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Old 07-05-2004, 10:56 PM
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squarebore vs spreadbore theory

OK. Simple question. Looking for opinions and, hopefully, real data.

It seems to me that a spreadbore carburetor would send a richer charge to the rear cylinders when running wide open. The vacuum on each intake runner should be the same. Therefore, the runners closest to the larger rear venturi on a spreadbore would receive more fuel. The greater the difference in size between primary and secondary venturi, the greater the difference between the front/lean and rear/rich condition. Or is there is enough air/fuel mixing to overcome this situation?

What brought this up is the fact that with a factory spreadbore intake and a Thermoquad my 440 would run a little rich on the rearmost cylinders, #7 and #8. Whenever I switched to a Performer RPM and a Holley 3310 this problem cured itself.

What do you think, or more importantly, KNOW?

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Old 07-05-2004, 11:16 PM
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I think the difference in charge would be negligible if any. You could say the same thing about squrebore carburetors too, generally the secondarys' jets are bigger than the primaries' are. Furthermore, you're presupposing the charge from the carburetor encounters 0 turbulence in the plenum of the intake, where it mixes. Also assuming 0 turbulence, the spreadbore would mix more evenly, considering the two primaries are probably too small to feed all four front slugs at full throttle, so they would draw off the secondaries and mix the charge.

All of this is theoretical. The real answer is this: Not all slugs are drawing charge at the same time, but the carb is feeding non-stop from all four barrels. This means that the two bores that are on their intake stroke at a time are drawing off all four barrels. Your question is assuming that all slugs are drawing at the same time. They're not.

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Old 07-09-2004, 08:10 PM
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According to Doug Roe's book on Rochester carbs, the secondary throttles on some Qjets are not exactly vertical at WOT because of the same fuel distribution issues.
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Old 07-10-2004, 04:48 AM
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I had a 440 with a 600 Vac sec Holley and it still fouled the rear plugs, 7&8.

Just because the blades are larger doesn't mean the mixture will be richer, I have never seen poor distribution solved by changing to a spreadbore or vise versa.
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