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Vacuum vs mechanical secondary #1195808 03/12/12 05:24 PM 03/12/12 05:24 PM
Joined: Jan 2003 Posts: 7,843 Spring Mount, PA MOPAULY OP master
MOPAULY OP master Joined: Jan 2003 Posts: 7,843 Spring Mount, PA	Need to purchase a carb soon for the 340 on my '72 Charger and am undecided between a vacuum or mechanical secondary 650. I previously ran a mechanical 650 but would like to squeak a little better fuel mileage out if possible; if there is no real gain I'll go mechanical again, what are you successfully running on the street? The motor is a 340 .030 over with the MP .484 HD cam, 904, and 4.30's in the rear. Last edited by MOPAULY; 03/12/12 05:25 PM.

Re: Vacuum vs mechanical secondary [Re: MOPAULY] #1195809 03/12/12 05:29 PM 03/12/12 05:29 PM
Joined: Nov 2003 Posts: 36,041 Lincoln Nebraska R RapidRobert Circle Track
RapidRobert Circle Track R Joined: Nov 2003 Posts: 36,041 Lincoln Nebraska	Quote: I previously ran a mechanical 650 but would like to squeak a little better fuel mileage out if possible; The simple solution would be to go w vac, you will gain mileage and I'd suggest one of the Holley clones w adjustable air bleeds etc so you can dial in each subsystem spot on & leave nothing on the table live every 24 hour block of time like it's your last day on earth

Re: Vacuum vs mechanical secondary [Re: RapidRobert] #1195810 03/12/12 09:55 PM 03/12/12 09:55 PM
Joined: Mar 2005 Posts: 3,152 Central NC gch master
gch master Joined: Mar 2005 Posts: 3,152 Central NC	You have the .484 cam and 4.30 gears and you want more mpg?REALLY?! Vacuum is better bet I would be looking a spreadbore anything.

Re: Vacuum vs mechanical secondary [Re: gch] #1195811 03/12/12 10:30 PM 03/12/12 10:30 PM
A Anonymous Unregistered
Anonymous Unregistered A	750 CFM 3310 Holley.

Re: Vacuum vs mechanical secondary [Re: MOPAULY] #1195812 03/12/12 11:19 PM 03/12/12 11:19 PM
Joined: Nov 2003 Posts: 2,715 closer to Canadian beer! torkrules I'm neurotic
torkrules I'm neurotic Joined: Nov 2003 Posts: 2,715 closer to Canadian beer!	Quote: Need to purchase a carb soon for the 340 on my '72 Charger and am undecided between a vacuum or mechanical secondary 650. I previously ran a mechanical 650 but would like to squeak a little better fuel mileage out if possible; if there is no real gain I'll go mechanical again, what are you successfully running on the street? The motor is a 340 .030 over with the MP .484 HD cam, 904, and 4.30's in the rear. Dare I say-750 Edelbrock Thunder series (AVS). The cruise setting is much more tunable (step up rods and springs), and the accelerator pump doesnt dump as much fuel, plus its mechanical that acts like a vacuum secondary (air valve on secondaries). If you were racing it I'd say a Holley style double pumper all the way.

Re: Vacuum vs mechanical secondary [Re: torkrules] #1195813 03/13/12 02:20 AM 03/13/12 02:20 AM
Joined: Aug 2003 Posts: 805 Eastern WA P ProStock1320 super stock
ProStock1320 super stock P Joined: Aug 2003 Posts: 805 Eastern WA	My vote goes to the vacuum secondary carb. If you go Holley, buy the pop-top pod and change springs if want to tune further. Let the engine tell the carb what it needs - especially for street use. My outside the box (or outside my mind ) suggestion would be a Thermoquad - small primaries for what little bit of economy you'll get with your combo & giant secondries to let that thing eat. If you really want to spend some coin, consider EFI - either thru a throttle body on top or multi-port.

Re: Vacuum vs mechanical secondary [Re: gch] #1195814 03/13/12 12:23 PM 03/13/12 12:23 PM
Joined: Jan 2003 Posts: 7,843 Spring Mount, PA MOPAULY OP master
MOPAULY OP master Joined: Jan 2003 Posts: 7,843 Spring Mount, PA	Quote: You have the .484 cam and 4.30 gears and you want more mpg?REALLY?! Vacuum is better bet I would be looking a spreadbore anything. The 4:30's will most likely be swapped for a set of 3:23's

Re: Vacuum vs mechanical secondary [Re: MOPAULY] #1195815 03/13/12 12:47 PM 03/13/12 12:47 PM
Joined: Jan 2003 Posts: 9,838 Detroit, Michigan, USA! peelerboy master
peelerboy master Joined: Jan 2003 Posts: 9,838 Detroit, Michigan, USA!	Running a purple stripe .484 cam will pretty much defeat any attainable fuel economy for that car & engine combo. (small V8 in a large car) That's an extremely inefficient cam for any sort of attempt at hyper-miling. There will be no discernible mileage difference between a mechanical and vacuum secondary square-bore carb. Although, you might see a pinch better economy with a spreadbore AND if your driving habits reflect a legitimate desire for success by keeping your foot out of it. (which I personally have a really hard time doing when driving my Challenger) Dale

Re: Vacuum vs mechanical secondary [Re: peelerboy] #1195816 03/13/12 01:05 PM 03/13/12 01:05 PM
Joined: Jan 2003 Posts: 7,843 Spring Mount, PA MOPAULY OP master
MOPAULY OP master Joined: Jan 2003 Posts: 7,843 Spring Mount, PA	I'm not looking for economy based on the setup obviously, but the most sensible setup for the car; vacuum seemed to be the way to go to limit the secondary's vs the mechanical.

