Technical CFM requirements for modded N54s

[fmorelli]

I'm looking for a bit of help. I've spent the past few nights digging around, trying to figure out a way to determine a reasonable CFM capacity for an N54 in the 700bhp zip code. I'd like to determine the intake side requirements. So that said, I won't bore everyone with lots of inane details. I'm not looking for "just run dual cones" or other duh kind of things. I'm not sure how I'd compute the value, since no clue what the volumetric efficiency of an N54 with turbos and mods would be.

If anyone here has experience with this, love to hear suggestions or data. Think of it this way: if you were to build a custom airbox and intake system, how would you size it? (and no, don't tell me you'd just make it big ... obviously I wouldn't as the question if that was the answer ) ...

Thanks!

Filippo

 

[fmorelli]

I'll pass along one piece of info, since I'm sure any manufacture engine has this computed. Here's information I found for the Hellcat (which would be in the 700bhp zip code):

1244 CFM - OEM stock filter measured (water 2.0 in H2O)
1293 CFM - OEM stock filter calculated (water 2.0 in H2O)
1297 CFM - supercharger rotor flow calculated (water 2.0 in H2O)
1380 CFM - Green Filter USA calculated (water 2.0 in H2O; 12 CFM per sq/in @ 115 sq/in)
1491 CFM - stock no filter measured

Filippo

 

[Rob09msport]

Don't single turbos rate how many cfm they flow would that be way too get in ballpark? Like see what cfm is rec for 6266 or something close to your power goal.

 

[Torgus]

Don't single turbos rate how many cfm they flow would that be way too get in ballpark? Like see what cfm is rec for 6266 or something close to your power goal.

Yes, both single and twin turbos rate it in LBs/MIN. IIRC that rating is the air flow rate at the turbochargers choke point (maximum production). You are VERY roughly looking at 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow on most engines/platforms. It is a good enough estimate which is why Precision can call a 6466 a 900HP turbo because of it's 90 lbs/min flow rate.

https://www.evolutionm.net/forums/evo-x-dyno-results/692309-turbo-flow-rates.html <- rates for many turbos.

What is funny is the N54 aftermarket turbo manufacturers refuse to provide this information. Only one who has is Hydra AFAIK. It would be much more useful than their BS WHP & TQ claims and glory pull dyno runs they post. People could make a much more educated decision on what to get for their power goals.

 

[Rob@RBTurbo]

What is funny is the N54 aftermarket turbo manufacturers refuse to provide this information. Only one who has is Hydra AFAIK. It would be much more useful than their BS WHP & TQ claims and glory pull dyno runs they post. People could make a much more educated decision on what to get for their power goals.

There is literally no one vendor in the N54 platform (or beyond) who is going to be able to provide any real world and accurate Lb/min for their turbos, especially in this day and age with a bazillion different custom wheel combos (5x5, 6x6, 7x7, 10,11 GTX style, etc).

What they will do however is look at a native flow map (which is generated with an OE wheel and housing) and make some approximations based on what their wheel design maybe for the better or the worse dimensionally and independent of compressor housing size/design/etc, but this rarely if ever is going to be correct unless they are a very great guesser. Also FWIW these ratings are obtained from compressor mapping only, which takes in zero consideration for hot side design.

We do agree that many vendors blow a ton of smoke up the asses of consumers, such as "buy these turbos and make 1,000hp all day long!". The N54 platform really eats that kind of stuff up.

Rob

 

[tisdrew]

Here's some ballpark cfm data for you:
https://www.n54tech.com/forums/showthread.php?t=30772

 

[Tiron]

no clue what the volumetric efficiency of an N54 with turbos and mods would be.

I'm quite positive that the "Boost Request Offset" table in TunerPro / MHD is actually VE at different RPMs / Loads (or maybe RPMs / VVT positions). This would put the N54 in the low 90% range stock, which sounds about right to me. Here is the table for anyone not familiar:
https://bmw.spoolstreet.com/attachments/bro-png.14440/
(borrowed from this thread: https://bmw.spoolstreet.com/threads/need-advice-with-stfts.3820/ )

Also, just for more information, my stock turbo car with catless downpipes dynod uncorrected 332whp (SAE 375whp) at 295 g/s airflow according to my data logs. N54 does not have a MAF sensor, so this value is calculated by the ECU, but on a mostly stock car should be pretty close. Double that gets you roughly to uncorrected 700whp on 600 g/s (or 78 lbs/min), but realistically it would take a bit less than this since the bigger turbo would be more efficient than the stockers. These numbers compare favorably against the number Torgus gives, 10 lbs/min equals roughly 100 crank hp. I've heard this number numerous times before, but its not true for every engine, many are worse.

