Calculated Load vs. Absolute Load

jcascel1 posted this data capture from his Mazda 3 in this thread :



And it got me thinking. Calculated Load is cool, but Absolute Load? I honestly never noticed this before, as I spend 90% of my scanning time in OEM data, and not Global OBD.

So, what's the difference? Google hooked me up with an SAE document which outlined the technical details. There's a lot of math involved (which I suck at), but here's the high points:

SAE wrote: Characteristics of LOAD_PCT are:

Reaches 1.0 at WOT at any altitude, temperature or rpm for both naturally aspirated and boosted engines.
Indicates percent of peak available torque.
Linearly correlated with engine vacuum
Often used to schedule power enrichment.
Compression ignition engines (diesels) shall support this PID using fuel flow in place of airflow for the above calculations.

Characteristics of LOAD_ABS are:

Ranges from 0 to approximately 0.95 for naturally aspirated engines, 0 – 4 for boosted engines,
Linearly correlated with engine indicated and brake torque,
Often used to schedule spark and EGR rates,
Peak value of LOAD_ABS correlates with volumetric efficiency at WOT.,
Indicates the pumping efficiency of the engine for diagnostic purposes.

I underlined the parts that really caught my eye. Indicates pumping and volumetric efficiency? Cool! We're very interested in engine breathing during diagnostics.

My question here is, should I be using this Absolute Load PID, instead of Calculated? Does one have more diagnostic value than the other? And are there situations where using both would be an advantage? Naturally, I decided to play with this on my own vehicle, but it doesn't have the Absolute Load PID. What's up with that?

So, I'm looking to you guys for help! I'd love it if someone has a known good vehicle with both Calculated and Absolute Load PIDs, and is willing to get some WOT data captures. Snap-On tooling would be cool (to ease data sharing), but not required. It'd be even cooler if you'd be willing to install an engine breathing problem (like an intake restriction) to see how the data responds.

Had a chance to play with these PIDs on a '12 Equinox, 2.4L.

www.scanshare.io/share/5nqWnZxps0umg8srqy7IKw#0,1,2,3,4,5,6,7

First thing I noticed was the Absolute Load reading of 100% at one point... That's an impressive volumetric efficiency! Actually, I think that's impossible on a piston engine... :huh:



And what does the volumetric efficiency calculator have to say about that?



86% VE is pretty good, but it's definitely not the 100% reading Absolute Load was telling us. OK, maybe that was a fluke, let's look for the highest airflow rate instead.



This says our VE is still a healthy 83%. Calculator says...



Nope. Still doesn't match. I could live with a percent or two off, but not like this.

First experiment with Absolute Load, and I'm not impressed. I didn't have the opportunity to install any problems into this Equinox, but I'll be looking for the next opportunity.

Just a quick reply to say I'm following and will definitely contribute when I can.

Tyler wrote:

First thing I noticed was the Absolute Load reading of 100% at one point... That's an impressive volumetric efficiency! Actually, I think that's impossible on a piston engine... :huh:

Agree

This a cool subject. I'm interested and learning. So can we actually state that Absolute Load equals the percentage of volumetric efficiency? When looking at the scan data and the VE calculator results, not really
I wonder what's missing here.

I have been mulling this one over I would think comparing the differences between the graphs of calculated load and absolute load on a test drive could be useful.

matt.white wrote: Just a quick reply to say I'm following and will definitely contribute when I can.

Thanks, sir! I welcome anything you can contribute.

Agree

This a cool subject. I'm interested and learning. So can we actually state that Absolute Load equals the percentage of volumetric efficiency? When looking at the scan data and the VE calculator results, not really
I wonder what's missing here.

Same. I mean, SAE obviously thought this PID had some kind of value, but I haven't seen it yet. :lol: To make things worse, I tried this same test on a '11 Buick Enclave, and it showed me an Absolute Load of 105%... I'll post that tomorrow.

I have been mulling this one over I would think comparing the differences between the graphs of calculated load and absolute load on a test drive could be useful.

I could possibly make that happen with ScanShare? I think it'll let me overlay graphs...

Tyler wrote:

Same. I mean, SAE obviously thought this PID had some kind of value, but I haven't seen it yet. :lol: To make things worse, I tried this same test on a '11 Buick Enclave, and it showed me an Absolute Load of 105%... I'll post that tomorrow.

Heuh?? 105% :huh: Now I'm really confused!

Round two! Got some more data off an '11 Enclave while test driving.

www.scanshare.io/share/iOuZpe1gy0KvSqqqbOnoqw#0,1,2,3,4,5,6,7



Yep, that's a reading of 105.5% Absolute Load. This thing ain't boosted, so that number is pretty optimistic. :lol: For giggles, I found the highest airflow rate and put the numbers into our VE calculator:



Pretty impressive, actually! :ohmy: I almost never see VE higher than 90%. Makes me wonder if this sensor data isn't accurate in some way...

I have been mulling this one over I would think comparing the differences between the graphs of calculated load and absolute load on a test drive could be useful.

Sorry, ScanShare let me down on this one. I set the graphs up in ShopStream a bit differently, check it out:



The part of that SAE document where it said that Calculated Load corresponds to engine vacuum seems to make sense, as I'd expect a vacuum gauge on the manifold to read zero while at WOT (this engine does have a MAP sensor, but I couldn't find it on the Global list :blink:). However, a plugged exhaust will also zero a vacuum gauge, so...

