MAF flow vs HP

[MSR]

from memory, i think the obd-II scanner hooked to my LT4 gave MAF flow in terms of grams per second. maybe some other units but that's what i remember. someone point out the flaw if one exists, please: i'd like to figure out what power levels correspond to what flow readings, assuming a certain brake specific fuel consumption. (i'm trying to do this in order to help determine a problem with my car, i'd like to know if it's just not drawing enough air to make power, or if it's drawing the air yet not utilizing it.) convert grams per second to pounds per hour, then use 13:1 as a ratio of airflow to fuel flow, then use the commonly available hp formula as a function of fuel flow and BSFC. will this work? thanks. -michael

[DOCTOR J]

It will work up to a point, but as you know the devil is in the details. I've tried to do
the same sort of thing by striking a fuel balance around the engine, but the absolute
accuracy of the result leaves a lot to be desired.

Remember that a typical engine dyno chart shows the RPM-dependence of the BSFC
number, under a constant load . A recent example was posted over on the TGO board:
http://temp.corvetteforum.net/c4/doc...x/engdynos.jpg


The shop BSFCs will not be completely accurate for a car under highway conditions.
I'm thinking here of the speed vs. aero drag loading, and also of the rotating mass/angular
acceleration loads. Neither will be constants in actual operation (with gear changes).

Not the smallest isssue is getting accurate BTU & specific gravity values for your fuel.
Weight shouldn't be a problem, but there is so much RFG blending going on (at least
around here) it's hard to guess what BTU value to asign to a tank of fuel any more. How
to relate your fuel back to that which was used to generate the BSFC numbers in the dyno
shop is a real unknown...


Except for all that it should be a piece of cake... :)


That being said, I suppose if you hold your testing to steady-state speed & RPM, many of
the variables above wash out. Is that the kind of testing you had in mind?


FWIW, I've settled on measuring output based on an acceletation transient (actual road HP)
and then making corrections for the drag terms. It helps some in evaluating hardware/
software changes. There is a little discussion of test methods & tools toward the end of
this page:
http://www.corvetteforum.net/c4/doctorj/cartest.htm

HTH

[MSR]

ok... yes, the difference between .455 and .555 for a BSFC would completely fill the hp difference gap i'm trying to measure. so it wouldn't really help me. i'll do it just the same, just for the science. i'll do a dyno pull and log MAF flow per rpm and compare to what comes out.

thanks.

-michael

[DOCTOR J]

Comparing air flow to dyno readings is probably the more accurate way to do it.

On a short-duration (5 sec) acceleration test, the effects of pump-shot, PE, and left over
fuel being swept off the internal surfaces of the manifold would - I believe - pretty much
mask the true BSFC. It needs a more steady-state test to be measured correctly, AFAIK.

I mentioned the fuel's specific gravity too - since of course the injectors are volumetric flow
devices, rather than mass controllers. As such, a change in fuel density (or caloric content)
also significantly skews the BSFC calcs, taking a big chunk out of their accuracy.

If you are going to an inertial chassis dyno, it would be interesting to see two test
results back-to-back. That would be measured power in two different gears. From that
data you could back out a correction for the rotating mass of the drive train. I have one
(blurry) reference point for that on a C4, but it would be nice to get another - with the gear
ratios included.

That or an actual coast-down test would also benefit science...

Good luck.

DrJ

[MSR]

That or an actual coast-down test would also benefit science...

this i've already done. at the speed limit of 4th gear it was ~30hp, and dropped as speed dropped. only 5hp more than a mustang with solid axle.

i'll do it again next time and save the chart so i can post it, along with a MAF flow vs rwhp chart i'll make. might be a week or two, though.

i also have runs in different gears, i'll try to remember to grab those too.

-michael

[DOCTOR J]

It would be interesting to see the data. If you get a chance post it, please.


This may be old news to you, but the site here has some good tech on dyno operations (toward the bottom of the page):
http://www.pumaracing.co.uk/mainmenu.htm

The author is a pal of Dave Vizard, IIRC.

Hope it helps.

DrJ

[Mo_Bandy]

Doctor J,

how are ya?

I thought I would pose a question for your opinion or observation...

My 96 is supercharged... so using the MAP readings for me are useless so Like MSR I have "played" around as well with this thought....

Using Autotap I log my runs.. make my adjustments in LT1 edit...

As very rough calulations

could you not take your Maf lbs/min * 14 to give you the CFM...
Then take the some what school of thought of 1.5 cfm ~ 1 horsepower?

So in my case for example at 5502 rpms I was pulling 48.75 lbs/min on the MAF ( my MAP at WOT is pegged at 30.4 and 103)

so I would have 48.75 * 14 gives me 682.5 CFM

682.5 /1.5 = 455 horsepower...

