How to convert lbs/min into CFMs...

[starscream5000]

As the title states I need someone more competent that me to figure this out...

What I'm doing is calculating the optimum piping diameter for my intercooler pipes matched to the specific turbo I will be buying. The turbo in question is a Stage 5 BOSS, 61 trim with a GTS cover. I asked S.S. the tech questions on this turbo since they do not post them up on their website and here's his response:

Chris,
Here is a link to ITS and it will show you the map for the GT61. This info was not done with the Boss housing on it and ITS said that our housing is much better that the one used for the maps. ITS also did a flow on the Boss housing and said the flow rate is 75 lbs per minute
The A/R with a stage 5 for the turbine housing is .81
We also are selling a new 4.5 stage turbo from ITS. It has the inducer of the stage 5 and the exducer of the stage 4. What ITS is saying is it has the power of the 5 and spool of the 4. I wave sold almost a dozen of them and they all are happy so far.
Need more help just ask me.
Thanks
Greg Clark
Lipp/SS

Once I figure out what ~75 lbs/min is in CFMs I will use this link: http://not2fast.wryday.com/gasflow/velocity.shtml (supplied by JDub ) to figure out the optimum piping diameter that I can find while getting as close to terminal velocity as possible.

Any help is apreciated, thanks!

Chris

 

[jdub]

1lb/min = 14.472 cfm

 

[starscream5000]

Awesome, thanks John!

Edit:: so that's 1085.40 CFMs... I thought Malloy's stage 5 was like 850 when you all did the calculations on that...

 

[Doward]

jdub's right I've always used 1lb/min = 14.47178 cfm - but I seem to have some weird problem with rounding numbers.

 

[starscream5000]

What's a good average temp to calculate with? The intercooler will probably be a style like Bigaaron's bigger spearco style, except with 3" inlets and outlets...

 

[jdub]

Yeah...just re-read the thread...I actually said 930 CFM. Looks like I used a density of 12.417...dry air at 0 deg C. The 14.472 number I posted above is more accurate for actual conditions. The premise doesn't change...Mike needed bigger pipes and a bigger IC inlet/outlet.

That does make me a bit suspicious...Mike's Boss was a 67mm inducer. They told him 75 lbs/min too...doesn't make sense.

Can you get them to supply a compressor map? I'm starting to doubt the GT61 you want to use flows that much.

I always use 75 deg for an average.

 

[starscream5000]

75 degrees was actually what I went with anyways . Plus, I emailed him back asking for the link to that compressor map he was talking about since he actually didn't post it up. With the numbers I have right now it looks like 3.35" piping is about as close as I can get to 300 ft/sec, but in reality I will not go any larger than 3". Even if it doesn't flow 75 ft/sec 3" should drop it down closer to where it needs to be if indeed it flows less... Time will tell, I'll give an update as soon as I hear back from him. Thanks for the replies!

 

[jdub]

The map will tell the tale

Even at 75 lbs/min...that is max flow. You won't be running it at that very often. Plus, exceeding critical velocity by a small amount is not a huge deal...you will have an increase in pressure as a result, but it shouldn't hurt you as long as it's not excessive like in Mike's case.

Keep in mind the stock TB is 2.36" in dia

 

[Doward]

Starscream, are you doing the calculations for all out 6500+ rpm? If so, just keep in mind that below your redline, you'll be under the velocity you seek.

Do a couple calculations, trying to approx your boost curve, to get a good idea of power output

 

[starscream5000]

Like I said earlier, even at 1085.40 CFMs with 3" piping velocity will be 368.4 ft/sec and I will very seldom try to max out this turbos output, actually, I most likely will never. I'm shooting for 450-500 HP built to handle 700. I'm hoping to achieve this at around 12-17 psi which shouldn't max the turbo to 1085.40 CFMs right?

 

[Doward]

The turbo may flow a certain amount of air, but don't calculate based on that.

