Lean effect
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Cargo Master
Joined: Aug 2007
Posts: 2,561
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From: New Zealand
Lean effect
I understand the benefits of cold air for power/ efficiency.
But on a carbed engine, what is there to prevent a leaning-out as vacuum drops in the venturi due to the ram-effect of higher road speeds?
But on a carbed engine, what is there to prevent a leaning-out as vacuum drops in the venturi due to the ram-effect of higher road speeds?
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
Power valves and metering rods. On a Holley or Motorcraft carb, a power valve at the bottom of the fuel bowl is held shut by high vacuum. Once vacuum drops below a threshold, the valve opens and effectively increases the inner diameter of the jets. Metering rods operate differently, but essentially are used to control the amount of fuel that is added to the main fuel circuit as the throttle plates open. Edelbrock carbs typically use metering rods. Metering rod carbs are typically easier to tune because they can often be changed without removing the carburetor and dumping the fuel.
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Cargo Master
Joined: Aug 2007
Posts: 2,561
Likes: 69
From: New Zealand
Thanks fmc400 - I've often idly wondered about this.
So, can both systems provide full protection against engine damage, even at prolonged WOT running? Is either system better, other than the ease of tuning? Thanks again
So, can both systems provide full protection against engine damage, even at prolonged WOT running? Is either system better, other than the ease of tuning? Thanks again
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The purpose of both systems is to regulate the air\fuel mix at open throttle, as such, they can serve to protect the engine from heightened combustion temperatures caused by lean air\fuel mix.
There isn't a whole lot of difference between the two in terms of performance in my opinion. One thing that metering rods have over power valves is that an intake backfire can blow out a power valve. Carbs with metering rods do not suffer from this weakness. However, most later model Holley carbs have a check valve to protect the power valve from rupturing in the event of a backfire. Motocraft carbs don't have this feature, but the power valves tend not to blow out as much for those.
There isn't a whole lot of difference between the two in terms of performance in my opinion. One thing that metering rods have over power valves is that an intake backfire can blow out a power valve. Carbs with metering rods do not suffer from this weakness. However, most later model Holley carbs have a check valve to protect the power valve from rupturing in the event of a backfire. Motocraft carbs don't have this feature, but the power valves tend not to blow out as much for those.
Elder User
Joined: Dec 2007
Posts: 842
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From: royal oak, mich
Carbs are calibrated at the factory without anything on them like a filter. The inlet system causes some restriction so any ram effect will help. A carb is adding fuel as it senses additional air coming in.
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
I don't follow that logic. Factory carburetors are intended to be calibrated with the air filter installed and that is specificed on most emissions stickers by the factory. The carburetor must be calibrated based on the environment in which it will be running; that's why it is always stated by the manufacturer to make settings with the filter on, with the transmission in gear, etc. A carb does add fuel based on the air it "senses" coming in, but it can only do so much based on the restrictions in its metering system, as definied by its jets and power valve vacuum\metering rod size. That is why all of the aforementioned items are sold in a plethora of sizes to match the wide range of air flow based on many factors such as engine displacement, maximum RPM, and altitude.
Elder User
Joined: Dec 2007
Posts: 842
Likes: 0
From: royal oak, mich
The only thing that I put on a carb was an air scoop for a Chrysler project. The emissions runs had everything hooked up. Both tests were done in a controlled environment. I'm glad they've put removable bleeds and feeds in them now. Those things were awful to change when you got them too big.
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