Did a search wondering if there is a minimum port velocity

[My427stang]

First, I apologize for a million questions, I am very interested in the math and behavior of airflow and want to do something different in my street car. I do plan to buy pipemax and some fluid dynamics books but right now I am burying myself in searches here

My question is: On a carbureted motor, assuming a nearly straight port from the plenum to the turn at the valve. Is there a minimum velocity required to keep fuel in suspension?

I am thinking if it slows to a certain point it'd drop out

I see a lot of talk about maximum velocity at a theoretical .55 Mach or so, but wondering what would be a "stalled" port or at least low enough flow to not adequately fill the cyl

Thanks

[Darin Morgan]

I am not good at Newtonian physics but here it goes. Everything falls approximately at 9.8M/S² if memory serves me correctly so the fuel droplets are always falling regardless of the velocity. There is no such thing as a minimum velocity to "suspend" fuel. There is however a minimum velocity to induce enough energy in the working fluid ( air fuel mixture) to fill the cylinder with both inertial and high amplitude wave super charging effects.You will always have some degradation of the air fuel mixture as it traverses the intake tract. By constantly accelerating the air and keeping the velocities both minimum and mean as high as possible (between .45Mach and .55 mach) you can minimize the de-atomization or fall out before the air fuel mixture reaches the combustion chamber. There is a lot more to this than just air speed.

The velocity of the air as it changes direction.
Boundary layer effects.
The over all velocity gradients through out the intake tract as well as at the entrance in the plenum area.

All these and many other dynamic effects have a great deal of influence of the homogeneousness of the air fuel mixture.

I am sure other on this board can add more but I have to get back to grinding.

[airflowdevelop]

Good post darin...I thought it was 9.8 Ft per sec/per sec...or maybe my memory is shot.

Are you concerned about minimum average velocity? Or at a particular area?

What makes you think this port is going to be a dog? It may surprise you!

Some of the fastest ports I have seen are straight shot large ports. If the port is design properly to the valve and bore size, there is no reason it should be slow.

Concentrate on designing the induction system to work properly, and the velocity should come with it.

Dennis

[My427stang]

I am still just trying to make some ryhme or reason about an issue I am still fighting with the 489 and the dual plane combo.

In some of the areas, the port of the dual plane has a calculated CSA of over 3 sq/in. When you compare that to the RPM (1000-1400 rpm depending on load) that I am having what I think is mixture distribution issues, the calculated velocity really takes a dive using online calculators, even with a 4.25 stroke / 4.277 bore

Add to that of #2 cylinder having to cross under the carb, and competing cylinders on that bank, I thought that it could be why my setup needs so much transition fuel but runs absolutely fantastic as rpm and load goes up, basically a stall at #2 port at low rpm letting fuel come out of suspension? or maybe just the corners it needs to take seperating fuel out at the wall?

I am considering modifying a very small port single plane to be basically a very fast tapered port with enough CSA at the head to feed the motor at 5500 rpm or so, but I need to understand it a bit more before I start carving LOL

Right now that second single-plane intake only has a CSA (narrowest point measured at the pushrod pinch with calipers) of 1.94 sq/in, which I dont think is enough for a 5500 rpm 489 cid motor, so I am trying to understand both what I am fighting with the dual plane and plan out a smart intake using the single plane as a starting point

If I am way off on my thinking, call me out, I am very humble about this LOL

[jacksoni]

I'm not going to touch the basic topic here but Darin is correct, at least partially. 9.8M/sec/sec (sec squared- I don't know how to do the superscript thing) is the acceleration of gravity on earth (definition of one gravity). Airflowdevelop is thinking of 32ft/sec/sec which is same thing in english units ( as opposed to metric). Problem is I don't think acceleration of gravity has much to do with the acceleration (and therefore the velocity) in the port which is being "pushed" by the weight of atmosphere trying to get into the relative vacuum created by piston motion in the cylinder. That happens in what, few milliseconds, at 6000rpm? To get the air/fuel to be going Mach .5 or so? Takes a lot of gravities, but not much force as the mass is so low and the time is short to reach those velocities. Anyway, all you engineers who remember this stuff better can expound.

[airflowdevelop]

you got me...yep 32.2 ft/sec/sec...my memory is drifting away!

[GREG K]

What size carby are you using..And are you using a spacer under the carb...

What type of boosters are in the carb..And how many holes are in the emulsion well..

[My427stang]

Carb is an 80512 Holley, vacuum secondary rated at 1000 cfm however, take that with a grain of salt. Dimensionally the venturis and plate are identical to an 850, even factory jetting is very close to an 850, it just has the HP series choke horn and the later style IFR on the top of the metering plate

Boosters are downleg boosters, I have run various spacers. From an open 1 inch phenolic, none, and finally the current setup is the open with a sheetmetal divider blocking off the two planes.

Good question on the emulsion holes, I have been all over the carb but I cant remember for the life of me how many in each channel. 3 in each channel I think?

However, the problem doesnt exist when I get on the boosters it runs great with any throttle, its at steady throttle just barely above idle.

I believe that #2 cyl is the "worst signal" if you will, and at low RPM its going lean, and/or low velocity is causing the fuel to puddle before it gets to #2 (or possibly one other cylinder on the plane is "stealing" if you will)

Think I am off base?

