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  #16 (permalink)  
Old 09-03-2009, 02:50 PM
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For one thing you will get better MPG with vacuum secondary and when it comes to CFM, vacuum secondary are more forgiving. You will never overcarb a motor. It will use so much CFM what the motor needs. You can adjust the opening rate on the sec. by changing the springs. Vacuum secondary carb are more street friendly.

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Old 09-03-2009, 03:02 PM
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Quote:
Originally Posted by lg1969
For one thing you will get better MPG with vacuum secondary and when it comes to CFM, vacuum secondary are more forgiving. You will never overcarb a motor. It will use so much CFM what the motor needs. You can adjust the opening rate on the sec. by changing the springs. Vacuum secondary carb are more street friendly.
Why doesn't everyone use them, then? That sounds alot more user-friendly.
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Old 09-03-2009, 04:49 PM
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Originally Posted by DeltaElite121
Why doesn't everyone use them, then? That sounds alot more user-friendly.
The take a little time to react. track racers will usually use a mechanical secondary, because it opens mechanically and most are also double pumpers. You cant really double pump a vacuum secondary carb as the the throttle position does not dictate when the secondaries will open, load does.

Many people have thought their vacuum secondary carb was not opening the secondaries because they sit in park and rev teh engine and the secondaries dont open. They never will.
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Old 09-04-2009, 08:10 AM
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I think in a lot of cases the DP just looks and sounds (appears) to be the way to go. Common sense and logic are thrown out the window.

On a 6-71 type application on the street, full throttle will cause a demand air/fuel flow nearly instantly that is far higher than an unblown application. Untill recently I thought that the vacuum sec might not work too well even though I've used the Carter AVS carbs on one blower application I did years ago. It worked very well on a 454 BBC with a manual trans in a Camaro. Edelbrock has a blown crate motor with one of these and it makes 500+ right out of the crate. Recently a friend who has a 502 BBC with an 8-71 and Holley vac sec. carbs ran it and ran 10.90 without even tuning it up after driving it all over on the street. Watching the carbs from the pass seat, the carbs open almost as quickly as directly linked. You can't feel the transition nor hear it. ( it has mufflers). I was concerned that you may get a backfire due to a momentary lean but the vac sec has fuel ready to be released on demand at a demand rate so as long as you are jetted properly they work great. Overdrv states very well just how the vac sec works and you can see that the very high demand that a blower will make on the primary will insure a very quick opening of the sec. A 50cc pump is usually run with vac sec so you can get extra fuel required quickly.

On a race application where you might have a bigger motor and run the blower in a large overdrive so you get a lot more boost (air flow) even quicker it might be better to run the double pumper so you can flood the AF ratio at the initial hit. This is what the mechanical FI units do. There is a lot of tuning in this area on these units but we are talking street stuff so we can leave that animal rest.

I really think the biggest problem running vac sec is that you will need to tune them yourself or do some dyno work to really get them set up properly. This is opposed the simply buying blower carbs already to go. These will only need minor jet changes for the most part.

As for mpg, well, you will be playing with the big boys now and mpg is way down the food chain. A streetrod buddy does get 20+ with his blown 3 window but he doesn't step on it very often. If you put on 3000 miles during the summer at 10 mpg it's $720 for gas. Not a lot considering what you have in the blower, engine and trans modifications. Pluss the fun factor simply is priceless. haha
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Old 09-04-2009, 08:22 AM
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^ Very insightful. Thanks for an awesome response.
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