I know there are a ton of variables here but I have found that the MAF for each of my runs seems extremely consistant and also have a great bearing on the run looking at knock retard and o2 reading...

Just a thought that I had....

Mo

[DOCTOR J]

Hey, Mo!

When did you put on the coil-overs? How are they working out for you? I can't recall if you
had those the last time we talked or not...


I don't see any major problem with estimating power based on airflow. My only point above was
that BSFC numbers are (AFAIK) based on equilibrium conditions, and you need to take them with a
grain of salt on a short-duration acceleration test.

I spent a few weeks chasing that line of thought myself, until I realized a (N/A) small-block is
only burning ~1 oz. of fuel/sec @ 6,000 RPM. Trying to decide if a test had used 1.5 or 2.0 ounces of
gas had me going cross-eyed. After slapping myself in the head a few times I decided it was better
to use kinetic energy to measure transients. My reasoning was that, based on my instrumentation,
it's easier to find an extra 100,000 ft lbs of energy than 0.25 ounces of fuel :). That gives me enough
repeatability between tests so I have at least one metric to evaluate engine & programming changes.

By way of background on the complexity of the fuel balance, there's an SAE paper here that
speaks a little to injector compensation, wall wetting, air flow and such - although it's more about
cold-start than power enrichment:
http://www.delphi.com/pdf/sae/1999-01-0553.PDF


There is another Delphi paper which deals with MAF accuracy & transfer functions -
http://www.delphi.com/pdf/techpapers/2000-01-0546.pdf

I don't recall anybody testing MAF transient behavior as such, but it's gotta be more accurate
than trying to use the fuel characteristic. My only caveat is that - at least in the older GM ECM's -
there is a lot of signal buffering that goes on before the sensor data is read by the CPU/sent out to
the ALDL line. You maybe need to tap into the raw data (before buffering/averaging) to get high-
quality MAF numbers for evaluation.

As to your calc, that's a litttle more gnarly. The industry rule of thumb (as you know) is to use 0.6
BSFC for supercharged engines. However as I read that, it's not-to-exceed design point that doesn't
take into account correcting the intake flow to standard temp & pressure. Your MAF should be reading
more like ACFM, so at first blush the BSFC ought to look more like a standard small block (albiet
one with very high CR). You are on the right track, but I dunno myself exactly where that takes
you for absolute accuracy of the result. But I know of the guy I would ask:

This board http://www.turbobuick.com/forums/ does a lot with forced induction, and there
is one guy named GNJones I see posting in the ECM section from time to time. He's at NASA in
Houston, and has done a lot with instrumenting his Turbo Buick. He (or one of the others on that
board) might take you right where you want to go.


Does that answer your question at all?

I have a couple of new things I'm trying to get ready for spring:

Picked up a Northstar 4-coil ignition pack. I think I know how to interface that with my ECM,
but I have to finish some trigger-wheel testing before I'll know for sure.

Also started trying to write some new code for the old 727 ECM. For that I need to finish my ECM
emulator for code testing:
http://temp.corvetteforum.net/c4/doc...atorboards.jpg

You ever work in Assembly language? That stuff is a real PITA.


Later,
Dave

[Mo_Bandy]

I put the coil overs on in July last year... I love them ... thank you for the insight and the white paper links...

Yes I'm aware of the .6 BSFC like everyone looking for the way of getting the right and best setups....

Keep me posted on your coil packs... I've been kicking that type of project around myself... :)

Mo

[DOCTOR J]

Mo-

Reading through the above I'm not sure my answer did justice to your calibration question.


Let's step back and look again at the equation you posted for your car:

1. An indicated mass air flow of 49 lb/min = 2940 lb/hr of air.

2. At a nominal AFR (AFR is a weight ratio) of 12.5;

2940 lb/hr air divided by 12.5 = 235 lb/hr of fuel.

3. At a BSFC of 0.5 (BSFC has units of lbs of fuel per hr per HP);

235 lb/hr fuel divided by 0.5 = 470 HP


What are the sensitivities?

A. The stock MAF is uncalibrated (against a known reference source) so you really can't tell
a priori whether its output is accurate in the region being tested. It does have the
advantage of reading something like actual cfm (ACFM) inlet flow.

B. The small block BSFC - as shown in the literature - has considerable rangability, based on
heads/cam/RPM. There are published engine dyno tests that show BSFC's anywhere from 0.3
to 0.6. The number will be at its minimum at/around the torque peak of the particular engine
being tested. Without a valid reference point similar to your set-up, applying this factor is a
crap-shoot. GM's normal design point for all the N/A small blocks seems to be around 0.5 average.

C. Fuel composition: Fuel blended for emissions is all over the map anymore. Around here
refiners are nominally adding 10% ethanol to the 'winter blend', which has the effect of dropping
heating value of the fuel by ~5%. That's a further potential error in the calc.

