Stock wideband useless? Not quite.

Let me begin by saying that I don't have any problems with people using aftermarket widebands.

 

Now, do a quick search here about the preturbo factory front O2 sensor and what you'll read goes something like this:

 

1) the factory wideband 'pegs' at .76 lambda or about 11.1:1 AFR

2) the factory wideband is not accurate because it is located before the turbo and is subject to high exhaust pressure.

3) x person compared a log of the factory sensor and an aftermarket one located farther down in the exhaust stream. Because they do not agree and the factory sensor reads richer, it must not be accurate.

4) Since it is not accurate, it is useless. You cannot trust it to determine whether your AFR is within some safe range.

 

The implication then is that a Legacy owner with only mild modifications and some form of a reflashed ECU must purchase an aftermarket wideband system to be sure that their is not running dangerously lean.

 

The purpose of this article is to argue, using literature from Bosch itself, that the factory wideband is like a watch that's set too fast. If your watch were not perfectly synchronized to an agreed upon value, you'd rather have it be a little fast so you're not late, wouldn't you? But does that make your watch useless? Do you need an atomic clock to get to work on time? No.

 

As source documents I will use the "mechanism and function" section of the 2005 Legacy GT service manual, and the Bosch technical document on the LSU 4.2 wideband sensor available here: http://wbo2.com/lsu/Y258K01005e03mar21eng.pdf . The LSU 4.2 sensor (or some minor variant) is used in most aftermarket wideband systems.

 

 

We know that the two sensors are similar in their function and in their overall signal outputs. I'm not going to go deep into the way a wideband works. To oversimplify some:

 

1) its operational principles have some similarities with hotwire MAF sensors

2) more current (often seen as the variable Ip) required = a leaner mixture, but the amount of current does not increase at a perfectly linear rate.

 

Now consider first this graph of the signal output range of the LGT wideband sensor:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75159&stc=1&d=1256523488

 

We don't have any numbers on that graph, but if you compare the shape of the curve it's not too far off from the Bosch LSU 4.2:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75162&stc=1&d=1256523831

 

It's reasonable to assume that these two sensors have a lot of similar characteristics, although numerical specs may differ some. Now take a look at what Bosch has to say about exhaust pressure's effects on the sensor's signal:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75160&stc=1&d=1256523831

 

You can see here that the higher the exhaust pressure, the more it increases the current signal. The more the current signal increases the leaner the sensor is going to read relative to what it would read with no pressure. How much leaner? I couldn't tell you exactly. It's going to change some with each car and I don't have the proper controlled testing setup anyway. As a side note, you can also see here that excessively high exhaust temperatures result in more current being required and thus a leaner reading relative to reference conditions.

 

Now, going back to the watch analogy: if someone told you your watch is fast but didn't tell you how much, you at least know you're not going to be late. Again, to sum up: the placement of the factory O2 sensor at the absolute worst will make you think your car is running leaner than it actually is--meaning the AFR tune is safer than it appears.

 

"But wait," you say, "I logged a few pulls with my factory wideband. It just pegged at 11.1:1 , there's no way that's right." Take a look at the 2005 factory and 2005 Cobb stage 2 (93 octane) target AFR tables:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75161&stc=1&d=1256524506

 

Yes, it really is that rich, or close enough. You can be sure that on a stockish car, under significant load Subaru is not going to run the engine leaner (leaner enough to matter) than their open loop targets because of the safety factor built into OEM tunes. If your factory wideband says you're at 11.1:1 or 12:1, you can be confident that you are not necessarily having a dangerously lean condition. Now that doesn't mean your timing is correct and your tune is safe, again it just means you probably are not running dangerously lean.

 

So can we use the factory wideband to give a precise AFR reading, especially at very rich mixtures? No. But can we use a good condition factory wideband to alert whether the engine may be approaching a dangerous lean condition? Yes. So is it useless? No.

 

On the other hand, should we automatically toss our aftermarket widebands in the trash? No, they still have their place.

If your factory wideband says you're at 11.1:1 or 12:1, you can be confident that you are not necessarily having a dangerously lean condition.

