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Discussion Starter #1 (Edited)
To big power valve / to small carb??

I was checking my vacum today at idle.

It was ~6 inch Hg at 700 rpm, and ~7 inch Hg at 1000 rpm (both with the first gear in, and holding the car with the brake)

The powervalve I have is marked 4.5
Should I change to a smaller one or not?

Carb is a Holley 650 dp (with milled choke horn).
 

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I would say your power valve is about correct. I run similar vacuum on my 385 with a 6.5 PV. Now mine is a 5 speed so my idle is around 900 so my vacuum is a little better about 8 inches at best. I really think your carb is to small. A 650 on a high winding 383 is not going to be big enough. I run an 825 Mighty Demon and feel it is just about right. I would not want anything smaller.
 

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Discussion Starter #3
Ok!
I was affraid that my pv was a bit on the large side.

Maby I should borrow a bigger carb and do some testing next time I go to the track.
 

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I was checking my vacum today at idle.

It was ~6 inch Hg at 700 rpm, and ~7 inch Hg at 1000 rpm (both with the first gear in, and holding the car with the brake)

The powervalve I have is marked 4.5
Should I change to a smaller one or not?

Carb is a Holley 650 dp (with milled choke horn).
You want the power valve to be stamped with a smaller number than your expected low vacuum reading. A typical "stock" Holley power valve is usually a 10.5, so if you had one of those, you'd be dumping fuel in all the time.

If the choke horn was properly milled, you probably picked up about 50 CFM, so I'd think you're probably OK on carb size.

It's generally much easier to over carb than to under carb.

Steven
 

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You want the power valve to be stamped with a smaller number than your expected low vacuum reading. A typical "stock" Holley power valve is usually a 10.5, so if you had one of those, you'd be dumping fuel in all the time.
Steven

It is a misconception to think a power valve dumps fuel in the motor. Other then rupturing it doesn't affect the idle or low speed running mixture.
The power valve is just another path around the main jet. When it opens the fuel just has another path around the main jet.
At idle the area behind the main jet, not the cavity that controls the power valve, the main jet curcuit is full of fuel that came through the main jet. This area is full at idle so a power valve adds nothing to it.
There is NO connection between the power valve and your idle circuit.
Other then a ruptured diaghram??? where the vacuum sucks fuel into the carb the power valve doesn't affect idle.
Your idle circuit takes you to about 2200-2500 while cruising and only running about 3000 can your main jets control the cruising mixture.
 

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It is a misconception to think a power valve dumps fuel in the motor. Other then rupturing it doesn't affect the idle or low speed running mixture.
The power valve is just another path around the main jet. When it opens the fuel just has another path around the main jet.
At idle the area behind the main jet, not the cavity that controls the power valve, the main jet curcuit is full of fuel that came through the main jet. This area is full at idle so a power valve adds nothing to it.
There is NO connection between the power valve and your idle circuit.
Other then a ruptured diaghram??? where the vacuum sucks fuel into the carb the power valve doesn't affect idle.
Your idle circuit takes you to about 2200-2500 while cruising and only running about 3000 can your main jets control the cruising mixture.
that's not entirely correct. i have done testing on my 406 with an oxygen sensor. i can get my idle circuit all the way to 4000 rpm. the transition between idle and main circuits is determined by two things, engine load and throttle position. both effect vacuum. if your vacuum is high, the idle circuit will be dominant in the final mixture. low vacuum and the main circuit takes over. it's kind of a ballet of supply. the power valve bypasses both circuits and dumps fuel into the mix at vacuum levels lower than the stamped rating. using a vac gauge is the best way to determine the propper valve needed. at idle in gear for an automatic is a great place to start, but don't forget to run the car down the road while monitoring the gauge. there may be times that vacuum drops low enough to open the valve when you may not want it. also the vac gauge is the best way to test the size carb needed for your motor. you want the smallest carb that will not build any intake vacuum at wide open throttle. if you start building more than 1 inch vac, it is time to buy a bigger carb.
 

