Tuning with g/rev airflow variable

[Rice Over Wheat]

I'm trying to add airflow to my tuning considerations as I push my turbo to its limit. The tuning guides don't really delve much into this.

I've read elsewhere in a thread that the following formula is used to calculate lb/min of airflow from g/rev off a logger:

(g/rev * rpm) * 0.00221

I'm looking at the following log (1st and 2nd gear pull, auto, 21 psi). Note that I am using PalmStein and 3.6/sec sample rate is the fastest I can log. OBDGauge fails to log airflow for some reason:


RPM / Timing / Airflow

2436 27 4.36
2628 23 6.12
2980 21 10.92....WOT to 1st
4836 0 22.16
6036 20 22.95
7084 21 21.46
7300 -2 22.40....to 2nd
5280 0 18.14
4888 12 18.84
5160 13 19.94
5468 14 20.80
5728 16 21.29
5988 17 21.59
6252 17 21.87
6508 17 21.85
6124 37 2.55....0% throttle
6036 37 2.56
5384 34 0.57

I don't understand what I'm looking at here. If I use the above formula for the highest airflow data point: 22.95 @ 6036 rpm, I get 306.14 ...so either the formula is wrong or these airflow numbers are not g/rev. Anyone have any idea what I'm looking at? I can't find a single reference online explaining what palmstein is logging and their manual doesn't explain either.

I'm trying to figure if my AGP BT28 is just blowing hot air at this point.

 

[spyderturbo007]

EDIT -> I just re-read your thread and wanted to let you know that your logger is giving you the airflow value in lbs/min, not g/rev.

Don't forget that you have to back calculate the logged airflow to remove the SAFC "molestation" factor before the conversion.

Do this first:

M / (1 - C) = A

Where:

M = Maximum airflow logged
C = Correction factor as decimal that corresponds to "M"
A = Actual Airflow

For example, if your max airflow is 25lbs/min @ 6500rpm and your 6500 rpm correction factor is -30%, then your equation will look like this:

25 / (1 - .30) = Actual ariflow

After you have your actual airflow, here is how you convert to g/min:

(A * 453.59) / R = Airflow in g/rev

Where R = rpm value corresponding to airflow and correction factor used in first equation.

Let me know if you need any additional clairification

 

[Rice Over Wheat]

That's some good stuff there, nobody explains this in the guides.

Ok, so then if my logger is showing 22.95 lbs/min @ 6036, and my SAFC correction at that rpm is -20%, then:

22.95/.8 = 28.69 actual lb/min
...or 2.156 g/rev

Ok, so now that I know this info, how am I supposed to use it when tuning? I've seen lots of comments by wisemen pop up about 2.06 g/rev and 16*, etc.

And btw, if this turbo is rated at 36 lbs/min then it looks like I have room to add more boost provided I can keep the knock down. Damn, I thought I'd be over 30 at least!

 

[FORMONTOYA]

The only time I look at the g/rev airflow variable is for my idle trims.........which should be at ~.27g/rev. Other than that, I've not had a reason to "fixate" on it.

 

[Rice Over Wheat]

So you don't use your airflow lb/min rate for consideration in tuning? I guess what I'm trying to do is see where I'm standing with my current tune and where the BT28 can go.

I've read speculation that the BT28 doesn't "come alive" until 21/22 psi, and I have to say that from my previous tune at 19 psi, 21 psi felt like a hell of a lot more torque when WOT. At full boost it's almost violent now for how suddenly the power comes on and the car surges forward -- in fact I was worried about that and haven't been doing this much for fear of breaking something. I'm wondering if I'm entering the efficiency range of the turbo, because I'm willing to push it to 23 psi next if I can do it safely and there is room left to do so.

 

[FORMONTOYA]

No, I personally do not. I'm on a 16PSI summer/20PSI winter type of setup and tuned accordingly. Although, I can understand where you're going.

