Volumetric Efficiency

Bob Bidonde

Bob Bidonde

Well-known member
Up to now, I have been using a 50% Volumetric Efficiency in my calculations for the Models "A" & "B" engines. I suspect it is actually lower. For those who are wondering what volumetric efficiency is, a 100% would mean that each cylinder would intake 50 cubic inches of fuel (fuel intake = 100% of displacement).

What volumetric efficiency do you engineering & inventive types use? I am guessing I should use 40%????
 
Bob, in 2018 you used 50% to calculate a max CFM of 100. BZR's flow tests a year later clocked an A block with a Weber carb at 84.9 CFM @ 2500 rpm, which would give VE of roughly 55. So possibly your original estimate was good?
 
Bob,

I am not sure that the terms "volumetric efficiency" and Model "A" should be used in the same paragraph, let alone the same sentence.

As I am sure, there are many critical factors in the design of an engine, CR alone does not make an efficient engine.

In my opinion, the basic intake port arrangement and crank layout makes it less efficient. And this is just one small aspect.

The stk "A" is rated at ~40HP, or .20 HP per Cu Inch displacement.
With some massaging, it can easily be raised to .50 HP/CID.

Most changes made have a point of diminishing returns, after CR, intake and Cam.

Enjoy and experiment, that is almost as much fun as driving them, John
 
Bob, ...yes some of us do care about VE, and we strive to increase the efficiency. Much akin that like some folks have studied Ohms laws and learned how electricity works and how to make it more efficient. Others just flip a switch and expect it to work without feeling the need to understand it. That is OK to be either way.

Just remember that your comment about 50 cubic foot at 50% VE gets skewed when fuel is added and elevation is factored in. The air pressure is about 14.76 at sea level, however the amount of air to fuel mixture affects things too. Optimum fuel mixture for economy will be around 15.5 parts of air to 1 part of fuel however to make the most power at sea-level, you need to be around 12½ to 1 ratio. Part of the other issue is the shrouded valves and small bowl sizes at come with 1.500" intake valve. These are all great restrictors which cuts VE. With higher compression heads, the roof of the chamber is often lowered which shrouds the path of the flow and generally hinders the cylinder being filled efficiently. That is why too much compression (-typically above 10:1 CR) does not make as much power as say 8:1 CR in an L-head engine.

Bill Stipe and I have discussed this at length before, and I feel pretty comfortable with free flowing port work, and a good chamber roof design, you should see in the 65%-70% VE range. Bill would be a good one to discuss this with further.
 
Why is it that there are always nay-Sayers who dislike a post and they just have to voice their negative opinions????? This is America. If you do not like the topic of my post, don't open it.
Being an engineer, my love of the Model "A" is in its engineering as well as its other aspects. If you don't like its engineering aspects, do not bother to open my posts.
Thank you to those who stayed on topic.
 
Bob and Brent I enjoy both of your posts and understand what you are talking about. I am a lifelong model A lover (65 years) so keep up the good work.
 
The volumetric efficiency has to vary by rpm. At low rpm the air/fuel has a longer time to enter the cylinders. At higher rpm the valve configuration will limit how much air/fuel gets in, assuming a larger carburetor and intake manifold than stock. The overhead conversions are meant to fix this.

As an old automotive engineer, this stuff is interesting to me. I have learned that not all persons think the same way I do. I am married to one. She has no idea what a piston is and doen't care.
 
Also, not an engineer...but curious.

Does a VE below 100% suggest that something in the intake is constricting airflow? Does that mean that when the intake valve closes when the piston is at the bottom, that the air pressure in the cylinder is lower than the current barometric pressure outside?

Ken
 
Excellent question and of course the one-star bandit tagged it immediately for it's validity.

The VE becomes a valid quantitative data tool if you are working with a properly calibrated flow bench or possibly with some over-the-top dynamic flow modeling software. Sans that, if you are looking backwards to calc it from existing dyno data it becomes clouded with outside indeterminate variables.

I'll add nothing further. Nothing says progress like watching a forum dumb down with bad post ratings and off topic replies...
 
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