Maf flow rate related to HP??

[BimmerWill]

I was talking to a buddy of mine with a TT who is an engineer at VT. He took an internship this summer up in Maryland for a company that designs and builds nozzles for just about anything you can think of, especially automotive. He mentioned that a guy he worked with had done some calculations to get a ball park figure of how much his boosted Saab was making.

Now naturally I was a little skeptical but these are two intelligent engineers that brought this to my attention. So I did a little digging and came across a few threads on the internet of various turbo vehicle owners using this same method to get a ball park figure.

I was wondering if anyone had also heard about this and whether or not it was feasible to have a similar calculation for our vehicles although naturally aspirated stock I would imagine there wouldn't be any hindrance of doing the same. Naturally a more "reliable" solution would be to simply use a Dyno but this has peaked my interest.

Any thoughts?

Sent from S4

[danewilson77]

I'm sure there's a correlation, as well as a proportionality of cold/dense air, hot/less dense air, atmospheric conditions, flow rate, etc...

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[Nivo]

Max I have seen out of my maf is 641kg/hr which equals to about 23.2 lbs/min.

[trancenation]

Looks like a thermodynamics problem. Look into the Otto Cycle and how the mass flow rate of air affects the Cycle to produce the work output (HP). Might need to consider the efficiency of the engine if you want to get to the nitty gritty.

[BimmerWill]

See I knew there was some much smarter people I'm here then myself. I'll look into it. Maybe figure out a form we could use ourselves if anyone is interested.

I know I can get the MAF readings fairly easily with my Ultraguage. What to do with the data after that I'll have to determine.

Sent from S4

[Nivo]

Did a run last night and got a peak of 642.18 kg/h or 23.59 lb/min which is 178.38 g/s. (Using BMW Logger)

The above was done with Traction and Stability turned off, level road, full throttle to redline.

my car had dynoed 205 wheel horsepower. Crank HP is 235.

1.5 cfm = 1 HP

1 lb/min = 14.472 cfm

going by most:

23.59 lb/min X 10 HP/lb = 235.9 BHP

178.38 g/s / .80 = 222.975 Andrew at Ross Tech like 10 years ago came up with this : "Your peak airflow should be roughly 0.80 times your horsepower."

178.38 g/s / .75617 = 235.9 BHP (Adjusted to meet the lbs/min demand of my actual engine)

a while ago when I worked at the dyno, the engine builder had an engine dyno and one of his built engines put out 507.93 Crank HP (with flex plate), his grams per second were 384.21. using this adjusted equation he did:

384.21 g/s / .75617 = 508.10 HP (using formula)

384.21 g/s = 50.82 lb/min (Actual)

50.82 lb/min X 10 = 508.21 (Actual)

now we know the car did 205 WHP, engine currently took in 23.59 lb/min, using this we can get basic drivetrain loss on my car. There are variables with air temps, atmospheric pressures but this gets my car pretty close to what my numbers are.

[BimmerWill]

Awesome write up thank you for taking the time for that. I do have one question? Where did you get the number for adjustment of the .80 number? I couldn't quite follow that.

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[Nivo]

Awesome write up thank you for taking the time for that. I do have one question? Where did you get the number for adjustment of the .80 number? I couldn't quite follow that.

Sent from S4

ok some tuners say that the peak mass flow in grams per second should be around 80% of rated crank horsepower power. This of course varies by engine design, atmospheric conditions, elevation etc.

So using the 80% figure we have 235 crank horsepower x .80 = 188 g/s. but I know my car did not pull 24.87 lb/min.

24.87 lb/min x 10 = 248.7 Crank horsepower not the actual air flow my car ingested of 23.59 lb/min.

I did 178.38 g/s or 23.59 lb/min, the percentage had to change based on actual air flow read on my car.

so 235.9 crank horsepower x .75617 = 178.3805 g/s = 23.59 lb/min x 10 = 235.9 Crank horsepower

if the car dynoed at 205.3whp and I know the airflow in lb/min is 23.59 then I can do my drivetrain losses. 235 x 12.97% = 205.303 whp

forgot that add that this only works if the MAF is unaltered as in the signal has to be original with nothing skewing it. guys using piggybacks and are altering the MAF signal directly will read lower as you are taking away MAF voltage.

Also just found a great site for the conversions:
http://www.translatorscafe.com/cafe/units-converter/flow-mass/29-2/

[wsmeyer]

Awesome write up thank you for taking the time for that. I do have one question? Where did you get the number for adjustment of the .80 number? I couldn't quite follow that.

