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Old 12-01-02, 01:34PM   #1
zedo
 
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Default too much VELOCITY causes port STALL, SONIC- interesting...

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"This is probably the single most limiting factor in making power in a production V-8 engine. I will now tell you why this is a problem.

When air travels through a tube, if the air velocity gets sonic, the speed of sound, the airflow through that tube is constant. In other words, once air velocity becomes sonic, you cannot flow any more air. Under sonic conditions, the only way to increase airflow is to either increase the area of the tube or increase the pressure on the feed side of the tube.

In an engine, this is also true.

The difference in an engine is that the air velocity at which flow "stalls", or no longer increases with increasing valve lift, is closer to mach .6 rather than mach 1. This is because things like friction get involved and as the port changes cross sectional area, the flow gets faster through this section.

In order to make HP you need air flow. An engine is just an air pump, the more air you pump, the more HP you will make. As you move more air, the air velocity goes higher and higher with increasing RPM. Once the air velocity gets too high, you canít flow any more air. Most V-8 production engines have very small intake ports.

There are two ways to talk about port size, most people talk about port size in ccs. This is fine, but what really matters is the cc's divided by runner length. I convert this to an average size diameter of the port. If you look at a Ford 390-428 head (or 352 to 428 head for the people that remember), the ports are very short so the cc's of the port are smaller, even though the average diameter of the port is still very reasonable. I will refer to port volume in average diameter rather than ccís. The average diameter of the ports is only 1.80" in diameter. This is calculated with a port volume of 187ccs and a port length of 4.5". This length is calculated through the center of the port to the valve opening. For comparison, the Edelbrock Olds heads have an average diameter of 1.71" and a set of ported Batten heads have an average diameter of 1.91". A stock set of "double camel hump" small block Chevy heads have an average diameter of 1.6" and a set of NASCAR Winston Cup heads have an average diameter of 2.04".

The limiting factor for most V-8 production engines making power is the intake ports on the cylinder heads. With the small port diameter, the air velocity gets very high and the intake ports stall. Most production V-8 cylinder heads stall around .550" lift. I have seldom seen a production V-8 intake port that still flows more air beyond .575" lift; most likely, you are not the exception to the rule. You cannot make the ports big enough due to several limiting factors. One is the fact that the intake port is sandwiched in between two pushrod holes. You can only make the port so wide because of this. In addition, one of the head boltholes is also located down in the port and prevents you from making the port wide enough. This is why raising the roof of the intake port is so successful, it adds port volume, slowing down the air.

There are also two different intake ports on some varieties of cylinder heads. The middle 4 cylinders have the same port layout and the outer 4 have a different port layout. This is due to the non-symmetric design of many cylinder heads."
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Old 12-01-02, 01:36PM   #2
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Default Interesting

Above article by a former Pro Stock racer, explaining his findings on port length/diameter/volume- and how he came to the conclusion that too much velocity was the limiting factor on factory head castings- it causes the port to go sonic and "stall"- limiting high lift flow and HP.

This statement is interesting: "This is why raising the roof of the intake port is so successful, it adds port volume, slowing down the air."

That's exactly what Pontiac did with the RA IV heads, and Edelbrock did with the aluminum heads. A lot of velocity isn't such a good thing after all.
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Old 06-15-08, 02:07PM   #3
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This would explain why the iron Pontiac d-port heads quit flowing at around .450-.480 lift. My #62 heads had a port velocity of around 430 FPS at .450 lift. This must be right around the sonic speed because they quit flowing about .480 lift. I guess some velocity is good but too much can be bad, which I kinda figured already but did not know what too much was. I wonder what the happy medium is betweem enough velocity and too much?
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Old 06-15-08, 03:15PM   #4
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But you also need velocity to pick air speed back up after the rpms drop on the shift. This is where velocity comes in handy and such a important factor now-a-days with bigger engines and higher rpms.
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Old 06-15-08, 04:11PM   #5
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Is velocity at all a factor in a power adder application? I know a bigger port will take less boost to make the same power as a smaller one. But it probably is still the same setup as the smaller head, just you can "turn it up" more before the heads blow off LOL. How about a nitrous motor, since it is more of a "chemical reaction" ? Was talking to a guy with a BBC 10.5 Camaro and he said everyone was raving about the profiler big chief style heads. He said he bought a set and ran no faster on spray than his old chiefs that he had done some work to. He then tried them on a buddies fast n/a bracket car and it blew away his heads.
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Old 06-15-08, 04:25PM   #6
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Default Boost?

Zedo,
Who are you quoting ?Just curious.

This is N A gasoline only theory with respect to ram tuning which includes the intake manifold runner volume & plenum
right?

Leaving everything else the same, with respect to the port limitation and engine displacement, NA solution is to throw a boatload of fuel / nitrous or nitro (Fuel + something with a more concentrated oxidizer/acceleraant in it) at the engine to pack more into the charge to compensate for the limited port/runner combination.

(Straightest runner possible but properly sized/contoured for displacement and the operating range of the engine.)

