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78 monte 09-18-2002 09:46 PM

Carb spacer theory
 
If you got a cast iron dual plane intake and you put an open plenum spacer(Q-jet to holley) what will happen???

Monte's theory:You will lose low end torque cause of the open plenum and still not gain upper rpm power cause of the dual plane low rise intake.

Correct???

4 Jaw????

deuce_454 09-18-2002 10:00 PM

i think you are correct monte, the only advantage you might have is a less heat to the carb gaining you about one half of a sleepy pony, and mabe the other half in the top end from more plenum volume, and when you were done fittng your spacer you would find that hood clearance was nonexistent from the dent in your newly painted hood hood from the carb stud.

4 Jaw Chuck 09-18-2002 10:35 PM

You end up losing all around because of the velocity drop, a four hole spacer that extends to the manifold carb flange is the best option. You might think that you could have the best of both worlds with this setup but because of the restriction caused by the dual plane design you never get the gains of an open plane design and end up losing bottom end because of the velocity drop in the spacer...it actually will cause fuel drop out on some installations because of this. One thing I have learned over the years is to avoid changes in velocity in the intake system because when fuel is mixed at the carb any change in velocity (slower) can cause fuel drop out. the exception is when there is a temp change, hotter air/fuel can handle a velocity change better than a cool charge can. Sometimes a HP increase (mild) can be seen on a dyno but it almost always is accompanied with a loss in torque at lower speeds. The end result is less area under the curve which means less total power and a weaker engine overall. Abrupt changes in velocity must be kept to a minimum no matter what in the intake system particularily with carburators. Some times a poorly designed manifold with bad distribution can be helped by this type of setup but these days the manifolds are so good it does not apply anymore.

KULTULZ 09-19-2002 02:56 AM

Spaces and Plenum Dividers

Spacers and plenum dividers provide an easy way to change the configuration of the intake tract and the relationship it has with the carburetor.

Adding a plenum divider to an open plenum manifold will help keep the left to right fuel distribution balanced for oval track applications when the car is in the corners. This is especially helpful on alcohol engines. These dividers usually do not have a measured affect on torque or horsepower.

Using spacers between the carburetor and intake manifold can produce dramatic results. The use of a four hole spacer can improve low end to mid-range by helping the carburetor draw and atomize fuel. An open center spacer increases the plenum area and can benefit the mid-range and upper rpm power. It is not uncommon to see combinations of spacer types or stacking of similar type spacers being used. It is also not uncommon to see results that are totally opposite from the general tendencies.

The actual results from any spacer or combination of spacers can only be measured during a test and tune session on the specific engine combination being run. This information can be a very useful tool when tuning to find the best horsepower or to change the power characteristics to suit a specific track condition.

Excerpt from -Barry Grant - Basic Carburetor And Fuel System Maintenance And Tuning-

slider in wa 09-19-2002 04:13 AM

So , if i understand correctly , my 7101 open plenum Eldbrock intake is all wrong for my street driven 350 eng? I run a 30/30 Duntov cam, 202 heads,and a small 600 cfm Eldbrock carb. I got the intake new ,never used in a trade. So i decided to use it. This could be hurting my low end toqure? I use a ton of gas in the truck.
Thanks
Slider in Wa.

KULTULZ 09-19-2002 04:53 AM

EDELBROCK 7101 Open Plenum?

The 7101 is a dual plain with separate plenums.

This intake should be perfect for hi-performance street use.

NAIRB 09-19-2002 06:02 PM

Using an open spacer on a dual plane manifold, will defeat the design of the dual plane manifold. Obviously, a dual plane manifold splits the carburetor in half and allows increased velocity at the carburetor's venturi booster, especially at lower engine speeds, the result of this is better low speed driveability, and improved low and midrange torque, the trade-off is less top end horsepower, as each cylinder cannot draw of the entire carburetor at high engine speeds, adding an open spacer to a dual plane manifold is kind of like putting a band aid fix on the whole deal. Just as Four Jaw said, today's purpose designed manifolds are so good, this type of thing is not necessary, unless of course you are restricted to some kind of factory type manifold, where I have seen the plenum divider removed, but most ot the time this is on a circle burner, that never drops below 3500 RPM. However, spacers are a great tuning tool, allowing you to adjust plenum volumes, or to straighten airflow when necessary.

NAIRB 09-19-2002 06:04 PM

Using an open spacer on a dual plane manifold, will defeat the design of the dual plane manifold. Obviously, a dual plane manifold splits the carburetor in half and allows increased velocity at the carburetor's venturi booster, especially at lower engine speeds, the result of this is better low speed driveability, and improved low and midrange torque, the trade-off is less top end horsepower, as each cylinder cannot draw of the entire carburetor at high engine speeds, adding an open spacer to a dual plane manifold is kind of like putting a band aid fix on the whole deal. Just as Four Jaw said, today's purpose designed manifolds are so good, this type of thing is not necessary, unless of course you are restricted to some kind of factory type manifold, where I have seen the plenum divider removed, but most ot the time this is on a circle burner, that never drops below 3500 RPM. However, spacers are a great tuning tool, allowing you to adjust plenum volumes, or to straighten airflow when necessary.


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