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Old 06-16-2003, 04:32 PM
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Mechanical Vs. Vacuum Secondaries

I have a 1965 Mustang, and I am wondering about the affect of having vaccum secondaries ad opposed to mechanical secondaries. Below I have listed my specs and the carb in which I am considering is a 670 street avenger. Can you guys give me any of your experiences or suggestions with this carb and how it should perform with my motor? i have the stock gears as of now, but will be running 373's or maybe even 355's. the car is basically a weekend driver but i will be putting it on the interstate for about 3 hours once every other month. i am asking about the seconday type because i came across a deal on a street avenger for 255 dollars brand new. it is a 670 with vacuum secondaries. i understand that the mechanical is the choice by most. but as far as just crusing and only racing sometimes. would the vacuum secondary carb really hinder me? if 2 exact same cars had the same setup only difference being vacuum as opposed to mechanical secondaries. how much of a difference would be the end result be if both were properly tuned and they went head up. also with my setup, what type of intake would you guys recommend? Thanks

347 Stroker Motor
3.70 gear ratio
camshaft .518 gross lift intake\exhaust
244 duration at .050 intake\exhaust
2-1/2 flowmaster exhaust system
long tube headers
roush 200 iron cylinder heads minor home pot and polishing
1.6 ratio roller rockers
t5 transmission

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Old 06-16-2003, 05:16 PM
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You are suffering from the old 'bigger is better' myth. A properly sized carburetor is a function of how much air your engine will pump. Period. An engine is, at it's most basic, an air pump. A carburetor is nothing more than a venturi inductor that depends on a certain air velocity from an air pump (the engine) to meter fuel into the air stream.

For a drag-race only engine which runs at WOT all the time, vacuum secondaries make no sense. The airflow generated by the engine is relatively constant and in the top range of the carburetor, so there is no need to modulate the carburetor. Thus mechanical secondaries are the logical thing to use since they are very predictable and simple to tune.

However, on a street driven car, the air pump will run at wide range of airflow rates, yet the carburetor is expected to meter fuel at the appropriate rate for the widely varying air rate. Vacuum secondaries are an extremely elegant mechanism to do that function. They open the carburetor automatically based on the air flow rate demanded by the air pump, thus the carburetor is receiving the proper air velocity past it's venturi at all times. Open the secondaries too soon as is likely with mechanical secondaries, and engine performance will be very poor.

As long as the ultimate size of the carburetor is near the ultimate demand from the engine, vacuum secondaries will out perform mechanical secondaries on the street every time and do a credible job on the drag strip. A 650cfm vacuum secondary carb is a very good match for a 350cu in motor.
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Old 06-16-2003, 05:22 PM
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For the street I prefer vacuum secondaries, they offer a little better mileage and smoother throttle response. I have been running a double pumper for some time after the vacuum carb and the mileage definetly suffered. While the double pumper does have a quicker throttle response, the extra fuel you use through the secondary squirter doesn't make up for the mileage drop in a primarily street driven car. For drag racing the double pumper would probably be my choice, but you can tailor the secondary opening on the vacuum model to come in pretty quick. Plus I like the sound as the secondaries open on the vacuum model.
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Old 06-16-2003, 06:48 PM
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I once built a really nice vacuum secondary carb with all the bells and whistles, but traded it for a Factory Holley Race carb with mechanical secondaries which I still have. I made the change because it was what I had always been told I needed. I wish I had the old one back. The vacuum secondary carb had perfect throttle response and I had it tuned perfectly. I have done my best with the double pumper, and have never gotten the off idle response as crisp as the vacuum unit had. The vacuum unit will not run as good out of the box, but if you get it set up and spend some money there is no replacement on the street, and the difference at the track is a moot point. For all out race, the double pumper has a slight edge, but not much.

Chris
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Old 06-16-2003, 11:22 PM
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I like the Avenger

i have a homemade setup of a 302 with, 512Lift/284Duration cam, underdrive pullies, RPM performer intake, 670 Holley Street Avenger carb., Long tube headers(of course with cutouts), 3.73 gears...and I love the way the Avenger (with the softer spring in secondaries) opens up when i floor it...its like it moves and then at 3k rpms or so it hits me with even more power boost i love that feeling. and after im done i can put in the stiffer spring and save a little gasoline with the secondaries opening up later.
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302HO converted non-computer with, no A/C, no Smog Pump ,Twisted Wedge Heads w/ TF RR's, CompCam XE268H ,LongTubeHeaders W/ 2 1/2" Dr.Gas X-Pipe ,2Chamber Flowmasters, Summit Radiator 31*19, 3.73's , ASP UnderDrives, Edelbrock RPM Performer, Holley 670 Avenger Carb, CraneCams Fireball HI-6 , Dual Electric 30" across Fans, LakeWood 70/30's 50/50's, Black Leather seats , Chrome Ponies, 2kCobra R Cowl Hood , Cobra Bumper Cobra Wing, Red on black Interior
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Old 06-16-2003, 11:38 PM
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vacuum vs. mechanical

For the street go for the vacuum secondaries. i run a 650 vacuum on my mild 351c [headwork, cam and 2500 stall] and have no problems, also uses a hell of a lot less fuel than other carbies i've used in the past.
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Old 06-17-2003, 08:56 AM
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Quote:
Originally posted by willys36@aol.com


As long as the ultimate size of the carburetor is near the ultimate demand from the engine, vacuum secondaries will out perform mechanical secondaries on the street every time and do a credible job on the drag strip. A 650cfm vacuum secondary carb is a very good match for a 350cu in motor.
What he said!

