I'm not sure if this should be under engine or Hi-perf, so I decided to place it here.

Ok, VE has me tagged, so help me out here. It appears as if you have to know what your VE is, or your CFM, before you can really calculate anything. If you don’t know your true VE you can’t calculate your CFM. If you don’t know your true CFM, you can’t calculate your VE, it’s a catch 22. With some help from you guys, I’ve got to the point below, but I am now stuck again.

For example.
454 Cu in engine divided by 8 = 56.75 Cu in per cylinder.
56.75 Cu in. divided by 1728 = .032841435 Cu ft.
The engine cylinder pulls every other revolution, so I will divide this number by 2 in the following equation.
At 5000 rpm you get.
5000 * (.032841435 / 2) = 82.104 cubic feet per revolution per cylinder.
Because of the speed of the piston and the quick opening and closing of the Valve, you need to multiply this value by four, to attain close to 100% VE. This is ignoring the fluid type dynamics of the fuel / air mixture flow. That’s beyond me at this point in time.
82.104 * 4 = 328.416 cubic feet is required for the flow of the head (per cylinder) to attain near 100% VE.
So, if I am still on the right track,
Let’s say you have a 781 oval port Cylinder head, which can flow a maximum of 272 cubic feet per cylinder at .600 lift.
To get the percentage of error between 272 cu in. and 328.416 cu in, we end up with 82.822.
Is 82.822 my VE?

I also expect that cam duration plays in here also, but I’m not sure how. It would make sense as duration increase, VE would increase. I’m ignoring overlap, as we are only talking about 5000 rpm.

Thanks for any help,
Fred.

VE is the efficiency ratio you get when you divide the actual engine air intake by the theoretical air intake.

Your calculation broke down when you divided by 2 and then only multiplied by 4. Each cylinder takes in 57 cu.in. per revolution. 4 cylinders go through the intake stroke per revolution. So, the engine would theoretically take in 227 cu.in. per revolution which works out to 657 cu.ft. at 5000rpm. This is not the VE though, it's just the amount of air that would theoretically flow through the engine.

If you're looking to pick a carb, you would multiply this number by the maximum VE you would expect from the engine. It seems that a typical rule of thumb is to use a VE of around 125% (x1.25) or 130% (x1.3). The VE will vary quite a bit over the operating range of the engine.

Peter

Fred,

Volumetric effiency is the result of the combination of carb, intake, heads, and headers/exhaust working together. Changing one component will change the VE. There is alot of physics involved in tuning components to work together.

For some good info, check out page 6 - Tuned port basics below

http://www.grapeaperacing.com/GrapeApeRacing/tech/inductionsystems.pdf

- John

Guys, thanks for the input.

Peter,
On the divide by two, this is only for one cylinder, as head flow is based on one cylinder. So I broke it down to one cylinder's flow per two revolutions. The times four is to attain what value is needed for maximum head flow on one cylinder. As I have been told, you have to have four times the cylinder flow, for maximum flow through.
So, if I go with your statement:
"VE is the efficiency ratio you get when you divide the actual engine air intake by the theoretical air intake."
I think that I have it, in the below equation, based on one cylinder. Even though I don't know the actual engine air intake, I know what the head limits it too. The (heads flow) actual versus what the engine really needs (calculated) at that RPM?

Thanks,
Fred.

I understand what you're saying. I had thought you had the cfm listed as your VE at the end there but you do have it as the ratio.

Head flow numbers really don't tell the whole story of the amount of flow that the head will allow when on the engine as you're trying to use it. Interestingly though, your times 4 number seems to be a decent starting value. The AFR 180cc heads are good performers for a 350 and they are 250cfm at 0.5" lift which is what your calculation gives for a 350 at 5k rpms.

Also look for a high exaust flow/intake flow ratio in your heads.

Peter

Fred,

I am going to bounce this over to Performance. I think you might get more responses over there.

R.J.

