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  #21  
Old 03-07-2012, 07:12 PM
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Originally Posted by hskr View Post
You are nuking it way too much. See my reply above. The drop in pressure on the back side makes for a better vacuum signal for the map and also more or less helps "pull" the air through the throttle body. If it was such a bad design to use a venturi then why are stock throttle bodies designed with a venturi effect? They would just use a straight metal tube with a simple valve in it.
There is a bit of misinterpretation of fluid dynamics in that.

Stock throttle bodies aren't designed with a venturi effect, they just happen to have one. It's due to the design requirements of having a single throttle body regulating air into the plenum rather than having an individual TB for each cylinder. As a result, a venturi effect simply happens because of the restriction followed by a large volume which allows the air to return to normal condition before being taken into each cylinder runner.

The TB is tapered to reduce turbulence so that the air inside the manifold is in a laminar flow rather than turbulent flow. The smaller TB compared to plenum is so that the individual runners can extract air from a common mass pressure within the plenum rather than differential pressures resulting from air entering a TB the same diameter as the entire plenum (nearly a 10" diameter TB) and traveling different distances to the cylinders with a varying velocity.

Your comment about the MAP sensor is very telling because it means that a modified TB creates an artificially low vacuum at the MAP sensor tricking the computer to think it has more airflow than it really does so it adds fuel. The same thing can be done by adjusting the A/F ratio with a predator.

By your explanation, the modified TB is little more than a trick to get the computer to change A/F ratio and has nothing to do with the venturi effect on the engine airflow other than it changes the vacuum pressure at the MAP. Similar to moving the IAT to the airbox so that it thinks the inlet air is cooler and more dense than it really is.

If that is the true explanation, then just get a predator and tune the engine properly with real data for maximum performance...for nearly the same price.
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  #22  
Old 03-07-2012, 07:18 PM
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Quote:
Originally Posted by Ddaddy View Post
There is a bit of misinterpretation of fluid dynamics in that.

Stock throttle bodies aren't designed with a venturi effect, they just happen to have one. It's due to the design requirements of having a single throttle body regulating air into the plenum rather than having an individual TB for each cylinder. As a result, a venturi effect simply happens because of the restriction followed by a large volume which allows the air to return to normal condition before being taken into each cylinder runner.

The TB is tapered to reduce turbulence so that the air inside the manifold is in a laminar flow rather than turbulent flow. The smaller TB compared to plenum is so that the individual runners can extract air from a common mass pressure within the plenum rather than differential pressures resulting from air entering a TB the same diameter as the entire plenum (nearly a 10" diameter TB) and traveling different distances to the cylinders with a varying velocity.

Your comment about the MAP sensor is very telling because it means that a modified TB creates an artificially low vacuum at the MAP sensor tricking the computer to think it has more airflow than it really does so it adds fuel. The same thing can be done by adjusting the A/F ratio with a predator.

By your explanation, the modified TB is little more than a trick to get the computer to change A/F ratio and has nothing to do with the venturi effect on the engine airflow other than it changes the vacuum pressure at the MAP. Similar to moving the IAT to the airbox so that it thinks the inlet air is cooler and more dense than it really is.

If that is the true explanation, then just get a predator and tune the engine properly with real data for maximum performance...for nearly the same price.
Uhh, no. If a ported throttle body is letting more air in, then the PCM will need to add more fuel for the increase air volume. So yes, you want the PCM to add more fuel to keep A/F ratios happy. I'd rather it be because the MAP sensors is properly reading the increased airflow(as shown by increased vacuum) than having to hook up a predator, data log, and hope I can get it right before I do any damage to the motor from making WOT runs running lean. You can use the predator to fine tune it though. And I really doubt the engineers just accidentally created a venturi effect with the TB when they designed them. But if that's what you want to say, then you are welcome to your own opinion.
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  #23  
Old 03-07-2012, 07:36 PM
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Quote:
Originally Posted by hskr View Post
Uhh, no. If a ported throttle body is letting more air in, then the PCM will need to add more fuel for the increase air volume. So yes, you want the PCM to add more fuel to keep A/F ratios happy. I'd rather it be because the MAP sensors is properly reading the increased airflow(as shown by increased vacuum) than having to hook up a predator, data log, and hope I can get it right before I do any damage to the motor from making WOT runs running lean. You can use the predator to fine tune it though. And I really doubt the engineers just accidentally created a venturi effect with the TB when they designed them. But if that's what you want to say, then you are welcome to your own opinion.
Uhh, yes. The TB can't let in more air than the engine is pulling, so a larger TB doesn't make any difference if the TB isn't restricitve to the maximum airflow...which the stock TB isn't.

