Mechanical or Vacuum secondaries - Chevelle Tech
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post #1 of 24 (permalink) Old Jul 8th, 20, 8:20 PM Thread Starter
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Mechanical or Vacuum secondaries

Hey Guys,

I looking for some good carb choices for my Chevelle project... Here what I'm putting together:

- 467 BBC 580hp, with hydraulic roller cam. Cam is [email protected]
- Muncie 4 speed
- 4.10 rear end

I'm hoping deciding between mechanical or vacuum secondaries would narrow down the choices.
I appreciate your expertise, Thanks in advance!
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post #2 of 24 (permalink) Old Jul 8th, 20, 8:33 PM
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Re: Mechanical or Vacuum secondaries

While VS do work pretty good, I only have one that works fantastic. Most of the time I use Double Pumpers.


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post #3 of 24 (permalink) Old Jul 8th, 20, 10:02 PM
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Re: Mechanical or Vacuum secondaries

I almost always recommend VS, in your a DP is almost called for. big engine, stick, low gears. Is this car street only, drag only or a mix?

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post #4 of 24 (permalink) Old Jul 9th, 20, 12:20 AM
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Re: Mechanical or Vacuum secondaries

Quote:
Originally Posted by Tom Mobley View Post
I almost always recommend VS, in your a DP is almost called for. big engine, stick, low gears. Is this car street only, drag only or a mix?
Agreed 👍
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post #5 of 24 (permalink) Old Jul 9th, 20, 2:53 AM
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Re: Mechanical or Vacuum secondaries

Get one with mech. 950 hp or 1000hp should work nice.
Mark Jones 467?

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post #6 of 24 (permalink) Old Jul 9th, 20, 8:23 AM
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Gene
 
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Re: Mechanical or Vacuum secondaries

Jake, there a big difference in mine DP vs vacuum. same setup as you, essentially. The DP is like a little nitrous hit. Vaccum you wait for. Good for a converter car, not for our "rats on a stick!" I'm still using an ancient 4781-2 850 DP.

I dont prefer the shiny ones but dont choke my car so .... for example : https://www.holley.com/products/fuel...parts/FR-82951

Been eyeing the 830 cfm annular booster "NASCAR" carb, refurb as well , but folks say it dont work that well at rpm. Annular boosters should work better than downlegs.

PS I did see a 850 on their refurb pages for under $300, but I know nothing about "Brawlers" . It has a choke.

https://www.holley.com/products/fuel...rts/FRBR-67214

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post #7 of 24 (permalink) Old Jul 9th, 20, 8:46 AM
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Re: Mechanical or Vacuum secondaries

Run whatever carb gives the best ET/MPH and has the best drive-ability for YOUR combo. Takes some experimenting and patience
Mine is an old 780 cfm vacuum secondary Holley. I don't have to "wait" for the secondaries to open,they are fully engaged before the 60 ft. beam and I can manage a 1.66 60 ft.
It works very well on street and will light the tires up at highway speeds. I think it actually gets decent MPG but I don't really pay attention


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post #8 of 24 (permalink) Old Jul 9th, 20, 9:21 AM
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Re: Mechanical or Vacuum secondaries

Quote:
Originally Posted by 427L88 View Post
Jake, there a big difference in mine DP vs vacuum. same setup as you, essentially. The DP is like a little nitrous hit. Vaccum you wait for.
Exactly. It's hard to come up with a way to put it any more simply than this.

I actually just swapped my carb, I was running a Holley 650 with vacuum secondaries and replaced it with a Holley 650 that has mechanical secondaries. The only real variable between the two carbs is the secondaries, and this perfectly sums up the difference in the way the car behaves. I could never get my vacuum secondaries to fully open unless I was already in 4th gear and held the pedal to the floor. Even with some "spirited" driving I couldn't get them to open even when flooring it in 1st through 3rd. This is after swapping out all of the different diaphragm springs and trying everything there was to try. I finally gave up and just figured mechanical secondaries were just a better match for my setup.

OP, the answer depends on how you drive your car and what you expect from it. But if you like to stab the pedal to the floor and get instant acceleration, go with mechanical secondaries.

