I am looking at carbs for my zz4 crate motor. Barry Grant dealer says I should use a 650CFM Speed Demon with vacuum secondaries. How about some help explaining the difference between vacuum and mechanical secondaries and the advantages of each. This is a 69 Camaro with a Fastburn 385 ZZ4 and a 2004R trans. Will be used for cruising most of the time and will see the strip probably twice a year.
Midlife Cruiser
Mechanical secondaries will open the secondaries under heavy throttle. This can be a problem on many engines if the accellerator is floored from a stand still. The work around is to modulate the throttle such that you don't open the secondaries until the engine has developed some RPMs.
Vacuum secondaries operate on venturi vacuum. As a result, the secondaries open once a certain amount of airflow is detected through the primaries. The opening is gradual as well. Especially on a car with an automatic transmission, vacuum secondaries will prevent bogging by gradually opening the secondaries as the engine developes RPM.
IMHO vacuum secondaries are the way to go for a street rod running an automatic transmission. This is a hot topic and I'm sure you'll find a lot of discussion in the archives.
-dnult
Vacuum secondaries operate on venturi vacuum. As a result, the secondaries open once a certain amount of airflow is detected through the primaries. The opening is gradual as well. Especially on a car with an automatic transmission, vacuum secondaries will prevent bogging by gradually opening the secondaries as the engine developes RPM.
IMHO vacuum secondaries are the way to go for a street rod running an automatic transmission. This is a hot topic and I'm sure you'll find a lot of discussion in the archives.
-dnult
Yes, with mechanical secondaries your foot opens the secondaries using mechanical linkage. With a vacuum secondary a vacuum signal pulls against a diaphram and a spring to gradually open the secondaries under heavy throttle conditions.
Double pumpers (mechanical secondary carbs) use A LOT more gas typically than a vac secondary carb when used on a street car.
I would only give 1/10 sec advantage to the exact same car with a double pumper carb. When set up correctly a vac secondary carb can run right there with a mech secondary carb.
With overdrive on a mostly street car I would go vac secondaries. I agree with Demon's recommendation.
Double pumpers (mechanical secondary carbs) use A LOT more gas typically than a vac secondary carb when used on a street car.
I would only give 1/10 sec advantage to the exact same car with a double pumper carb. When set up correctly a vac secondary carb can run right there with a mech secondary carb.
With overdrive on a mostly street car I would go vac secondaries. I agree with Demon's recommendation.
Im thinking of stepping down from my Holley 750cfm now too. Its loading up at idle alot, smells gassy and I dont need the power for street and highway use.
I might go to my Qjet that I got instead.
For a standard 350 Im hearing the 750's are a bit too much. Interested in mine John?
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I might go to my Qjet that I got instead.
For a standard 350 Im hearing the 750's are a bit too much. Interested in mine John?
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