Airflow calcs under boost

This is taken from the T04B Housing Help thread. I threadjacked and thought this topic could use it's own dicsussion, since it's pretty important to turbo selection.

(Copied from previous thread- Gassed 260 told me airflow rate of 347CFM at 15psi boost, 5000 RPM, and 80% Volumetric efficiency. I got different numbers and am looking to figure out why)

Hmmmnn.... I get 467.7 CFM. How are you arriving at that?

Here's what I did:

(cid*RPM*.5*Ve)/1728 for N/A flowrate. It comes to (200*5000*.5*.8)/1728 = 231.48 CFM

Pressure ratio = (Patm + Boost)/Patm = (14.7+15)/14.7= 2.02

N/A flowrate * Pressure ratio = Boost flowrate = 231.48 * 2.02 = 467.7 CFM.

Is ther a critical assumption I've missed someplace?

To avoid further thread-jacking, I'm going to start a new discussion topic on "airflow calcs under boost"

(end copy)


--mikey

I get the same numbers you did for a 200 @ 5,000 rpms and 80% VE and using 0.075 for standard density.

This would yeild about 35.078 lbs/min of flow at 15psi... correct?

That's about 55% efficiency on a S-3 trim TO4B. It's also at 72% on a V-trim compressor. H-3 trim shows over 74% efficiency.

I'd like to see how things change when you figure that a log head probably isn't anywhere near 80% VE.

Just for grins, I plugged in the same figures for the 200 (15psi, 200", 5Krpms), but this time I only used a VE of 50%. I think a log head is going to be a lot closer to 50% than it is to 80%.

On all three compressor trims I listed above, the map shows you well past the surge line. Just like I predicted in the original thread.....

50% VE (200" @ 5000 rpms) and 15psi yeilds 21.9lbs/min of flow.

If I ever get the time/money, I'll be working with an Oz 250-2V head.

I've got the head (uninstalled), just no cash to move forward with other mods.

Is there any way to estimate volumetric efficiency using flowbench numbers from the head? I can (eventually) get the head flow tested. Knowing a real VE number would be preferable- for a lot of reasons.

At least your numbers and conclusions are the same as mine on paper. Makes me feel a little better. Glad I'm only in the daydream planning stages.

--mikey

Well, that's always going to be the key. Boost isn't always going to solve an inherently bad design.

As for finding true VE:

http://www.installuniversity.com/instal ... 012000.htm

Maybe this can be done on a non-efi/MAF equipped car in a dyno facility?

Two things John:
1.The log is probably closer to 60-65%; 50 is really bad.

2. Your calcs are for 15psi; I doubt you'd move enough air to get that high. He's looking at maybe 8-10 psi tops, unless serious mods are made. The heat at 15 is exponentially higher than at 8psi; this becomes a major limiting factor in the engine's ability to support combustion.

3. Most guys over-turbo and over-cam. It usually starts with a turbine hsg that is too big.

Realistically, if I were to go for a turbo, it's be relatively low boost, probably 8psi max. The initial issue of the thread stemmed from radically differnt flow number calculations based on the same set of operating assumptions.

The issue at hand now, and preferably there's some way to measure on a dyno- is what is the VE of the engine?

I think to do a turbo 100% right by the numbers, you'd need VE numbers on the particular engine in question in whatever trim you intend to use for the turbo (cam, head porting done, carb or EFI, etc.) Basically, get everything else dialed in and measured, then figure out the turbo based on that engine configuration. Anything that effects flow will impact VE. A bit anal-retentive, but it'd be the best by-the-numbers way, IMO.

An easier, possibly better way would be to learn from others' experiences, and just buy what works well without re-inventing the wheel.

mostly, I was hoping to sort out why the calculations came up different. The discussion on actual VE of the motor has been a worthwhile bonus.


--mikey

Mikey,

What I think you're missing in your calculations is you have to multiply pressure ratio X density ratio. I have not found an easy to understand calculation but basically its a reduction in corrected, boosted airflow due to compressing of the air and the efficiency of the compressor heating the air and making it less dense. That's why intercoolers work so well. Especially air to water - using ice water. Ray Hall Turbocharging has a nice website. I find "Pick a Flow Map" gives you alot of information. You have to scroll down to find the instructions. Click off intercooler if you're not running one. Also, you'll have to convert farenheit to centigrade. I think if you play with it you'll have fun. Also, try the effects with an intercooler. If you search around there is also some calculations for compression, horsepower and drag strip times. Ray Hall's site is http://www.turbofast.com.au/turbomap.html.

This may be threadjacking but due to the beating up of the log and talk about volumetric efficiencies on this thread I feel acccording to generally accepted calculations and actual drag strip times which give you actual or more horsepower, mildly ported heads with larger exhaust valves and mild cams have to be around 80% volumetric efficiency or more otherwise the numbers don't jive. Have fun.

Jim