blow or suck

[cspeier]

[color=#000000]I've always wanted to do this, but never got around to it. Has anyone back-to-backed a exhaust port being flowed either way? IMO, pulling air thru an exhaust port, won't give you the same answer your looking for. But, that's why I'm asking. Super Flow says it won't matter. I'm looking for some past experience on this subject. I've been flowing heads for 15 years but never really tested it. Below is the quote from SF. Thanks for any comments.

Should it blow or suck?
For a typical flow test, mount a cylinder head onto a flowbench. The flowbench will either blow or suck air through the cylinder head. A common question is, "Well, don

[larrycavan]

[color=#000000][quote="cspeier"]Should it blow or suck?
For a typical flow test, mount a cylinder head onto a flowbench. The flowbench will either blow or suck air through the cylinder head. A common question is, "Well, don

[Thomas Vaught]

As some of you know I would for this little company in engineering. We do flow testing once
in a while. Some might have seen our little bench posted in the forum.

We actually have four different "cylinder head" flow stands. Two in the Engine Engineering
building are "Suck only" flow nozzle benches. Very large capacity benches.

We have the flow stand seen in the forum pics that can do "suck and blow". The main reason why it is a "suck and blow" bench vs like the older "suck" benches is that it is fully computerized and without changing any of the bench set-up you can do a complete 2 valve, three valve, or 4 valve head intake system and exhaust system. Time is money. One set-up and the test technician can just push the button.

With the other flow stands the techinician has to have the fixturing to do the intake testing and then reset-up the fixturing to do the exhaust testing. More time. The thousands and thousands or flow tests that my company has done over the years has always shown basically the same flow numbers at the same test pressures.

If you trick the testing by adding a 10" piece of pipe to the exhaust port vs flowing without the piece, sure you will get different numbers. A lot of guys use the "stub" to improve the exhaust numbers.

Super Flow says it makes no difference, Smokey Yunick sucked through the exhaust port or header, My company has done the same as mentioned above.

But what do we know.

JMO

Tom V.

[cspeier]

I understand what you are saying. You are correct, it makes no difference. Bernoulli principle proves this. Thanks for the reply.

Chad

[Rick360]

[larrycavan]

[bruce]

[Tony]

Yes Bruce, I must buy a camera !!

Anyhow,

I cannot see how it can be said that reversing the direction of airflow through a port can make no difference to measured flow ??

Something can be streamline in one direction and have MASSIVE drag in the opposite direction. I am thinking of something like the cups on an anemometer, or a deliberate reversion step in a pipe.

We all know that placing a flared bellmouth entry on the inlet of a pipe will significantly increase flow. Placing a flared exit on a discharging pipe end will do rather less.

I am baffled, what am I missing here ?

[cspeier]

I thought the same thing also, but I'm wrong! I've been emailing Darin Morgan to help me understand, and now I do. First I read up on the Bernoulli principle and then I fully understood this email comment from Darin. Read it close. Again, my thinking WAS wrong.

Darin comment:
A pressure differential is just that, a difference in pressure. It does not matter how the difference in pressure is formed or on what side of the port it is formed on. When dealing with pressure there is no such thing as " pulling" or "pushing". There is only a difference in pressure. There is no such thing as a Vacuum there is only low pressure. Have you ever considered the fact that there is a name for low pressure (vacuum) but none for high pressure???? That's because its a made up term used to describe low pressure so laymen could understand it.

Stop thinking in terms like pull and push. Start thinking in terms of low or high pressure.


Makes total sense!

[larrycavan]

If the pressure decreases as velocity increases then the fastest part of the port has the least pressure.

Why then does the floor of the intake have the least pressure and the roof of the exhaust have the least pressure?

What is it that creates the variance in pressure differential within each port being opposite in location [floor/roof]?

If there is no directional imact of energy during the exhaust stroke from the piston and only pressure differential combined with shape to determine the location of highest and lowest pressure, then what is it that causes the shift in low pressure locations between intake and exhaust ports?

Is it not tied to direction of flow?

The cylinder volume does not change between strokes. The engine speed does not change. Therfore would there not be an equal measure of pressure differential created by the piston moving either up or down in the cylinger?

Is it related to the fact that the exhaust valve being smaller than the intake valve, generates greater velocity? That seems perfectl logical BUT..The greater velocity alone does not explain the switch of low pressure location reversals.

In one direction of air travel, it likes the shortest path in. In the opposite direction of travel, it likes the longest path out.

Mass and Newtons law seem to be the only logical reason for the shift in low pressure location reversals.

So far, all I've heard is pressure differentials are all that exist. Nobody has been able to explain the reversal.

[larrycavan]

[Tony]

Ah ! I knew there must be something really simple I had missed. Just could not see it. Duh.

[Tom]

[larrycavan]

[Sir Yun]

interesting but like larryc to me it just seems off.


Shure the pressure differential will be the same (the flow is after all calculated by a pressure drop across a standard resistance).
and the flow as measured will be the same.
but the density of the air / amount of mass and it's distribution will (shurely??) be differnt.







that said..

sucking through a header is so much easier for me that