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It is a misconception to think a power valve dumps fuel in the motor. Other then rupturing it doesn't affect the idle or low speed running mixture.
Let's assume that you have a 10.5" power valve installed in your Holley.

If your engine idle vacuum is greater than 10.5", then you're correct - the power valve will be closed, and it won't affect the idle.

If your engine idle vacuum is less than 10.5", the power valve will open, and the power circuit will feed fuel through the power circuit orifice into the intake manifold (which is what I meant when I said "dumps").

The power valve is just another path around the main jet. When it opens the fuel just has another path around the main jet.
That's right - the power circuit supplements the main jet circuit. If there was no power valve circuit, you'd have to jet up between 8 - 10 jet sizes to compensate.

At idle the area behind the main jet, not the cavity that controls the power valve, the main jet curcuit is full of fuel that came through the main jet. This area is full at idle so a power valve adds nothing to it.
I'm not quite sure what you mean by that.

There is NO connection between the power valve and your idle circuit.
Other then a ruptured diaghram??? where the vacuum sucks fuel into the carb the power valve doesn't affect idle.
I didn't say there was - however, if idle vacuum is lower than the power valve set point, the power circuit WILL add fuel to the engine through the power circuit orifice independantly of the idle circuit.

Your idle circuit takes you to about 2200-2500 while cruising and only running about 3000 can your main jets control the cruising mixture.
That statements sounds a bit too generalized to me. The actual progression from the idle circuit to the main circuit isn't as clear cut as you imply.

Assume engine RPM increases slowly and gradually, thereby minimizing the effect of the accelerator pump circuit as well as the power valve circuit (just for clarity's sake). The main circuit will be enabled when sufficient airflow past the venturi's provide a large enough pressure differential to overcome the resting inertia of the fuel. The fuel will then begin to flow through the venturi's and into the intake. The idle circuit will continue to supply some fuel to the intake, but as engine RPM continues to climb, then it supplies less and less fuel relative to that supplied by the main circuit (the idle circuit is severely limited in its flow capacity, so it can't compete with the main circuit). Additionally, as air flow through the venturi's increases, the differential flow around the idle circuit orifices tends toward zero, further limiting the flow from the idle circuit. As the throttle valves continue to open, those two flow phenomena continue to diverge until the main circuit is supplying essentially all the fuel required by the engine, and the idle circuit is supplying essentially none of the fuel required by the engine.

At idle, the opposite happens. There is very little air flow past the venturi's, and the nearly closed throttle plates cause a high air flow condition near the idle circuit orifices, creating sufficient differential pressure to cause fuel to flow past the idle circuit orifices.

Under rapid engine RPM changes, the accelerator pump circuit and the power valve circuit assist in adding more fuel to the intake to compensate for the dramatic change in pressure values in the carburetor venturi's, giving the main circuit a chance to establish sufficient flow to supply the fuel to the engine.

I'm not real sure if we're that far apart, but I do know that an improperly sized power valve will contribute to poor idle quality (and a host of other problems). The original poster said he had a 4.5" power valve, and reported measuring 6" at 700 RPM (which I'm assuming to be idle). In this case, the power valve shouldn't have any adverse effect on his engine. All I was trying to point out was that IF his carburetor had a more standard 10.5" power valve, THEN it would have an effect on his performance (by dumping fuel through the power circuit and into the engine).

Steven
 

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Discussion Starter #10
you want the smallest carb that will not build any intake vacuum at wide open throttle. if you start building more than 1 inch vac, it is time to buy a bigger carb.
So if using this method I can found out if my carb is to small, is this a common method to check carb size?
Newer heard of it before!
 

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So if using this method I can found out if my carb is to small, is this a common method to check carb size?
Newer heard of it before!
It's kind of a variation of the SAE standard method for verifying a carburetor's flow rate. A 650 CFM carb is tested on a flow bench, at STP (Standard Temperature & Pressure; typically 68F and Sea Level), using dry air. A pressure drop of 1" is then used to measure the flow rate of the carburetor.