Hi Fred,
I moved this whole answer over here that I posted on your other thread that got a bit lost & it probably will get more attention here even though it is related more to your cylinder head answer than just VE ;)

Originally posted by SS4speed:
JakeS,

The heads will only flow 190 cfm at .300 inch lift. They will flow 272 cfm at .600 inches of lift. I have seen the same heads ported, (but not polished) and they flowed close to 284 cfm per cylinder. Based on a VE of 85%, which is the best most modified street cars will do, I would calculate the following.

@5000 rpm you need 279.152 CFM

@5500 rpm you need 307.068 CFM

So, if you plan on going over the 5k mark, you should look for a different set of heads or you will need larger valves. Expect larger valves to cost around $600 plus. That's if you do the bowl work and Port/polish. Remember that if you cam is smaller, you will have less flow.
On the pistons, stock pistons will give you too low of a compresssin ratio, you will need to go with something around 20 to 30 cc's more. This will give a around 9.? to 10.0 compression. Then depending on the cam, it will be dropped to 8.0 to 9.?. These are just two of the things that I have started to look into.

Fred. =
Fred,
I am not following your logic :confused:

You seem to be referring to two distinctly different things in your post.
VE is a measure of the efficiency of an engine in relation to what it could make if the cylinder is completely filled or over filled (more than 100%).
In other words, VE is simply the amount of air actually kept or used in the cylinder vs what it could hold under the same weather conditions.
You can have terrible VE & still RPM but you will be down on power.
You may not be getting the air in the cylinder or you may be dumping it out the exhaust or causing intake reversion. either way the VE number will be down but it will not effect the RPM capability of the engine just the power.

Air flow & VE are related but you cannot relate just RPM & VE.
VE is a calculated number & is constantly changing & is usually highest around peak torque.

The size of the port in square inch area will determine at what RPM the engine will stop making power assuming you have enough airflow in CFM to support the power you wish to make not VE.

I have posted this before so I will just paste it here.

I guess it really depends on what you are building but here's my thoughts,,,,,,
A smaller good flowing port with higher velocity will almost always be a better street head but,,,,,,,,,
There is more to a port than just airflow numbers
The minimum cross section area is also very important.
For your engine to make power to XXXX rpm you need a minimum of XXX sq inches in area for a flat tappet cam, less for a street type roller & even less for a real good roller.
Otherwise at about 690FPS the air tends to get very unhappy. :D

The volume of the port in cc's is not as important as the cross sectional area of the of the port but volume in cc's is a pretty good indicator of heads of the same type though & a lot easier to measure.

Generally speaking, a smaller port (in cc's) that flows more air is going to be a better port.
BUT ONLY IF THE PORT IS BIG ENOUGH TO SUPPORT THE RPM YOU WANT TO RUN THE ENGINE TO!!

If the port has a minimum restriction that is too small the port will stall.
Unfortunately, most head suppliers don't publish this spec.
Wish they a would along with cc's as it is a pretty important number ;)

These are some of the reasons you cannot just say a 325cc head or a head that flows XXX amount of air is going to be a better head on a certain size engine,, it is very dependant on what your plans are for the whole deal.

A 500" engine will probably be very happy with a 270cc port with say 2.2 sq inches until you start leaning on it then somewhere along about 5000 or so rpm you are going to be done.

This is an example,, I don't feel like doing the math :(

Put a bigger head on & you now have the capability of going further.
Works just like overcaming an engine with small heads,, all you do is shorten your good torque curve.
A too small head with a cam that wants to run from 5000 to say 8000 just defeated the purpose of having a small head. The cam doesn't work at low speed & the head can't work at hi speed.

Done the other way, Small cam, large ports,, the heads are lazy at low speeds & the cam cannot support the rpm the heads want to be efficient at :confused:

So I don't think you can take just a VE, cfm & RPM number & relate it to anything.
You need to consider cylinder size, port size, air flow & torque.
Then you will have your VE numbers at differing points in your torque curve for the engine in question.

Just some of my rambling thoughts ;)


Now,,,, please no comments about me sounding like Mr. Ed even though I think some of this tends to follow his thinking on cylinder heads :D