I guess you're in the realm of "baby steps" now, just make any corrections at this point tiny since it sounds like you're getting closer to the edge....keep a close eye on the knock and A/F ratio and turn the boost up slowly until the air flow starts to drop.

 

[scottsee]

The only time I look at the g/rev airflow variable is for my idle trims.........which should be at ~27g/rev. Other than that, I've not had a reason to "fixate" on it.

You do mean .27g/rev right? Thats for around 70deg F.

Ok, so now that I know this info, how am I supposed to use it when tuning? I've seen lots of comments by wisemen pop up about 2.06 g/rev and 16*, etc.

well.. Couple ways. Our engines use their calc sums bassed off g/rev, not lb/min. Knowing the exact g/rev your engine is using will tell you what fuel/timing map you are in. Your top fuel/timing map starts about 2.1g/rev and enriches fuel by +55% stocho over 4500rpm's. If your dipping over/under 2.1g/rev you'll be cycleing between diffrent fuel maps making it hard to stay consistent from run to run.

That airflow seems pretty darn low for 21psi on an T28, even for 2nd gear..

 

[FORMONTOYA]

You do mean .27g/rev right? Thats for around 70deg F.

Whoops........you're absolutely correct. That's a big difference when I forget to put the (.) in there. Thanks!

 

[Rice Over Wheat]

well.. Couple ways. Our engines use their calc sums bassed off g/rev, not lb/min. Knowing the exact g/rev your engine is using will tell you what fuel/timing map you are in. Your top fuel/timing map starts about 2.1g/rev and enriches fuel by +55% stocho over 4500rpm's. If your dipping over/under 2.1g/rev you'll be cycleing between diffrent fuel maps making it hard to stay consistent from run to run.

That airflow seems pretty darn low for 21psi on an T28, even for 2nd gear..

Yeah I know, I don't know what the deal is. I'm not sure how accurate PalmStein is when calculating airflow.

Going by 1 lb/min per 10whp, this turbo's rating of 36 lb/min or 360hp means I should be pushing quite a bit more than I am.

My last tune on a dyno with stock ic and injectors was 242 whp 270 wtq @ 15 lbs with the stock actuator. Now, that would be around 24 lb/min of flow right? My current setup is much better now and I'm at 21 psi but I'm only netting 28.7 lb/min of flow? WTF About how much additional airflow should I be seeing per psi....1 or 0.8 or something?

I'll be getting Link in january before upgrading this turbo as I just cannot abide this guessing game using a logger. I need to see knock and I need to see accurate parameters.

 

[Hal]

The lb per minute is NOT calculated by the logger.

It's simply showing you the actual number reported by the ecu.

 

[Rice Over Wheat]

I see, thanks for clarifying that, Hal. You mean that's the post-AFC-correction number it's seeing right?

 

[Hal]

I see, thanks for clarifying that, Hal. You mean that's the post-AFC-correction number it's seeing right?

That's correct

 

[CanadianTSi]

and my SAFC correction at that rpm is -20%

-20 Up top with 650's would make you really rich.

 

[Rice Over Wheat]

Everyone keeps telling me that, but it was my understanding that the more airflow I push with boost, the more I would have to richen from base correction.

For example, if you are running 15 psi on 450s, then you add 650s, base correction would be around -30.8%. But isn't that all relative to the same boost level before going to larger injectors? So if you increase boost to 21 psi, you'd be flowing more air and therefore would need more fuel right? So I just sort of assumed that my settings were in the right neighborhood for 21 psi at -20% HI. That's how I've been removing knock in my logs...

If that's not the way it works, I must be way off the ranch in my reasoning.

 

[Black95TSIawd]

Yes and no. You have to understand that fuel pressure also goes up with boost via the FPR.

 

[CanadianTSi]

Base correction stays the same regardless. The ECU see's the additional airflow and injects more fuel.

I'm at -33 with 650's at 24psi flowing 45lbs/min.

 

[Rice Over Wheat]

I must be running pig rich this whole time, damn. I'm going to try tuning from scratch again starting again from -25%. I was going in the wrong direction. I'm such a friggin noob at this.