If the PCM is adding more fuel, it's doing it since the vacuum (pressure) reading is telling it something is happening (that actually isn't) because there has been a change in the system that makes it appear that more air mass is flowing (when it actually isn't).

They didn't "accidentally" create a venturi effect, it just happens anytime you pull or push air through a restriction. It also happens moving through the valves into the combustion chambers since the valves are smaller than the cylinder volume. It's not a design, it's a fluid dynamic effect resulting from the fundamental physics. The "venturi effect" just describes the fluid dynamic behavior through the restriction and provides a mathematical explanation for the phenomenon.
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Old 03-07-2012, 07:42 PM
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I like kerosene. It burns much longer than gas.

Some more fuel for the fire.

http://en.wikipedia.org/wiki/Velocity_stack

There is more to it, and "venturi effect" isn't exactly what "it" is.
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Old 03-07-2012, 08:02 PM
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A velocity stack has to do with turbulence and wall effect. Venturi effect has to do with velocity/pressure of a fluid moving through a restriction. Both are related and there are mathematical explanations for the fluid dynamic behavior of each.

It is rather simple to understand the venturi effect. The velocity of air moving through a 3" opening will be greater than a 4" opening and the pressure will be lower to compensate. The total mass of air moving through each is the same. The 3" opening does not restrict airflow, it just changes the behavior of the air moving through it. The velocity stack simply changes the turbulence of the air entering the opening, smoothing it out.

The PCM has a set of parameters it uses to calculate mass airflow based on IAT and pressure at the MAP sensor based on the stock design of the system and it adds fuel accordingly. If you change the input readings by changing the system, it only knows it's original design parameters and adjusts A/F acording to what it thinks is happening.

Change the TB diameter or pressure behavior (tapering) and you change the pressure reading at the MAP and the PCM adjusts A/F according to it's original programming thinking there is more air flowing than there actually is. You are simply tricking the system into thinking something is happening that actually isn't.

Fluid dynamics aren't an "opinion".
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  #26  
Old 03-07-2012, 08:11 PM
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have to agree with ddaddy on this one...fluid dynamics is a hell of a beast few fully understand
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  #27  
Old 03-07-2012, 08:31 PM
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sorry guys but is there a write up on installing these i just got mine tonite
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  #28  
Old 03-07-2012, 08:40 PM
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sorry guys but is there a write up on installing these i just got mine tonite
Huh?

No one is trying to talk anyone out of getting one, just trying to understand how they seem to provide apparent gains.

I'm not sure how a write-up on installation adds anything to the discussion.

Perhaps you can clarify?
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  #29  
Old 03-07-2012, 10:40 PM
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Well, the best performing throttle bodies made for the 5.2/5.9L Magnum motors used in the Dodge trucks are made by a engineer who designed them using the fluid dynamics flow testing and venturi design. Maybe he should talk to ddady and he can improve his tried and true design. http://www.fbthrottlebodies.com/ Feel free to let him know his designs don't offer any improvement.
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  #30  
Old 03-08-2012, 02:42 AM
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My 52mm f&b on my 5.9 sure makes a difference. I actually wish f&b made HEMI throttle bodies, cause I would definitely have one on the Charger.

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