Volumetric Efficiency

[Max Headroom]

I learned to write some VB6 code in my early 50s (about 20 years ago) and when I got to be an advanced beginner, I started a progam called "Basic Performance Calculations".
I got pretty far into it and then life (mostly wife, kids and work) got complicated and I put the project on the shelf. (I posted some pics of what it could do when i stopped working on it) Anyway, I retired and life has slowed down enough so that I have started working on the program again. In the meantime, my understanding of many things "Hot Rod" has expanded to where I see that some of what I put in the program needs a more detailed or complete explanation. I have also come across a lot more content I would like to include in the program.

Anyway, a couple of weeks ago, I started going throught the program from the beginning, making sure that the content was accurate and understandable. So one of the very first things I originally put in the program was a way to calculate Volumetric Efficiency and from that approximate Carburator Sizing for a given engine air capacity. My original results were based on info I found in performance mags, which it turns out is complete "ca ca". It is much more complicated than Car Craft and Hot Rod made it out to be 20 years ago.

So, in the process trying to gain a better understanding, I have been reading people who really know wtf they are talking about (mostly big buck racers). As you might expect, opinions vary but all agree on the physics involved. Having been on this forum for a long time, I know we have some Smokin Aces who have probably forgotten more than I will ever understand and I will be posting questions about things I don't really get in the hopes that some of you can make me understand (at least better than I do at present).

So, the biggest thing I don't understand about VE is that some are saying that an engine can only have 100% VE (because the space available in the cylinder for the air/fuel is fixed). That by itself is true but I think that there are dynamic circumstances at play that can vary the VE, including to more than 100%. First of all, if I understand correctly, one atmosphere is the pressure used to determine VE. But I have heard it argued that valve opening and closing points, exhaust pulse scavenging, intake air temp, injected or inducted fuel rates and some other exotic factors that I have to look straight up to see go over my head, can contribute to calculating VE and I assume that somewhere in there change the calculation to something other than just the one atmosphere calculation.

I would really appreciate any replies, for or against that anyone might have. I would like to get this program complete enough so that people could use it their projects go a little faster and maybe, easier. I iwill be posting more pics as time allows.

 

[64nailhead]

There are many variables to be factored into determining VE on paper. But measuring it in terms of fuel consumption and HP/TQ production is much easier imho.


The old wive’s tale of having ‘too big’ of a head in terms intake runner volume defines how the thinking and testing have changed since I’ve been around hot rodding.

I feel that whatever can be done to lower the amplitude of the intake and exhaust pulses is extremely important. The lower the amplitude of the pulses, then the lower the X Tau of the fuel puddling on the intake and intake runner. The lower the X Tau factor is at a given HP and rpm range, then the more efficient the engine is. And the same concept applies to exhaust pulses. Obviously the higher the rpm, the fuel clinging occurs less.

Why do long runners make more torque at a lower rpm, and why does an open plenum make more HP at a higher rpm? VE is my answer at a given rpm.

More of a question for me is did the OE’s
discover this via mathematics or trial and error testing? I usually use GM as an example. In the mid 80’s GM used old school head design and added a 17” runner to make ground pounding sub 4500rpm torque with a TPI. Then came a Vortec head that outflowed the older head design that didn’t require an insane intake runner length to dull the intake pulses with a relatively small head intake volume. Then came the LS1 that took both concepts, long intake runner with much better low rpm head intake runner flow. And then the same LS1 head intake runner design coupled with these giant, ugly truck intakes that made HP and torque. Every step of the process improved VE at normal daily driving rpm ranges.

My thoughts. Trying to calculate VE is well
over my head - good luck with your project.

 

[Max Headroom]

There are many variables to be factored into determining VE on paper. But measuring it in terms of fuel consumption and HP/TQ production is much easier imho.


The old wive’s tale of having ‘too big’ of a head in terms intake runner volume defines how the thinking and testing have changed since I’ve been around hot rodding.

I feel that whatever can be done to lower the amplitude of the intake and exhaust pulses is extremely important. The lower the amplitude of the pulses, then the lower the X Tau of the fuel puddling on the intake and intake runner. The lower the X Tau factor is at a given HP and rpm range, then the more efficient the engine is. And the same concept applies to exhaust pulses. Obviously the higher the rpm, the fuel clinging occurs less.

Why do long runners make more torque at a lower rpm, and why does an open plenum make more HP at a higher rpm? VE is my answer at a given rpm.

