Anyone have any good data or calculations for airflow and VE ?

I'm having trouble finding the mathematical relationship between swept volume, and airflow requirements as tested at 28" of water.

For example, an online calculator would suggest that a 412 cubic inch engine, would require 237cfm per cylinder at 6,000 RPM at 100% VE.

However, a simple math calculation shows that the swept volume of a single cylinder at 6,000 RPM if 89.5 CFM.

I'm trying to understand the formula used to arrive at the number of 237cfm (at 28" water) vs the swept volume.



-- Joe

 

you must have found a bad calculator or you're reading it wrong. the first calculator i found

http://www.4secondsflat.com/Carb_CFM_Calculator.html

says at 100% ve a 412 gets 715.27 cfm @ 6000rpm. 715.27/8 = 89.5

or maybe i'm reading it wrong.

 

That is a CARB CFM calculator.

I'm talking about head flow at the runner.

Think about it. If all a 412 cid motor needed was 89.5 CFM heads, we would be using 305 heads.

http://www.wallaceracing.com/flow-rpm.php

-- Joe

 

i need to work on my reading comprehension

 

Head CFM is only one factor on how much air can flow into a cylinder. The engine is an air pump. As the piston is pulled down into the cylinder, it creates a vaccum which pulls the air/fuel mixture into the cylinder. The mixtures need to be pulled through the carb, intake runner, head then finally past the intake valve before filling the cylinder.

As with any kind of flow numbers, a restriction creates pressure and reduces flow. Having heads that flow more air doesn't help if the valves are still too small or the cam grind doesn't open the valves enough or at the proper time. A high flowing head may flow more air than an intake runner does.

For a typical street car with a typical street or mild cam, it's unlikely you'll ever see 100% VE especially with a naturally aspirated engine. Plugging numbers into DD2000, my current cam should produce 107% at around 7000 rpm. There's a different camshaft I would like to try that can bump the VE number to around 115% at 7000+ rpm. My entire induction system is capable of providing a lot more air than the engine can use.

As for head flow, max CFM is only a part of how well they perform. You need to see how well they perform through the entire operating range of the engine. Large volumes can flow a lot of air but since the piston is pulling the same amount into the engine each time it goes down into the cylinder, a larger diameter opening has less velocity than a smaller opening. Too small an opening and air gets restricted enough that it can't be pulled through fast enough.

Here's a lot of information about different heads and how they flow.

http://users.erols.com/srweiss/tablehdc.htm

 

I've read a lot of that, including a lot of books specifically on cylinder heads.

What I'm trying to understand, is the relationship between swept volume and port flow.


We know from calculating, that a 412 cubic inch motor like mine, at 6,000 RPM needs 89.5 CFM per cylinder.

So why do all the head formulas and calculators recommend 237 cfm..

-- Joe

 

Though i dont know how the online calculator figures its numbers I do get what your asking. If a cylinders swept volume at 6k is roughly 90 CFMs why does it require heads that flow up to 237 CFMs? At least that my impression of what your asking. The reason is this the cam shaft. See ideally cams would just be open and closed with no lifts in between. However since we cannot have a square cam lobe they do the best the can with the most aggressive lobe shape possible. Which is why when heads are flowed the list all the different flow rates at various lifts. So with that in mind lets look at a real world application. The goal is to obviously fill the cylinder however if you look at the real flow of the head with a cam installed the valve will be opened from 0 to the max and hit all values in between for some amount of time. This means that what the head actually flowed would be the sum of what it flowed at .1 lift times the amount of time it spent there + .2 lift times the amount of time it spent there and all the values in between from 0-max lift (this is also know as the area under the curve or more technically known as an integral type equation). Not only that but all thoes flow rates are at 28 inches which in a motor is not always the case obviously. This means that the actual amount thats flowed in a revolution is the sum of all the air flowed at all valve lifts given the amount of vacuum exsists in the cylinder. Now with something that complicated how would you make an approximation of what is needed? Whats done i imagin is they calculate some sort of average flow for a runner based on a decent cam and realistic cylinder pressures for a given max flow rate. So in your case a head with a maximum flow of 237 CFM may roughly equate to 89.7 CFMs in a real world situation. Course all thats just a guess as I dont know how their calculator works or even if i did ide have no way to substantiate if how the calculated it is an acceptable way to do it.

 

That is about the best explanation I've seen thus far. I guess if you mapped out the lift points over the intake valve open duration, you would need substantial flow capability for the short window that the cam is open near max lift, to make up for the very limited airflow at lower lift points.

I guess, mathematically I could probably map out the total amount of airflow since I know the lift CFM rates, although I'd only be guessing at how fast the ramp is to determine lift at every degree..

Guess that is why they use a rough formula like you suggested to point an average..


So the real cliff notes answer is, the total airflow us 89.5 CFM, however the rate during the intake stroke could be anything from 1 CFM to 237 CFM as the valve is variable over the duration.

Thanks for the brain stimulation!

-- Joe

 

Perhaps it's because a cylinder is only taking in air 1/4 of the time, but in a port flow test, it is flowing 100% of the time. In "real world" flow, the flow and pressure is not constant, but in a port flow test, both the pressure and flow are constant.

Just a wild guess. I've never tried to understand the details that you're going into here.

 

It's not like I don't have software and already know the answer of what flow to use for what cam and what size engine.

I'm just overly curious about the physics and engineering math behind it.

Starting to get bored with the hobby I guess.. Trying to entertain my brain rather than buying another stupid expensive toy

-- Joe

 

Well i have always said in discussions such as these cars are our hobbies and its a lot cheaper to talk about them than actually do the stuff we talk about lol. Plus when you have discussions like this some times you come across some interesting truths that will help you out later and once you start to get into the physics of how cars in general work its almost mind blowing. It really goes to show why it takes teams of engineers specialized in a wide variety of areas to put together a reliable car let alone something like the new Z06. It also goes to show how so many "mistakes" can be made when designing a new car lol. Its easy to overlook details when there's just much involved.

 

Yep.. So true.

After a while.Years and years.. It's all the same.. Get a body, figure out weight, gear ratio, trans, clutch / converter, build motor and cam for desired operating range, pick heads, calculate, wrench, drive, sell, start over..

Did a few boats and watercraft in between. Sick of those now.

We all know roughly what we want for cam specs and heads when blueprinting an engine, but sometimes it's nice to be able to go a little deeper and look at the relational math involved.. I've got all the fancy software, dragstrip analyzer all that I can let the computer run computations and decide what components work best for the vehicle weight and everything, and for the most part it's all self explanatory stuff. But the relationship between swept volume of a cylinder vs required head airflow - I really would like to know what the formula is that the software uses, and why that formula is used. Because even the most simple calculators that calc CFM for HP and peak RPM based on displacement, don't take cam specs which seem rather important on determining the total airflow over the duration of the valve being open and it's associated lifts..


-- Joe