 

Typo? 12:1 is pretty lean for WOT. 11.2:1 or even "less than 11.5:1" sounds more reasonable to me.

 

That said, your thesis seems basically correct to me, especially for people with TMICs and and stock injectors.

 

With FMICs, there's usually a rich dip that appears a second or two after a throttle stab (regardless or RPM or airflow, and even with tip-in set to zero) that won't show up since it all occurs well below the 11.1 limit.

 

LBGT and I both found that with big injectors the fuel table doesn't correspond well to measured AFRs. I only found lean spots from that but I wouldn't want to bet that the weirdness doesn't work in both directions.

 

At 2.0 bar EGBP, the sensor is off by 15%. So if you're reading 11.1 you could be as rich as 9.5. Tuning with the stock O2 sensor seems sketchy for that rason. My WBO2 only goes down to 10.0, but I think I'm getting knock at around 9.5 (my timing is fine for normal AFRs, but see above about the FMIC rich dip).

 

I'm pretty sure you can get more useful readings from the stock O2 sensor by putting it in the downpipe, pre-cat. I moved mine but haven't gotten around to comparing the readings with my wideband. After I discovered the 11.1 limit, I kinda lost interest.

Old news.

Unless you are going to monitor pressure and temp at the sensor, and then have a computer running a real time calculation to offset the actual reading.... you still can't reliably use the stock sensor in the stock location.

Unless you are going to monitor pressure and temp at the sensor, and then have a computer running a real time calculation to offset the actual reading.... you still can't reliably use the stock sensor in the stock location.

 

Who watches gauges in real time? That's what Excel is for.

 

Why temperature? The sensor has a heater, that's what it's for.

 

There's a guy on NASIOC who posted some interesting EGBP graphs, he just used a manifold-style pressure sensor with a long thin metal tube between the sensor and the exhaust, so the sensor doesn't get too hot.

 

Of course if you're going to go through that much trouble you might as well just install a wideband.

Properly tuning the AFR with a wideband can be good for 10-15 HP compared with stock or OTS fuel maps. $180 for 15 HP? That's not a bad deal I just wanted to emphasize that point.

-Franz

 

Why temperature? The sensor has a heater, that's what it's for.

 

The sensors heater is there to bring it up to it's operating temperature faster after a cold start. Your EGT's are well above the threshold of operation of the sensor. The info YOU posted states that as the temperature of the exhaust gas rises above it's mean operation temperature, the readings become offset.

 

 

I would also like to point out the "note" on the bottom of the picture. "Note: These characteristics are valid for the specified test gas only. They may be different for real engine exhaust gas and must be determined separately. "

 

So even if you programed temp and pressure offsets into a computer, it wouldn't be right based of these graphs.

The sensors heater is there to bring it up to it's operating temperature faster after a cold start. Your EGT's are well above the threshold of operation of the sensor. The info YOU posted states that as the temperature of the exhaust gas rises above it's mean operation temperature, the readings become offset.

 

I see. You may have point there.

 

On a related note, last night I finally did some logging to compare with WBO2 with my post-cat stock O2 sensor. Unfortunately the only computer I own that has Excel installed, is toast. I'll post scatter plots when I'm able to.

I see. You may have point there.

 

On a related note, last night I finally did some logging to compare with WBO2 with my post-cat stock O2 sensor. Unfortunately the only computer I own that has Excel installed, is toast. I'll post scatter plots when I'm able to.

 

Very interested in those.

Yeah, me too.

Related:

http://www.iwsti.com/forums/cobb-street-tuner/36914-stock-narrow-band-o2-vs-lc-1-wideband-o2-lots-data.html

http://www.bescaredracing.com/sti/data/nb_vs_wb/06.JPG

 

I'm not sure what car that came off of, but note that the factory LGT sensor is not a narrowband, not at all. That's a derisive term people use. It uses a pump cell and a nernst cell like every other wideband. The factory sensor is NOT a narrowband. Anyway, it's graphs like this (from an earlier link) that actually prove my point. We don't have any real quantitative data about the factory wideband, but everything we know says that the stock sensor should read LEANER than the aftermarket one! LEANER. Yet it reads richer. The exhaust pressure and the heat increases current. Increased current makes for a lean reading. Hmm. Maybe it's because the stock sensor is

 

1) closer to the engine, which increases accuracy. Even on n/a engines, the location in the exhaust pipe has a big effect under high load conditions. Tailpipe testers are the worst because of air mixing effects, but that's beside the point.

