Inliners International Forums General Discussion Hi Performance Manual or Vacuum secondaries

I had planned to use Offenhauser tri-power intake with thre single throat carbs on my eventual street-driven 292 but have changed my mind to a single 4-barrel, which would be much easier to deal with although it does not have the "eye candy" effect. Leo Santucci recommends a Holley 600 CFM vacuum secondary dual feed carb or an Edelbrock 600 CFM for optimum results. Wouldn't manual secondaries be a better choice because when one hits the accelerator pedal, the manuals respond instantly, the vaccums take their good natured time. Or am I oversymplifying matters?

IMHO 600 for a mild engine is too big, the last 100 CFM does nothing.
292 @ 80% VE at 4,500 RPM is about 335 CFM.
However: just as important as size is the fuel curve, which is not information available from the mfg.
Some results reported as due to change in size are instead due to the existing air bleeds etc. producing a closer mixture with the "wrong" size carb than the right one. A correctly jetted 600 (even if by accident) produces more power than a badly adjusted 500.

A 600 vacuum secondary will work but a smaller carb would drive better. I recommend vacuum secondaries as the secondaries only open enough to maintain a strong signal at the boosters. With mechanical on the street it is possible to open all barrels too quickly such that you lose a strong signal at the boosters, causing a bog. Don't believe me? Try it.

I'd recommend the 390cfm Holley 4 brl, # 8007 or the Edelbrock 500cfm. For a street driven engine either carb will be plenty big enough and driveability will be excellent. The Holley secondary opening rate can be adjusted with Holley part #20-59 & #20-13.

I dynoed a 302 SBF with the Edelbrock 500cfm and the engine made 347HP so either carb will work great at your HP level.

Ron

Even tho it is for racing, I have consistantly used a Vacuum Secondary Holley 600 CFM carb for the last 10 years. If the carb is set up correctly it is a lot smarter than I am. I did work with the dash pot springs to get the optimum control for my use. It was originally built by the Carb Shop in Ontaria Ca. and since been refreshened by Sean Murphy in Huntington Beach.

From my experience, stick shift cars can get away with mechanical secondaries but automatics perform better with vacuum especially off the line. Good Luck

From my experience, stick shift cars can get away with mechanical secondaries but automatics perform better with vacuum especially off the line. Good Luck [/quote]


Amen on that..!!!

George,
See "Performance Enhancements", Snowman's post, I just ended up discussing the mechanical vs vacuum issue though these guys covered it well.
I've seen Panic's situation many times, where my best running carb on the clock was really too big, but the engine just "liked" the way it was tuned. But a carb that is the right size AND tuned right will be best, whether mechanical or vacuum.

Vacuum carbs are a lot more forgiving of too big of a carb, because they will just "decide for you" to only open the secondary part way since that's all the air the engine wants. Their disadvantage is the same - without careful tuning, they can open the secondary too early (an air bog) or too late (starving the engine for air/fuel). Mechanical carb is more "certain" but you can't just floor it and go - YOU have to put your foot where the engine wants it which is not as convenient as a vacuum carb.

I like and actually prefer mechanical secondaries on a stick car, but I like a well-tuned vacuum carb on an automatic - once it's set up properly, you're done.

But, if I find either type for under $40 and it's a good runner, it "becomes" the right carb, for the time being at least.

Thanks, guys. Much appreciated. I'm going to have to do some thinking and more researching about what you recommend.