I've also heard that 3" at WOT throttle is sufficient for a street engine, so I suspect you'll have to just go out and mesure what your car is doing.

I have wondered why your engine vacuum is so low at idle - what are the current engine mods that you have?

Steven
 

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I have a sectioned holley sitting in front of me. There is NO connection between the idle circuit and the power valve.
The main jets feed a hole behind them that leads to the venturies. As the air passes through the throttles gas is sucked through the venturies and into the motor. Without air flow no gas is sucked into the motor.
It requires air flow to suck this gas from the main jet circuit.
The power valve is just another path that gas can take bypassing the main jets. This main jet circuit is full of gas all the time while on idle and low speed running. Weather the power valve is open or closed makes no difference.
If the car is idling air flow is low so no gas is sucked through the mains and the circuit remains full.
Cruising at low speeds still doesn't increase the air flow enough for the main jets to add to the mixture so the car cruises on the idle circuit, still the power valve open or closed still doesn't add fuel to this condition since air flow over the venturi's isn't high enough to create a vacuum that sucks the fuel out of the main jets.
There is no exact point where the main jets take over and contribute fuel to the mix but it is somewhere in the 2200 - whatever range but by 3000 most setups are on the main jets.
I too run a LM-1 wide band oxygen sensor and by 3000 I am on the mains.
Air bleeds can be used to tune where the mains come in but most of us just accept what is in our carbs already.
So if you are idleing and the power valve keeps opening and closing due to poor vaccum it doesn't change the idle mixture.
The power valve only comes into play when the main jets are flowing fuel.
A ruptured power valve is another thing. Once the diaghram??? ruptures raw fuel is sucked into the motor by it's own vacuum and the mixture is totally destroyed.
 

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A power valve has a cavity right behind it that vacuum from the motor acts on. The power vavle is drawn closed by this vacuum and a spring tries to open the valve. It the vacuum is above a certain value the spring is compressed and the valve is closed.
When the vacuum drops the valve opens and there is a metered hole in the jet block that is uncovered allowing fuel to flow through this reamed hole added more fuel then the main jets flow. You have 2 sources of fuel flowing to the venturi.
Shut the motor off and what happens? The power valve opens allowing fuel to flow through this reamed hole but since the area behind the main jet is already full of fuel on fuel flows through the power valve.
The fuel level in the float bowl is equal to the fuel level behind the main jet and the power valve.
When you start the motor the power valve is open but closes due to vacuum and you do not pump this fuel dry from behind the main jet, the fuel remains at the height of the float level behind the main jet waiting for the venturi air flow to start sucking it out.
If there is NO connection between the power valve and idle circuit and we know the main jets are not flowing fuel because air flow is too slow over the venturi how does this power valve fuel get to the idle circuit to ruin the idle mixture?????
 

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Your idle circuit takes you to about 2200-2500 while cruising and only running about 3000 can your main jets control the cruising mixt

No there is no clear cut point where I can say the main circuit has taken over or rather adding fuel to the mixture but using a wide band oxygen sensor you can find the exact point but so what?? Why do you need it.
With normal 60 mph cruising and road friendly gears you are cruising on the idle circuit. I have to downshift a gear and bring the rpm to 3000 before I can read the main jet's with my LM-1 sensor. I can't read it at say 2200 rpm.
If someone wants to find this cut off point you need the wide band oxygen sensor. Jet the car say 4 or 6 sizes too rich. This will give a reading in the 12 range while cruising. I am also assuming you set the idle mixture at about 14-1.
Bring the rpm to about 4000, might need to downshift alot to keep the speeds reasonable. Slowly decrease the rpm watching the mixture ratio. It will be very rich at first, in the 12 range but slowly you will see a change, the mixture climbs towards the 14-1 ratio since you are dropping back to the idle circuit. There is a crossover point where you rich mixture created by the over size mains turns into the lean idle circuit.
You can not just drop rpm and coast down. You need to drop say 250 rpm, cruise at a steady state for say 10 sections watching the reading, drop agian 250, cruise at a steady state but you will find the crossover point.
So you find this point??? What are you going to do with it???
It can be changed but you need to replace, modify the air bleeds??
I set by car for 13.8-14 all the time except under full throttle where I want it about 13-1. I know 12.5 is safer but maximum power is still in the 13 range but more chance of detonation.
This is getting out of the power valve topic and more into the wide band oxygen sensor. Sorry.:thumbsup:
 