 

2) is DESIGNED for its location instead of being a universal part for mostly VW applications, like the LSU sensor. We have no quantitative date on the backpresure and heat effects for this sensor, but the basic principles tell us that these effects if anything would make the stock sensor agree with the leaner readings of the aftermarket one.

 

3) programmed with OEM grade heater control, instead of the LC1 which burns out (done it), the AEM which will keep on trucking if you unplug it for thousands of miles (done it) and not tell you anything's wrong despite not even agreeing with real 1 volt narrowbands, etc... Remember that if you have very effective heater PID control you can handle much hotter exhaust by responding quickly to the conditions. It is the temperature of the sensor ceramic that matters most, and that is a function of heat transfer. And heat transfer is a function of the starting temperature and the temperature of whatever is supplying the heat (Newton's Law of Cooling). The OEM heater control is partly why Rx-8's have factory widebands that never burn out (rotaries have insanely high EGTs) but when you put an aftermarket one on there you have to locate them so much farther back in the exhaust stream to keep the sensor alive.

 

4) more closely matching the factory lambda tables (on stock or nearly stock cars) much more closely than aftermarket sensors do. The factory PCM models the engine pretty well. If anyone here's done any hardware-in-the-loop work you'd understand. For a project in college I did some work on a GM project where we did simulations for an experimental hybrid diesel setup (EcoCar project). You'd be amazed how much they know about the engine and vehicle before even a prototype is built

 

What's more accurate, the AFR estimates of a computer made by the manufacturer that models the engine, or people turning wrenches in their garage and posting on the internet?

 

Again, there's nothing wrong with aftermarket widebands... I've owned several. But they have their own set of problems, especially reliability and self-diagnosis capability if something goes wron. Aftermarket sensors' self-diagnosis is pitiful compared to all the CELs you can get for factory sensors (slow response codes etc). When you install an aftermarket wideband and it reads lean, the part automatically justifies itself in the mind of the purchaser. But that doesn't automatically make it "right." How can anyone here know? Who has a five-gas analyzer in their garage http://www.autoshop101.com/forms/h56.pdf? Factory and aftermarket widebands are just tools. There's so many other methods out there, such as old-fashioned plug reading.

On the topics of exhaust temperature and narrow vs wideband signal curves:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75400&stc=1&d=1257008095

 

This is from the 2005 LGT service manual, page FU(H4SO)-17 . You can see that narrowband sensors directly produce voltage (aka electromotive force) in something close to a step function. Since it is located behind the cat, it is used to measure catalyst efficiency.

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75401&stc=1&d=1257008095

 

You can see that your typical monolith-type converter warms up around 300 C/572 F, and that's right when the LGT narrowband sensor warms up. This is no coincedence. Now, take a look again at the section on the factory LGT wideband:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75399&stc=1&d=1257008095

 

The stock wideband is optimized for the high preturbo temperatures, it is not even fully accurate until the heater or the exhaust itself gets it to 700 C/1292 F (more than twice as hot as the heated narrowband in the rear). I think relocating the factory wideband is pointless. It's clearly designed for the stock location and it should stay there. The high temperature sensor design and the OEM-grade heater control logic keeping it from burning out.

 

The backpressure issue exists, but its effects cannot be quantified without further data. The backpressure is not that big of a deal, because if it had such a big effect then the factory sensor would read leaner than the aftermarket ones people are installing. Remember that more pressure = more current , more current = leaner reading according to the Bosch article I cited above.

 

Remember that the OEM's spend huge dollars on their emissions control systems. If it won't pass emissions and meet cat life requirements, it cannot be produced.

Remember that the OEM's spend huge dollars on their emissions control systems. If it won't pass emissions and meet cat life requirements, it cannot be produced.