D. The blower: My first thought was the blown 0.6 BSFC was a de-rating mechanism for heat of
compression. The more I think about it, it could also be a reflection of the compressor power loss.
Moving the BSFC up from 0.5 to 0.6 really seems too high for either. Your best bet is to pick up
one of the standard blower engine books & do a little research - isn't there a Corky Bell text that
everyone references? That's one area I've not looked into at all, and I don't ever recall seeing
an engine dyno report for a blown application.. There must be some around somewhere.


There is a site here that shows some primer info on sizing air compressors (Turbos in this case).
It's not too hard to follow, and the general design criteria apply equally to any centrifugal compressor:
http://www.gnttype.org/techarea/turbo/turboflow.html

I don't think I've ever looked at air compressor power consumption for a car blower - need to add
that to my to-do list..

Michael's idea of correlating the logged MAF & RPM values to a dyno run has a lot of merit.
It establishes a base line reference for that engine that can be used to evaluate later changes.


Does that help?

Dave

[Mo_Bandy]

Doctor J

Thank you, yes I have the corky Bell book on supercharging. What I have kind of used for my "baseline" if you will is I have probably close to 100 normally aspirated runs. Like MSR I have logged them into a spread sheet with the Normally aspirated RPMs with the MAF throttle and injector pulse widths.

When I went to the Vortech, I have used those as a reference to see what the pressure ratio I was running by comparing the MAF, throttle and injectors.

It would sure be nice to have it where you could program in the fuel pressure the Injector timing, with the CFM and have some reasonable idea as to how the AFR looks without relying on the O2 sensors... Rapidline has sort of a way of doing it but it is pretty cluggy as it seems to always wants to change the BSFC to a normally aspirated value...

Looks like maybe time I do a little code writing as well... HEH

Thank you again

Mo


[Modified by Mo_Bandy, 2:24 PM 3/3/2004]

[DOCTOR J]

It would sure be nice to have it where you could program in the fuel pressure the Injector timing, with the
CFM and have some reasonable idea as to how the AFR looks without relying on the O2 sensors... Rapidline
has sort of a way of doing it but it is pretty cluggy as it seems to always wants to change the BSFC to a normally
aspirated value...

Well, you are only lacking one tiny bit of info to make the spreadsheet you want, and there are at least two
ways to get the missing piece:


1. Measure your fuel injectors-

Bowling & Grippo published an easy design here for a programmable injector tester:
http://www.bgsoflex.com/FI_tester_doc.pdf

With a spare fuel pump, a pressure reg, and a little hardware you can characterize your injectors exactly
for pressure & pulse width. The difficulty in measuring injectors is in their flow modulation (which occurs
during their opening/closing times). Wells briefly describes the phenonemon here:
http://www.wellsmfgcorp.com/counterp...v5_i1_2001.pdf

A small graphic of the main point of the article is here:
http://www.thirdgen.org/techbb2/atta...postid=1590404

As you see, high impedance injector flow is much more sensitive to pulse width & injector latency than one
might expect. Measuring the little buggers accurately looks like a royal PITA.


Building one of those test benches is on my long-term list of weird things to do. If I had one, I'm sure I could
set a track record at Lime Rock... :)

2. Just measure the AFR directly-

I went with the less exotic method & built a Wide Band, so I could see first-hand what the AFR was doing.
The details are on my CF page. It's calibrated against free-air & the GM narrow-band O2 sensor, and has
surprisingly fast response times (out past the limits of my data logger).

Measuring the AFR directly has the advantage of telling me whether or not the injectors are working up
to snuff. As a side benefit, it also shows ignition problems on the same chart...


Let me know if you decide to build the injector tester <VBG> The new LSx injectors are supposed to be
much faster than the old Bosch style, but I haven't run across a real analysis anywhere...


Dave

[Mo_Bandy]

Dr J,

You know that is a most EXCELLENT Idea!

Once I get back home, I just might put that on my front burner....

Thanks again for the excellent info

Mo

[MSR]

I have logged them into a spread sheet with the Normally aspirated RPMs with the MAF throttle and injector pulse widths.

interesting. do you have those handy?

i might have some dyno time this weekend. if not, next weekend for sure.

-michael

[MSR]

As you see, high impedance injector flow is much more sensitive to pulse width & injector latency than one
might expect. Measuring the little buggers accurately looks like a royal PITA.

i've seen many times in tuning cars that the injector opening/closing times must be properly accounted for. when you get that right, seems like all the other tables just fall right into line.

-michael

[Mo_Bandy]

MSR,

I'm on a biz trip this week but I'll be more than happy to shoot you a couple runs for comparision. once I get back home Friday.

Shoot me an email to remind me if you would please

mo