In what situations would you use a double pumper instead of a vaccuum secondary? I thought that double pumper meant an accelerator pump for primaries and secondaries so when you mash the gas it squirts into both. Why this over a regular vaccuum secondary, performance-wise, excluding gas mileage? The only thing I can really see is the mash off the line and if you had a manual tranny, when you hit the gas after a shift you would want the extra squirt. Anyone fill me in?

Now, though, realistically, how many miles per gallon less does a double pumper give, in general? 1-2? I doubt it'd matter what engine, just in general..

Thanks...

Double-pumper = manual secondaries; you floor it, they open

Vaccum secondaries = secondaries operated by primary venturi vacuum (different from manifold vacuum, it is the same as the "signal" that pulls fuel into the primaries, it increases in proportion to air flow through the primaries)

Vacuum secondaries are better with an inexperienced driver; the kind that just stomps the gas to the floor while the car is sitting still. People get used to being able to do that with small stock carbs with an air valve or other means of regulating air flow, i.e. the carb protects itself from that particular driver error. But if you use a big enough carb to get max HP out of a motor, odds are, when you do that at low RPM like off-idle, the amount of air that will come into the intake is SO HUGE in proportion to the engine's demand for it, and the air flow SO SMALL because the RPM is low, that it will go horibly lean right then; bog; stutter; hesitate; maybe backfire through the carb; and worst of all, run much slower than it would if the driver knew how to drive it.

If the driver knows how to drive a car and can avoid that temptation, which is easier to avoid in a stick car, then the man sec carb will almost always go faster.

As far as gas mileage, if the driver never uses the secondaries, the 2nd pump never operates; and gas mileage is the same as it would be with an otherwise similar vac sec carb.

With your foot to the floor the Vac carb opens the sec. as the engine needs the extra air. DP opens the carb all at once usually over carbing the engine at anything but very high RPM. Guys think that DPs make more power because in lower gears the shock to the tires will cause them to break loose. Actual acceleration will usually be better with a vacuum carb on anything but a true race car.

Originally posted by Mike Feudo:
With your foot to the floor the Vac carb opens the sec. as the engine needs the extra air. DP opens the carb all at once usually over carbing the engine at anything but very high RPM. Guys think that DPs make more power because in lower gears the shock to the tires will cause them to break loose. Actual acceleration will usually be better with a vacuum carb on anything but a true race car. Exactly what Mike said..just because the secondaries can be opened sooner with a double pumper doesn't mean it's faster.My mild combo actually ET's the same and MPH's slightly higher with a 750 vacuum 3310 vs. a 4779 double pumper
I plan on making this comparison again now that the car has 3.31's

From Holley's web site:

"For high performance engines a carburetor with mechanical secondaries has
an inherent advantage over a carburetor with a "controlled" secondary
system (air valve or vacuum diaphragm). This is possible because a
controlled secondary carburetor, until it reaches wide open throttle, will
not have as great a pressure drop below the throttle plates as would a
mechanical secondary unit. The greater the pressure drop below the
throttle plates the more dense will be the fuel/air charge to the engine
and, hence, the more output."

More:

"As a rule of thumb, vacuum secondary carburetors work best on:
Relatively heavy vehicles
Street gearing
Automatic transmission
Engines built more for low-end torque

Conversely, mechanical secondary carburetors seem to work best on:
Relatively light vehicles
Strip gearing (4.11 or numerically higher)
Manual transmission
Engines built more for top-end horsepower"

My personal experience, having tried both on my Chevelle, is that vacuum secondaries work better on a car with a torque converter that stalls at 2400 or less. Better streetability and better/more consistent 60' at the track.

I will agree with Mike and Chris, and also Holleys info...

If you have a car that can use full throttle all at once (light car, manual trans or steep gears, maybe launching at high rpm AND the ability to hook up), then the DP would be a good carb... But I'd still lean toward the VS for a predominantly street-driven car. While that DP might give a little better performance (overall, but for all-out racing), it will not be significant.

Where a true race car might be searching for that extra 0.01 second gain, a streetcar still has so many places that aren't optimized (tires, suspension, engine) that it doesn't make much sense to go to the DP...

Many enthutiasts also tend to over-size their carbs... Basically, It you have a carb that flows 750 cfm, you need an engine that needs that 750 cfm... On the street, at lower RPMs, pretty much NO car needs 750 cfm... The VS allows the carb to open as it is needed, where the DP is opened as the "driver" desides... On the racetrack, most cars are launching at RPMs that require almost the full ability of the carb... On the street, this usually isn't so... For any car, even a 2-barrel will usually work well, until the cfm requirement is exceded, usually only at higher rpms...

It is very hard to drive a DP and NOT get into the secondaries a little, which will shoot some extra fuel each time, so mileage will usually suffer...

I look at the Double Pumper on most street cars and think of the stickers/coffee can exhaust/rear wing you see on the ricers... totally unnecessary...

I thought that double pumper meant an accelerator pump for primaries and secondaries so when you mash the gas it squirts into both. Why this over a regular vaccuum secondary, performance-wise Taylor, you are correct on the "Double Pumper" designation. Any carb works off the vacuum signal it receieves from the engine. The pump shot is used to cause the engine to accelarate, which in turn will increase the vacuum and therefore pull more fuel into the engine... The engine generally needs the pump shot to increase speed at a reasonable rate.

I am not an expert, but have raced a few cars and tinkered with my fair share of carbs...

