CFM requirements for N54 power

fmorelli

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This is an old subject ... the hot rod guys running 600-1,200cfm carbs on old V8's. Our 3 liter, turbo engines run 164CFM air intake, according to aFePower. Their Momentum GT for our cars up this a paltry ~20%. Can someone explain why - on say a 600whp N54 (so that's like ~700hp @ the crank) - that a 3 liter motor with forced induction wants to spin to 8,000rpm on CFM numbers anywhere near a few hundred CFM? Yes I realize lots of people stick cone filters in the engine compartment as a quick solution, picking up whatever dirty, hot air happens to be there.

There are any number of calculators and discussions online. Google will surface them. Anyway the numbers below (~200CFM) seem waaaaaaay off to me. Most of the calcs seem to say 800-1,000CFM for 700HP at the crank. What am I missing? Thanks.


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Filippo (note @Seb335i )
 

rac

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Yes, the bit that your missing is the measured at 1.5". The reality is there is a much bigger pressure drop occurring at full tilt. The turbo makes up for it but has to work a lot harder to end up at the same boost.
 

ShocknAwe

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My guess is that there's a velocity and pressure component when factoring intake CFM requirements to feed FI as opposed to NA intake manifolds. Great question Filippo!

So I'll stick around here to find out the answer from someone who knows what they're talking about!
 

mikeseli

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Fillipo where did you get this info, please provide the link.
The stock N54 and N55 panel air filters (There are 3 sizes) should flow about 560-700cfm @ 1.5"
 

Torgus

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iminhell1

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Just adjust the boost number, pressure ratio is calculated automatically.
This will be crank HP approximately. Or maybe it is close to wheel, looks closer to wheel. I forget how I made the calcs.
 

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rac

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maybe what is being misunderstood is that your power potential is dictated by air mass rate (lb/min), not air flow rate (cfm) to use US units.
It's convenient for n/a guys to talk about flow rate in cfm because minor density changes are not going to change the big picture much. If the intake manifold is undersized in a n/a application it acts like a partially closed throttle - creating a pressure drop across it. The situation is self limiting so you must have an appropriately sized intake.

with forced induction the 1.5" (pressure differential) H20 cfm measurement isn't as relevant - its just telling what the cfm is with virtually no pressure drop. As the flow rate goes up so to will your pressure differential across the restriction. Those big hybrid twins with stock intakes will have a magnitude of order greater pressure drop from turbo inlet to atmosphere than 1.5". The turbo then has to work harder because it needs a greater pressure ratio to make boost target. The turbo is effectively pulling a vacuum across the sh88ty intake system until it's efficiency falls off a cliff or you reach choked flow at the restriction. Which is what all sorts of motorsports use to limit turbo power (restrictors pre-turbo). They put an upper limit on total power potential b/c they restrict the total possible mass flow rate - so your forced to make huge amounts of torque at low rpm and adjust gear ratios to suit.
 
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fmorelli

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@rac thanks for the explanation of relationship. So in gross terms, everything in front of the turbos can create varying restriction, which makes the turbos work harder. This would include the air filtering system (feed, filter). So how does one determine what's reasonable on the intake side? I understand it is a system, and everything in front of the turbo matters, and that air velocity at the turbo matters in particular. Sorry I'm not trying to put fairies on the head of a pin, just trying to get an idea of how to think about the air intake system in terms of, as you say, effectively creating a restrictor plate.

Filippo
 

rac

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the pressure at the turbo inlet is the one that you want to be concerned about, and you want to maximise it (excluding ram air discussion), you want that inlet pressure to be as close to atmospheric as possible. Assume you want to run 14.7psi of boost (29.4psi absolute), your at sea level with no intake at all - there is 14.7psi absolute at turbo intake. The turbo needs to operate at a pressure ratio of 2. Now you put on a restrictive system and at WOT there is 1.7psi pressure drop from atmosphere to turbo inlet, making 13psi available there. The turbo operation at a PR of 2 is only going to make 26psi absolute (11.3psi boost). To make target, the PR has to go up (29.4/13 = 2.26). The turbo heats the air up more because the PR is up, and you get more back pressure because the WGDC goes up to.