The "Author" says that the only "other" way to get around the problem is to go positive displacement on the inlet side.
That = BOOST.
Lots cheaper than spending thousands of $ and 100's of hours trying to make a silk purse out of a stock casting.

So if you are running boost when do the ports stall out, when will they go "sonic" or "super sonic"? When does that become a factor again ?

I know that PSI is a referenced # above atmospheric pressure that is caused by a resistance to flow.
I know that there is a point at wich you can only get so much through a given length port and cross sectional area even in extreme boosted conditions .There is a point of dimishing returns.

So if you are running boost when do the ports stall out, when will they go "sonic" or "super sonic"? When does that become a factor again ?

Air traveling @ high velocity will actually go "super sonic" and "pop" when it is trying to negociate a tight radius bend or an actual sharp corner.
Like wing tip vortices in a high speed turn.
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Old 06-15-08, 11:26PM   #7
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All good stuff. GTOFREAK. Where did you measure the 430 fps? Is the pitot tube location scewing the results? Here is something to look at. Lots of ported Pontiacs do the same "stall" or go "turbulent" at around that lift.

Quote:
Air traveling @ high velocity will actually go "super sonic" and "pop" when it is trying to negociate a tight radius bend or an actual sharp corner.
Like wing tip vortices in a high speed turn.
Like Sprint said here, the air is not negotiating the corner (SSR) to well. If you put your finger on the floor of the port about 1/2 inch to an inch in, does the depression drop and then you can pick up flow? If it does, you are helping upstream of the port to negotiate that corner. Just like raising the port ( but not allways).

Quote:
This statement is interesting: "This is why raising the roof of the intake port is so successful, it adds port volume, slowing down the air."
Slowing it down so it can turn inside the port if it is not shaped correctly.
During that lift there is a transition where the valve helps the air to make the turn to the wall/floor. If that area sucks then the port will go turbulent and not flow any more.

Check your seat angles too. Make sure they are no more than 15 degrees between them.

Quote:
The difference in an engine is that the air velocity at which flow "stalls", or no longer increases with increasing valve lift, is closer to mach .6 rather than mach 1. This is because things like friction get involved and as the port changes cross sectional area, the flow gets faster through this section.
Still is, .50 to .55 mach seems to be the # to shoot for. Higher and there seems to be pumping losses. This is what I gathered through others.

That number with velocity that the Pro Stock author uses is for the whole tract with a cirtain size motor, and avarage csa and rpm. It is a number calculated from 1116 fps.

Example. Avarage cast head. Lets say avarage csa 2.4. 455 pontiac. 5500 rpm
(4.155* 4.155*4.210*5500*.oo353)/2.4= 587.9FPS /1116=.52
.52 mach Thats where that comes from.

As for forced induction, I want to someday test the fact that the design for forced would be the same as NA only slightly larger to work with the extra volume of air and heat that goes with it.
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Old 06-15-08, 11:43PM   #8
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Also, GTOFREAK, the pushrod pinch point is probably your smallest area and may be also the limiting factor in the total flow. Plus the airspeed there being high upstream may be affectng the air turning over the SSR. At .400 lift the valve window area is greater than the pushrod pinch area. So the valve wont be the restriction but the pinch. Wonder if that is why the Pontiac engineers decided to go with a 2.11 valve and .408 lift at the valve back in the day. Things that make you go MMMMMMMMMMM
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Old 06-16-08, 10:43AM   #9
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Quote:
Originally Posted by sleeper67 View Post
All good stuff. GTOFREAK. Where did you measure the 430 fps? Is the pitot tube location scewing the results? Here is something to look at. Lots of ported Pontiacs do the same "stall" or go "turbulent" at around that lift.
It was just above the short turn radius, tested with a velocity probe on a SF-600 with computer readout.
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Old 06-16-08, 11:16AM   #10
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Just food for thought...I have filled the floor of a factory d-port with as much as 3/8" of clay before any change was seen on the bench. This was without an intake bolted up just a radius on the intake face.
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Old 06-16-08, 11:29AM   #11
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It has been proven that a later intake closing event can help heads to rpm higher and make more power that would normally choke at a much lower rpm. My D ports heads are now pulling 6400 rpm with a medium sized solid roller whereas with the smaller solid flat cam they were starting to choke at 5900.
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Old 06-16-08, 05:46PM   #12
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Quote:
Originally Posted by Mb125 View Post
Just food for thought...I have filled the floor of a factory d-port with as much as 3/8" of clay before any change was seen on the bench. This was without an intake bolted up just a radius on the intake face.
I did the same also tried to mimick the finger with clay and no difference. Has to be something with the hand in the proximity of the opening also.

GTOFREAK, did you try slowing the air down? Or is this head long gone.
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Old 06-18-08, 04:04AM   #13
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More good reading.

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Old 06-18-08, 10:36AM   #14
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Quote:
Originally Posted by sleeper67 View Post
GTOFREAK, did you try slowing the air down? Or is this head long gone.

No, I didn't. And yes, they are still around. Matter of fact, I am still working on them from time to time.
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