Calculating CFM used from Dyno sheet ?

[swampbuggy]

Thanks to all, now what about the second part of my question a out MCSA determining air speed? Mark H.

[Scotthatch]

Warpspeed---The CSA and or the MCSA (Minimum Cross Sectional Area) have to be designed or sized to get the air SPEED about where it is needed to be when the engine is at full song, is this correct ? Mark H.

Yes and no

Again this is more about porting

You do not control airspeed in a engine if you change the cross section the rpm will change to balance it out....

when porting you need to look at airspeed more inline with how the port is moving the air and where it is heading if the airspeed is too high as you head into the short turn it will not make the turn ...a lot of times when you open up the cross section just before the turn the flow will only change a little but the quality goes up ...

Years ago I remember seeing a diagram of a bubble on turns in a pipe and it stating that's what it took to maintain the speed in the turn .. always stuck with me


Minimum cross section is more about what you need to move the air ... one of the problems with porting is the work to put back something you cut to big so in a problem spot it helps to know how much you need to open up ... I have one here now that I need to push the pinch point back .120 but sonic checking says there is only .126 in the wall ... knowing what I need let's me plan how to get there

From a HP standpoint the flow is what you need to make the power as long as the port is stable .... with that said there is a fine balance on cross section you make it big enough to move the air clean but not so big that at low rpm or part throttle it's sluggish ... some of the old Ford heads where really bad about to much cross section to much cc you had to fill it to make it work

The math part of cross section is a little different then the flow work on the bench

[swampbuggy]

Scott, I bet when you said old Ford heads you were talking about the Cleveland intake port from appx. 1970 ?? Mark H.

[Scotthatch]

Why would you dyno your engine without measuring the engine ACFM?

Actual cubic feet per minute as measured on a dyno can be used to figure VE and other things but is useless as compared to a flowbench cfm as there is no corresponding pressure drop to equate the cfm number

I didn't suggest that the engine's actual air consumption was something to be used for calculating more theoretical values. You are making all these calculations and asserting a given efficiency of this air pump, so why would you not measure the actual air consumption on the dyno.

I have seen little use for VE data from a dyno run .... when I was young the magazine articles would rav about how this build got to almost 100 VE then came along a race engine and it was 103% next one was 107% now there are engines at as high as 127% so the number could be anything from 70 % to 130% and I don't see how that tells me anything about what I built ... now the interesting thing is that formula is about VE but that's not what I use it for

One of the problems with the turm cfm is that air is like silly putty it stretches and compresses and moves around a one inch hole can move all sorts of cfm depending on temp density and pressure drop across it so to truly use a cfm number in a math problem it has to be in context so the question becomes how can I use a formula ment for VE with airflow that is from a flowbench? That's because though the context for the cfm number is different because it is from how it was measured on the flow bench how the air will behave is the same so the Z factor from the formula works on both

In my world I use the flow bench to measure my work on porting a head .... the guy comes and gets it ... I tell him something like I got the cfm up from 240 to 280 fixed the pinch point and smoothed out the short side so the quality is pretty good.... first thing he will say is something like "cool" then "so what do you think it will make in hp ?". That question is what I try and answer with this ...

Flowbench cfm times the Z factor from the chart times superflow conversion of cfm to HP gives me what it will make in HP before it is built ....

If you look that's working backwards from the formula . It is meant for you to plug numbers in and spit out a Z factor (I also call this port efficiency)

Since I saw little use in solving for Z the first thing I did was turn the formula around and solved for rpm ...I did this because most of the time I had parts but needed to know what rpm it would peak at ....

[Scotthatch]

The formula as it is in the superflow manual

15266624958120.jpg

The formula as I use it now

15266625802481.jpg

The first thing to note is it solves for rpm not Z factor ...this is because rpm is the one thing I do not control with a given set of parts .... this let's me take a cam ,head flow @ 28 and intake and find out where peak HP is going to happen ... this is a big help to me when the short block is only good to 6500 rpm and I find the breathing is good to 7800 rpm ....

Second thing I redid is average inlet area ... this was giving me problems because it is based on a ideal airflow in that given area and most of the stuff I port will never be ideal because you can't cut where you need to . So I realized to make it ideal I could take the cfm it was going to have and calculate the area based on a given airspeed and that worked

[Scotthatch]

A few things to note in the formula is the relationship between cid of the cylinder and runner length ... if either go up the rpm goes down with is very consistent with what we see in real life but now you can calculate by how much also the relationship between the constant and the Z factor as the camshaft intake closing point is moved out you can see that it pushes the rpm up by increasing the constant ...

[DrillDawg]

Intake tract length tuning

2nd pulse

((1100x 1\2intake can duration )x .96)\rpm

3rd pulse, same but use .705 instead of .96

4th pulse, Same but use .538 instead

[Scotthatch]

Intake length tract tuning

2nd pulse

((1100x 1\2intake can duration )x .96)\rpm

3rd pulse, same but use .705 instead of .96

4th pulse, Same but use .538 instead

I think the pulse thing may be a joke ...I have never seen anything that showed a extra peak caused by a pulse lining up ....

This is inertia ram ... or the the effect that moving air has mass and will keep going .... every 4 cycle engine has been using this effect since we found out you can close a intake valve latter than bdc

[SchmidtMotorWorks]

I think the pulse thing may be a joke ...I have never seen anything that showed a extra peak caused by a pulse lining up ....

This is inertia ram ... or the the effect that moving air has mass and will keep going .... every 4 cycle engine has been using this effect since we found out you can close a intake valve latter than bdc

I develop manifolds that often have series of runner lengths, there are definitely benefits and losses from the reflected waves.

[DrillDawg]

Chrysler may use this very formula using the 2nd pulse to design their infamous long ass cross ram on their 413, lol.

[Scotthatch]

I think the pulse thing may be a joke ...I have never seen anything that showed a extra peak caused by a pulse lining up ....

This is inertia ram ... or the the effect that moving air has mass and will keep going .... every 4 cycle engine has been using this effect since we found out you can close a intake valve latter than bdc

I develop manifolds that often have series of runner lengths, there are definitely benefits and losses from the reflected waves.

But in the inertia ram length it changes it too ....

If a wave was doing something then as rpm changed would you not see the wave pulse in the output power as the power drops on either side of the wave pulse ?

[SchmidtMotorWorks]

But in the inertia ram length it changes it too ....

If a wave was doing something then as rpm changed would you not see the wave pulse in the output power as the power drops on either side of the wave pulse ?

Yes, that is what you see when you test enough lengths.

[Scotthatch]

But in the inertia ram length it changes it too ....

If a wave was doing something then as rpm changed would you not see the wave pulse in the output power as the power drops on either side of the wave pulse ?

Yes, that is what you see when you test enough lengths.

Do you have one you can show me?

[DrillDawg]

As the intake valve closes, a pulse is sent up through the intake tract and as the pulse hits the opening of the intake tract the pulse is bounced back toward the intake valve, time it correctly and you help increase your ve, that's why motorcycles are using variable length velocity stacks now days. Works on the exhaust, works on the intake.

[Scotthatch]

As the intake valve closes, a pulse is sent up through the intake tract and as the pulse hits the opening of the intake tract the pulse is bounced back toward the intake valve, time it correctly and you help increase your ve, that's why motorcycles are using variable length velocity stacks now days. Works on the exhaust, works on the intake.

I understand the idea .... just not seen data to support it and as I keep checking dyno numbers with inertia ram and it hits the peak number right I have trouble believing that is what is going on... I have looked for dyno sheets that have a power bump before peak power that might show a pulse showing up a little early from peak power as in not tuned to peak and I am still looking