I think I'm better off sticking to whatever Load PID the OEM data gives me. :lol:

Tyler wrote:

Pretty impressive, actually! :ohmy: I almost never see VE higher than 90%. Makes me wonder if this sensor data isn't accurate in some way...

I just wonder in what way :lol:

Dylan wrote: I just wonder in what way :lol:

I wonder if the Air Flow PID is a bit 'optimistic'? I'm not sure how the Global version calculates it. It may not be entirely based on the MAF sensor.

Anyway, I tried again with an '06 GMC Yukon. I could post the data, but the short version is that it doesn't support the Absolute Load PID. :blink: Calculated Load is there, as always, but not Absolute. jcascel's '05 Mazda had it, why not this '06? Both engines have MAP sensors, so that's not the determining factor.

Conclusion: Absolute Load is kinda useless. :lol: There's a reason no one in the industry teaches it's use. Not every vehicle has it, and it's not terribly useful on the vehicles that do. Oh well, it was fun to explore! This is how we develop diagnostic methods.

Tyler wrote:

Conclusion: Absolute Load is kinda useless. :lol: There's a reason no one in the industry teaches it's use. Not every vehicle has it, and it's not terribly useful on the vehicles that do. Oh well, it was fun to explore! This is how we develop diagnostic methods.

Agree Wish it would've lead us to some 'new discovery '... But hey this how it goes in our business :whistle:

Thread bump outa nowhere! :woohoo:

Ended up taking a class that helped answer some questions that came up in this thread.

1.) The 105% absolute load value I saw on the Enclave is perfectly normal. I'm told this is a product of VVT, and not unusual at all. Some have observed 110% on other engines at WOT. :cool:

2.) The OTC volumetric efficiency calculator is incorrect. I'll be comparing it to other brands of calculators in the near future, but for now I'm taking off the Service Info thread. This is why I got all kinds of wacky readings earlier... There IS a software issue, just not on the vehicle side! :blush:

3.) Absolute load is a mandated PID from 2012 onwards, but appears on some vehicles before that when the OE decided to use it.

So like I said earlier, I had an instructor inform me that the OTC calculator is NOT accurate. I believe him, but I wanted to see for myself.

Last go-round with the OTC VE calculator, I came up with this on a '11 Enclave:



That's not a bad reading, by any means. But what happens when I put it into another calculator? I found VE Calculator on the Android Play Store for free. Same values:



Different result:



This is closer to the 105% value we saw from the Absolute Load PID in Global OBD. The 5% difference didn't matter in this case, but what if you had a genuine low power complaint? That difference might end up leading you down the wrong path...

And here I was thinking that VE was a standardized calculation, and not open to interpretation. :lol:

What's that app? I've been disappointed with the play store offerings of VE calculators...

Tyler wrote: Thread bump outa nowhere! :woohoo:

Ended up taking a class that helped answer some questions that came up in this thread.

1.) The 105% absolute load value I saw on the Enclave is perfectly normal. I'm told this is a product of VVT, and not unusual at all. Some have observed 110% on other engines at WOT. :cool:

2.) The OTC volumetric efficiency calculator is incorrect. I'll be comparing it to other brands of calculators in the near future, but for now I'm taking off the Service Info thread. This is why I got all kinds of wacky readings earlier... There IS a software issue, just not on the vehicle side! :blush:

3.) Absolute load is a mandated PID from 2012 onwards, but appears on some vehicles before that when the OE decided to use it.

Oh man that sucks! In a good way ... So it's the calculator itself that's throwing us off

Although I saw different videos on YouTube where the OTC VE calculator was used... :S

Also have a VE calculator from the Android playstore. There's also one I saw on the Autonerdz channel. Only if you have Windows.

Anyway I'm excited to see this topic is back!

Noah wrote: What's that app? I've been disappointed with the play store offerings of VE calculators...

I found it under VE Calculator. Maybe it'd be fun to take several for a spin? See how much variance there is out there.

Another tidbit I got over the weekend is how valuable Absolute Load is for forced induction engines, particularly Ford Ecoboost platforms. Somewhere between 155 and 185% is a good value on those engines.

This is the video of that VE calculator I saw on Youtube:

I have a screenshot of a Tech 2 screen with the VE pid vs VE calculator and it is really close. I'll grab it off my other computer tomorrow.

gav09 wrote: I have a screenshot of a Tech 2 screen with the VE pid vs VE calculator and it is really close. I'll grab it off my other computer tomorrow.

Cool. I'd like to see that screenshot

gav09 wrote: I have a screenshot of a Tech 2 screen with the VE pid vs VE calculator and it is really close. I'll grab it off my other computer tomorrow.

I've seen that before! Very nice of GM to add that PID. But not all have it?

Decided to play a bit more with Absolute Load, this time on an '08 Escape. 3.0L engine, no VVT.



89% at the peak. This kinda makes sense compared to the '11 Enclave I tested earlier, which is supposed to use it's VVT at WOT to improve VE.

Still trying to build a baseline for what this PID should be doing on different engines. Maybe 80 - 90% for non-VVT engines, 90 - 105% for VVT?