Hey advanced guys,

I figured this is the best place to go for this. I want to compute on paper--in the theoretical, not actual--how much air is flowing through my enginer at a given RPM. From what I read, this has something to do with VE (volumetric efficiency).

I figured each cylinder = 43.244 cu. in or 708.645 cc.
This is what the motor sucks in with each intake stroke. So to compute the total volume being taken into the engine, I would multiply (cu. inches * number of cylinders intaking per RPM) * RPM.

So first, how many cylinders are on intake each revolution of the engine? Is it one cylinder per revolution? One cylinder per two revolutions? Or am I totally going about this the wrong way?

I know a lot of things have an effect on the amount of air entering the engine. I'm just tyring to compute the volume, not density or anything like that. Just a simple air pump. Just for ****s and gigiles, I'm trying to see how the opening on my airbox (5,674.5 mm^2) and also the size of my MAF (7,530 mm^2) flows in relation to the engine's capacity.

Thanks guys!!!!!!!!!!


Jim


EDIT: Also, how do I compute the velocity of air?? If this is all to detailed and complicated to answer here, I totally understand guys, really.

 

Very basically:

4-stroke engine needs 2 revolutions to complete a cycle, so on a V8, 4 cylinders on intake per revolution.

Volumetric Efficiency is % of theoretical displacement engine actually injests.

Example: 350 in^2 engine @ 4000 rpm with 85% VE

350in^3 x 4000rev/min/2 x .85 x 1 ft^3/1728 in ^3= 344 ft^3/min (CFM)

Obviously 1728 converts cubic inches to cubic feet.

For a highly tuned 358 in^3 race engine with intake tuning giving 105% VE @ 8500, it would be about 925 CFM.

Using total flow and area of MAF (which looks big to me), you should easily find average velocity.



You might research engine basics a bit more on your own for more info.

 

If you want, I can e-mail you an Excel spreadsheet that does what you want, including both volume and mass flows, with inputs for:

-displacement
-RPM
-volumetric efficiency
-throttle blade diameter
-throttle blade quantity
-MAF diameter
-MAF free area ratio
-air temperature
-barometric pressure
-inlet manifold pressure

and outputs for:
-air flow: ACFM, ci/min, cf/sec, #/min, grams/sec, grams/min
-air density
-velocity through TB
-velocity through MAF

An 8-cyl 4-stroke engine has 4 cyls on the intake stroke for each revolution. Rather than calculate each cylinder, simply take the total displacment of the engine and multiply by the RPM/2.

 

[img]
http://www.slowcar.net/VE.gif
[/img]


That is what I have been using on LS1's, since the VE tables actually have an effect in MAF mode for some reason.

It's actually a ration of mass as opposed to volume, but since density will be the same for either under a given set of conditions it really doesn't matter. The 2 term comes from 2 intake pulses per stroke. This combined with the 1/8 term gives us the same 4 intake pulses per revolution as mentioned above.

 

Thanks! So correct me if I'm wrong here:

4 cylinders per revolution on intake * 43.244 in^2 per cylinder = 172.976 cubic inches taken in per revolution.

Therefore, at 4000 rpm, my engine is ingesting 691,904 in^2 (400.4 CFM). Correct? And assuming an 85% VE ratio, I'm taking in 340.34 CFM. Is that right???

given your calculations:
350 in^2 @ 4k rpm = 344 CFM
346 in^2 @ 4k rpm = 340 CFM?

So that 4 cube difference equates to 4 CFM @ 4k rpm at an 85% VE ratio?


Thanks so much, you've really helped me out a lot!!!
You're right, I have a lot to learn by searching the internet, but most the stuff I found confuses me and the only way I know how to learn effectively is for me to ask questions. But I should be good from here on out. Thanks a lot, I really appreciate it!



Jim

 

Thanks Injuneer!

That's actually what I was trying to set up last night, but got stuck when trying to figure out how many times the engine is "sucking in" per revolution.

If it wouldn't be any trouble, my email is jchrisos@hotmail.com

Thanks again, you guys are great!


Jim