CFM Calculations and VE

SON240sx Automotive
1

Hey guys,

When you are running through compressor map CFM calculations, what do you typically use as a VE value for a mildly cam’d SR20DET.

Is 90 about right or does that feel high?

2

Ve of the engine or of the turbo??

3

That doenst make sense.

Dude this is a pure FA question

I think some guys had CFM readings for differnet boost levels iirc reading them sometime on FA. But its gonna vary a lot depending on cams and exhaust/turbine

Generally just be conservitive and go with 20% more flow capability than you think you’ll need

4

Play nice boys. This is your only warning.

5

I have no clue but I would think that 90 is a bit high on a boosted application especially when you are getting into higher pressure ratios.

What are you trying to figure out? Looking to switch to a different turbocharger?

6

OF course it makes sense… ’ dude’… I have no idea to what end the final number is to be used for. VE is represented in allot of parts within the turbo system. VE for a turbo Piston car is generally between 55-65, VE on a turbo, it varies from moment to moment, VE in an IC varies again moment to moment and effects the overall final value for ’ in cylinder’ boost greatly.

It’s not as simple as “20% over what you think”… ?? what kind of answer is that!!

That doenst make sense.

Dude this is a pure FA question

I think some guys had CFM readings for differnet boost levels iirc reading them sometime on FA. But its gonna vary a lot depending on cams and exhaust/turbine

Generally just be conservitive and go with 20% more flow capability than you think you’ll need[/quote]

7

So then lets assume it’s for the motor… what is your opinion?

8

I was just doing some calculations with the compressor map for a GT2871R and trying to figure out if there was any benefit to increasing/decreasing the boost pressure at varies points throughout the RPM range in order to keep the turbo on its most efficient “island”

But right now I am using theroretically “made up” numbers.

I also posted this question on FA and based on a link to Garrett…90% may be an okay guess

· Engine Volumetric Efficiency. Typical numbers for peak Volumetric Efficiency (VE) range in the 95%-99% for modern 4-valve heads, to 88% - 95% for 2-valve designs. If you have a torque curve for your engine, you can use this to estimate VE at various engine speeds. On a well-tuned engine, the VE will peak at the torque peak, and this number can be used to scale the VE at other engine speeds. A 4-valve engine will typically have higher VE over more of its rev range than a two-valve engine

9

you have to factor in compressor and turbine losses, therm eff. losses and a whole slew of other losses… also you have to consider that any change you make at the turbo takes time to be realized at the inlet to the chamber. So just like in closed loop if you try to over control the 02 reading by setting your read time to fine you get surging… you can get similar undesired effects by trying to over control the turbo pressure …

controlling it gear to gear is a good idea for racing if you have the ECU to do this with… even allowing for a braod curve within the power band would, after testing on YOUR system , probably bennifit you with a cleaner power curve… but it’s going to require testing on your setup becuse thier are so many variables to input into the equation to get the answer to your question…

Keep in mind though that a turbo ( unless your using a VANT ) is not variable in that you can’t modify beyond a small range , the maX thermal EFF / compresson eff. so any modifiacations you do maxe are going to involve comprimises .

10

Thanks BAS - yeah it was more of my just sitting at work bing bored and thinking about it.

In order for this to work it woud probably take days of actual dyno time.

11

i would gues the VE for the SR would be in the 70-75% range

Adam

12

as a starting point you can make a chart based on ideal pen and paper numbers.

  1. FUEL FLOW (pph) = HP x BSFC

  2. REQUIRED VE = ( 9411 x HP x BSFC ) / (DISPLACEMENT x RPM)

  3. AIRFLOW (pph) = 12.5 (Pounds-per-Pound) x FUEL FLOW (pph)

  4. 100% VE AIRFLOW (scfm) = DISPLACEMENT (ci) x RPM / 3456

use the numbers gained from these equations particularly the 3rd. one, to X reference with your compressor map.