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Old 12/30/2004, 12:17 PM   #1
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Best e.t. : 11.042, Fastest speed: 124.08, Best 60' : 1.615, 0 to 100 in 6.98

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I was wondering if anyone has some experienced advice:

is a holley 750 double pumper (mechanical secondaries) too much for a 347 stroker making 400 horsepower?

Right now i have on a holley 670 street avenger with vacuum secondaries and bigger jets making it about a 700.

Do you think switching the 670 vacuum for the 750 mechanical will make a difference in power?

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Old 12/30/2004, 12:49 PM   #2

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There is a mathmatical calculation that will tell you quite accurately what is the optimal CFM for your engine.

I remember a long time a go an article in a old car magazine about this subject and the gist of the story was that most of the engines out there are OVER carburated. The BOSS 429 ( or maybe it was the BOSS 302 ) came with a 780 CFM and the car actually made more horse power with a smaller carb.

Do a google search for carburator cfm calculation and see what you come up with. I would bet fine tuning what you have will yield more power and better driveability than just bolting on a carb that flows more. Good luck!

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Old 12/30/2004, 02:23 PM   #3
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Rule of thumb, a smaller carb. (within reason) will give better torque which is what gets us off the line. To much carb will hurt horsepower and torque. I think the 670 is enough. I also haven't had good results with a double pumper in a car with an automatic tranny. I don't know what you have, but something to think about.
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Old 12/30/2004, 08:09 PM   #4
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David, Back to the carb discussion. First, the jetting has nothing to do with the CFM of the carb. The CFM is determined by the size of the venturies of the carbs (barrels) and the amount of vacume the engine creates to suck in air/fuel mixture. The Jets supply the fuel to the barrels(if you look into a carb when someone pumps the throttle you see gas coming out of the Jets). So, your 670 is a 670 unless you rebored the barrels.

On the subject of size, as was sed b4, there is a formula that will tell you what is the correct size. Holley or any other carb company can tell what is best. Like sed b4, most engines are over carbed, not under carbed. That is the discussion of putting parts together that work well. Changing the carb may mean changing the intake, heads, or cam to get the use of the additional CFM.

When we built our 302 punched out 30, we looked at the formula for carb size. As I remember 570 to 575 is all our engine would handle for max torque/hp. We chose the 600 holley because we thought we could get away with the extra given we went alm single plane intake, ported and polished and headers. Thus, hopefully enuf flow for 600 cfm. The 600 cfm carbs are a nearly a perfect fit for a hipo 350. That is why holley and most other carb companies make a 600, to fit the chevys. Just as a side note, if you get a chance and can get hold of a 600 cfm carb, try one on the dyno. It may surprise you to know your engine is today over carbed.

If you put too much carb on your car, it will litterally fall on its face off the line. Just not enuf air flow to take all of the fuel coming in from the jets. It will bee way too rich. As stated above, under carb engines will tend to run stronger than over carbed engines. Some danger at hi rpm as the under carbed engines will eventually run lean.

My suggestion is, if you dont think you are not getting enuf carb, check the spring setting on your street avenger. If it is stock, you probably can drop down one or two spring strengths and get the secondaries to open more quickly. That will give you a lot better fuel and air flow in the power band. That would be my first "try it". If that will not get you enuf feul, then think about more carb. But frankly, I cant imagine your engine needing a 750 double pumper. FYI, I run a 750 double pumper on my 428, bored 30 over, performer RPM alum intake, heads, cam really pretty straight through pipes. the 750 is plenty. Makes over 450 hp, over 520 tq.

My 2 cents worth.

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Old 12/30/2004, 08:48 PM   #5
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Brian, very well put. I'm running a Holley 750 with vacume secondaries on my 390 HI PO and it's more than enough. For some reason everybody thinks the bigger you go the more power you get. WRONG. It doesn't always work that way.

Don :)
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Old 01/01/2005, 06:00 PM   #6
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Best e.t. : 11.042, Fastest speed: 124.08, Best 60' : 1.615, 0 to 100 in 6.98

Location: San Fernando Valley, California
Joined: Jul 04
Posts: 703


Thank you guys so much for your info. I will definetly try putting different springs into the vacuum secondary to get them to open faster...sounds like great advice. Thanks again and happy new year.
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