to calculate the air velocity at a given rpm.

 

Velocity at the TB, velocity of the intake port, velocity at the valve, velocity of the exhaust port... exhaust.... ??

We will need a little more information. There is more than one equation envolved to solve this.

 

sorry about that. it's the one where you figure out the speed of the air flowing through the intake.

 

To Calculate CFM:

(Displacement in Cubic Inches)x(Engine RPM)/3456=CFM

 

not that one. there was another one that said how fast the air was traveling. i'm looking for SPEED not VOLUME of the air.

 

That has alot more to do than with the info give, you need area, shape etc...

 

that's getting too specific. there's a generic formula to calculate this. i've seen it before and used it once in a post of mine to convey my meaning about airflow and the chance of fuel puddling at 2000 rpms when using a wet kit. and i've searched and haven't found my post.

 

You have to know the cross sectional area at the place you want to figure the velocity. That's the only way it can be done.
You need the CFM and the cross sectional area to calculate velocity.
600CFM moves slower through a 3" hole then it does through a 2" hole.

 

you were probably looking at the velocity of the MAF/throttle body area.

 

To figure out approximate average velocity through a crossectional area, the area of the crosssection and divide it into one square foot, then multiply that value times the CFM flow rate.

Example a 8 inch diameter tube flowing 543cfm.

radius = 8/2 = 4 inches

crossection area of tube= 3.14159 * 4 * 4 = 50.26544 sq inches

1 square foot = 144 sq inches

vel = (144 / 50.26544) 543 = 1555.5817277238595742919986376325 feet per min.

 

The generic formula is:

Volumetric Flow rate = Velocity * Area

Rearrange for velocity: Velocity = Flow/area

This is pretty much what Connclark is saying above but this is the formula it comes from. Make sure your units for flow and area are the same(ft, in, mm, etc.) and you'll get avg velocity.

Al

 

If you want to understand airflow in the induction path of internal combustion engines, Bernoulli’s law, which relates airflow pressure and velocity under specific conditions, can be helpful. But it can also be misleading if not applied correctly. The branch of physics involved, fluid dynamics, is notoriously complex so knowing when Bernoulli’s law does and does not apply is difficult. Science alone can not provide expertise in this area. You also need intuition and experience.

 

ok then, let's say i want to figure out the speed of the air going through an LS1 intake at 2000 rpms.

 

Which particular portion of the intake? The TB opening which is about 77 mm diameter would be a fairly easy calculation..if you knew the CFM @ 2000 for the engine condition you are interested in. Of course, the minimum crossection of the TB about 1 inch away is smaller, so velocity there is higher.

Max power @ 2000? or part-throttle cruise? Stock or modified engine? It's about 165 cfm @ 2000 with WOT (wide open throttle) producing max power thru headers and open exhaust. That's about 55 ft/sec (or 37.5 mph) at the intake opening for the WOT condition quoted above.

Why 2000? Just what is it you are really interested in?

 

this is getting too specific.

i'm interested in how fast the air is moving through one runner of an LS1 intake at 2000 rpms at WOT. stock motor.

it's another one of those how low of an rpm is it safe to activate nitrous.

there was a post some time ago where i used a simple basic formula to figure out how fast air was moving through a basic one cylinder motor. then you would multiple by 8 to get a more total volume of air for a v8 motor.

this reminds me of a test i took back for my first responder course. (pre-EMT class)

Q: person isn't breathing and unconsicous. what do you do?

A: CPR

unfortunately, doctors would fail this question more than an average person because they thought too deeply into it. they kept bringing variables into the equation. is the person bleeding? does the person have a fractured skull? neck injury? leg cut off?

i really appreciate and understand how thorough you guys are trying to be. but, i'm just looking for a basic formula. like the one ProdriveMS put up. hell that might be the one i'm looking for, but i think there was a couple more variables in the one i'm looking for and posted about here once a while ago. and yes, i've used the search function to try and find my old post. hell, it could've been on another site.

 

this will probably do for what i need it for.

 

Oh, come on, tell us why you want the information....and I'll give you a real answer.

 

i guess when i said this, it wasn't a good reason?

a little more specific. have a tiny thread about activating nitrous below 3000 rpms. as usual, i'm the proponent of so long as the motor and drivetrain can handle it, then do it.

then, i get this:
and this:
come back with this right off NX site:
and this:

 

Sorry, I missed that post. No offense intended, but if post #15 were the initial post, things might move along quicker.

In the stock LS1 I referenced, (from a good engine simulation program) average intake port velocity was about 80 feet per second (fps) @ 2000 and increased linearly with rpm. (160 fps @4000 and 240 fps @ 6000).

Intake flow (CFM) is not linear with rpm as you might expect, but it's fairly close. If a "150" shot injects the same amount of nitrous and fuel independent of rpm, it could easily be too much for a slow rev from 2000. In first gear engine speed is increasing over 1000 rpm/sec.

 

...don't forget the volumetric efficiency of the engine.

CFM = ((Displacement in Cubic Inches)x(Engine RPM)/3456)xVE