Carb questions

[AdamInChains]

K this is a noob question but what exactly is the difference between Vacuum secondaries and a Double pumper?? Advantages/Disadvantages of each?? I have a 570 Holley Street Avenger i believe is a vacuum.

 

[fvike]

A double pumper has a direct link to the secondary butterflies. This makes all 4 open at the same time when you press the gas pedal. A Vacuum secondarie carburator has vacuum operated secondaries. Hence the name. This means that the secondary butterflies will open to demand - gradually as the vacuum increases.

The double pumper is usualy used on manual transmission cars, while automatics use vacuum carbs most of the time.
Advatages/Disadvantages can be discussed, some people won't touch anything else than a DB, others prefer the Vacuum sec. carb for it's fuel economy, etc. This does not mean that a vacuum carb isn't a preformance carburetor.

To determine what carb you have, look for this: Does it have a vacuum pod on the left side? Does 2 or 4 butterflies open when you pull the throttle lever open? If your answer is "yes" and "2", then you have a Vacuum carb. If "no" and "4", then you have a double pumper.

 

[D.Hearne]

A double pumper Holley or it's aftermarket clones (BG carbs and others that use the Holley design) is a mechanical secondary carb, the secondaries don't open though in tandem with the primaries, but rather it's got a progressive linkage, at about 1/2 throttle, the secondaries start to open and are fully open when the primaries are.The reason it's called a double pumper carb, is because of it's two accellerator pumps, one each for the primary and secondary. Now some new guy's confuse the dual feed (or line) Holley with a double pumper carb. A duel feed carb can be either, this is a feature that's tied to the bowl design. Holleys have used 3 basic fuel bowls on the 2300, 4150 & 4160 series carbs. These 3 are the side inlet (also called a side pivot, as the float inside the bowl pivots from one side). Next is the center inlet of which there are actually two designs, both receive fuel from a dual feed line on the passenger side of the carb but one has the float that pivots at the center of the bowl, midway up the bowl. The other pivots from the center but at the top of the bowl.A 570 Holley is a vacuum secondary carb that's also a dual feed, center pivot bowl carb, where the float pivots at the top. The center pivot, center hung bowls were only used on a very few specialty carbs in the 60's. The ones I've seen were for Shelby's.

 

[Max Power]

This means that the secondary butterflies will open to demand - gradually as the vacuum increases.

Actually, the secondaries open with a drop in vacuum, not an increase.

Vacuum secondary carbs are the perfect variable venturi carb. Flow based on demand. Issue a lot of performance fans have with it is that they can't "feel" the secondaries open like they can with a double pumper. If they can't feel it, they are not sure it is happening. I can't tell you how many times over the years I have had people complain that their secondaries weren't opening when something else was really the problem.

 

[D.Hearne]

Actually, the secondaries open with a drop in vacuum, not an increase.

Vacuum secondary carbs are the perfect variable venturi carb. Flow based on demand. Issue a lot of performance fans have with it is that they can't "feel" the secondaries open like they can with a double pumper. If they can't feel it, they are not sure it is happening. I can't tell you how many times over the years I have had people complain that their secondaries weren't opening when something else was really the problem.

You got is bass-ackwards MP. Vacuum increases as the engine rpms go up. This is what pulls the secondaries open on vac. sec. carbs. Depending on what spring is in there, say the stiffest, the engine's going to max out on the primary side, at that point it's got the highest vac signal with 2 bbls, then that vacuum pulls the secondaries open.

 

[danny clemens]

Vacuum secondaries are more street oriented than mechanical secondaries. You use springs with different tensions to get the secondaries to open when you want like D. Hearne said.

 

[AdamInChains]

Thanks everyone to for your replies! its much more clear to me now. So am i losing much power off the line usig a carb with with vacuum secondaries as opposed to a double pumper. I'm have a 570 with vaccums and i'm my motor is a 347. Some people think my motor is slightly under carb'd but it actually runs very well.

 

[D.Hearne]

With a 570 on a 347, depending on what sec. spring you have, it's likely pulling the secondaries open almost immediately. Yea, I agree that it runs well, (under carbed) the only place you might be leaving power on the table is in the upper rpm's. If you ever have the chance, take it to the track and make a few passes with the 570, then bolt on something like a 700-750, you'll see the difference there. With the triple Holley 250 setup on my 331, the Ranger would get up to about 90mph on just the center carb. Throttle response on just that center carb was fantastic. Then once you maxed it out on the center carb, and nailed the other two open, you really understood was was left out.

 

[AdamInChains]

With a 570 on a 347, depending on what sec. spring you have, it's likely pulling the secondaries open almost immediately. Yea, I agree that it runs well, (under carbed) the only place you might be leaving power on the table is in the upper rpm's. If you ever have the chance, take it to the track and make a few passes with the 570, then bolt on something like a 700-750, you'll see the difference there. With the triple Holley 250 setup on my 331, the Ranger would get up to about 90mph on just the center carb. Throttle response on just that center carb was fantastic. Then once you maxed it out on the center carb, and nailed the other two open, you really understood was was left out.

Very interesting thanks. I've considered upgrading to the 670 Street Avenger but i have yet to take my car to the track. If the 670 wont offer me any advantage off the line i prolly wont bother with the upgrade. Do these carbs come with 2 springs or something?? One for better gas milage and one for performance??

 

[D.Hearne]

I don't know what they come with, I never buy new carbs. I buy the used Holleys that the Edellbrock fans abandoned. You can buy the secondary spring assortment at most of the big chain parts stores. The pack comes with 7 different springs. The pack I have , I bought at Advance Auto Parts. Seen em at Autozone and O'Reilly's too.