How much vacuum does an intake port really see?

[plovett]

How much vacuum does an intake port on an actual running engine really see? I realize that is a pretty vague question so I understand the answer may be pretty general, too.

The reason I'm asking is that I was thinking about cylinder head flow testing. It's almost always done at 28" H2O. I understand that could just be the convention and it makes comparisons easier if everybody tests at the same vacuum, but is that the best representation?

Then next thought I had was that an engine with a properly size induction system usually sees less than 1.5" of Hg in vacuum in the intake tract. So I looked up the conversion from inches of mercury to inches of water. It is 13.6185 so 1.5" of Hg = 20" of H2O.

So is 28" of H20 a good measure?

Maybe testing at a lower vacuum would more accurately recreate events in a running engine?

Maybe testing a higher vacuum would more readily show deficiencies in an intake port and more readily show improvements?

Am I even making sense?

paulie

[OldSStroker]

How much vacuum does an intake port on an actual running engine really see? I realize that is a pretty vague question so I understand the answer may be pretty general, too.

The reason I'm asking is that I was thinking about cylinder head flow testing. It's almost always done at 28" H2O. I understand that could just be the convention and it makes comparisons easier if everybody tests at the same vacuum, but is that the best representation?

Then next thought I had was that an engine with a properly size induction system usually sees less than 1.5" of Hg in vacuum in the intake tract. So I looked up the conversion from inches of mercury to inches of water. It is 13.6185 so 1.5" of Hg = 20" of H2O.

So is 28" of H20 a good measure?

Maybe testing at a lower vacuum would more accurately recreate events in a running engine?

Maybe testing a higher vacuum would more readily show deficiencies in an intake port and more readily show improvements?

Am I even making sense?

paulie

Paulie,

Your are rediscovering the wheel, but you are thinking ahead of many folks. A lot has been written here about this topic. Start by searching for posts by Darin Morgan and maxracesoftware (Larry Meaux). You'll find this topic discussed.

In an operating engine the intake sees pressure over and under atmospheric way more than 28 inches H2O. At times there may be 5-7psi more pressure in the intake runner than in the cylinder. Do the conversion to in. H2O for that. It will open your eyes.

[SWR]

In an operating engine the intake sees pressure over and under atmospheric way more than 28 inches H2O. At times there may be 5-7psi more pressure in the intake runner than in the cylinder. Do the conversion to in. H2O for that. It will open your eyes.

And if you don't care to do the math;7.35 psi is about 200" H2O. 5 psi;about 135" H20.

I really need a bigger flowbench...and/or a direct connection to the HT wires outside.

[plovett]

How much vacuum does an intake port on an actual running engine really see? I realize that is a pretty vague question so I understand the answer may be pretty general, too.

The reason I'm asking is that I was thinking about cylinder head flow testing. It's almost always done at 28" H2O. I understand that could just be the convention and it makes comparisons easier if everybody tests at the same vacuum, but is that the best representation?

Then next thought I had was that an engine with a properly size induction system usually sees less than 1.5" of Hg in vacuum in the intake tract. So I looked up the conversion from inches of mercury to inches of water. It is 13.6185 so 1.5" of Hg = 20" of H2O.

So is 28" of H20 a good measure?

Maybe testing at a lower vacuum would more accurately recreate events in a running engine?

Maybe testing a higher vacuum would more readily show deficiencies in an intake port and more readily show improvements?

Am I even making sense?

paulie

Paulie,

Your are rediscovering the wheel, but you are thinking ahead of many folks. A lot has been written here about this topic. Start by searching for posts by Darin Morgan and maxracesoftware (Larry Meaux). You'll find this topic discussed.

In an operating engine the intake sees pressure over and under atmospheric way more than 28 inches H2O. At times there may be 5-7psi more pressure in the intake runner than in the cylinder. Do the conversion to in. H2O for that. It will open your eyes.

Thanks. I'll try searching.

paulie

[plovett]

In an operating engine the intake sees pressure over and under atmospheric way more than 28 inches H2O. At times there may be 5-7psi more pressure in the intake runner than in the cylinder. Do the conversion to in. H2O for that. It will open your eyes.

And if you don't care to do the math;7.35 psi is about 200" H2O. 5 psi;about 135" H20.

I really need a bigger flowbench...and/or a direct connection to the HT wires outside.

Well, I think it's safe to say 135-200" of H2O is a lot more than 28". If everything was proportional I'd suppose that wouldn't be too big of a deal. I'm guessing that's not the case.

paulie

[Cogburn]

A little off subject but the local fan company uses a 455 olds that is hotted up some to run up the new fans.

Who needs power companies?

[A Atwood]

A little off subject but the local fan company uses a 455 olds that is hotted up some to run up the new fans.

Who needs power companies?

Oh no! Did he say 455? Shouldn't it be 400 or less.
Sorry, couldn't resist.
ARN

[Rick360]

Then next thought I had was that an engine with a properly size induction system usually sees less than 1.5" of Hg in vacuum in the intake tract. So I looked up the conversion from inches of mercury to inches of water. It is 13.6185 so 1.5" of Hg = 20" of H2O.
paulie

Your thinking about the vacuum in the intake is backward. The 1.5"Hg vacuum is differential pressure across the carb, not the port.

