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  #1 (permalink)  
Old 04-15-2010, 06:26 PM
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Default Dual Quads (Holley) on a Small Block Questions

I'm pondering putting a dual quad setup from Carl's Ford Parts on my 302 crate engine and have a question or two for anyone who's installed a similar setup on their small block.

I called Carl this morning - and surprising to me - he recommended his 450 cfm mechanical secondary carb setup, not the setup using the 465 cfm vacuum secondary carbs. He said that the mechanical secondary carbs would provide better performance while the vacuum secondary carbs would result in a bog/dead spot.

I'm afraid that ~900 cfm of mechanical secondary carb on my 345 horse Ford Racing engine would not be optimum and I'd be way over-carbed. (I've currently got 10,000+ miles on my engine with zero problems and am using an Edelbrock Performer RPM intake and Holley 600 cfm vac. secondary carb. It's just that the dual Holleys look so COOL...)

Seems to me that for a street engine like mine, the vacuum secondary carbs would be the way to go.

Any comments, experience or recommendations?

And although they might perform great, I'm not crazy about the Edelbrock (intake and carb) dual quad setups. To me, for the look I'm going for, it'd have to be dual Holleys if I go down the 8 barrel path.

Thanks in advance.
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Old 04-15-2010, 06:59 PM
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Jim,
I hope you are looking a dual quads on a SBF for looks and not for an increase in power. They will rarely make more power compared to a properly sized and tuned single quad, and if they do make more it will be not much and way up in the power band.
But they do look good, and I agree Holleys look the best.
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Old 04-15-2010, 07:02 PM
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You are right. You don't have enough displacement, cam, head or exhaust for 900cfm mechanically delivered.

I have 1308cfm feeding 427 inches with big cam, 320cfm heads, 2.250 intakes but vac secondaries meter that demand perfectly. Only on WOT does all 1300 come in.

Stay with vacuum.
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Old 04-16-2010, 01:09 PM
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I won't comment on the recommendation you received from Carl, other than to say that we've never had problems getting a pair of properly sized and calibrated vacuum secondary Holley carbs to work well on engines of this type. Personally, I would opt for a pair of the Holley #8007 390 cfm carbs as a starting point. They'll require some recalibration, but nothing too complicated or beyond the understanding of the average gearhead. Venturi air speed would still high enough for efficient fuel atomization good throttle response, and also result in sufficient signal strength to the vacuum pod so that the rate and timing of secondary throttle opening could be well controlled.
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Old 04-16-2010, 03:35 PM
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Good advice here.

I called a different vendor earlier today prior to Stan's post and we agreed that 390 cfm carbs would be the preferable choice. So they're going to provide me a quote for a complete setup including linkage, fuel log and air cleaners. I'm hoping to get the quote early next week and if the price is right, pull the trigger.

And this project isn't in a hope for more power, it's just for the visual factor.
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Old 04-16-2010, 03:47 PM
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Jim...I personally like the look of multi carb systems as I've had several combinations over the years.

I agree with Chas on the vacuum and Stan with the 390 cfm holleys.

What's nice about vacuum is they only open up enough to meet the airflow needed by your engine, so with your 302 345hp engine the 390 dual quads might not open all the way...but they''ll open enough to meet the max airflow of your 302.

The only thing is, Carl's 465 cfm carbs are probably already setup for the factory style 427 dual quad linkage and the universal 390 holleys are not...so some tweeking will need to take place.

I've bought dual quad carbs from Carl's and they were first rate, although in this case you don't need 900 cfm, one last thing... the 427 carb linkage he sells was much better quality then others I have purchased.
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Old 04-16-2010, 07:36 PM
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I ran dual 600s on my Boss 302 for quite a while and the only thing I didn't like was stumbling in corners while autocrossing. In a straight line it went great, posted the fasted quarter it ever ran with the dual quads in fact.


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Old 04-17-2010, 04:06 AM
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I run dual 660's on my 460 and love them.

While I'm pretty good with Holleys, I finally had to take the car to a shop with a dyno to get them dialed in. The 660 is a race only carb with mechanical secondaries, 4 jet center squirters, and no power valve in the front metering block.

It took him about 5 hours to get them to work right. We ended up swapping out the front metering blocks for ones with power valves. He drilled up a couple passages also. Since the secondaries use metering plates, not metering blocks you cannot use jets and the selection of sizes for the metering plates did not match what we wanted, we installed Quick Fuel jetted metering plates so you can use jets.

Personally I prefer mechanical secondaries. I want my right foot to control the opening of the secondaries, not a vacuum diaphram and spring.
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Old 04-17-2010, 09:53 AM
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I have an original dual quad setup (from a 66 GT350 Shelby Mustang) on my HIPO 289. The engine is now in my Unique Cobra but is not yet finished. I ran the engine in a 65 mustang for many years and loved the performance from it. When I first got the setup it did not have the linkage, so I made my own and not knowing any better, connected both carbs to operate at the same time. The car always had a short lag in response (from too much fuel) and then came on like a rocket, when the vacuum secondaries opened it was like you mashed the throttle again. I originally thought that over 900 cfm was going to be way too much but other than the lag problem it worked fine. I now have the linkage set up right and am anxious to see how it runs now. My 289 had stock heads with 12-1 pistons, oversized valves and the Ford C7FE camshaft.
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