No matter which one you choose, keep in mind it's easy to swap out carbs if you decide to try something different.

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post #9 of 24 (permalink) Old Jul 9th, 20, 9:56 AM
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Re: Mechanical or Vacuum secondaries

The math says you need 810 CFM at 6,000 RPM and 100% Volumetric Efficiency.
You won't go wrong with an 850 Double Pumper.
Needless to say it has to be jetted properly.

Chas

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post #10 of 24 (permalink) Old Jul 9th, 20, 2:04 PM
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Re: Mechanical or Vacuum secondaries

Your 4 speed and gear, I would go with a AED 950HO. If you want to have a choke, then a FST 750 RTX vacuum secondary.

I have a 630hp MJ engine in a 71 Nova. 3.42 gear, 27" tire, 2500 10" converter. I run a AED 950HO Modified, and 770 street avenger (actually a 750 after measuring the throttle bores and venturis). The double pumper hits way harder, and with normal street tires they don't have a chance, especially since I changed the converter from one that would flash to about 2200. The few more rpm this converter flashes to made the difference that the tires light up instantly. That includes a Hoosier drag radial that with the other converter would stay right on the edge of traction, hazing the tires, but staying hooked up.

Now with a tire change to a regular Nitto NT555G2, I put on the avenger to gain a choke which is nicer for cold starts, and give the tires a chance to hook. They still light up at will but I can roll into the throttle fairly aggressive, and keep them hooked. With the AED carb, as soon as the secondaries would open the tires would light up.

I have ran the car at the track with both carbs. The avenger is only 1-2 mph and about .1 to .15 slower. And that is using the stiffest black secondary spring.
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post #11 of 24 (permalink) Old Jul 9th, 20, 2:52 PM
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Re: Mechanical or Vacuum secondaries

Interesting Kerry, assumed it was more noticeable with a clutch car rather than a viscous coupling car. PS, I am comparing Qjets to the 850DP, so...... with enough motor, like mr 4 speed's, its doesnt matter as much I suppose with a well tuned Holley VS carb.

Dont know if this helps, but oddly, my MJ467 was dyno'd with an 850 with EXACTLY the same jetting I run. 77 pri 80 sec.

To round it out, 085 front PV, 045 back. Blue pump cam sec, custom white in front. Runs like a scalded ape, ALTHOUGH has a little part throttle thin spot right around 2000. Kerry said it may need some air bleed tweaking as they all kind of do, but I dont really want to mess with it.

I think its as old as the car!

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post #12 of 24 (permalink) Old Jul 9th, 20, 3:52 PM
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Re: Mechanical or Vacuum secondaries

I am a former Holley factory Pro-Stock carb tech and race assistant.

Automatic: vacuum secs
Manual: double pumper

Only issue is, getting the timing right for either version, and getting the carb dialed in correctly. MOST setups that don't work are either the wrong type for the trans, like a D-P for an automatic, or just plain poor carburetor setup/tune. Before getting to the carb, the ignition curves MUST be done correctly, or the carb will NEVER come around.
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post #13 of 24 (permalink) Old Jul 9th, 20, 4:16 PM Thread Starter
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Re: Mechanical or Vacuum secondaries

Thanks for the replies!!

Yep - it's an MJ 467, can't wait! I've always used a DP so was curious how the vacuum secondary would perform. If I decided on a vacuum secondary I would go with a 770 Street Avenger. Otherwise probably an 850 DP. I enjoy reading all the replies, you guys are a wealth of experience!

The car will most likely be a street cruiser, but can't rule out an occasional run down the track.
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post #14 of 24 (permalink) Old Jul 9th, 20, 5:00 PM
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Re: Mechanical or Vacuum secondaries

To me it is almost all in the tune. You have to be able to jocky carb and ignition tuning to get the most.
I have 950 AED mechanical and no choke. cold start up and I can get out of the car in less than 15 seconds and it never stalls.
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post #15 of 24 (permalink) Old Jul 9th, 20, 7:17 PM
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Re: Mechanical or Vacuum secondaries

My 850 AED runs very good. Same thing, 20 sec or so no choke and stays running. Best carb I’ve ever owned right out of the box.
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