 

Also remember that the ECU completely ignores the O2 sensors when you're under boost. Subaru doesn't care about sensor accuracy under boost.

 

Also, just for the record, temperature was not a consideration when I moved the stock sensor to the downpipe - I only did that to minimize the effects of EGBP.

 

That said, if we assume that pre-turbo EGBP correlates predictably with manifold pressure, it may be possible to correct for it and get reasonably good AFR information. That would just require some data logs showing the stock O2 signal, aftermarket/post-turbo WBO2 signal, and boost (plus RPM, time, and throttle in case the boost/EGBP relationship varies over time and RPM). Those logs would have to come from a car that's tuned to run 11.5 or so, which is a little leaner than most people tune for, but not unreasonably so.

Also remember that the ECU completely ignores the O2 sensors when you're under boost. Subaru doesn't care about sensor accuracy under boost.

http://www.legacygt.com/forums/attachment.php?attachmentid=75407&stc=1&d=1257019672

You can see that the vehicle will stay in closed loop under 4000 rpm, 58% (or 90%? don't understand that table) throttle, and 1.60 load. Load over 1.0 is of course boost. There are some variations to this logic based on gear etc.

 

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75408&stc=1&d=1257019672

 

Not on a completely stock car it doesn't, only under WOT or near it. And a stock car driven by typical drivers almost never sees WOT.

 

If Subaru ignored the front O2 sensor under boost

 

1) LGT's and any other car with a factory turbo would spew out emissions in any kind of city driving.

 

2) there would be no need for the cost and complexity of a wideband sensor, when a 4-wire heated narrowband will do the job. Even narrowbands are used during boosted closed loop operation on other non-Subaru cars. I have observed this before.

 

That said, if we assume that pre-turbo EGBP correlates predictably with manifold pressure, it may be possible to correct for it and get reasonably good AFR information. That would just require some data logs showing the stock O2 signal, aftermarket/post-turbo WBO2 signal, and boost (plus RPM, time, and throttle in case the boost/EGBP relationship varies over time and RPM). Those logs would have to come from a car that's tuned to run 11.5 or so, which is a little leaner than most people tune for, but not unreasonably so.

I honestly think the backpressure effects are of limited concern. If they were that big of a deal, under low boost closed loop the artificially lean reading would result in mixtures that are in fact too rich for full catalyst efficiency. That would result in unacceptable amounts of HC and CO for a production car. But clearly the sensor skews more lean as exhaust pressure increases, there's no denying that. But we cannot quantify its effects on the actual AFR reading.

 

The backpressure might skew your readings lean a bit, with a more conservative tune being the result. But widebands are just tools anyway, they have their flaws and they are only one indicator of what's going on with the engine. Some people tune off dyno tailpipe widebands and then start complaining about factory O2 wideband accuracy (on Subarus and other cars), which makes no sense to me. Clearly it's accurate enough to meet emissions in low boost conditions. Tailpipe widebands are better than nothing, but they are nowhere near as trustworthy as the factory sensor.

 

Unfortunately, correcting the factory sensor signal is not possible for the following reasons:

 

1) we don't have the current-to-AFR conversion function for the stock sensor or even for the Bosch sensor. It's not like a narrowband where lambda is always at .50 volts. I'm sure that diagram in the FSM had some smoothing.

 

2) we don't have the actual pressure correction function for the stock sensor or the Bosch sensor

 

3) we don't have the actual temperature correction function for the stock sensor or the Bosch sensor

 

You need to know what current corresponds to what AFR, and then you need to know how the various conditions change that current signal. I'm sure that's all proprietary manufacturer data.

 

 

I just tuned off the factory LGT front O2 sensor the other day, modifying the Cobb 93 octane stage 2 map with AccessTuner. The lowest AFR recorded on the factory wideband I think was 11.3:1 when I was done, right near or right above where the signal is clamped. The gas smell under boost (factory main cat still) went away after leaning it out. Before tuning, the FLKC table had read -2 degrees in one spot that might have mattered. After tuning it didn't change, and I didn't even touch Cobb's timing map (but I may later). Logs showed no meaningful knock events, and DAM stayed at 1.0 . The car runs great.

