Thoughts On Volumetric Efficiency

2649 Views 16 Replies 10 Participants Last post by zja, Jan 23, 2004

mobiuslogic

Discussion Starter · Jan 16, 2004

2

So like the title says, I couldn't sleep and started think about volumetic efficiency and more importantly, how to improve it. Now the volumetric efficiency is of course the ratio of manifold pressure to absolute pressure. The piston will always take in the same volume of air, but the pressure relative to absolute (ambient) pressure dictates the volumetric efficiency.

And so I thought on....with our V6 engines, 2 intake valves will always be open on the intake cycle at the same time. Thus the intake charge will always be split almost equally in half. Now if the pressure has already dropped in the manifolds due to obstructions like the air filter, MAF, throttle body, and of course the mythical e-ram

, then the best way to increase VE is to relieve these obstructions. But how much is needed? And that became the basis of my though process for the half hour preceeding this.

Now the intake valve head has a diameter of 40mm, so unless you go to bigger valves, 40mm diameter is the best possible size to hope for for the intake's diameter. 40mm diameter has an area of (pi(r^2)) which is (3.1416(20^2))=1256.6mm^2. Double this for the 2 valves open at a time which = 2513.27mm^2. So logically, we should have an opening and all openings to the upper manifold with an area of 2513mm^2. Now if we work this backards and divide 2513 by Pi we get ~800 and then take the square root and we get a 28mm radius, or 56mm diameter. Pretty damn close to the stock 55mm design.

Now all this makes me wonder. Why do our stock intakes have what seems to be a big restriction when they are really just the right size? Why does Jason's intakes at 80mm and an area of 5026mm^2 give such big gains, or do we even know how big exactly the gains are? Do his intakes actually increase VE or do they just make the engine work easier?

And it's now that I realize that all this has really been 1 long incoherent rambling on due to the fact that I really just gotta stop drinking soda so damn late. And I guess to answer your question Patrick from quite a while ago:

QUOTE

Do you sit around your bedroom trying to think of the most whacked out way to squeeze 2hp out of your vulcan?[/b]

I guess it's apparent that I really truely do.

-mobiuslogic

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biteableniles

· #2 · Jan 16, 2004

You're thinking in physics world, man. Real life is much more different, and much more complex. In real life, if you were to increase the circumference, the volume would be exponentially higher-- leading to far less frictional effects on the air. Plus, think of a funnel. I know that even on large funnels, I can still stop the water up and make it pool if I don't angle the water spout directly into the hole. The water would flow much easier if the hole was bigger, even if the source hole was the bottleneck. This was a bad analogy.

Eh. Physics world isn't real world. Increasing the size decreases the effect of flaws.

sixfofalcon

· #3 · Jan 16, 2004

Originally posted by biteableniles@Jan 16 2004, 04:33 AM
Increasing the size decreases the effect of flaws.

Exactly.

You can go with a bigger-than-optimum intake system and the effect of interior flaws is reduced, or you can simply P&P an optimum-size intake and leave it at that. Obviously, the latter is a much better choice, since the former would not eliminate the turbulence introduced by the interior flaws (roughness) altogether--only reduce its impact on the airstream.

I like to think the JK intakes' "huge gains" were largely b/c of the P&P, and hardly due to the bigger TB. They still had quite a bottleneck right after the TB, nullifying most of the potential gains.

And, BTW, physics IS the real world. But we are only discussing about .01% of the physics that enters the equation here. The fact is, common sense is something that we all understand (OK not everyone, but you know what I mean,) whereas even a highly-trained physicist can only approximate about half of the physics involved. And most of the time, hours, days, or weeks later, the physicist will arrive at the same conclusion that we got in a few minutes by using common sense.

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DayOff

· #4 · Jan 16, 2004

Originally posted by SixFoFalcon+Jan 16 2004, 08:29 AM-->QUOTE (SixFoFalcon @ Jan 16 2004, 08:29 AM)

<!--QuoteBegin-biteableniles

Click to expand...

@Jan 16 2004, 04:33 AM
Increasing the size decreases the effect of flaws.

Click to expand...

[/b]

My wife said this exact thing to me last night.

:moon:

baby sho

· #5 · Jan 17, 2004

either you or me must be missing somthing...(probbaly me)...if you have a vulcan, you have exactly one (1) intake valve at XXmm and if you have a duratec, you have exactly two (2) intake valves at 35mm. So your calculations may be a little off. But the theory is the same.

