Intake port air speed.

[randy331]

comp 317 solid roller 282/292 @ .050 .704 lift 106 lsa 13.1:1 compression.

Is this by any chance a 20* misprint?

Randy

[BowtieNut]

No that's at .050. This is just a bracket motor to go race on. That is a cam that we had laying around. We have used them in several engines in the past. They work well I'm the 400s

[Rick360]

At the moment I'm working on a 380 inch sbc. 4.165x3.48. Heads are the old dart iron eagle 215s. I've done some clean up work but haven't tried to burn the world down with flow numbers. They flowed 299 @ .700. Air speed on the bench was around 350. This is just a work horse bracket motor. Running the old comp 317 solid roller 282/292 @ .050 .704 lift 106 lsa 13.1:1 compression. I was just asking in general has anyone found where X airspeed in most cases will cause one to fall over. I figured there wouldn't be an exact number due to port shapes will determine the point at which airflow goes from laminar to turbulent. I know there's and old formula cu-in of 1 cyl/minimum CSA X RPM/360 will get you theoretical airspeed. In this case with this 380 it shows 385 ft/sec at 7500.

So which are you inquiring about? flowbench speeds or using your formula? I am unsure where that formula came from, but I don't think numbers from that would be comparable to actual engine airspeeds.

Rick

[Belgian1979]

Any idea what the choke speed is on a Dart alu pro 1 215 ?

[Brian P]

Just to touch again on the importance of stating a frame of reference any time you specify a velocity ...

I earlier stated mach 0.5 - 0.6 referenced to peak piston demand (at peak piston speed).

Someone then stated 400 fpm without stating how this was referenced and there was some subsequent discussions around this number give or take some.

If you make the ASSumption that this is referenced to the "nominal velocity averaged over a 180-degree intake stroke" then the peak piston demand (referenced to peak piston velocity) is very close to 3.14159.../2.0 = 1.57... times higher, which is neglecting the effect of the con-rod swing (rod length / stroke length ratio) but since this effect is small let's proceed to ignore it for the moment.

So 400 fpm average over a "nominal" 180 degree intake stroke corresponds to 628 fpm at the point of peak piston demand.

That's Mach 0.56 based on standard atmospheric conditions.

So despite using different numbers, it's possible that we are actually in agreement.

I state again ... don't state a velocity without stating a clear frame of reference for that velocity, because without doing so it is unclear and ambiguous.

[MaxFlow]

I would be more concerned with velocity profile than any particular velocity. The better the profile the higher the speed the port will tolerate.

The compromise is where the men and boys are defined. Do I compromise size to create a better profile or ?

[mag2555]

x 2 with max flow.

[twl]

Just to touch again on the importance of stating a frame of reference any time you specify a velocity ...

I earlier stated mach 0.5 - 0.6 referenced to peak piston demand (at peak piston speed).

Someone then stated 400 fpm without stating how this was referenced and there was some subsequent discussions around this number give or take some.

If you make the ASSumption that this is referenced to the "nominal velocity averaged over a 180-degree intake stroke" then the peak piston demand (referenced to peak piston velocity) is very close to 3.14159.../2.0 = 1.57... times higher, which is neglecting the effect of the con-rod swing (rod length / stroke length ratio) but since this effect is small let's proceed to ignore it for the moment.

So 400 fpm average over a "nominal" 180 degree intake stroke corresponds to 628 fpm at the point of peak piston demand.

That's Mach 0.56 based on standard atmospheric conditions.

So despite using different numbers, it's possible that we are actually in agreement.

I state again ... don't state a velocity without stating a clear frame of reference for that velocity, because without doing so it is unclear and ambiguous.

When I wrote my post using the 400 figure, I prefaced my statement by saying "using that formula" that Bowtienut(the OP) defined in his post, and I did that to give it context to the way that he was doing things and would be within his frame of reference for better understanding.

To put it into your context, yes it is .56 mach peak, and yes we are in agreement on that figure.