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Old 06-17-2003, 09:10 AM
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Re: I like the Avenger

Quote:
Originally posted by sum87mustang
I love the way the Avenger (with the softer spring in secondaries) opens up when i floor it...its like it moves and then at 3k rpms or so it hits me with even more power boost i love that feeling. and after im done i can put in the stiffer spring and save a little gasoline with the secondaries opening up later.
sum87; I think you will find that your engine will pull even stronger and turn significantly better 1/4 mile ETs if you put in a stronger spring in your carb. That 'boost' you are feeling is the airflow kicking in once the engine finally catches up with the secondaries opening too early and killing the flow signal. Your secondaries are opening too early and you are actually bogging down and losing performance before that 'kick-in'. You should feel a constant strong pull from off-idle to top speed. I suggest you start installing stronger springs until that hesitation goes away, then your secondaries will be tuned for max power and max fuel economy.
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Old 06-17-2003, 10:04 AM
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I agree, you should not "feel" the secondaries coming in. If you do then it is bogging.

Should be a seamless rise in rpm and power, do your testing with a stopwatch not the seat of your pants.

BTW I happen to like double pumpers but only on manual trans cars, on an automatic it's a waste. You need to drive it differently compared to a vacuum secondary car.
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Old 06-17-2003, 10:38 AM
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another question would be...with the size of the cam i am using, does anyone think i may have a vacuum problem at idle? and with my specs listed in the first post, can anyone estimate my horsepower potential with the 670 avenger on top? and also, for street use, what is the ideal selection for intake? high-rise, dual or single plane, low-rise? thanks guys
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Old 06-17-2003, 10:40 AM
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head help

ok i think i'll go and put a stiffer spring...but i have a question now...i need to know what valve size is a good size to put on my E-7 stock heads after i port them....im not to sure what size valves would do some good for me?? thanks
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302HO converted non-computer with, no A/C, no Smog Pump ,Twisted Wedge Heads w/ TF RR's, CompCam XE268H ,LongTubeHeaders W/ 2 1/2" Dr.Gas X-Pipe ,2Chamber Flowmasters, Summit Radiator 31*19, 3.73's , ASP UnderDrives, Edelbrock RPM Performer, Holley 670 Avenger Carb, CraneCams Fireball HI-6 , Dual Electric 30" across Fans, LakeWood 70/30's 50/50's, Black Leather seats , Chrome Ponies, 2kCobra R Cowl Hood , Cobra Bumper Cobra Wing, Red on black Interior
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Old 06-17-2003, 02:18 PM
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Quote:
Originally posted by lakecityg
another question would be...with the size of the cam i am using, does anyone think i may have a vacuum problem at idle? and with my specs listed in the first post, can anyone estimate my horsepower potential with the 670 avenger on top? and also, for street use, what is the ideal selection for intake? high-rise, dual or single plane, low-rise? thanks guys
You are confusing the various vacuums in your engine. Manifold vacuum at idle has a bearing in how your power brakes will work and the vacuum advance of your distributor but has nothing to do with the carburetor secondary opening diaphragm. The vacuum source for the secondaries is the venturi in the primary side of the carb. At idle, there is virtually no air flowing through the primaries so there is little pressure drop. That is why you need an idle circuit in the carb with an opening below the throttle plates so manifold vacuum can suck some gas into the engine.

As you open the throttle plates, air begins to flow through the primary venturis, the pressure drops and gas is sucked through the primary jets. At the same time a vacuum is formed in the secondary actuating diaphragm chamber. This vacuum is resisted by the secondary spring so the secondaries are prevented from opening. As the throttle plates are opened more, flow and pressure drop increases in the primaries until you reach the point that they can't flow enough gas and air to serve the engine's growing needs. Vacuum in the venturi/diaphragm grows to the point that the tension in the spring is overcome and the secondaries begin to open, giving the engine the air and fuel it needs to keep increasing in power. That is if the spring is properly sized; your current spring is too light and your secondaries are opening too soon and killing the flow in the primaries, thus the bog. In fact, the secondaries on your engine with that size carb will probably never even come close to fully open if the spring is designed properly.

Incidentally, your cam is fairly mild so manifold vacuum should be fine. Hard to beat an Edelbrock Performer 180deg manifold for all-round street/strip performance.
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Old 06-17-2003, 05:12 PM
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i looked on edelbrock's sight and tried to find that 180 degree manifold that you mentioned. i wasan't sure which one it was. can you help me out and give me a part number. thanks
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Old 06-17-2003, 08:32 PM
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http://store.summitracing.com/defaul...fold&x=11&y=11

I assume you engine is a SBC. The above page shows all the Performer manifolds that Summit sells for SBCs. I assume #2101 will be your best bet. #21011 is the same thing only polished.
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Old 06-18-2003, 05:23 AM
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thanks for all the help so far willys36. but as far as myself, I am running a 347 in a 1965 mustang. I am a die hard ford fan. and wouldnt have it any other way. is the manifold you sent me the link for, basically the same look as the one for a ford? if any other question I have may arrive, would you mind me e-mailing you at your aol adress or even sending you an IM? wondering because I also have AOL. it would be a great help. thanks.
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