Load over 1.0 is of course boost.

 

That is not how "load" works.

 

A NA car will see about 1.5 load IIRC. It is just g/s/rev, or something like that. It is not a calculation based on atmospheric pressure.

We're not NA cars and on our ECU, anything over 1 is boost.

 

 

Pressure in the Up Pipe under WOT is around 60psi. That's gonna make a sensor read funny.

Manifold absolute pressure over 1.0 is boost, but load over 1.0 just means there's more than 1 gram of air entering the combustion chamber for each cycle. EDIT: I will acknowledge that I don't often see loads over 1 g/rev without boost, but I've got a log in front of me right now that shows 1.12 load at -0.33 psi.

 

"Under boost" was a simplification, but it's not very wrong. It's very easy to switch from closed loop to open loop while accelerating, or even just going up a steep hill on the freeway, at constant speed. I don't know if Cobb lets you datalog the 'fueling mode' parameter, but RomRaider does.

 

There's a good thread at romraider.com about the logic the ECU uses to determine when to transition from closed loop to open loop. In a nutshell, there are various conditions which must be held for a specified time. The primary consideration is the open loop fuel table - if it's richer than the open loop threshold (14.41 in my stock tune), the countdown starts. (If you set the delay to zero, the other conditions cease to matter and the open loop table is the sole deciding factor, which makes datalogging for AFR errors (MAF scaling, etc) a bit simpler.)

 

Anyway, any cells in the fuel table that call for AFRs richer than 14.41 are candidates for open loop operation, and that includes everything above 4000 RPM. From this I deduce that the EPA tolerates considerably worse emissions when cars are accelerating hard. For most drivers, that's probably a fraction of the total time spent driving, so it seems like a (surprisingly) reasonable compromise (for a government agency to make).

 

The point of the datalogging that I mentioned was just to reverse-engineer the transfer functions of the stock sensor. I suspect that's how Bosch figures them out in the first place, though they probably have more advanced reference sensors than our aftermarket widebands.

That is not how "load" works.

 

A NA car will see about 1.5 load IIRC. It is just g/s/rev, or something like that. It is not a calculation based on atmospheric pressure.

 

Load calculations are standardized by SAE J1979. I have no idea where that g/s/rev comes from, it is an oversimplification of the calculations and sounds like something propagated by internet forums and tuning shops. The factory Subaru tables are measured in SAE absolute load, it's a more complicated formula than that. Absolute load is essentially the ratio of measured air in the cylinder to hypothetical max air at standard conditions.

 

It is listed as [air mass (grams/intake stroke) ] / [1.184 (g / intake stroke) * cylinder displacement in liters]

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75414&stc=1&d=1257033314

 

that's SAE J1979v2 page 117. I'm not making this up.

 

Technically there are two types of load: calculated load and absolute load. When most people say load, they are referring to absolute load. Calculated load is normalized such that n/a and boosted engines always read from 0 to 1:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75415&stc=1&d=1257033314

 

that's SAE J1979v2 page 103

 

A NA car will see about 1.5 load IIRC.

Generally speaking this is only possible with an intake manifold that produces a supercharging effect through tuning of the intake manifold runner length and plenum volume to improve volumetric efficiency (Helmholtz tuning). As the valves open and close pressure waves are produced, and the proper intake manifold design can take advantage of those waves to improve pumping efficiency. The non turbo DSM 4G63 intake manifold is one such design:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75416&stc=1&d=1257033702

 

that's the 1990 Eagle Talon technical information manual, page 11-2

 

That's why n/a engines can reach more than 1.00 load sometimes. On a turbo motor, load will be over 1.00 when the engine is in boost.

Load calculations are standardized by SAE J1979. I have no idea where that g/s/rev comes from, it sounds like something propogated by internet forums and tuning shops.

 

It's g/rev, not g/s/rev. You can calculate g/rev by dividing MAF by RPM. Open source loggers will do it for you with a 'calculated parameter' (it logs MAF and RPM and does the math for you) or you can do it in Excel afterward.