[Ron E]

Does the #2 plug read different? i'm just wondering how you arrived at #2 being suspicious. Dual planes are wierd to me. Years back I tested (for 350 2bbl LMS) the spec performer intake on 351 C-2V,351N, bowtie, and W2's. In each case the low-approach port flowed a bit better than the high one did. Of course, that's one hole at a time, so it doesn't address any robbing issues. If it is a velocity problem in the intake, making it good at off-idle vs. 6K RPM will be a hell of a trick.Normally, dual-planes seem to work better with big runners.
Does it get noticably better with heat? Just wondering, as I think you'd suffer less at 6K with a heated manifold than you would with a CSA under 2"sq.

[My427stang]

I have another post in here from a few weeks back.

#2 plug looks lean, and has in normal driving.

What I did, after much troubleshooting and expeimentation, was richen the IFR slightly, close down the idle air bleed from .073 to .062 and then closed the idle screws to get a clean idle

What this did was allow the transition fuel to come on a little harder and be a little richer at 1000-1500

It idles good, runs pretty good, but still bucks a little at very low rpm. Before richening transition fuel, it bucked HARD with any carb at 1800-2000, now its livable, but not right, with a buck at 1000-1400 or so

I have gone through all the normal tuning, vacuum leak checks, manifold checks, etc. This thing just loves a lot of transition fuel and I dont know why

My theory is that #2 isnt getting fuel at very low RPM and by fattening it extremely, I am bandaiding it

I have reviewed literally 40+ dyno sheets of various FEs with the RPM intake and EVERY ONE has a dip in the torque curve at some point between 1500 and 3000 depending on how its configured.

I think it has to be the manifold, but my alternative doesnt look like it will feed the motor, so I figure with some thinking, I can port the single plane to do what I need it to do

Take a look at the dip on this older Edelbrock test (scroll to the bottom, 2 charts)

As a rule, the bigger the motor, the lower the RPM for the dip, but from 390's to 511 FEs they all show a dip in the torque curve, look at the drop at 3500 It doesnt bother some guys, they shift around it, but as a street car its a pain. This one is 50+ cubes less than mine, which could account for the dip being higher, but just the same, it exists on every graph I have seen with an Edelbrock RPM on an FE Ford

http://www.edelbrock.com/automotive/sto ... omber.html

FYI it is an unheated manifold, and at initial fire up it seems better. Once it reaches temp it seems more prevalent (which seems backwards to me)

[GREG K]

Another thing is..What air cleaner set up do you run..

Dual-planes can be weired and wonderful at the same time..And they can be quite reactive to things like..end gearing,throttle position vs ground speed..The position of the throttle blade to the bore of the carb,when you have certain size gearing,and then change the gearing and now the throttle position changes to keep the same ground speed...Dual-planes sometimes react to this very stangely,and the flatter the dual-plane is, the more reactive they can be...An interesting test to do is to graph your a/f ratio,your ignition timing and your manifold vacuum all on the same graph,as you are driving in top gear,at the mph that you are experiencing the problem,and see what it looks like..Now while your driving along,add more timing or reduce the timing,does this change what your experiencing..Because,the timing that you have at that point,will not be right for all cylinders,especially with a dual-plane that has distribution problems..And perhaps changing it might help the cylinder that has the worst distribution problem..

Getting low rpm cylinder efficiency is hard,because of many factors,and one of the main ones is intake dilution..There are many contributors to this condition,and two of the main ones are firstly poor fuel atomization,and secondly,slower intake port velocity..Couple those conditions with exhaust backpressure and intake reversion,and it makes for a nasty condition..

That is why,down lower in the rpm you need to have a richer mixture,to help compensate for this condition..But the richer mixture,needs to be a quaility richer mixture,atomized correctly...The more larger fuel droplets that you have,along with more clumps of liquid fuel that you have,will not help the situation,it will make it worse...It really needs to be of a good atomized state..And thats where we get back to that manifold,the vacuum/pressure inside that manifold ,and the a/f mixture quaility...

Find out what that throttle angle is at ,when you are cruising at the speed where the problem is,take the carby off and have a look at the throttle blade in relationship to the bore,and remember,that the nearly closed throttle blades,are slowing the air speed that comes throgh the carb...
And forcing it towards the sides of the bore..This can be the most difficult time to get good fuel atomization,hence a lot of the fuel can plate out and turn back to larger droplets and deliver a leaner mixture...Sometimes the fuel can enter the runner in a stream effect,and not mix properly..There are many ways that you can get a lean mixture,just to ONE cylinder..

[My427stang]

Everything you say makes perfect sense to me and seems to apply to what I am seeing

We are planning to put it on a hub-driven dyno prior to the manifold swap to see exactly what everything is doing when it bucks. Hopefully mid week next-week

What I wish I had, was individual cylinder EGT and mixture but I dont have that luxury. I will have a wide band O2 sensor on each bank though, hopefully its sensitive enough to show a single lean cyl out of 4

I think that the small port single plane intake spare I have may be a very good match for this motor, however, as it is right now, I think its too small. If I get that far, I'll try to open the intake to an extent that it's still a fast port, but basically peaks at my max rpm (I only want 5500-5700 rpm out of this motor)

That will be the next step, but this dual plane runs so well other than that basically almost-closed throttle buck that I'd love to figure it out

[My427stang]

I run a drop base 14 inch open element, but it does it with or without the air cleaner

(maybe slightly worse without it, but you know, tough to measure the buck)

[Ron E]

While I'm sure you've already considered it, what about a 'medium" sized single plane manifold. Just thinking with the displacement and RPM range you've set it up for, torque is probably less a problem than traction. But, I'm not familar with the offerings for an FE. I think it's a cool choice though.

[Windsor377]

Out of curiosity, what is your ignition curve?...my appologies, if this has been discussed before.