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Dual Quad dilemma...too much carb?
Guys,
Does anyone have a suggestion regarding the use of an original BJ-BK 600 CFM dual quad setup for a good, practical driver? I drive a '67 GT500 soon to have a mild 428 w/Edelbrock heads. The stock dual quad set-up is too much carburation for the motor. How can I avoid dumping too much fuel into the motor and causing a rich condition and premature wear etc.? I guess I could re-jet the carbs? Any ideas would be welcome. MC |
Vacuum secondaries I assume?
I just dyno'd my 428FE with a pair of 725cfm Demons. A/F ratio was great all across the band. But yes, you could jet down along with some other tricks. |
In case you didn't notice this thread: http://www.clubcobra.com/forums/shop-talk/74335-harm-overcarburetion.html
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I run dual 600cfm Edelbrocks on my small block. I disconnected the secondaries, not because of overcarburation, but rather because the flapper valves would open too soon and cause a flat spot during accelleration. Anyway I wonder the secondaries open at all with a nominal 1200cfm carburation on a small block.
The plaugs are rather white than black, because of the general lean jetting of Edelbrock carbs. I think you can't go wrong with dual 600cfm vacuum secondary carbs on a 428. http://img221.imageshack.us/img221/4...edual4s8eu.jpg http://img484.imageshack.us/img484/9984/dscf01232vu.jpg -Simon |
Add a little weight on the conter weights on the flapper valves (drill and peen some lead in) to help with them opening too soon.
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Quote:
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They did come stock, you are correct, but when I run a calculation the numbers seem to show a smaller amount of carb required.
I.E. 428 x 6000 (rpm) / 3456 = 743 CFM, even if I go to 6500 RPM, we get 800 CFM or so. The dual quad set-up is cool to look at and talk about, but I think that a well thought out single four would perform better. In my case for the sake of originality I need to maintain the dual qual set-up. Mike C. |
Just put the two fours back on. When I was in High School my daily driver had two 600 Holleys vacuum secondaries on a lowriser 427 with an automatic. No problems.
I have the same set up on my 427 Medium riser. No problems. It will run fine. They are vacuum secondaries and they don't open unless the engine needs it. Good luck |
SFM5S159, first of all, I'm not sure what size the Bj/BK carbs are, but I believe they're a bit smaller than 600 cfm; could be wrong, but 535 cfm comes to mind. Doesn't matter, vacuum secondaries only open when the engine can use the extra fuel/air, and only open as much as the engine can use; so even if they're rated at 600 cfm, they won't supply that much cfm on your engine. Run those puppies!
Dan |
Mike,
I just sent a PM. These systems can be adjusted to be very smooth. Dan |
Listen to Naumoff. He's been running stock FEs since he was a baby. I think of him as the "FE-Whisperer." When he talks to them, they always seem to behave.:)
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Mike,
Your forgetting the efficiency factor in your calculation. Most engines run 80 to 85%, with real good intake, cam profiles, heads and headers you can reach 90 to 95% but thats about it untill you go to forced induction. Your 743 CFM X .90 = 669 CFM. 743 CFM X .95 = 706 CFM Go to www.csgnetwork.com and plug in the numbers. It does the calculation for you. Two 500 CFM Edelbrock's, progressive linkage, lots of power with no flat spots. Ray |
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Dual fours on my 427, work good.
As for 'reading plugs' bear in mind a Multiple Spark Discharge type system (typical MSD) tends to make the plugs run a lighter color (even white) regardless of other factors (like a rich mixture). So it can be a bit misleading. |
Also, high end motors can and do achieve better than 100% VE. From the old cross rams to modern engines. As a matter of fact, Hughes Engines has a couple dyno tests on street engines on thier web site with better than 100% VE.
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That "Calculator" is missing some things. A dual plane manifold needs a larger CFM carb than a single plane on the same engine, because only half of the carb can be drawn through by any one cylinder. Whoever wrote that calculator program seems to be unaware of that.
I wouldn't trust it. Kevin |
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