 

It is indeed widely propagated by internet forums and tuning shops. I have no idea how Cobb's tuning software does things, but I'd be quite surprised to learn the table axes shown in RomRaider are in fact using something other than g/rev.

 

Example:

 

http://i193.photobucket.com/albums/z151/Legacy_NSFW/Tuning/v049FuelTable.png

I've never used Rom raider and of course I didn't write it, although I'm sure I'll get the opportunity to use it eventually. When logging, if it is somehow deriving load by doing a g/rev calculation it is doing you a disservice. That's not how the ECU calculates it. I just posted the formula. Subaru has to comply with SAE standards just like everybody else.

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75417&stc=1&d=1257037045

 

This factory Rx-8 timing map has absolute load on the x axis. Nobody tries to do g/rev calculations, but then again there really aren't many DIY Rx-8 tuners (most people buy Cobb protune maps).

 

Furthermore, "absolute load" and "calculated load" are OBD II PIDs (Parameter ID's). They can be viewed/logged with any higher-end OBD II scanner. I have an Actron and have read load calculations on my Infiniti before when I was checking some other stuff. The Factory subaru tool can log it too. No derivations are necessary.

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75418&stc=1&d=1257038584

The SAE defined the OBD2 PIDs, but the SAE did not prescribe the ECU's table definitions, nor the parameters that can be logged via the SSM protocol (which is Subaru-specific, and is what RomRaider uses). I think it is a mistake to assume that the PID definition is used for anything other than generic OBD2 logging.

You guys bring up some very interesting points. And I applaud Boostin, as you seem to know a fair amount, good to have you around:

 

Please feel free to prove me wrong, but I am pretty convinced I am correct.

 

The load I am referring to is the load we see in our tables, and can log direct from the ECU. FWIW the NA rom I have goes up to 1.3 load in most tables (OEM ROM)

 

I have logged load direct from the ECU, and then done the calculated load function in airboy's sheet (from g/s and rpm) and they are (+/-2%) dead on.

 

The calculations has nothing to do with engine size, what happens if I had a stroked 2.7 liter? The engine then would easily flow an extra 10% at the same pressure?

 

On on slightly different note, if 1.0 load = 14.7 psi (absolute), then wouldn't (not factoring in efficiency etc...) 2.0 load be double that? As in 14.7 psi relative (assuming sea level)? Wouldn't 3.0 load be almost 30 psi? Most of our cars can easily hit 3.0 load, and they are only at about 18-19 psi to do so. Heck I can hit 3.7 load at about 22 psi, shouldn't that be (14.7 *4-14.7) 44psi relative?

 

It just doesn't add up to me.

First of all, I do want to say that the labeling of the load axis is really kind of trivial in practical terms, and perhaps I've made too big a deal of it. The whole original point was that closed loop logic exists for boost conditions (manifold pressure greater than atmospheric) and the O2 sensor is not always ignored. The front o2 sensor was designed to be used when the engine is in boost, just not WOT boost. In my earlier post you can see the stock closed loop control table extends up to a load of 1.60, where the target lambda is .95 or about 14:1 AFR. You're not going to reach 160% pumping efficiency (1.6 load) without boost.

 

Now, back to theory: all boosted conditions occur when engine load is greater than 1.0. But not all engine loads greater than 1.0 are boosted conditions, and that has been noted here already. Manifold absolute pressure is an indirect measure of restriction in the intake tract, relative between two points. Consider this diagram of engine vacuum in a carbureted intake tract:

 

http://www.legacygt.com/forums/attachment.php?attachmentid=75431&stc=1&d=1257085618

 

That's from a Mazda carburetor repair manual. A lot of those principles apply to a fuel injected engine, which don't have a venturi in the intake tract.

 

The calculations has nothing to do with engine size, what happens if I had a stroked 2.7 liter?

