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#1
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carb selection vac or mech secondaries
What's the deciding factor to go vacuum secondaries or mechanical secondaries? I've always stuck with mechanical and never had any problems. Whats your thoughts?
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#2
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On the drag strip...with the proper gearing and converter..a mech secondary should be faster..
On the street...depends on gearing and converter...weight of car...in other words how fast does the rpm on the engine come up.....if you got 2.76 gears...still with a vacuum secondaries... |
#3
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Mechanical secondaries all the way. Really fed up with vacuum operated secondaries on Holleys. They stick, then pop-open and bog, or they open too slowly and never open enough. Go ahead and play with all the Holley springs. You'll never be happy with any of them, if you're fussy about carb tuning. Air valve secondaries are good (Carters, Edelbrocks, Rochesters) and provide extra tunability, particularly AVS carbs. Just don't go too big with double-pumpers. A 650 cfm is good for most small blocks and a 750 cfm is good for most big blocks, except really "raced-out" engines. On GM products a 750~800 cfm Rochester is very hard to beat. Lot's of Pontiac guys succesfully race 400's and 455's with Rochesters. I've never run a Thermoquad. The only ones that ever show up in junk yards are pretty much trashed beyond repair. My opinion,
Mark H. |
#4
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I have good luck with T-Q's myself. Holleys are a bit tempermental IMO though I had great results with my 4150 DP 600 on a 318.
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#5
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Vac. secondarys will absorb a over-carb situation. Mec. secondarys need to be selected more accurately to the size and tune of the engine. That`s why they are built on a 50 cfm increment vs. only a few vac. sec. sizes. That doesn`t mean vacs. don`t run. A buddy of mine runs 10.90-11.20`s with a 3310 holley on a 454 chebbie in a `69 camaro.
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#6
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i run both a 750 Dp and a 750 Vacuum carb at the race track..both have the ProForm main body upgrade.
the vacuum carb even has a Quick Fuel Technology adjustable vacuum secondary cannister...has a needle and seat to adjust the opening.. based on my experience the DP is usually about .15 faster then the vacuum carb...but one thing i like about the vacuum carb is once i set the tranny brake and come up on the two step (3600 rpm)..it is only running on the primaries..and after it launches (5000 rpm) the secondaries will open.... |
#7
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We had a 3310 in our 4 door, and once tried a dominator instead. Sun just the same with both. Vacuums can be made run well and they are way more forghving than the dp's. They feel softer because the secondaries open slowly, but when adjusted right are not considerably slower than a dp in an average street & strip car. A vacuum carb gives the engine only the amount of air it needs, it's kind of a "variable size" carb and is a good choice for dual purpose cars.
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#8
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Rallye,
The transmission (if auto, the convertor plays the main role), the rear gears & the rear tire diameter play a big role. Can you tell us this information on your setup? The 340 will run well with a DP if your (auto:convertor) tranny, rear tire diameter/gearing are matched well. |
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