carb question (mechanical or vacuum)

[SS Shaun]

so my uncle is thinking about upgrading his carb to a bigger carb since it was having some fuel starvation issues. he wants to go from a 650 to a 750 so he asked me today which one is better. i wanted to ask you guys for it being a 100% street car whats better for it the mechanical or the vacuum secondaries?

[Mike 1972SS]

Shaun, more information on the engine/trans/rear-end ratio is needed to make an educated choice.

In general, most would recommend a mechanical (double-pumper) for a manual trans car, where a vac -sec carb usually suits an automatic better.

Mike

[SS Shaun]

355ci
th350
3.73 gears
I read online that the auto for vacuum amd mechanical for manual is a big myth amd untrue we are looking for performance but not loose that much mpg i think vacuum is the way to go because it will open when needed and the mechanical will open even if you dont need it. Am i correct?

[Mike 1972SS]

You are right about the vacuum secondary/mechanical secondary carb operation. If that 355 is not spinning past 6000 rpm, it would seem that the 650 VS carb he has, should be just about right. You say he has fuel stavation problems. Has he checked the basics, fuel line size, fuel filter, ect.? Has he tried tunng the carb? Maybe just a simple carb rebuild would help?

I'm running an AED 750 mechanical sec. carb on my 68 Camaro, but it's a manual trans car. You might want to try giving them a call for a recommendation, not cheap, but they really know their stuff.

http://www.aedperformance.com/Carburetors.htm

Mike

[Hank70SS]

I agree with Mike on the size of the Carb. On a small block street car what he has should do the job just fine, I will disagree a little on mech vs vac. For a street car, regardless of trans, I would say vac secondary.

I'm running a 770, vac secondary on 4 spd 454. It's a street only car and not a high reving engine. It feeds the engine just fine no with fall off through out the rpm range.

[Hank70SS]

On a side note, how you doing Mike? Unfortunately I'll be in California during the Dream Cruise/Back to the Bricks. You going to bring the 72 up to Autofest in Frankenmuth? Sure would like to see it.

[Mike 1972SS]

Originally Posted by Hank70SS

On a side note, how you doing Mike? Unfortunately I'll be in California during the Dream Cruise/Back to the Bricks. You going to bring the 72 up to Autofest in Frankenmuth? Sure would like to see it.

Hey Hank, I'm doing great. Been driving the Chevelle to local cruise nights mostly. I'd love to get up to Frankenmuth this year, can't stay the whole weekend because of work. (You remember what that was, don't you )

Take care,
Mike

[FlintTony]

I pulled a double pumper, mechanical secondaries, off my 427 and put a 750 vacuum secondary on it. The car has much more driveability on the street. I haven't noticed any power loss. Holley recomends the vac sec for street cars. They have a chart for CFM too, and the 650 should be plenty of carb for a 355.

[SS Shaun]

yeah i spoke to a guy yesterday and man he broke it down for me big time he went very scientific..lol here is his formula he showed me for my 454

Principle items in baseline carb CFM calculation are displacement and RPM. Secondary factors are volumetric efficiency and intake manifold efficiency (manifold factor--MF. MF ranges from about 1.1 for a racing manifold to about 1.5 for a poor-flowing stocker.) 3456 is a constant that accounts for the number of cubic inches in a cubic foot; and the fact that this engine has a four stroke cycle.

Cubic inches X RPM / 3456 X VE X MF = baseline carb CFM requirement

454 X 4000 = 2270000

2270000 / 3456 = 656.83

656.83X .75 (projected VE) = 492.62

492.62 X 1.4 (projected manifold factor) = 690 (rounded) CFM


so he told me a jetted 670 a 700 or a 750. he told me to get the 750 to play it safe and i can always jet it down. he really broke it down for me big time. after doing my camaro calculation we came up with 647 rounded CFM so he told me we need to play with the jetting on the 650 thats on it now

[Bullcrappy]

600 or 650 is right on for a 350... play with jetting, get the right power valve in there for how much vacuum your pulling along with reg adjustments.

fuel starvation? fuel pump good? fuel pressure is at?