to expect you'll hear the sound of detonation before harm to your engine has occurred; it's not exactly quiet inside a Miata at anything over forty. If you can actually hear detonation under boost it's probably really bad for the life of your pistons.

Joe, my name is Jim. Should you feel compelled to call me an idiot again you now know how to preface the description. BTW, the title should read, "...is it safe", not "...it is safe". I don't know if super moderator status allows you to change that, but if you could that would be great.

Last edited by Thucydides; 09-06-2008 at 11:51 PM.
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Old 09-06-2008, 11:46 PM   #10
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Just wondering... why did you pick Zoom3 over MS or other ECU?
My primary reason for choosing the Zoom3 was to be able to pass California emissions without the need for an add on a fuel pressure regulator, high pressure fuel pump, and a means to retard ignition timing at boost. The Zoom3 takes care of both fuel and timing in one package. Ultimately I'll end up installing a MSPnP and larger injectors, but I also want a system I can quickly and easily install once every two years for the mandatory smog check. We have two '95 Miata's and one Zoom3 will serve both when it's needed.

So I guess the short answer is because theoretically Zoom3's a plug and unplug proposition without adding possible fuel leak sources, and that appeals to my sense of laziness.
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Old 09-07-2008, 01:30 AM   #11
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I'm not Hustler, for god's sake!
You just redeemed yourself right there! I believe that this one will do OK. (also, title has been changed per your request.)

A lot of us, myself included, started out with piggybacks of all different types. My previous setup on the current car was an EMU (eManage Ultimate) which, like the Zoom3, works by modifying the stock ECU's injector and ignition signals. Unlike the Zoom3, it also has the advantage of directly reducing injector duration by a specified ratio, thus allowing you to run relatively large injectors, such as 440 or 550 cc/min, while still retaining good idle characteristics. Thus, the unit can provide sufficient fuel for higher levels of boost (12-15 PSI) without the need for an FPR.

It's not a bad way to do things, and I'll be the first to admit that Mazda probably put a lot more time & effort into tuning the off-boost tables than I ever will. Still, there are certain things about piggyback tuning that I find annoying. Perhaps the biggest is that you find yourself spending a lot of time working on the transition point from vacuum into boost, where suddenly the unit springs into action. I know that when I made the jump to a full standalone, it took a while to grow accustomed to the fact that there was no more "transition point" anymore- going from vacuum to boost was a non-event, no more battling to try and force the stock ECU to go open-loop, just nice, smooth operation.

Anyway, a piggyback, and specifically a tuner-supported pigyback, is a nice easy way to get started. If nothing else, you spare yourself having to worry about things like getting your idle right. I'm slightly concerned by the fact that you're choosing it for reasons of CARB, however. In order to be legal with a Bell kit, you must be using an AFPR and an aux fuel pump. It sounds to me as though you intend to use the Zoom3 in lieu of those parts. If you do this, then even if you remove the Zoom3 for your inspection, the system will technically be noncompliant due to the absence of these pieces. The Zoom3 is not a CARB-approved device. And ironically, not running an AFPR is forbidden so long as you're running a turbo kit whose paperwork says that you are supposed to have one.

Whether your particular smog tech will call you on this is another matter- technically, you're also supposed to run an aux injector manifold (a remnant of an older design), though Bell reports that people have been passing without it. They also keep them available for loan should you ever need it. Here is a copy of EO D-349-1, which is the document covering Bell's '94-'97 turbo kits: http://arb.ca.gov/msprog/aftermkt/de...eo/D-349-1.pdf

Of course, when I lived in CA I passed with a Greddy-based system that wasn't even remotely close to compliant from a technical standpoint. I was simply fortunate to find an inspector who was somewhat casual about the visual portion of the test, so long as you passed the sniffer.

Your setup does look nice. I really like the look of the ceramic-coated manifold. Every time I see one of those I feel self-conscious about my rusty old manifold and turbine housing...
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Old 09-07-2008, 12:59 PM   #12
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Still, there are certain things about piggyback tuning that I find annoying. Perhaps the biggest is that you find yourself spending a lot of time working on the transition point from vacuum into boost, where suddenly the unit springs into action.
Ahhh, just another one of the joys of turbocharging Miatas. Hopefully this will be good practice for doing the same thing on a much larger scale with a stand alone ECU.

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I'm slightly concerned by the fact that you're choosing it for reasons of CARB, however. In order to be legal with a Bell kit, you must be using an AFPR and an aux fuel pump. It sounds to me as though you intend to use the Zoom3 in lieu of those parts. If you do this, then even if you remove the Zoom3 for your inspection, the system will technically be noncompliant due to the absence of these pieces. The Zoom3 is not a CARB-approved device. And ironically, not running an AFPR is forbidden so long as you're running a turbo kit whose paperwork says that you are supposed to have one.

Whether your particular smog tech will call you on this is another matter- technically, you're also supposed to run an aux injector manifold (a remnant of an older design), though Bell reports that people have been passing without it. They also keep them available for loan should you ever need it. Here is a copy of EO D-349-1, which is the document covering Bell's '94-'97 turbo kits: http://arb.ca.gov/msprog/aftermkt/de...eo/D-349-1.pdf
I'm told, and believe, BEGi is pursuing CARB certification for the Zoom3 system. And so beyond that it's a small leap of faith on my part that the certification process will be completed before my next smog test one and a half years from now. If the cert's not completed I'll be doing what I've tried to avoid with the Zoom3; installing the components specified in the existing E.O. So it's a bit of a gamble, but either way I will get my car smogged when the time comes.

Right now the plan is to get the Zoom3 up and running and able to pass the smog test. Once that's done I'll get to work on a MSPnP and larger injectors to give me whatever the little GT2554R is comfortable with; say around 225 BHP at about 12 psi. I'll install a clutch, and whatever else is required to handle the greater power and call my wife's commuter good. Then it's on to my own '95 where my goals will probably be a bit higher.

But you know the scenario; project cars are never done. Anyway, thanks for the great information Joe, and thanks for fixing the title.
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