Paulyy

There has been some funny claims on a facebook post..
As far as i'm concerned the stock AFM is limited to ~300-320 hp.

With out using names, someone is claiming 600hp out of an AFM and there's others with 400-450 hp. with a stock afm.

Possible or not?

Also one person is so convinced that your turbo will spool better with a AFM vs MAF if all variables are the same.

I know my answer and i know this has been bashed. but anyone with a dyno with back to back AFM vs MAF tests?

Dougs951S

In a word Paul, and without going into too much detail: Let me get some of what they're smoking. No chance in hell you're forcing 600 hp worth of air through that trap door. Not if your life depended on it.

Paulyy

What every their smoking. mustn't be good for you LOL.

if the afm can only read 5v at around 320hp. and these guys claim 400+hp with a fuel controller. how the hell do they control fuel if you're getting a 5v flat signal.

IF they are using something like a AFMLink system which is a afm intercept unit and and use it to delete the AFM. i can see how you would get 400hp as it used a MAP sensor to understand airflow

Dougs951S

Right then..

And on with the details...Paul, I am unfamiliar with the AFMlink system. Is it like other standalones that rely solely on a MAP signal, or does it retain the AFM for some function, which seems a bit redundant? Engine displacement, RPM, and MAP are all an ECU needs to do a rough derivation of airflow. If you add in air temp and throttle %, that calculation gets pretty darn accurate, much more so than the stock AFM allows. This is whats commonly called speed/density and is used on lots of factory cars, lots of older honda's for example and most standalone ECU. Now, as far as the factory AFM goes..

Its obvious for anyone who has ever examined one, its restrictive. How much? The inlet measures 65mm X 49mm, but that's just the area on the flange where air can enter from the airbox. In reality, even with the barn door all the way open, the actual channel for air to pass through is 64mm X 45mm, with a large obstruction in the path of the airflow in the form of the adjustable clockspring mechanism that sticks 6mm out into the air stream. The outlet side is 73mm diameter where it meets the J boot, but again, has large obstructions in the way and the true cross sectional area of the smallest section of the outlet is only 50mm x 70mm!

Now for some math. I'm going to greatly generalize here, and surmise that to push 600 hp, you'd need to move 60 lb/min of air. That's actually being generous, since if it was 600 rwhp you'd need at least 12% more air. So, we're trying to cram all that air through a 28.8 cm^2 orifice.

The Bernoulli differential equation relates pressure, area, and volumetric flow. Lets make up some numbers, lets say that you have a HUGE FMIC so even though you're making 600 hp, on boost your air charge is 150*. Lets also say you are at sea level, and that your air charge is at 0% humidity. The density of air under these conditions is ~.0647 lb/ft^3. That works out to a volumetric flow of 927 CFM under those same conditions. Things get a little hairy here actually, depending on if we assume the flow through the AFM to be laminar or not (hint: it isnt). Very roughly speaking, when talking about incompressible fluids and laminar flow, pressure differential across the "flow area" is a function of the velocity squared, meaning here that doubling the mass flow rate (or roughly, "horsepower") means that the velocity of the intake air inside the AFM also doubles, and that the pressure differential between the atmosphere and the intake side of the AFM quadruples. This all comes from the Darcy-weisbach equation. I say "very roughly" because this applies less to compressible air and more to fluids like water and oil, but to make my point it will suffice without busting out some much more serious math.

In a nutshell, trying to flow 927 CFM through a 28.8 cm^2 orifice would require that air to be traveling at some serious velocities; many hundreds of feet per second...not going to happen. Does air reach some pretty high velocities in the intake track? Sure, but only after the air has been compressed by the turbo and is flowing into a manifold with vacuum in it. No way intake air will reach those velocities given the pressure differential between atmosphere and the AFM output! Not to mention the pressures inside the AFM would be huge!

Edit: Just wanted to add one more thing, a bit more food for thought. Its worth bringing up the fact that restrictive as it is, the inlet of the AFM still has more area than a stock throttle body, by more than 15%. Despite that, people DO make 400+ hp on a stock throttle body. Again, the huge difference is that we are cramming air into the TB under pressure...and still we find it restrictive. You'd be hard pressed to move 60 lb/min of air through a stock TB even at pretty high boost levels (and again, the real limitation is that eventually the air is forced to travel at ridiculous velocities and everything breaks down). 600 hp with an AFM is absolutely, completely impossible even if you fibbed the ECU to inject enough fuel. 400 hp is stretching the realms of possibility even. You can tell those guys on facebook to crack a physics book before making outlandish claims...or you can just let them think they're right

Willard Bridgham 3

I saw Jon Milledge get 400 hp from a stock afm on a dyno.

H.F.B.

According to JMG Porsche co.uk. Tuning the 944 Turbo Part Two ----> The air flow meter (AFM) also known as a VAF Meter (Volumetric Air Flow Meter) limits to 340 bhp and 400 ft-lbs torque.

So I guess you are right and they are wrong.

blade7

The picture I saw of a 968 RS with a AFM looked like there was a temp sensor mounted behind the throttle body, maybe that was for a MAP setup.

Dougs951S

Not calling you a liar, but I doubt that. Reason being is...yes, its possible, but only with a VERY efficient, super built engine where everything is working perfectly. The reason is, under those idea conditions the required volumetric flow rate to make XXX hp is reduced, very possibly low enough that the AFM could flow it. Buuuut, when you're building an engine like that, it seems unlikely to me that such an engine would use stock engine management hardware IE stock motronic and oldschool AFM. Not saying its impossible, just unlikely. To be clear though, 600 is definitely impossible under any circumstances, period.

adrial

NASCAR makes what, 600+HP? According to google, they use a restrictor plate with 4 x 0.875 - 1in diameter holes.

Somebody do some math to compare the areas... But that'll give you a real world data point to compare to.

Dougs951S

Interesting point, but the carb's used by Nascar have a maximum flow rate of 830 CFM with no restriction in place. The plates reduce the airflow further. Nascar engines make a ton of power, but in actuality they are not moving as much volumetric flow as a 600 hp 951 engine would require. Additionally, with four 1" holes, the area is 31.9 cm^2, almost 11% more than the area of the AFM inlet on the 951. Doesnt sound like much, but every .125 cm^2 increase in area is worth about 50 hp at the crank on a nascar engine.

mikey_audiogeek

It is still possible to tune the ECU even with a maxxed out AFM. Just takes some understanding of how the AFM signal defines the load zones, and which cells are actually never used, or hardly ever. An AFM that maxes out at flow equivalent to 340hp will still allow the ECU to be tuned to around 500hp or more. I could explain more, but there's plenty on the web about this - particularly in the BMW arena.
Cheers,
Mike

mikey_audiogeek

...even though the pressure drop is likely to be huge, as mentioned above!!!

Willard Bridgham 3

Pressure drop is not an insurmountable problem with a turbo......just increase the pressure.

Weston Dillard

A builder I trust a lot told me years ago you could get 400 through the stock barn door. My 3L made 380 WHP with a stock barn door. Since most dynos seem optimistic, that may be 400 crank, who knows. Interestingly, in a back to back run with a MAF, it made exactly the same HP/TQ. The MAF was old school and wasn't tuned perfectly, but it was an interesting experiment. Moving to standalone soon.

West