Yes and no. You have to understand that fuel pressure also goes up with boost via the FPR.

Understand, being the operative word. I don't have a solid grasp on that concept just yet. I think if I understand it right, the AFPR raises my pressure 1:1 with boost over 43 psi, my base setting. If that's correct, then no wonder my logic was wrong and why I was tuning richer instead of leaner...arrrgh!!

Thanks for bringing that up.

Is Link far more intuitive to someone like myself who is new to tuning? I'm thinking $1000 spent would be a bargain just to ease my frustration.

EDIT: I reset to -25 HI across, did several pulls and I got lots of pulled timing around 5000 to 6500. Very noticeable loss of power. I'm restoring to my previous rough tune at -20%. I don't know why but my car just doesn't seem to like going any leaner. I think I'm going just wait until I get DSMLink so I can see what's really going on in there. For now I'll run rich and be safe until January.

 

[spyderturbo007]

Ok, so then if my logger is showing 22.95 lbs/min @ 6036, and my SAFC correction at that rpm is -20%, then:

22.95/.8 = 28.69 actual lb/min
...or 2.156 g/rev

Correct. But........

Ok, so now that I know this info, how am I supposed to use it when tuning? I've seen lots of comments by wisemen pop up about 2.06 g/rev and 16*, etc.

At 2.1g/rev the ECU will place you on it's highest airflow map. This means that if the ECU "sees" <-Keyword, 2.1g/rev, your timing will be capped at 16 degrees.

Remember, that is the airflow that the ECU sees, not what we just calculated. With your example, the ECU is only seeing 22.95lbs/min, or 1.72g/rev because you are pulling out 20% of the airflow before the value is passed along to the ECU.

That's the hitch. So, even though you are actually flowing 2.16g/rev, the ECU never sees that number.

And btw, if this turbo is rated at 36 lbs/min then it looks like I have room to add more boost provided I can keep the knock down. Damn, I thought I'd be over 30 at least!

Your airflow numbers do seem really low as scottsee said, but since you couldn't log airflow until now, I could only go off your timing values.

From what I can see, your timing looks good, although more data points would help. Now we just have to figure out why your airflow is so low.

 

[Kevin Jewer]

I see some familiar material here... There is a thread either here or on automotive forums that I put a good bit of detail into. The main purpose of calculating airflow/rev is to back asswards figure out if you are on a "known" timing map and then do the math to figure out your knock retard, for poeple that can't read knock, as Nathan mentioned.

 

[andymoraitis]

I see some familiar material here... There is a thread either here or on automotive forums that I put a good bit of detail into.

Yup, I learned a ton from reading your articles Kevin. It's good stuff.

 

[Kevin Jewer]

Holy crap, it is on my site. Go figure. Good thing I upload some of this stuff before my dumb ass forgets where I posted it.

 

[Rice Over Wheat]

Thanks for that link and info, I'll be studying up on that stuff.

 

[Rice Over Wheat]

I do not believe AGP's inflated numbers of 36 lb/min for this turbo. I think it's more around 32 maybe. Of course, that's just an uneducated guess based on what I'm getting. Does anyone think a large fmic can affect the airflow numbers of a relatively small turbo like the Big T28? Or does ic volume have little impact on what the turbo can push through?

The only exhaust restrictions I am aware of are:

1. Ported turbo matched to NON-ported O2 housing.
2. Exhaust piping: there is a bend before the muffler going over the axle. This 8 inch section is actually 2.5" piping compared to 3" for the rest of the system. I have been meaning to eliminate this but haven't had time yet.

Thing is, I didn't think this turbo is big enough for the above considerations to matter.

 

[CanadianTSi]

Your gonna need alot more boost if you wanna come close to the Max Airflow of the Turbo...

 

[Rice Over Wheat]

I aint scared. But I think I'm going to put DSMlink in before I go to 25 psi on this guy.