The cylinder size is used in the calculation for the express purpose of preventing what you are talking about here. To use a perhaps oversimplified analogy, consider this: You have two measuring cups. One is a 2 liter measuring cup and one is a 3 liter cup. You pour a liter of water in. You want to know how "full" each cup is in terms of a percentage. That's volumetric efficiency or absolute load. You can't tell how full the cup is just by saying "ok, I put a liter of water in there, it must be x percent." To know what percent full a measuring cup is, you MUST know the size of the cup. And that's why the swept volume of the cylinder must be known. The smaller cup is going to be 50% full and the larger cup is going to be 33% full.

 

Now when you start going back and forth among pressure, mass, and volume, it can get confusing. Continuing with our example, how much more water you can cram in there depends on the size of the cup and how full it is. But if it were possible to compress the water entering the cup (more than atmospheric pressure), you could fit more water in there, and you could have 120% or 150% volumetric efficiency (absolute load). But knowing only the amount of pressure applied to the water wouldn't tell us how much water is actually in there (mass) or how full the measuring cup is.

 

But if we know 3 out of 4 of temperature, pressure, volume, and mass, we can solve for the fourth. That is the ideal gas law. From wikipedia:

 

"The state of an amount of gas is determined by its pressure, volume, and temperature. The modern form of the equation is: http://upload.wikimedia.org/math/4/3/f/43fa535941b0be935b3b173e1ce20338.png where p is the absolute pressure of the gas; V is the volume of the gas; n is the amount of substance of the gas, usually measured in moles; R is the gas constant (which is 8.314472 JK−1mol−1 in SI units[4]); and T is the absolute temperature."

 

 

This mathematical relationship is the basis of the Bosch D-Jetronic speed density (MAP) engine management system, originally introduced in the Porsche 914 and recently used on Hondas, the new Taurus SHO, and Mazda Rx-7's. Let me oversimplify again. On a speed density system, it goes like this:

 

Volume: we know the volume of the cylinder

Pressure: we measure the pressure with the MAP sensor

Temperature: we measure the temperature with the IAT sensor

Mass: we calculate the mass using the ideal gas law.

 

 

After calculating the intake air mass, we take all that information and compare it to how much mass would fit into that sized cylinder in "ideal conditions," the standard temperature and pressure listed in that SAE document. That's our load or pumping efficiency when using the standardized SAE formula. On a Bosch L-Jetronic based (MAF) system, we directly measure the mass in the cylinder and compare it to the "ideal" mass that could fit into that particular cylinder under standard temperature and pressure. That's our load or pumping efficiency.

 

Knowing grams per revolution doesn't tell me how full the cylinder is. It's just a measure of mass flow over time. The units don't work. It would have the exact problem that everyone here is pointing out. It doesn't tell us the pumping efficiency (absolute load) because we aren't incorporating the size of the engine into the load calculation. And it can't be telling us how hard the engine is working (calculated load) because calculated load cannot exceed 100% or 1.0 on any engine according to SAE. So grams/rev is a bastardized unit from a theoretical perspective.

 

Again, in practical terms the exact formula doesn't matter that much for our purposes of looking through datalogs. But using the wrong formula for load calculations when the ECU itself is making calculations isn't going to fly on a production vehicle that has to meet rigorous emissions standards.

I have logged load direct from the ECU, and then done the calculated load function in airboy's sheet (from g/s and rpm) and they are (+/-2%) dead on.

 

The calculations has nothing to do with engine size, what happens if I had a stroked 2.7 liter? The engine then would easily flow an extra 10% at the same pressure?

 

On on slightly different note, if 1.0 load = 14.7 psi (absolute), then wouldn't (not factoring in efficiency etc...) 2.0 load be double that? As in 14.7 psi relative (assuming sea level)? Wouldn't 3.0 load be almost 30 psi? Most of our cars can easily hit 3.0 load, and they are only at about 18-19 psi to do so. Heck I can hit 3.7 load at about 22 psi, shouldn't that be (14.7 *4-14.7) 44psi relative?

 

It just doesn't add up to me.

 

Can you explain the above then?

 

(I understand your last post, and it totally makes sense. In fact I always kinda wondered it)

 

But the above about 3+ load doesn't add up to me. Also, the calculated load we use lines up almost exactly with load direct (as logged) from the ECU.