IF you had a mechanical secondary carb with NO secondary squirter, then when you opened the secondaries, the signal to the primaries would be reduced and you would get a bog as there would be far more air than the engine needed... That secondary squirt also allows the extra air mixture to not be as lean, until the secondary venturies vacuum signal begins pulling the fuel into the engine...

Vacuum secondary carbs don't need the second pump, because the secondaries gradually open allowing the proper amount of air into the engine, and therefore pulling in the proper amount of fuel also...

Pat you have to be kidding. Do you really believe your quote. If it were true every street car would run better with a pair of dominators.

Originally posted by Mike Feudo:
Pat you have to be kidding. Do you really believe your quote. If it were true every street car would run better with a pair of dominators. Pat's quote (really Holley's quote) begins with "For high performance engines..."

99% of street engines probably don't qualify as "high performance engines" in that context.

.

Originally posted by Motor Martyr:
Maybe YOURS doesnt! That peice of ***** couldnt fall out of a tree. Brian,
I believe that statement is TOTALLY uncalled for, serves no constructive purpose, and it NOT the type of attitude that is welcome around here. :rolleyes:

for the street i would always go with a vac sec. if you've got some gear and convertor and you're interested in picking up some ET then go with the dp. i've done several back to back testing on a 355, a 400sb, and a 454, and the dp ALWAYS ET's better. not as much of a difference on the 355 as it was on the 454, but still better on everyone of them. the biggest difference was always in the 60ft time. my car had 4.10 gears with each engine, and at least a 3000 stall. the difference on the 355 was only .04 in ET so i elected to use the 3310 instead of the 750dp since i mostly drove it on the street. the 454 picked up a 1/4 of a second going from a 3310 to a 750dp. it wasn't a mild 454, but it wasn't exactly wild either. that's just my experience with it. and i spend much more time trying to get the 3310 to ET what the dp did.

.

It ok, we all know hes a teenager. When he gets through puberty he'll grow up!

So, based on opinions and experience, what would you put on a heavy Chevelle with a 454 and 4.10s and a manual tranny?

.

Soccerguy045,
No matter what the opinions were here, if I had 454, 4.10.s, manual I would be getting a double pumper. And with my 454, 3.55's. Auto, after reading these posts, I will be sticking with Vac sec.

I just bought a 750 Speed Demon to test against my 750 Holley 3310. Maybe I have made a bad investment after reading this post but I will see what the Demon has to offer and if I can get a better tunability than the Holley. This is a street car, so Hopefully out of the two one will give tell which is better for MY combo.

brian,
i think you have some words that are spelled wrong on your web page.no offense just wanted to let you know.
shane
was checking out your combo

Originally posted by Motor Martyr:
</font><blockquote>quote:</font><hr />Originally posted by Georgia69:
</font><blockquote>quote:</font><hr />Originally posted by Mike Feudo:
Pat you have to be kidding. Do you really believe your quote. If it were true every street car would run better with a pair of dominators. Pat's quote (really Holley's quote) begins with "For high performance engines..."

99% of street engines probably don't qualify as "high performance engines" in that context. </font>[/QUOTE]Maybe YOURS doesnt! That peice of ***** couldnt fall out of a tree. </font>[/QUOTE]Ain't you the fella who butted into the BG tech thread with an unrequested opinion about Holley Vs BG, turned it into a pissing contest, then started to whine for being called immature and demanded respect? You cry about wanting respect and then act like a 10 year old. This is the impression you have given to a complete and unbiased stranger.

I'll give you this advice, and you can do what you want with it. A man's good reputation is all he really has. He works to earn it, No one can take it from him, but he can give it up willingly. If you want some respect for your 22 years of age, show some respect and act mature.

What kind of suspension? What's the powerband? What kind of E.T.'s are you looking to lay down? What cam, heads, intake, etc.?

Probably wouldn't be too difficult to pick up a 750 VS and a 750 DP off of Ebay. Something cheaper that needs a rebuild. Go thru them both, find out what you like...and then sell the other or keep it as a spare.

Probably cheaper than buying a brand new version of either carb. Just a thought.

Originally posted by DEEBOO:
I just bought a 750 Speed Demon to test against my 750 Holley 3310. Maybe I have made a bad investment after reading this post but I will see what the Demon has to offer and if I can get a better tunability than the Holley. This is a street car, so Hopefully out of the two one will give tell which is better for MY combo. Don't second guess yourself. I made the same swap last season and the car went faster with the Demon right out of the box. The Demon's quality is great and they are easy to tune -- if you even need to.

.

.

Brian,
I've just gotta ask one question...

Why are you hanging out here on a CHEVELLE forum? :confused:

Pssst...Dude, you drive a CAMARO!

I'm not dissin' F bodys...I've owned them before.
BUT...
Leaf sprung and probably 400 lbs lighter, not much in common with the A bodies that are so truely loved around here.

You're an "orange" in our "apple" orchard.

Wouldn't your vast knowledge and expertise be much more useful and appreciated when applied over on Team Camaro or NastyZ28 ?? ;)

BTW...no one here appreciates their beloved Chevelle being called "junk"! :rolleyes:

.

DEEBOO,
your the one I want to hear from. I was going to make the same move and step into a speed demon in place of a 3310. I mostly hear the DEMON IS very touchy in dialing in, but than ERIC68 claims out of the box performance. I would enjoy that if thats the case. Again most say THE DEMON takes more work than dialing in a holley.

.

Rich

I have a speed demon 650 mech/sec on my motor. Out of the box the thing runs great. Even on a mild geared car like mine.

David