Blow through intake manifold flow testing

[turbo2256b]

looking at the pic tape looks like it maybe sucking not blowing. The bench i used could reverse the flow usually suck flowing intake side head or intake, blow through on the exhaust.

[cjperformance]

I understand testing on a head is a chore but why not take a port mould and a chamber mould, then make a hard mould of them so you have a port and chamber that you can bolt/attach to the intake and quickly shift from port to port, not as good as a head and i know a lot of stuff has 2 different runners per head etc but may be a quicker and more accurate baseline before bolting to the actual head. *disclaimer- i dont have a flow bench but am familiar with how they are setup/work so it 'sounds' great to me!

[Carnut1]

looking at the pic tape looks like it maybe sucking not blowing. The bench i used could reverse the flow usually suck flowing intake side head or intake, blow through on the exhaust.

Bench was off. Blew tape off right away.

[MadBill]

How about tightly wadding shop rags into seven of the ports from the plenum end? The worst that could happen would be turning the manifold into a tee shirt cannon...

[user-23911]

IF you are doing blow through instead of sucking myself would never do that.
Also because of pushrod locations the left side of the intake needs to be worked different than the right side.

Why would it matter if you suck or blow? Pressure differential is the same.

Of course it matters, air flows past obstacles differently depending on the shape.
Reverse the direction, the shape changes to the airflow.


Or better still, try it both ways, get 2 sets of numbers, compare them?

[Carnut1]

How about tightly wadding shop rags into seven of the ports from the plenum end? The worst that could happen would be turning the manifold into a tee shirt cannon...

I used some plate aluminum over the tape and clamped them to the manifold. Looked ugly but worked well. I would not want the wadded rags to change the flow in the plenum. Thanks, Charlie

[mag2555]

Balance for a blow thru set up does not matter much as long as each runner can pass more air then the head does .

[Carnut1]

IF you are doing blow through instead of sucking myself would never do that.
Also because of pushrod locations the left side of the intake needs to be worked different than the right side.

Why would it matter if you suck or blow? Pressure differential is the same.

Of course it matters, air flows past obstacles differently depending on the shape.
Reverse the direction, the shape changes to the airflow.


Or better still, try it both ways, get 2 sets of numbers, compare them?

I am not changing the direction of flow if I blow into the manifold plenum and still use 28" depression. I figure I will get two sets of numbers. Blow through and then sucked on through the head. Thanks, Charlie

[Carnut1]

Balance for a blow thru set up does not matter much as long as each runner can pass more air then the head does .

That is what I was thinking too. Manifold is lagging 20 + cfm on some ports vs. Usable lift on my Darts. I am not sure I can improve this manifold enough to get it close. Thanks, Charlie

[Carnut1]

0527180758.jpg

Feb 2008 Hot Rod mag article on blow through bench testing followed by dyno test. I think it is flawed because they get 300 cfm from a performer rpm that is ported. I have a Weiand Airstrike that is fully ported that I will be testing soon. On my Dart 215's they knocked the flow way back. Thanks, Charlie

[MadBill]

IF you are doing blow through instead of sucking myself would never do that.
Also because of pushrod locations the left side of the intake needs to be worked different than the right side.

Why would it matter if you suck or blow? Pressure differential is the same.

Of course it matters, air flows past obstacles differently depending on the shape.
Reverse the direction, the shape changes to the airflow.


Or better still, try it both ways, get 2 sets of numbers, compare them?

He's talking about blowing through the plenum vs. sucking from the port; the flow is from plenum to port regardless.

[Greg Long]

Charlie, I tried the same thing as you several years ago on 3 or 4 intakes and made notes on each test. Then I turned them around with the air being pulled in as it normally would be. Every test in the blow through method gave higher numbers. I think this is due to the way the air acts at the runner entrance areas, but I am not sure.
A simple way to block the ports is to use a straight 1x4" wooden plate with a piece of high density foam and 2 quick clamps and make it long enough to cover any head you may test
I have made an adapter for the bench but I don't have the computer skills to send pictures, sorry.

[groberts101]

Why would it matter if you suck or blow? Pressure differential is the same.