Brad

EDIT: I forgot....

also if you are talking about the duratec, the intake valves open at very diffrent times, thus the point of the 2 valves (longar intake duration)

Patrick Norris

· #6 · Jan 17, 2004

Good theory but you are missing several key elements of air flow dynamics. First off since an engine is in a state of varying vacuum standard air flow characteristics don't apply but for the sake of arguement your numbers work fine. Second, like every other part of an engine or car for that matter the parts are at the happy medium of things. An engine is a very versitile device that has a wide range of operating ranges. The reason duratecs and shos have secondaries is to take advantage of the better VE available at different engine speeds and loads.

Short large runners are good for air in quick but long narrow runners are better for larger volumes of faster moving air needed for torque. The vulcan uses a middle or the range runner but the sho v6 has both. I think someone posted the measured VE of all the engines before if someone know where they could post it again. VE is pretty much a given in an engine there isn't alot that can be done to improve on it.

On another note bigger is not better. Optimum is best and no bigger. Smaller is better then bigger. Air moves faster through a small passage then it does through a large one. P&P is the best way to get to optimum but for the most part the engineers have used the best size and P&P just gets rid of the extra casting left from mass producing a part with a 10-20% error margin both ways.

Like spyguy I don't get what you are saying with 2 intake valves. You have a vulcan which has one valve per cylinder and only fires one cylinder at a time.
But if you are refering to a duratec it uses 2 smaller valves and runners to accomplish more then one big valve.

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mobiuslogic

Discussion Starter · #7 · Jan 17, 2004

Because it's a V6 there is always going to be 2 valves open at the same time, each on a different cylinder and at a different point on the intake cycle. Regardless, it was late but I got the answer I think I was looking for in that:

"it's better going with a P&P on an optimum sized intake than going to a bigger sized intake or TB"

Like I said, it was late. Oh well.

-mobiuslogic

Patrick Norris

· #8 · Jan 17, 2004

Originally posted by mobiuslogic@Jan 16 2004, 08:45 PM
Because it's a V6 there is always going to be 2 valves open at the same time, each on a different cylinder and at a different point on the intake cycle. Regardless, it was late but I got the answer I think I was looking for in that:

"it's better going with a P&P on an optimum sized intake than going to a bigger sized intake or TB"

Like I said, it was late. Oh well.

-mobiuslogic

Because it is a v6 has nothing to do with valves being open. It only fires one cylinder at a time so every cylinder is at a different point in stroke. There may be valve overlap but not 2 at the exact same time.

P&P is the best way to increase for runners and valves but the stock TB is still too small for max air flow. IMO

mobiuslogic

Discussion Starter · #9 · Jan 17, 2004

Originally posted by Patrick Norris@Jan 16 2004, 08:49 PM
valve overlap but not 2 at the exact same time.

Thats the term I was trying to imply. Yes 2 valves will be open, but correct not at the sametime. However it will still be drawing air in threw 2 valves at all times, regardless of what point of the stroke they are in. And I do agree the 55mm TB and MAF are still too small.

-mobiuslogic

G

· #10 · Jan 17, 2004

Bore and stroke need to be included in this if we're going to talk volumetric efficiency. Remember, and engine is nothing more than a pump, and can only ever hold the volume dictated by the bore, stroke and combustion chamber.

I think this discussion is more toward the theory of intake velocity. Maybe it's just me. It's been a long day, maybe I'm not thinking correctly, but all the formulas in the first post dealt with area, not volume.

sfblacksel

· #11 · Jan 17, 2004

mobiuslogic, you should of drank a few beer and called it a night

.
Anyways, it is people like you that have invented so many goodies that are present in our everyday life. I was thinking the same thing that Bob said; if you increase the size, you will lighten the load on the engine to suck in air, thus working less hard and allows the engine to be more "free revving". It does however reduce velocity of the air, but allows more air in altogether. I think all this really does is allow the engine to go into the power band sooner, and not add any true horsepower.

I am by no means a scientist, and if the world was perfect, Ford would of made a perfect engine by now. Hmmmmm, 450HP, 500 TQ, 150 MPG on 87 octane......

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mobiuslogic

Discussion Starter · #12 · Jan 17, 2004

3

QUOTE

drank a few beers[/b]

I'm only 19, but what dad doesn't know won't hurt him.