Re: Vacuum vs mechanical secondary [Re: MOPAULY] #1195817 03/13/12 01:14 PM 03/13/12 01:14 PM
Joined: Jan 2003 Posts: 9,838 Detroit, Michigan, USA! peelerboy master
peelerboy master Joined: Jan 2003 Posts: 9,838 Detroit, Michigan, USA!	Vacuum actuated secondaries do nothing more than slow the speed at which the rear butterflies open under full acceleration. Comparing apples to apples (square bore mechanical 650 vs. square bore vacuum), there will be no mpg difference while practicing conservative driving habits. But if you were to turn up the heat to a slightly "spirited" style of driving, the vacuum setup with definitely squeak out superior economy because it's not delivering as much fuel. At the same time, the car will be less responsive. It's all a series of trade-offs. If you're not seeking every last ounce of performance from this car, I'd go with the carb with vacuum secondaries.

Re: Vacuum vs mechanical secondary [Re: peelerboy] #1195818 03/13/12 01:35 PM 03/13/12 01:35 PM
Joined: Jan 2003 Posts: 7,843 Spring Mount, PA MOPAULY OP master
MOPAULY OP master Joined: Jan 2003 Posts: 7,843 Spring Mount, PA	See that's where I don't like the compromise - and makes me want to stick with a mechanical and just deal with it...changing the gears alone should help some.

Re: Vacuum vs mechanical secondary [Re: peelerboy] #1195819 03/13/12 01:37 PM 03/13/12 01:37 PM
Joined: Jan 2003 Posts: 27,421 Balt. Md 3 383man Too Many Posts
383man Too Many Posts 3 Joined: Jan 2003 Posts: 27,421 Balt. Md	I would not go to 3.23 gears with the .484 cam in the car. Ron

Re: Vacuum vs mechanical secondary [Re: 383man] #1195820 03/13/12 02:08 PM 03/13/12 02:08 PM
Joined: Jan 2003 Posts: 7,843 Spring Mount, PA MOPAULY OP master
MOPAULY OP master Joined: Jan 2003 Posts: 7,843 Spring Mount, PA	I have the rear handy so it's an easy swap to see how it works/doesn't work, although 3:91's or 4:11's would probably be more appropriate long term.

Re: Vacuum vs mechanical secondary [Re: MOPAULY] #1195821 03/13/12 07:33 PM 03/13/12 07:33 PM
Joined: Feb 2009 Posts: 207 People's Republic of Mass. B Belvedere2 enthusiast
Belvedere2 enthusiast B Joined: Feb 2009 Posts: 207 People's Republic of Mass.	Vacuum w/ 4 corner idle. Wouldn't do the 3.23s either. Then again I run a thermo, W2s, and 4.10s. Just make it work.

Re: Vacuum vs mechanical secondary [Re: 383man] #1195822 03/13/12 07:57 PM 03/13/12 07:57 PM
A Anonymous Unregistered
Anonymous Unregistered A	Quote: I would not go to 3.23 gears with the .484 cam in the car. Ron Really? I've had good luck using both in combo. Ran a little 318 into the 14's at 95mph on street tires and pulled down 17 mpg at the same time. Not stellar numbers but it was a wore out, re-ringed 318 that probably had 7.5:1 CR if I was lucky! I always had plenty of low end torque, even with a single plane M1 intake.

Re: Vacuum vs mechanical secondary #1195823 03/13/12 11:12 PM 03/13/12 11:12 PM
Joined: Jan 2003 Posts: 27,421 Balt. Md 3 383man Too Many Posts
383man Too Many Posts 3 Joined: Jan 2003 Posts: 27,421 Balt. Md	Quote: Quote: I would not go to 3.23 gears with the .484 cam in the car. Ron Really? I've had good luck using both in combo. Ran a little 318 into the 14's at 95mph on street tires and pulled down 17 mpg at the same time. Not stellar numbers but it was a wore out, re-ringed 318 that probably had 7.5:1 CR if I was lucky! I always had plenty of low end torque, even with a single plane M1 intake. Was that with the older style .484 MP cam ? Did you have a loose converter ? I would be surprised if the older grind .484 had much low end with 3.23's. I ran that .484 in a 383 with 3.91's and I loved it but it just seems it would not like 3.23's with its duration and LSA. It usually likes at least 2000 or more rpm in the ones I have seen. Thats why I feel it would be a little doggy with 3.23's but I have been wrong before. Ron Last edited by 383man; 03/13/12 11:13 PM.

Re: Vacuum vs mechanical secondary [Re: 383man] #1195824 03/13/12 11:24 PM 03/13/12 11:24 PM
A Anonymous Unregistered
Anonymous Unregistered A	It was late 90's and I had a 2800 stall convertor. 3,260 lbs. of car and 195 lbs of me.

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