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You actually want the power valve to be 2 points under your reading of ~7. But then you might get to where it is not opening soon enough.

As to carb size requirements. You have to have your whole system setup to use a bigger carb. The intake manifold is the first point of restriction. High HP motors can use a smaller CFM carb on a single plane intake manifold because of the plenum volume and the fact that each cylinder is drawing off all 4 barrels. I recently went to one of the biggest volume single planes made "Motown" and my 434 is not lacking for carb CFM.

My 383 like your 388 depending on you built it and how lucky you feel is basically limited to @7500 rpm. So i used a Weiand "Team G" smaller plenum single plane and my 383 would just power into the rev limiter.

If I was going to make your Vette faster. I would ensure that the motor could breath. So forget the carb and look at correctly sizing the intake, heads, headers, exhaust system.

You really need a full three inch exhaust and probably up the TQ stall. I ran generally 3500 stall 9.5 inchers.
 

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There are all kinds of recommendations, including those from Holley and Smokey etc.

After running with a vacuum gauge, I usually always use a 2.5 PV that opens under very heavy load and/or 3/4 to WOT.

I found it suits my driving style.

Drive with a vacuum guage, great tool.:D
 

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Discussion Starter #19
My 383 like your 388 depending on you built it and how lucky you feel is basically limited to @7500 rpm. So i used a Weiand "Team G" smaller plenum single plane and my 383 would just power into the rev limiter.

If I was going to make your Vette faster. I would ensure that the motor could breath. So forget the carb and look at correctly sizing the intake, heads, headers, exhaust system.

You really need a full three inch exhaust and probably up the TQ stall. I ran generally 3500 stall 9.5 inchers.
What is the pros/cons with a singelplane like Team G make, comparing to my dualplane PRM?

I have 2500 stall, don't know anything about the converter (except that it's color is dark green). I know, I should have taken a closer look when the engine was out, but I didn't.

If checking my narrow band lambda when driving, the air /fuel mixture seems to be to fat.
I have changed the jets to 64/75, but the mixture is still to rich.
Should I go down even more ??
Or can this indicate that my power valve is to big?
 

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What is the pros/cons with a singelplane like Team G make, comparing to my dualplane PRM?

I have 2500 stall, don't know anything about the converter (except that it's color is dark green). I know, I should have taken a closer look when the engine was out, but I didn't.

If checking my narrow band lambda when driving, the air /fuel mixture seems to be to fat.
I have changed the jets to 64/75, but the mixture is still to rich.
Should I go down even more ??
Or can this indicate that my power valve is to big?
Let me ask you a question what is the rated rpm range of your dual plane RPM intake. Also remember this rating is for a your typical 350 cubic inch motor. Like if a manifold is rated for 2000-6500 doesn't me that it shuts off at 6500 it means that they are just past where they function well. This achived at a lower rpm as ci goes up

I have not used a dual plane in my Vette for about 20 years. The team "G" shortest single is rated up to 7200 rpm. The higher RPM models are just taller. I used a wood open spacer to raise the plenum volum and be a thermal blocker.

Cruising down the road should be high vacuum. I would lower the primary jetting. Also you need to keep the secondary jetting @ 8-10 sizes from the primary
 
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