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Double pumper vs. Vac. secondaries


labrat

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Is all the hype you hear about double pumper carbs really based on fact? Is the vacuum secondary carb mainly for fuel economy like i've heard? I basically threw fuel economy out the window when I did the swap, and my buddy swears by the double pumper holleys. Is there any real difference besides the addition of accellerator pumps in the rear barrels? I'm just curious if it's worth the extra money, i've already got 2 vac. sec. 650 carbs (one holley and one edelbrock), is it worth upgrading to a DP? Thanks fellas....

 

Bill

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Guest Anonymous

The only issue with double pumpers is, yes you will lose mileage definitely, you have another squirter when you kick the secondaries. That said, its only bad mileage if your in the secondaries, otherwise not much change IMHO.

 

As far as Vacuum secondaries, I used to believe it was the 'street' choice because of what Hot Rod and Car Craft etc recommended.

 

Well they deal with heavy american cars. In a car as light as the Z, you don't need the vacuum secondary IMHO because the car's reaction to the gas is so abrupt and there is so much less load on the motor thats its ready to go, it ain't laboring with low vacuum trying to push 1.5 tons around and will bog if you put the gas to it to fast.

 

The math I've seen shows the actual CFM a 350 requires is like 670cfm or there abouts. If I were to do it again, I'd buy a 700 mech secondary carb and jet it down, I'd rather go a bit big, than a bit small. :D

 

Again, thats a opinion, but I really don't think these cars need a vacuum secondary, to light.

 

Regards,

 

Lone

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In my experience, the double pumper can make for significantly more low speed torque, at the expense of fuel economy. As long as you don't actuate the secondaries, it uses no more fuel than a vacuum carb with the same size primary throttle bores, booster type, jetting and idle mixture. Lone is correct that a 350 only "needs" about 650 cfm, but they usually run faster with more carb until law of diminishing returns is hit. IMO, this makes the Holley Classic 750 double pumper the ultimate hot rod carb. PS, mine has been tweaked by Barry Grant to flow 1040 cfm with no diminshed throttle response or poor carburetion. My observations have been that this type of carb is more effective than the Demon Carbs that BG sells now instead of their worked over Holleys. Even without a choke housing, the engine starts easier and idles better with their 4 corner idle circuit. (Ran actually, carb is off for a rebuild as 8 years of light usage have caught up with it :rolleyes: )

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  • 2 weeks later...
Guest Anonymous

Umm. I am pretty sure that you guys are wrong about the acutation of the carb. The terms "into the secondaries" apply only to vacumn carbs.

 

A double pumper is just that. All 4 butterflys open at the same time. You are always using all 4 barrels.

 

At least that is how they work on my '72 Corvette with a 750 Holley DP.

 

Mike

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A few points here:

 

Try a light (pink) spring in the vac secondary and it opens quicker - not as quick as a DP though.

 

Only the race carbs (Dominators and maybe others) have all 4 barrels opening all the time. Most DPs, (4776, etc.) don't start to open the secondaries until 40-45 degrees of primary throttle opening.

 

I may well move to a 650 DP. Jamie and Mike Kelly have talked me into just doing away with vac sec in the Z - the old light car arguement.

 

I agree that the mags cater to the heavy muscle car set when they talk about carbs, cams, etc. I think you can take much of their recommendations about cam size and go larger in a V8Z, and move away from vac. sec carbs to DPs and still remain streetable. Light weight is our savior :D .

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Guest Perry

Without much experience in these matters I'll stick my neck out and offer a theory.

Vacuum sec are probably better (throttle response and torque) at low end on milder engines where a DP might work better on higher performance applications and higher displacement engines.

Here's why. Assuming std Holley stuff with straight leg boosters you need to maintain a good amount of airflow in the venturies to get a good booster signal and draw/atomize fuel properly. A vacuum secondary will to some extent maintain primary side airflow in a range where the boosters work well meaning a better running car at lower RPM unless the engine is bigger displacement or higher performance and pulls a lot of air at low RPM. a DP will, as soon as your foot hits the floor, crank open to full capacity and if the engine doesn't pull enough air right away the booster signal drops and the mixture runs lean (same thing as too much carb).

So unless you do some carb work to get more booster signal or you really size that DP right you may get a bog with a DP.

Also, Vac sec carbs don't have a secondary accelerater pump because they don't open as fast. They open a bit slower allowing use of the transition circuit in the carb to work properly until booster signal brings in the main circuit. Therefore the second pump is not needed.

Perry

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Mechanical secondaries carbs (not all are double pumpers, double pump means two accelerator pumps) do NOT open all of the butterflies at the same time. They have a linkage. Only after reaching a certain point of throttle opening on the primary does the secondary start to open. At that point the linkage accelerates the opening of the secondaries so that at WOT, they are both just that. That is what you call "being into the secondaries". On a vac carb, the vac is only actuated after a certain point of throttle opening. Then the airflow through the priamary venturi (not engine vac) actuates the vac diaphragm which opens the secondaries. Because this is slow and linear, it is not neccessary to have the second accelerator pump which hides the "hole" created by the near instantaneous opening of the secondaries when you "punch it". The bog from most carbs when flooring it is by too LITTLE gas being applied from the accelerator pump and not by too much gas as most people believe. Thus if you are puttering around with a double pumper and stay "out" of the secondaries, it should get the same fuel economy as a vac secondary assuming they have the same size throttle bores, primary jetting, and idle mixture qualities.

