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Discussion Starter · #1 ·
Which is the best choice, I have a holly 600 mech sec on my 350 now and I was considering going with the holly 670 street avenger vac sec, would this be a wise move? I was looking at the 670 street avenger out of the box and noticed that the secondaries don't open when you throttle,I know it has to be hooked up first, how well does it throttle when hooked up, does it rely on the distributor for vaccum or is it a self contained unit. Is vac sec better or mech sec??

Greg


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[This message has been edited by 1967^Malibu (edited 05-05-2001).]
 

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You will not get the secondaries to open unless it is on the car and under load, not sitting still and hitting the throttle. These seem to be nice carbs, I would definately consider one if I were in the market.

Steve R.
 

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Greg, I think it really depends on your driving style and whether or not your engine combo will be best served with alot of initial WOT fuel pump shot (mechanical double pumper) or a gradual feed (vacuum secondary). Speaking in generalities here, vacuum secondaries better serve numerically lower gears, heavier vehicles, and stock or close to stock stall converter sizing. If the rest of your combo is closer to what I described above, then you 'might' stand to benefit from a vacuum secondary.

BUT, a quick question...are you experiencing any kind of low-end bog or sag when using the current Holley 600cfm mechanical carb? Or are you going after the extra CFM? Just a little curious as to why the switch in the first place.

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Discussion Starter · #6 ·
I'm going with a new carb because the 600 is a pain in the ass to keep adjusted correctly and the power valve factor, although I must say the carb has been doing fine lately but I know it won't last very long. I have the worst luck with carbs, I never seem to be able to tweak them correctly, By the way I have a M-21 trans if that matters. Will I be able to tell the diference (performance wise) with the new carb?
Greg
 

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Wish I could tell you with confidence that the new carb will outperform the old one, but I'm not sure that your engine will respond to more CFM from the newer carb. Assuming you're spinning it to 6,500 rpm's and using a .85 ve ratio, you only need 560cfm thru the intake (in theory at least).

Yep, double pumpers can be a hassle to tune properly but once you get the hang of it and they're dead on, they'll run circles around an equally prepp'd vac secondary piece.

Not trying to talk you out of a new carb purchase, just not sure that you'll see an appreciable difference between the new piece and what you have now. Especially since you mentioned that it's running good as is right now. Out of curiousity, what rear gearing are you using?

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Discussion Starter · #8 ·
I believe it's a 3:73 rear, the only thing I have to go by is that I'm running 3000 rpm at 60 mph, people in this forum told me it's probably 3:73. I appreciate all of your help with this, is the 600 dbl pump mech sec better than the vac sec and is the new holley 670 a dbl pumper ? My main concern is dependability, not speed.
 

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Greg, that newer Holley carb is a vacuum secondary carburator. Is it better than a mechanical secondary? I know you hate to hear this but it depends on the purpose. You'll likely get better fuel mileage with it, if that's a concern. A mechanical secondary requires a little more tuning than the Street Avenger and the newer piece does have some extra 'features' built-in to make it more user friendly. Heck, if it comes down to an ease of use decision and I wasn't concerned with getting as much pump shot acceleration down low as possible, then I'd lean toward the Street Avenger.

You definitely have enough rear gear for that double pumper though.
 

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Before I shelled out 360$ for a holley in your situation with no tuning experience I would buy the Carter or Edelbrock 650 vac sec carb for 230$. They run great out of the box on 350's and they're the most trouble free carbs I've ever dealt with. Besides saving you a 100$. I would go this route.
 

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What size tire do you have, diameter if you know it, or I can calculate from the size listed on the sidewall.

If diameter is ~ 26" then you should have 3.55 gears.
If diameter is ~ 27" then you should have 3.73 gears.

Anytime someone posts the question "mech vs vac secondary", I'll always reccomend vac secondary. While it is true that just about nothing will make more power at WOT than a properly set up double pumper - it is also equally true that most enthusiasts seriously lack the tuning knowledge to make this happen.

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