More of a question for me is did the OE’s
discover this via mathematics or trial and error testing? I usually use GM as an example. In the mid 80’s GM used old school head design and added a 17” runner to make ground pounding sub 4500rpm torque with a TPI. Then came a Vortec head that outflowed the older head design that didn’t require an insane intake runner length to dull the intake pulses with a relatively small head intake volume. Then came the LS1 that took both concepts, long intake runner with much better low rpm head intake runner flow. And then the same LS1 head intake runner design coupled with these giant, ugly truck intakes that made HP and torque. Every step of the process improved VE at normal daily driving rpm ranges.

My thoughts. Trying to calculate VE is well
over my head - good luck with your project.

Thanks very much for the reply. I don't have any advanced math but what I am getting from your reply makes sense and matches with my cruder understanding of what I believe is happening to cylinder filling.

It's 4:30 am and I can;t sleep thinking about this. What I have come up with so far is this: 100% VE is optimally what can be obtained when intake filling is measured at atmospheric pressure (14.7 psi). However, there are conditions (many of which you mention above) that can support filling at marginally greater than the 14.7 psi (one atmosphere). This means that the cylinder contains more a/f mixture than it would at 14.7 psi. (charge mass at greater than one atmosphere) In other words "boost" although certainly not in the sense of forced induction. However, this line of thinking definitely supports the idea that forced induction can generate VE well in excess of 100%.

In the immediate sense, what I am trying to come up with is a way to equate VE with intake device sizing in CFM. (carb, tpi, tbi, etc.) The process I have decided on is to calculate Theoretical Air Capacity based on thhe engine's physical characteristics and then let the user input their own VE to arrive at a calculated intake air flow requirement.

I will leave the discussion of the finer points of VE for a later time. Please let me know what you think.

 

[Built6spdMCSS]

Forced induction definitely does generate more than 100% VE because air is forced into the cylinder. VE on NA setups is based on vacuum to get the air in as well as how fast the exhaust is going out.

I've only had one NA Engine hit it 100%+ VE values in the chart while tuning it, that was a big Head, big Cam, Single Plane Intake setup with big Exhaust. The large Cam overlap and scavenging did this. 7800 RPM peaks may have been a factor.

As mentioned above the LS Engines were designed taking advantage of the full power band, and they do. Decent Cam in stock stuff I'll hit 98% VE values.

 

[Max Headroom]

Forced induction definitely does generate more than 100% VE because air is forced into the cylinder. VE on NA setups is based on vacuum to get the air in as well as how fast the exhaust is going out.

I've only had one NA Engine hit it 100%+ VE values in the chart while tuning it, that was a big Head, big Cam, Single Plane Intake setup with big Exhaust. The large Cam overlap and scavenging did this. 7800 RPM peaks may have been a factor.

As mentioned above the LS Engines were designed taking advantage of the full power band, and they do. Decent Cam in stock stuff I'll hit 98% VE values.

I agree with what you said. VE tends to go up with an increase in rpm and is capped out by the limitations of the air flow components.
Thanks for the comment.

 

[Built6spdMCSS]

Yup that's exactly it there, literally the friction loss of the air moving through the ports. The valves are your limiters here as in what allows and stops the air flow. Air is a fluid so you have to apply fluid dynamics here.

 

[Max Headroom]

Yup that's exactly it there, literally the friction loss of the air moving through the ports. The valves are your limiters here as in what allows and stops the air flow. Air is a fluid so you have to apply fluid dynamics here.

The fact that air acts as a fluid under many circumstances can be very confusing to those who don't takek the changed dynamics into account.
I am going to delve a little deeper into VE a little further down the road. Any comments to those (or any) posts would be welcome.

 

[joesregalproject]

A good place to start would be the actual formula for VE for a compressor. Here's the actual formula for VE.

 

[Max Headroom]

A good place to start would be the actual formula for VE for a compressor. Here's the actual formula for VE.

View attachment 235712

Hadn't seen that anywhere. I'll run some scenarios and see what I come up with.
Thanks very much!!

 

[Built6spdMCSS]

With air being a fluid, another aspect is density. You're not going to change the actual volume an engine has in it unless you open up cylinders and ports. The denser the air the more friction you'll have as it's moving through an engine. Probably a minimal effect when you factor in heat soak and dwell, but it's a factor as altitude changes.