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Premium Member
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Discussion Starter · #1 ·
Hi Guys,

As a spin off of my fuel mileage question - I am like a more lower end pop. I am thinking about going down in size from 670. Becuase our cars weigh so little - would it be better to use mechanical secondaries?

Thanks!

PJ
 

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6,092 Posts
Welcome to one of the most often discussed topics on this forum. :001_rolleyes: You will get lots of opinions on both sides.

I've had both on my 306 powered Mk3. Started with an SA570 vacuum secondary, and in spite of a lot of tinkering was never happy with it. Changed to a Quick Fuel SS-650 mechanical secondary, and the car really woke up. I was probably under carbed with the SA570 and AFR heads (as others have pointed out) but still for me there is no comparison. Light, high powered, manual transmission = mechanical secondary.

Here are a few other threads. There are lots more...

http://www.ffcars.com/forums/17-factory-five-roadsters/181864-mechanical-vs-vacuum-secondaries.html

http://www.ffcars.com/forums/17-fac...orner-idle-650-vs-750-cfm-pls-educate-me.html

http://www.ffcars.com/forums/40-200...2115-double-pumper-vs-vacuum-secondaries.html
 

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Senior Charter Member
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i agree with Ed above.

i initially bought a Holley 670 SA, but i swear i couldn't get the secondaries to open. went to the lightest spring, tuned and checked vacuum readings, did the "paperclip" trick to test it and no go. i think this car is just too light.

went to a Holley 4150 600 CFM with mechanical secondaries and i'm very happy. i can feel it now, but gas milage has gone down (had to, considering the previous "2 barrel" carb) but it hasn't been horrible. on the freeway in 5th gear, driving with the flow i can get up to 17 mpg.

good luck!
 

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2X on the QFT SS650. Switched from a 670SA (on a 351 bored to 363) and have been very happy with the QF. Much better throttle response and much better tunability. Scott
 

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I have a QF 750 MS on my 408W - it's just awesome - snappy throttle response. I have the roadrace version, and autocrossed it a couple of weeks ago - never missed a beat.
 
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