if the whole intake system only had 1.5" of water pressure drop (0.054psi), its obviously negligible. the thing you want to know I guess is at what point should you be worried about it. losing 1.7psi is like driving at 3400' elevation - no good. so that's another way to think about it / visualise it. losing 0.2psi is closer to 400' elevation - not so bad.

if you search other forums from many years ago when the whole bolt on intake pipe were the rage i'm pretty sure i remember some of the sellers/developers doing flow rate v pressure drop testing. you might find something of interest. otherwise you'd need to rig up your own car with sensors at the turbo inlet, get the data and experiment.
 
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mikeseli

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The chart comes from AFePower.

Filippo

If you call AFe and ask them to explain you that flow chart, I'm sure they will not be able to explain anything based on those numbers. I'm 100% certain if this air intake system is placed on a flow bench and you read what each 2.25" pipe flows at 1.5" of H20 it would be about 300 cfm. Therefore the both about 600-650cfm.
 
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Torgus

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Your turbo has a snout. Make sure your inlet pipe is not smaller than the snout and at the end of it is a good flowing filter and you will be fine. It can even be bigger than the snout aka a Velocity Stack.

What matters is lb/min and efficiency islands with turbos. After that the AKI of your fuel. I wouldn't get hung up on intakes and filters.
 
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there‘s a nanny table in the n54 for 569 g/s. And the intercooler have an influence on this as well via another table.
 

RSL

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Those are for calcs, setpoints, etc. There's also one for air filter loss and another for loss through cats.

Without AFE's methodology, no way to know what their numbers include, but there's clearly a difference between their testing just filters and their intakes. Anyone can hypothesize what the differences are, but if anyone just went by flow numbers at face value, they'd never sell an intake.

Momentum GT as above:194/164 stock
N54 Pro 5R drop in: 765/583 stock

Their GTR stuff:
Takata full intake: 124/100 stock
Pro 5R drop in: 333/197 stock (x2 filters)
 
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Coupes66

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Whether this helps or not, I am not sure but this is an extract out of a book "Turbo: Real World High Performance Turbocharger Systems" by Jay K. Miller.
"Use this formula to calculate how many square inches of K&N style filter you need. The formula is courtesy of K&N filtration.
Square inches of filter required = (lbs boost / 14.7) + 1 x CID x Max RPM / 20,839
For example: at 10 lbs of boost, a 3 liter engine (183 cubic inches) that's designed to create max power at 6,000 rpm will require 88.5 square inches of filter.
(10 / 14.7) + 1 x 183 x 6,000 / 20839 = 88.53 square inches.
The filters are pleated to allow more surface area within a given diameter for packaging purposes. Now, to help you choose a filter, determine the diameter that will fit your installation, and then use the following formula to determine the filter length (or height, depending on orientation). (Note that this calculation is for round filters. For cone shaped filters, simply estimate the average diameter, which should be about half of the larger diameter plus the smaller diameter.)
Filter height = (inches of filter / Filter Dia. x 3.14) + 0.75
Consequently, in the above example, if you had room for a 12 inch diameter filter it would require a filter height of about 3 inches.
(88.5 / 12 x 3.14) + 0.75 = 3.1 inches
If this seems large to you, then you now understand the value of a properly sized air filter assembly and the value of knowing how to design your own turbo system.
Once you have captured the air, it's time to route it to the compressor inlet. If you have a few feet to navigate, keep your tubing as large as room will allow. This reduces tubing line loss. Unfortunately, air likes to slow down before it is redirected, which means you'll want a smooth track with as few bends as possible."

I used this calculation to determine whether an AFE DCI was large enough for my requirements and it came out too small so I designed my own dual cone with larger cone filters.
 

mikeseli

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So on my setup each turbo has it’s own air filter. The front one has a 8” long x 4” diameter cylinder filter and the rear turbo uses the 740i air intake box.
189C372A-1659-465A-9E1F-16DA0D6FACFB.jpeg
 
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