[BowtieNut]

At the moment I'm working on a 380 inch sbc. 4.165x3.48. Heads are the old dart iron eagle 215s. I've done some clean up work but haven't tried to burn the world down with flow numbers. They flowed 299 @ .700. Air speed on the bench was around 350. This is just a work horse bracket motor. (unning the old comp 317 solid roller 282/292 @ .050 .704 lift 106 lsa 13.1:1 compression. I was just asking in general has anyone found where X airspeed in most cases will cause one to fall over. I figured there wouldn't be an exact number due to port shapes will determine the point at which airflow goes from laminar to turbulent. I know there's and old formula cu-in of 1 cyl/minimum CSA X RPM/360 will get you theoretical airspeed. In this case with this 380 it shows 385 ft/sec at 7500.

So which are you inquiring about? flowbench speeds or using your formula? I am unsure where that formula came from, but I don't think numbers from that would be comparable to actual engine airspeeds.

Rick

Seems to me it would being swept volume cross sectional area and engine speed are the 3 main variables in the formula.

[Stan Weiss]

I see some confusion here (and many places on the net) between flowbench airspeeds and engine airspeeds. On the flowbench you can't tell if a port is too big or small for a given engine by measuring airspeeds. This will only tell you where the air is flowing in the port and how balanced or unbalanced it might be.

For airspeeds on an engine you can calculate air speed based on rpm, ci and size of the port. These calculations can be used to determine an rpm limit for that combo. The max speed of a port can vary greatly depending on the type of head and how well it is ported. The best ports can tolerate much higher speeds.

What type head/engine are you working on?

Rick

At the moment I'm working on a 380 inch sbc. 4.165x3.48. Heads are the old dart iron eagle 215s. I've done some clean up work but haven't tried to burn the world down with flow numbers. They flowed 299 @ .700. Air speed on the bench was around 350. This is just a work horse bracket motor. Running the old comp 317 solid roller 282/292 @ .050 .704 lift 106 lsa 13.1:1 compression. I was just asking in general has anyone found where X airspeed in most cases will cause one to fall over. I figured there wouldn't be an exact number due to port shapes will determine the point at which airflow goes from laminar to turbulent. I know there's and old formula cu-in of 1 cyl/minimum CSA X RPM/360 will get you theoretical airspeed. In this case with this 380 it shows 385 ft/sec at 7500.

Why done you try this calculator on my web site.

Calculate an Engine's Port Limiting Velocity or Minimum Port Cross Sectional Velocity

http://users.erols.com/srweiss/index.html#jcalc

Stan

[RAS]

Not that many people just flowing air through their heads. Most are flowing fuel and air. Air and fuel have weight and thus inertia. Turn that with velocity! Always flow test on a cold day with low humidity and low barometric pressure. The numbers are higher! All aside, I'd shoot for a great mid lift number (like a Kaase head design) and except everything else as the cost of doing business.

[user-9274568]

Not that many people just flowing air through their heads. Most are flowing fuel and air. Air and fuel have weight and thus inertia. Turn that with velocity! Always flow test on a cold day with low humidity and low barometric pressure. The numbers are higher! All aside, I'd shoot for a great mid lift number (like a Kaase head design) and except everything else as the cost of doing business.

Totally disagree, it's a myth with a orifice style bench.

[user-9274568]

I would be more concerned with velocity profile than any particular velocity. The better the profile the higher the speed the port will tolerate.

The compromise is where the men and boys are defined. Do I compromise size to create a better profile or ?

This is the truth..

The things I have learned with my 23º heads vs velocity profile vs pinch size, validates this statement.

There is a point and time where the velocity through any area is KING over that raw size.

IMO, the most overlooked and most important area in the head is the apex. Before it and after it. That velocity will define the head.

[Mike Kalbfeld]

Since the minimum cross sectional area in a port is often the throat and considering that the throat diamter should be some percentage of the intake valve diameter (I hear 91% quite often), I would imagine that one could calculate the required intake valve diameter as a function of the cylinder swept volume and desired max HP rpm. Does anyone do this with success?

After applying a little algebra to the formula (that is based on average piston velocity) I get the following:

Intake Valve Dia = 100/throat % * sqrt( (4/pi)*(CV/MPV)*(rpm/360))

Where:

throat % = the ratio of the throat diameter to the valve diameter (%) - 91% is often used
CV = Swept Cylinder Volume (in^3)
MPV = Max Port Velocity (ft/sec) - 400 ft/sec was suggested

I just thought this might be a useful tool to understand if the intake valve diameter is a governing restriction.

[Brian P]

Plugging in the numbers for an application that I know into the formula that you just gave, suggests that it's not far from the truth ...