Its hard to get CFM out of this since mass flow is not the same as volume flow. Mass flow is what you actually need, volume is pretty irrelevant. The volume required to flow a certain mass changes based on your altitude (baro pressure), humidity level, and any other factors that change air density. Assuming an air density at standarnd temp and pressure (roughly sea level at 75°F), you'd need a volume flow of 1020 cubic feet per minute. Keep in mind the Hellcat numbers above (~1300 CFM) that the crank 700hp does not include airflow that generates power lost turning the supercharger.
Air density at STP: 0.0765 lbs/ft3
Convert lbs to ft3: 78 lbs / 0.0765 = 1019.6 ft3
Mass flow rate was in min, so: 1019.6 ft3/min

You still can't solve for a pipe diameter until you determine an air speed, but once you do this calculator looks like it can give you a pipe size:
http://www.1728.org/flowrate.htm

 

[Tiron]

For another example, on the same dyno day my brother had has supercharged LT1 C7 Corvette on the dyno. Uncorrected was 491whp (SAE 555), data logs show 460 g/s airflow (this vehicle DOES has a MAF sensor, so this should be accurate). This gives 60.6 lbs/min, and 792 CFM at sea level (we are not at sea level). The Corvette has a 4" intake pipe feeding the supercharger, which gives a sea level airspeed of 151 ft/s according to that flow calculator. Note that this car has a baro pressure sensor in the MAF, and as such we can read that the supercharger is pulling a vacuum on the intake pipe. I did not log that on this dyno run, but I believe it was about 6 or 7 kPa below actual baro pressure in other logs I have made of this car -- almost a full 1psi loss.

My car was on factory inlets with an aFe DCI. I believe the factory inlets are about 1.5" in diameter, or an area of 1.77 in2 each. This gives 3.54 in2 area for both, or approximately the area of a single 2.12" inlet. This gives me an airspeed of 346 ft/s using the same calculator. Couldn't tell you how much vacuum this generates, but I'm sure its substantial.

Long story short, N54 badly needs inlets. Seeing how big a 4" pipe is, I have doubts it would fit under the hood on a BMW. I don't think you could realistically make your intake "too big". There is probably power on the table even when you go as big as you can fit.

 

[Panzerfaust]

Yes, both single and twin turbos rate it in LBs/MIN. IIRC that rating is the air flow rate at the turbochargers choke point (maximum production). You are VERY roughly looking at 9.5-10.5 horsepower (as measured at the flywheel) for each lb/min of airflow on most engines/platforms. It is a good enough estimate which is why Precision can call a 6466 a 900HP turbo because of it's 90 lbs/min flow rate.

https://www.evolutionm.net/forums/evo-x-dyno-results/692309-turbo-flow-rates.html <- rates for many turbos.

What is funny is the N54 aftermarket turbo manufacturers refuse to provide this information. Only one who has is Hydra AFAIK. It would be much more useful than their BS WHP & TQ claims and glory pull dyno runs they post. People could make a much more educated decision on what to get for their power goals.

There is literally no one vendor in the N54 platform (or beyond) who is going to be able to provide any real world and accurate Lb/min for their turbos, especially in this day and age with a bazillion different custom wheel combos (5x5, 6x6, 7x7, 10,11 GTX style, etc).

What they will do however is look at a native flow map (which is generated with an OE wheel and housing) and make some approximations based on what their wheel design maybe for the better or the worse dimensionally and independent of compressor housing size/design/etc, but this rarely if ever is going to be correct unless they are a very great guesser. Also FWIW these ratings are obtained from compressor mapping only, which takes in zero consideration for hot side design.

We do agree that many vendors blow a ton of smoke up the asses of consumers, such as "buy these turbos and make 1,000hp all day long!". The N54 platform really eats that kind of stuff up.

Rob

I would say some vendors offering twin setups are pretty forward about the design and why they make claims like they do. I get what Rob is saying about the 10k different combination things though and I think that even applies to a lot of companies selling Garrett/PTE/BW turbos considering they still modify or cast their own housings.

People love to complain about N54 vendors for some reason, but any vendor selling performance adding products on any platform will always claim theirs is best, and typically still only use in-house testing to show why they say that. I'm not sure if everyone here came from 90s foxbodies or something where there's been parts for decades and as such proven independent work and testing, but most "newer" car's communities are about the same as the N54s.

 

[fmorelli]

Funny you say that. I did some pretty heavy Fox body stuff way back in the inception years. That market defined horsepower claims. Ever seen a 400HP 5.0 with bolt ons only? If one added up all the claims back then ... :-D

Filippo

 

[Panzerfaust]

Hey, nothing wrong with dabbling in American muscle imo - I had an LS3 Camaro for a while when they were ~3years old at max, which is why I know that all platforms before a certain age are the same as the N54 is right now. We're at the "teenage" years for the N54 right now imo - the cash-grabbers who hopped on the bandwagon are mostly gone or were chased away and we're mostly stuck with people who have a real passion for the platform and want to continue improving it.

People keep comparing our platform to the EVO platform because they're both 3L turbo engines. But the EVO hasn't evolved very much in multiple generations, so much of the info available is still from two decades ago added on top o

 

[Clean WHP]

the lbs/min is my favorite guideline to look at when comparing turbos. Not sure why its hard to find that info on precision turbos, but Garrett and Xona Rotor provide that info readily available.

Xona even names the first 2 numbers of their turbos in the lbs/min they flow. I have a "motiv 800 kit" but the important fact is that its a xona 82-67, and flows 82 lbs/min of air. the Xona 95-67 flows 95 lbs/min, and 105-xx flows 105 lbs/min. lots of people don't realize this and freak out as soon as they hear any numbers that arent the size in mm of the compressor inducer lol