(displacement in ci * rpm / 3456) * VE (%) * Absolute Pressure Ratio = cfm you will be flowing.

Plot that (you may have to convert to lbs/min - just divide by 14.472) vs the pressure ratio you chose, on the compressor map to determine the efficiency at that point.

That CFM flow can also be used to figure out your best-sized pipe, at that point.

Myself, I like to take approx 85-90% of my maximum rpm potential (that I am making power at), use a VE of approx 90%, and try to land that point as my 'max power' point. This should leave you with a sizing that is neither too small for the high end rpm, but also neither too big for the low/midrange (where you spend most of your time anyway).

Just my .02, and how I tend to do things

 

[jdub]

That's dead on

Here's an example of what Doward is talking about...different motor, but same idea. Shows it plotted on different maps and why/why not the turbo is optium for the motor.

http://www.mygen.com/users/dbruce/m...Engine Air Flow and Turbo Compressor Maps.htm

 

[starscream5000]

:aigo: ... I'll have to stare at those equations for a while to figure it out. It seems like this is a lot better way to determine what CFMs I'll be flowing for a set amount of boost at what RPM... Yeah, this helps a lot, thanks John!

 

[Doward]

jdub, is your name John, too?

starscream, you're looking at why 90% of people just go with what someone else has proven works. Getting into the physics of it all is time consuming - but ultimately will give you the best all around performance you can get.

If you'd like, post up more details of your setup, and what you are trying to achieve - I'm sure the vast majority of us on here would be more than happy to help ya out

 

[jdub]

jdub, is your name John, too?

Yeah...John, JW, JDub, hey sh**head...LOL, I answer to all the above

The equations are not that tough...stare at them for a bit and the way you plot on the map. It is the best way to get you to the set-up you want. Once you know, you build from there

 

[Doward]

Ha. John here, too, that's why I was like 'wtf - how'd he know my name?'

The equations are not that tough, but I remember when I got them thrown at me, it was info overload Always willing to help!

 

[starscream5000]

Yeah it was a little bit too much sensory overload at first (Haven't take a math class in years). Basically what I need to change in those equations is the displacement for the supra, I found a calculator to convert liters into cubic inches, but I'm having problems searching through here finding the bore and stroke of a stock 7M.

Now I understand that cid in the link jdub provided is the cubic inches, but what is the EV for? And why is it all divided by 1728? Also, on down to the next equation, what is the 14.7 stand for? Sorry if I ask too many questions on this, but I want to know why I'm using these set numbers .

Oh, I finally got that link to the compressor map for ITS GT61 turbo (bear in mind this is an apple to orange comparison between the GT61 and a stage 5 BOSS 61 trim). http://www.innovativeturbo.com/images/compressor_maps/cm_t61.jpg

 

[jdub]

Stock bore = 83mm
Stroke = 91mm
Displacement = 2954cc

EV = VE = Volumetric Efficiency
Like Doward said, 90% is a good number to use for the 7M.

Pressure ratio is expressed:
PR = Pa + Po / Pa

Pa = inlet (ambient) pressure
Po = outlet pressure

The 14.7 is the ambient pressure (Pa) the guy in the example used...it was for sea level altitude pressure (std day) in psi. This calculator will give you the answer for the altitude you choose:
http://not2fast.com.com/turbo/glossary/pressure.html

This will also help to do the calculations...read the definitions
http://not2fast.com/turbo/glossary/turbo_calc.shtml

 

[Doward]

WHOA - 152mm stroke you say?

91mm, last I checked

I've used 181ci for the 7M for the longest time!

 

[jdub]

WHOA - 152mm stroke you say?

91mm, last I checked

I've used 181ci for the 7M for the longest time!

You know...you're right...I hate brain farts. :3d_frown:
The C - C rod length is 152mm. Stroke is 91mm...fixed!

Actually if you convert: 2954 cc = 180.26414 cubic inch on a stock motor.
:runaway: ....LOL