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Intake port air speed.

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Intake port air speed.

Postby BowtieNut » Fri Apr 25, 2014 10:20 am

Does anyone have a general idea of about what point the airspeed in a intake runner gets to high and makes the power lay over. I know that's a bit of a weighty question. I'm sure it depends on where the maxim airspeed happens in a port. Example.. I'm sure airspeed being the highest at the pushrod pinch would be less detrimental than being at the top of the short side. Though I have seen cases where turbulence upstream at the pinch upset the air at the turn. Just a general where abouts is what im looking for.
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Re: Intake port air speed.

Postby Lockwire » Fri Apr 25, 2014 12:27 pm

That will depend on the cylinder head type and if the port job is correct.Air doe;s not like corners necessarily.
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Re: Intake port air speed.

Postby Brian P » Fri Apr 25, 2014 3:17 pm

I've heard (and believe) 0.5 - 0.6 Mach based on instantaneous peak demand from the piston at peak piston speed, and that is an important clarification.

This is not the air speed that you measure on a flow bench with a fixed (low) depression, and it's not the nominal average over the whole stroke.

If the intake port has turns in it or the shape of the port otherwise doesn't lend itself to recovering some of that velocity into pressure as it flows towards and through the valve, it's going to start dropping off well before that. I would expect that a number in that range is only going to work for an engine in which you can look straight down the port and see almost the entire valve(s) ... No swirl ports, no 90-degree turns, no jogs around pushrods, etc.
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Re: Intake port air speed.

Postby BrazilianZ28Camaro » Fri Apr 25, 2014 8:15 pm

BowtieNut wrote:Does anyone have a general idea of about what point the airspeed in a intake runner gets to high and makes the power lay over. I know that's a bit of a weighty question. I'm sure it depends on where the maxim airspeed happens in a port. Example.. I'm sure airspeed being the highest at the pushrod pinch would be less detrimental than being at the top of the short side. Though I have seen cases where turbulence upstream at the pinch upset the air at the turn. Just a general where abouts is what im looking for.



At 400 fps the air flow is choked into the port.
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Re: Intake port air speed.

Postby randy331 » Fri Apr 25, 2014 10:51 pm

BrazilianZ28Camaro wrote:At 400 fps the air flow is choked into the port.

????? :?:

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Re: Intake port air speed.

Postby wil8115 » Sat Apr 26, 2014 12:45 am

BrazilianZ28Camaro wrote:
BowtieNut wrote:Does anyone have a general idea of about what point the airspeed in a intake runner gets to high and makes the power lay over. I know that's a bit of a weighty question. I'm sure it depends on where the maxim airspeed happens in a port. Example.. I'm sure airspeed being the highest at the pushrod pinch would be less detrimental than being at the top of the short side. Though I have seen cases where turbulence upstream at the pinch upset the air at the turn. Just a general where abouts is what im looking for.



At 400 fps the air flow is choked into the port.



How do you figure that?

speed of sound at sea level is 1125ft/s.
(.5) is 562ft/s / (.6) is 675ft/s. both much faster then 400fps.
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Re: Intake port air speed.

Postby mag2555 » Sat Apr 26, 2014 6:52 am

For a race only motor with a 2000 rpm wide power band I use a number of 375 ft per sec max anywhere in the port.
You should never see a higher pressure appear than what you are pulling thru the bench except at low lift through the flow window formed by the valve and seat.
If you are talking about air speeds around the push rod bulge in a 23 degree bow tie head what needs to be done is to grind the window at that point wider at the top if you are going for flow numbers greater than 270 cfm@ 28".
On street strip motors that do not use race rear gearing and spend most of there time in street mode, port air speed numbers of over 400 fps make for far better throttle response and tuning ease!
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Re: Intake port air speed.

Postby digger » Sat Apr 26, 2014 8:00 am

mag2555 wrote:For a race only motor with a 2000 rpm wide power band I use a number of 375 ft per sec max anywhere in the port.
You should never see a higher pressure appear than what you are pulling thru the bench except at low lift through the flow window formed by the valve and seat.
If you are talking about air speeds around the push rod bulge in a 23 degree bow tie head what needs to be done is to grind the window at that point wider at the top if you are going for flow numbers greater than 270 cfm@ 28".
On street strip motors that do not use race rear gearing and spend most of there time in street mode, port air speed numbers of over 400 fps make for far better throttle response and tuning ease!


60" depression at peak valve lift is not unrealistic
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Re: Intake port air speed.

Postby Rick360 » Sat Apr 26, 2014 9:08 am

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?

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Re: Intake port air speed.

Postby cspeier » Sat Apr 26, 2014 9:22 am

The key is to measure the air speed at the choke at the lift point of max piston speed. Then the experience tells you how to correlate that data to a running engine. Then the data is all based on 100%VE. What happens when your VE is at 120%? That is the confusion...
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Re: Intake port air speed.

Postby BrazilianZ28Camaro » Sat Apr 26, 2014 8:43 pm

randy331 wrote:
BrazilianZ28Camaro wrote:At 400 fps the air flow is choked into the port.

????? :?:

Randy



Sorry, I should be more specifc:

On hi performance engines air speeds in excess of 400ft/s on the choke point will loose pk hp and the rpm band after hp pk will nose over sooner. This is port shape dependant.

Hope this helps.
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Re: Intake port air speed.

Postby randy331 » Sat Apr 26, 2014 10:26 pm

BrazilianZ28Camaro wrote:Sorry, I should be more specifc:

On hi performance engines air speeds in excess of 400ft/s on the choke point will loose pk hp and the rpm band after hp pk will nose over sooner. This is port shape dependant.


You have tested this your self and found the 400fps to be a valid limit on port speed? Over 400 fps and the power has nose dived on the engines you've dynoed ?

Is this a locally measured speed, or a speed average for the "choke point" ?

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Re: Intake port air speed.

Postby mag2555 » Sun Apr 27, 2014 7:18 am

Sorry,I wanted too say grinding the push rod bulge area out to a trapizoidal shape.
You can do the math that will tell you at what rpm your motor will reach the .60 sonic choke point, but just good old spending a lot of time velocity probing and mapping the whole port progressing in half inch steps length wise down the port from the flange and to the first bottom cut of the valve job is the eye opener in a lot of cases,as is having a good working port and slapping a intake manifold on and having all by in large go to hell!
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Re: Intake port air speed.

Postby BowtieNut » Sun Apr 27, 2014 7:42 am

Rick360 wrote: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.
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Re: Intake port air speed.

Postby twl » Sun Apr 27, 2014 8:30 am

BowtieNut wrote:
Rick360 wrote: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.


Using that formula, but in a different engine design, we find 400f/s avg to be the approximate vicinity of peak hp, and begins to decline above that.
Not sure if that will be of help with your engine, but those are our observations with ours.
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