He's talking about blowing through the plenum vs. sucking from the port; the flow is from plenum to port regardless.

Right but maybe think about it like this too. Is the air sucked into the engine?.. or forced into it by higher atmospheric pressure when the cylinder is in depression?

I do know many/probably most flow tests are done with the head mounted to the bench's cylinder and then manifold attached to hang out in free air.. but have always wondered if that is really the best scenario from a dynamic flow standpoint or if it's generally boiled down to the time and attachment concerns related to doing it in the opposite manner? Maybe the numbers go down and we don't like to see that happen?

My disclaimer is that I've never actually flowed an intake in any way shape or form, but physical logic makes me imagine slightly more realistic turbulence within the plenum if pressurized flow bench air is forced into the open plenum(via open carb mount pad) and mandated to more forcefully navigate off the floor and around dividers, even creating more realistic dead flow or chaotic area's as the bench's resistance to its true flow capability builds/backs up, in a more havoc induced manner like a real running engine would experience. Well.. not even close but at least some small derivative of such chaos as the port being tested flows in a less than linear manner than compared to a single active port pulling from such a large available open flow window area. And I'd also guess that including the carb may smooth out that tested runners airflow into a more linear manner as well, possibly slightly reducing or less mimicking of the crazy chaotic nature of pressurized air being trapped or diverted before passing into its perspective runner.

Crap.. now I have more questions than before I logged into this thread! LOL I wonder/wish if Larry, Bill or others could chime in here to add perspective.

So my brain can better understand what may be going on inside an intake manifolds plenum.. let's use another "more chaotic open space" example like a chambered muffler attached to the flow bench. And let's assume that bench has the capability to flow 500 cfm @ 28".

Does it flow exactly the same when air is sucked through it vs pushed through all that open space? In other words.. does one method saturate the available space moreso than the other just reaching the point of what the internal obstacles and entry/exit tube diameter is capable of?

Is the pressure buildup/resistance to flow through the open chambers space identical either way its installed on the bench?

[groberts101]

Charlie, I tried the same thing as you several years ago on 3 or 4 intakes and made notes on each test. Then I turned them around with the air being pulled in as it normally would be. Every test in the blow through method gave higher numbers. I think this is due to the way the air acts at the runner entrance areas, but I am not sure.
A simple way to block the ports is to use a straight 1x4" wooden plate with a piece of high density foam and 2 quick clamps and make it long enough to cover any head you may test
I have made an adapter for the bench but I don't have the computer skills to send pictures, sorry.

Now see.. that makes some bit of sense to me.

Pressurizing the available dead space in the plenum forces the floor of the ports entry to accept more air than may be normally occurring if the plenum is just left hanging in open air. But wouldn't that also be assuming that the entry area was probably slightly too big to begin with compared to the cylinder heads flow potential? Basically livining up flow in the lower portion of the too big port that may have not been as high if pulling air through the port?

We obviously know that the ports entry needs to be larger at the plenum on a real running motor.. but more curious about the affects of flowing in each direction and how it may be attributed towards a more realistic number.

[MadBill]

The current most common flow bench test pressure is 28" H2O/2.05" Hg./1.00 psi. Standard sea level air pressure is 408" H2O/29.92" Hg./14.7 psi. Air flows as a result of pressure differential, so testing at sea level, when sucking through a manifold runner the plenum entry pressure is 408" H2O/29.92" Hg. and the runner end pressure is 380" H2O/27.87" Hg., a delta of 28" H2O/2.05" Hg.

Now let's move the bench to a higher altitude, one at which standard pressure is 2.05" Hg. or 28" H2O lower, per this chart: https://www.avs.org/AVS/files/c7/c7edae ... 54f87b9e.p , 27.82" Hg. @ ~ 2000 ft. This time though, we will blow through the plenum, so the inlet pressure is now 27.82 + 2.05 or 28.87" Hg./408" H2O and the runner end pressure is 28.87" Hg./380" H2O, a delta of 28" H2O.

Hmmm... The inlet pressures are the same for both tests and the outlet pressures are the same, so the delta is the same. I wonder if perhaps the flow will also be the same?