QUOTE

Anyways, it is people like you that have invented so many goodies that are present in our everyday life[/b]

I'll take that as a compliment, I think...Heh.

QUOTE

Hmmmmm, 450HP, 500 TQ, 150 MPG on 87 octane......[/b]

I'm working on it.

:zoomsmiley:

Ok but seriously. Yeah, I thought about it some more and the area of the 2 valves compared to the MAF is right on. However because of casting irregularities, the intake velocity is reduced. A port and polish should be fine because the engine doesn't have to work against the irregularities. However going to a larger MAF/TB will slow down the air coming in. So the real question is which upgrade gives the most power:
1) P/P upper and lower intakes with stock MAF and TB
2) Upgrade MAF and TB only.
3) P/P and bigger MAF and TB

Of course logic says #3, but are they really necessary after the P/P. I could see them being needed with forced induction or even N2O, but are they really a big deal after the P/P on a NA engine?

-mobiuslogic

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G

· #13 · Jan 17, 2004

The stock Vulcan MAF is 65mm. As you said before, the stock TB is 55mm.

IMO, you don't need a larger MAF unless you are running forced induction. The TB could be a little larger.

mobiuslogic

Discussion Starter · #14 · Jan 17, 2004

Originally posted by Bob Gervais@Jan 17 2004, 06:16 AM
The stock Vulcan MAF is 65mm. As you said before, the stock TB is 55mm.

No wonder I can't find a 55mm MAF adapter. Thought they were the same size.

-mobiuslogic

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ken_23434

· #15 · Jan 18, 2004

Larger MAF will result in lower air velocity in the MAF. The cross sectional area of the valve (or valves) is still the same, so the air velocity going through the valve will not reallybe affected by the change in MAF.

Mathematically Q=A*V

Q is the mass flow rate
A is the cross sectional area
V is the mean velocity through the pipe (or MAF, or Valve)

Q will be a constant in the system. Since you cannot create mass, the flow rate through the air filter will be the same as the flow rate through the MAF which will be the same as the flow rate through the valves. A larger area will result in lower air speed in that part. The smaller area will result in a higher velocity through that part.

A larger intake path will result in less head loss (pressure loss, losses due to friction, what ever you want to call it) in the system. The lower head loss will mean the system will be able to flow more, for a given differential pressure.

So, if you take into account less systems losses for using a larger intake path results in a larger mass flow rate, then the larger MAF will actually speed up the air going through the intake valves.

The advantage that most people talk about with high air speeds for the intake of an engine deals with the air flow being turbulent. If the air "tumbles" as it passes into the engine, there is better mixing of the air and gas in the cylinder. This will result in better fuel burn, and therefore more power, better fuel economy.

This is why performance engines usually do not have a mirror polish on the intake runners. On the exhaust side, different story.

My $0.02

Ken

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jedhead

· #16 · Jan 18, 2004

I remember some writings about the VE and formula that is used to calculate it. The article I read involved the SHO engine. VE is over 100% at two rpm points in the engine. They also coincide with the two torque peaks 2900 and 4900 rpms with the 2900 having a higher VE in the stock engines. The VE is over 100% because of the pressure waves in the intake mainfold traveling between the tanks and intake runners. At the torque peak the pressure wave help pressurize the air going into the engine for a slight supercharged effect allowing the engine to exceed 100%VE. Some of the new designed variable length intake mainfolds allow this effect over a greater rpm range. The two stage in the SHO gives two rpm peaks.

Bob

zja

· #17 · Jan 23, 2004

it's been a while since i posted in here, but here it goes..
go to www.eng-tips.com. they have some very interesting forums on intake design concerning bmw's m3 engine. apparently the plenum size of the intake is 14-18 liters, and this was achieved by port throttle bodies. In theory you want the air in your plenum to be moving slowly for your "tuned" runners to operate as designed. if we try to create a large plenum using only one throttle body, we get aweful throttle response because the engine doesn't see the pressure drop for a while because the throttle body is too far from the cylinders. I guess this is how tunnel ram works on racing engines, they don't care much about low-end with 3500rpm stall converters. anyway, check out this link, because there's a lot of good info there but don't take everything as law. i have thought about doing this for our cars but it seems to be a waste of money. even if you could concieve some sort of port throttle system, the MAF wouldn't see increased flow for a second when you stomp on it.

eng-tips.com

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