If the intake manifold is 1.5"Hg below atmosphere it still has an absolute pressure of 28.41"Hg (sea level, std conditions) remaining to "push" air into the cylinder. The piston is creating lower pressure inside the cylinder. That is the potential differential pressure across the intake port.

How much actual differential pressure between the cylinder and the plenum, (thats what causes the flow and determines the velocity), depends on how well the port keeps up with the piston demand from the downward motion of the piston in the cylinder.

I don't disagree at all with the other responses, just thought this should be cleared up. The test depression on a flowbench is an important consideration and the trend is to higher depressions trying to more closely model the actual velcoity in the port. Engine Pro software (developed by Patrick Hale and sold on Speedtalk) actually calculates predicted diff press across port at different valve lifts to help determine test depressions that should be used on a head.

Creating a large depression across a port flowing a large amount of air requires a lot of power.

Rick

[MadBill]

What Rick says is very true, however my Dynomation program, which I believe has decent correlation with real engine physics, shows the peak pressure difference (induced by the way by the negative pulse from the exhaust) of as much as 7 psi in the overlap period, where valve lift is perhaps only 20%-25% of max. This would mean you wouldn't need quite such a huge bench to simulate flow at these high depression flow conditions.
Nitro2 could share some insights, but his info may be proprietary..

[randy331]

Bill, if the ex. created a negitive preasure in the chamber during overlap, and that negative preasure was still present at ex. valve closing, wouldn't the in. port now be exposed to the negative preasure created by the ex. plus the increasing negative preasure created by the desending piston? It seems to me that would increase the depression on the in. port, requiring more depression on the flow bench to simulate.
What am I missunderstanding?

Thanks, Randy

[OldSStroker]

What Rick says is very true, however my Dynomation program, which I believe has decent correlation with real engine physics, shows the peak pressure difference (induced by the way by the negative pulse from the exhaust) of as much as 7 psi in the overlap period, where valve lift is perhaps only 20%-25% of max. This would mean you wouldn't need quite such a huge bench to simulate flow at these high depression flow conditions.
Nitro2 could share some insights, but his info may be proprietary..

200 inches of water can move a lot of wind thru a .050 - .100 open valve. Larry Meaux once quoted a figure.

As Rick said, it is differential presure that moves air, whether it is in an engine or in the earth's atmosphere. The stronger the high and low pressure areas are on the weather maps, the stronger the wind between them.

[MadBill]

Bill, if the ex. created a negative pressure in the chamber during overlap, and that negative preasure was still present at ex. valve closing, wouldn't the in. port now be exposed to the negative preasure created by the ex. plus the increasing negative preasure created by the desending piston? It seems to me that would increase the depression on the in. port, requiring more depression on the flow bench to simulate.
What am I missunderstanding?

Thanks, Randy

The suction pulse from the exhaust is quite short duration. By the time the piston starts making substantial downward progress, it has passed as well as being cancelled by the inlet flow it induced. Ideally the exhaust valve closes at the crucial moment to prevent back flow of burnt gasses and the escape of unburned mixture out the exhaust. (Quite a juggling act when you look closely at it...)

[nitro2]

The suction pulse on the intake stroke can easily be in the range of 5-8 psi below atmospheric pressure.

During valve overlap it is possible to generate a suction of 7 psi or even lower in the exhaust port. However, I wouldn't count on it. The amount of suction in the exhaust port depends on many things. Bolting on a set of headers with a tuned length just gives you a tuned length and consequently an rpm band where the header works better than at other engine speeds. It by no means dictates that you will have 7 psi of suction nor does it guarantee how many degrees of how much suction you will have.

Same thing goes for the intake side of things. There is a lot happening in both systems with resonance, influence from other cylinders, porting, cross sectional area changes and so on. We have often been asked (by people that already own our analysis equipment) if we think they should adjust this or that before doing the first test. The answer is always no. Trying to guess what the port pressures are going to be ahead of time is a waste of time, because 9 times out of 10 they will look different than your best guess, unless you have already seen data from another engine with very, very close to exactly the same setup.

One other consideration is that unlike a flowbench an engine is not operating with steady state flow. It is possible to have 5 psi (or some other value) suction in an intake port and have the flow going out of the cylinder through the intake port in the early part of the intake stroke at certain engine speeds. It is also possible to have 7 psi suction in the intake port and very little flow into the cylinder right in the middle of the intake stroke. Obviously these conditions are not sustained indefinitely during the intake stroke as the wave action changes with crank angle position. Several degrees of flow in the wrong direction or flow with minimal velocity despite high suction can easily occur. It just depends on the wave action.

Looking at it from this perspective it is easy to see that the flow rates at some points in the intake stroke have to be very high in order to fill the cylinder. It would seem that flowbench testing would ideally need to be conducted at much higher pressure differentials to simulate the peak flow speeds in an engine. Perhaps testing at 2 or 3 pressure differentials (high, medium and low) to get a better overall rating?

Clint Gray
TFX Engine Technology Inc.
(Combustion/Intake/Exhaust Pressure Analyzers)
www.tfxengine.com

[SWR]

Hmm... Flowing at 10", 40" and 160" so you can see if it doubles up the cfm's every time?

[MadBill]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?