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Mike is right on the money. The secondaries on a double pumper do not open with the primaries, only open when you really put your foot into it.

 

Mike

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  • 4 weeks later...

Thanks for all the info, this site is great! What I want to know now is, how much of a power hit will I take by running vac secondaries rather than a double pumper? I traded my 650 edelbrock for a 750 edelbrock (thanks TomahawkZ!)and I still have the 650 holley, both with vac secondaries. Which carb should I use? They're both in good shape. Or should I just sell em both and get a double pumper? Any input here is vastly appreciated, this things going back in the car in a week or two...

 

Bill

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Guest gprix1

Your last suggestion,

Or should I just sell em both and get a double pumper?
would be the one I would go with - if it were me :D

 

cheers.gif

 

- Rick

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Larry, where were you a few weeks ago when I needed a good 650DP!! Just kidding.

 

In the past month, I've had a 750 vac sec (3310), 600 vac sec (1850), and a 650DP on my 327.

 

The 600 vac sec was a very sweet, responsive around town carb. It had NO problems. I never did fully tune the secondary opening, but it was coming open too soon and even still, it never had the top end power that the 750 vac sec carb had.

 

But the 750 vac sec had a HUGE off idle / off cruise hole in the response. It might have been that the idle fuel restrictions were too small, I dunno. Mostly, the primaries were just too large (venturi diameter) for the motor.

 

Today, my boss told me to stay home (putting in WAY too many hours) so I took the 650 DP I just got in the mail from a nice Chevelle dude up in Canada, and tore it apart. Lot's of cleaning, etc., spent the whole day cleaning and working on it. I set it up with the jets, etc. that it would have came with from the factory (71 primary, 76 secondary, 6.5 power valve)

 

Got the DP on the car this evening, tuned the idle mixture and speed. I thought to myself "I wonder if that hole is gone?" and hammered the throttle very quickly full open then closed, with no stopping. I about had to clean my pants out!!!! malebitchslap.gifrockon.gif

 

After that I rushed around getting tools out from under the hood, put on the air cleaner (and a stub stack I got of ebay). I could hardly wait to get it out on the road.

 

I have three words for you:

 

DOUBLE PUMPERS RULE!!!!

 

Note that it's only a 650 DP. It drives almost as nicely as the 600 vac sec when taking it easy (I probably need to lean the jets down, and open the PVCR's to help around town character), but man if you're in the first 4 gears above 2500 rpm, hit the pedal ALL THE WAY TO THE FLOOR, there's just no comparison between the Double Pumper and the Vac Sec.

 

It's a light car, with a big engine.

 

PUT A DOUBLE PUMPER ON IT.

 

Just my opinion.

 

Vac Sec carbs are for bigger, heavier cars with low numerical gears and automatic trannies.

 

Mike, Jamie, Dave - thanks for continuing to preach the Double Pumper gospel to me. I know I'm hard headed, but I finally listened and I'm in love with the DP!! 2thumbs.gif

 

I have a feeling I'm ready for the test and tune at 75-80 Dragway (Monrovia, MD) this Friday!

 

The INSTANT kick in the back and the torque that I gained was nothing short of INCREDIBLE.

 

(Note to self: Hmm. Where is that Road Dyno thing I bought anyway?)

 

Sorry so long, but DO THE DOUBLE PUMPER! rockon.gif

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So I guess you are happy with the double pumper? :D

 

Glad to hear it is sooo good. Now I'm very curious, but don't have the funds for yet another carb. I'll stick to the 3310 750 for now. Thanks for the info, Pete.

 

Davy

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Fuel economy? What fuel economy, My v8Z gets pathetic mileage either way. Around town maybe 8-10 mpg. Highway maybe 12-14 and I have a 700r4 with 3.54 gears. I must say with 750 double pumper vs Edelbrock 750 my mileage is 4-6 mpg off.

 

But performance is much better with the Holley!

Oh and thats with 70 jets in front 74 in rear.

 

Mike

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  • 1 month later...
Guest Anonymous

Ok, I guess that I was wrong about the DP actuation. But on my Corvette, they all open at the same time equally. They also did that on my '70 Mustang.

 

So, why is that?

 

Mike

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Holley made a "center squirter" (650cfm?) that had the accelerator nozzles in the center post of the carb instead of the sides of the center section. This carb did have both venturies open equally, but this is a pretty rare carb, and was an aftermarket one. It was basically a race only product.

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Guest Carlissimo

I have one word to describe the response of the mechanical secondaries on my 750DP AFTERBURNER

 

I added a second spring to the linkage so that I can feel the primary stop with the gas pedal. That way I can floor it with just the primaries. Then, just when my passenger thinks they can start breathing again, I push the pedal a little harder and light up the afterburner ! Gets 'em every time.

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