I was kinda wondering that myself. You're already using a conversion of 1.5cfm = 1HP which I assume is a general conversion for gasoline internal combustion, so why the .80 "correction"?

[Nivo]

I was kinda wondering that myself. You're already using a conversion of 1.5cfm = 1HP which I assume is a general conversion for gasoline internal combustion, so why the .80 "correction"?

Some guys use the 1.5cfm per 1hp unit but it believe it is not valid unless you know your BSFC, air fuel ratio and atmospheric pressure. If I was just to do dummy math using the cfm formula I would be at 227 HP. Or 22.7lb/min

the simpler g/s x .80 or in my case .75617 gets me close to my real horsepower.

[BimmerWill]

Some guys use the 1.5cfm per 1hp unit but it believe it is not valid unless you know your BSFC, air fuel ratio and atmospheric pressure. If I was just to do dummy math using the cfm formula I would be at 227 HP. Or 22.7lb/min

the simpler g/s x .80 or in my case .75617 gets me close to my real horsepower.

Okay so you got the .75617 due to already having a dyno number? You just plugged in your hp from Dyno and then solved for x which have you the .75617?

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[Nivo]

Okay so you got the .75617 due to already having a dyno number? You just plugged in your hp from Dyno and then solved for x which have you the .75617?

Sent from S4

Now the first time I saw the grams per second turned into crank horsepower by a divider equation was back in I think 2002 or 2003 by the guys at Ross-Tech which do VAG cars. That was based on a 200HP crank car that did 151 g/s.

Basically I am showing 2 formulas to get the same result. Not my formulas btw...

Known from my data:
MAF measurement using BMW Logger is 642.18 kg/h. This translates to 23.59607592165 lb/min or 178.3833333334 g/s.

easy formula:
23.596 lb/min x 10 (hp per 1lb/min)= 235.96 HP at the crank

Other formula using grams per second in which I found my variable to arrive at 23.596 lb/min which is a known:

178.3833333334 / 0.755973156690567 = 23.59607592165 at the crank

^^^This_________^unkknown^________=^^^This

So in short no, I didn't use the dyno to come up with the percentage, I came up with the percentage to use grams per second to get crank horsepower using my already known flow in lb/min.

The massflow in lb/min and the dyno is used to come up with a drivetrain loss percentage.

2 ways to cook the pizza....in the end it is still...well.... cooked

I found this just now on a WRX forum:

"I wouldn't consider anything other than 'peak' g/sec to be very conservatively 'close' to actual whp.

My WRX on pump was doing ~318g/sec @ 350whp and ~336g/sec @ 370whp on meth injection. My whp numbers were on a 'average' reading dyno."

By looking at his information and being that the car is AWD the info seems right.
We have 336 g/s convert that to lb/min we get 44.44lb/min or 444.4 Crank HP.
He is awd so we guestimate between 16-18% drivetrain loss. Using 16.7% drivetrain loss we get his 370 AWHP.

his 350 AWHP is 318 g/s converted to lb/min is 42.06lb/min or 420.06 Crank HP. miraculously that translates to 349.90 AWHP using the same 16.7% drivetrain losses. without hitting the dyno and upping the boost he will know his wheel horsepower time and time again. Unfortunately he didn't know that bit of information.

[BimmerWill]

Amazing. Thank you for taking the time to write that explanation up. I'm a little slow sometimes :)

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[BimmerWill]

Okay finally got around to doing a pull tonight and ran my numbers. Although the tool I used does not log so this was the highest number I got visually of 159.6 g/s. Using the converter I came out with roughly 21.03 lb/min. Multiplying that by 10 yielded me with 210 HP at the crank. Factor in a drivetrain loss of 13% yields 182.7 whp. Granted I cannot account for exact drivetrain loss so I'll go off of crank.

Supposedly my motor yielded 235 at the crank from the factory, being a non-zhp 330. Total loss of crank HP is 25.

With a logger I might have slightly higher numbers but I feel that a total loss of 10.6% over the course of 12 years and 213k miles I feel that's not a bad drop. I could be wrong however. That seems pretty acceptable to me. Considering motor is still completely stock save a final muffler delete.

Any thoughts?
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[Hornung418]

ZHP was rated at 235hp. 330 was rated at 225. European market was rated at 231hp.

GS3 + TT4

[BimmerWill]

ZHP was rated at 235hp. 330 was rated at 225. European market was rated at 231hp.

GS3 + TT4


You are correct. My mistake.
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