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Old 01-07-2008, 02:16 PM
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Vacuum Vs. Mechanical Secondaries

I'm stick on deciding which type of carb to go with. The vacuum secondaries will have better gas mileage, but the mechanical secondaries will have more power, right? Well anyway, if this is true how much more power over gas mileage will i have? I'm looking for a 650 cfm that will go on a 383.

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Old 01-07-2008, 02:21 PM
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There will be no power difference as long as the vacuum secondary is properly tuned and opens fully at w.o.t. A double pumper is a great race carb when the right size is selected they are usually better suited to light manual transmission vehicles.
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Old 01-07-2008, 02:23 PM
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But will i notice a huge gas mileage difference?
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Old 01-07-2008, 02:52 PM
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No, mileage will depend on how well you tune the primary metering which is the same between either type.
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Old 01-08-2008, 05:58 AM
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If milage + performance is the goal a Q-jet is hard if not impossible to beat.
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Old 01-08-2008, 06:00 AM
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Mechanical secondaries do not belong on a street driven car unless you own a gas station. There is no performance gain from running mech secondaries on a street engine, quite the contrary in fact.

Vince
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Old 01-08-2008, 06:12 AM
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Mighty

What type of transmission are you running in your car?
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Old 01-08-2008, 09:29 AM
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Years ago (early 80's)I ran a 30 over 327 with 292turbo casting heads 11-1 comp and a holley 750dp which was my "daily driver". And for the sake of saving gas I switched and ran with a 600 vac sec. The diff in mileage was 1mpg. I ended up switching back for the performance factor and tuning able. I eventually then went bbc and was not concerned as much (obviuosly) with economy. On the holley dp the secondary side starts to open at about 1/3 throttle with there standard link bar. If you went 1to1 when the pri and the sec causing them to open at the same time you would certainly keep opec happy. My friends run in super street,super gas,super comp they launch or leave against the limiter so the accelerator pumps are used up because there foot is already to the floor so they are working off the mains.
Here is a interesting read http://www.classictrucks.com/tech/cl...tor/index.html

And as always your results might vary
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Old 01-08-2008, 03:34 PM
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I'm runnin a 379 with a TH 350.
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Old 01-08-2008, 05:32 PM
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Unless you're running a loose converter you're going to get better drivability with a vacuum secondary carburetor.
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Old 07-08-2008, 09:18 PM
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there is always both... no body seemed to mention this, you could buy vacuum seconday carb like the holle 4160 and bend the linkage to make it shorter betwen primaries and secondaries to make it partially mechanical, u can have the secondaries open sooner than the vacuum would if you want more performance or take the linkage out if ur looking for economy and let the vacuum do its job
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Old 07-09-2008, 10:03 AM
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Quote:
Originally Posted by 000882334
there is always both... no body seemed to mention this, you could buy vacuum seconday carb like the holle 4160 and bend the linkage to make it shorter betwen primaries and secondaries to make it partially mechanical, u can have the secondaries open sooner than the vacuum would if you want more performance or take the linkage out if ur looking for economy and let the vacuum do its job



Actually this is a really bad idea....

First, you won't have any secondary accelerator pump shot when you manually pull the rear butterfly's open which can cause drivability issues.

Second, and much more important is that the secondary link on a vacuum secondary carburetor is designed to physically close the secondary butterfly's when you take your foot off of the throttle. If you shortened the link up the secondaries could hang causing all sorts of havoc.
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Old 07-09-2008, 11:10 AM
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383 cubes worth of TQ + 3.79 gears = for normal driving mpg you "should" be operating 100% on the primaries only with a darn good rate of acceleration....

strictly depends on how well you can control your right foot....with either carb type

a simple way to "not" get into vacuum secondaries is mount a vacuum guage on your dash...control your foot and don't let the hg fall below about 10 when accelerating

a simple way to "not" get into mechanical linkage secondaries is rig a second spring on the linkage that adds pedal resistance at the point that the linkage is about to start moving to remind your right foot....

LOL, 90%+ of my driving is in town stop and go....I have both on my car
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Old 07-09-2008, 11:38 AM
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Quote:
Originally Posted by CarbGuy


Actually this is a really bad idea....

First, you won't have any secondary accelerator pump shot when you manually pull the rear butterfly's open which can cause drivability issues.

Second, and much more important is that the secondary link on a vacuum secondary carburetor is designed to physically close the secondary butterfly's when you take your foot off of the throttle. If you shortened the link up the secondaries could hang causing all sorts of havoc.

im sure your right, but mine seem to be working pretty well the way it is, the only thing is like a half second delay of the secondaries closing, which i have no complaints about
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Old 07-09-2008, 11:41 AM
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Quote:
Originally Posted by 000882334
im sure your right, but mine seem to be working pretty well the way it is, the only thing is like a half second delay of the secondaries closing, which i have no complaints about

Great way to get dead in an emergency...
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