Quote: Originally posted by doc How do we convert cfm to gms/sec??? I know how to convert lbm/min to gms/sec. gms/sec = (453.5/60) * lbm/min So I need the weight of a cubic foot of air??? DAH! Who can help me? AND is there a published article on the above flow rates? 1 cf/air = 0.0747 lb @ 72f & sea level I'll leave the rest as an exercise for the reader. RBob.

Thanks Rbob,

So I get for our stock TPI MAFs w/o screens 658.4cfm = 371.8gms/sec

Interesting

My '99 MAF car flows a max 320gms/sec, so this equates to 567cfm

SOOOOO, I conclude that a screenless TPI MAF (658cfm in above table = 371gms/sec) will flow enough air for my L98, but the MAF table value is limited to 255gms/sec,,, which is much less than what it can actually flow at WOT. (Big oversight by GM!!!)

BTW: my '99 MAF LS1 car makes 400RWHP with about 320gms/sec air flow (567cfm).

Any comments on this logic, or am I all screwed up.

hhmm yet agian the limit is 5 volts not 255grm/sec.by reducing flow restrictiuon you drop the voltage which drops resoluytion which drops voltage which increase aviable table resolution.stop thinking 255 start thinking 5voltsd.

255 is just what the table tells the computer is at max (min voltage on the wire). So, all you'd need to do is make a dynamic flowbench for the 90mm and match its voltage outputs to the smaller one at given air flows and re-write the maf tables to correspond to the new flows...piece-o-cake.

This is just an FYI post on the CFM ratings. As most know 2bbl carbs/TBI are rated at 3" of pressure drop (Hg). Four bbl carbs are rated at 1.5" of pressure drop. The conversion is to multiply/divide the one rating by 1.414 to obtain the other rating.

So I started thinking 'what is this 28 inches of water.' Nice guys, another pressure drop that isn't even in the same units as the carb ratings. So I found this:

1 psi = 27.712" water = 2.041" Mercury

(Machinery's Handbook, 20th ed., p. 2317)

Hrummph, now another CFM rating at a different pressure drop. To convert this rating to the 4bbl rating divide the 28" H2O rating by 1.166

So the Stock 4.3/5.0/5.7 2bbl TBI complete -- 574.1 cfm (dry) will flow:

574.1 / 1.166 = 492 CFM @ 1.5" Hg

Still don't understand why another pressure drop is being used. The 28" of water does seem to be a standard of some nature as I've been seeing it used quite a bit for intake manifolds and head port flows.

RBob.

hmm if i was going to use this MAF on a $64 calibration i would cut the injector contsat down by about a 1/3 from actuall. then rework the grm/sec valuces in teh maf tables. undereporting airflow to reach the best tune. i would also expect to work on the timing tables and other things that are in fact based on grm/sec and lv8. its not perfect but its doable.

if i were gonna run $32b or $32 i would still expect to have to rework the timing and fueling tables. but the beauty is you can leave the maf tables alone.

anyways i could get a response from bosch this week they told me to call back monday.

OK, think 5 volts as the limitation.

Still 5 volts converts to 255 gms/sec flow.

I dont view this as a restriction value anymore, because the MAF will actually flow more and the ECM will go into PE mode, so I can add the extra fuel that is needed which is not gotten from the MAF table.

In other words, the MAF value limit is 255 gms/sec, that is the max number the ECM will ever see from the MAF. But the MAF will be flowing more than that, say 300gms/sec (at WOT). I will configure the ECM to go into PE mode before the ECM sees 255 from the MAF. In this way, I should be able to add the required fuel via the PE table.

Also, according to the above flow rates, my 52mm TB should be plenty large enough, even for a 383.

ok there in not really a code limit of 255grms sec. but the ad convereter can only scale 5 volts and the proccesor only see maf value as 8 bit so 255 decimal is the limit.glad thats cleared up.now using a larger maf will drop the vout per cfm so let say this is how it works

560cfm = 5vout 75mm maf stock tpi

now the new bosch uinit could look like this

560cfm =3.6 vout on a 90mm maf. so the idea is to reduce the voltage by using a larger maf to gain code resolution.

now if i was better at coding i would look into switching the ecm over to a FQ based maf and rework it to read in 16bit values from the maf tables. add maf tables and use an lt1 maf. but im not that good yet. maybe sometime down the road i will be but thats a long ways off.

so in the mean time this is the basic idea.