88_Import_Slaye

What is the difference between a double pumper, and...non double pumper? I read that someone on the boards went from a 600cfm to a 650cfm DP and dropped .7 off their 1/4. Why is that?

7plagues

iTrader: (1)

something that i want to know as well, im goin to switch from an edel 600cfm to 650 holley dp.

five7kid

iTrader: (14)

Double-pumper = mechanical secondaries. The flow through the secondaries is completely controlled by mechanical linkage. Only Holley and Demon (to the best of my knowledge) are currently making true double pumpers.

All others, including the air-valve secondary AFB, Performer, and q-jet, and the vacuum secondary Holley/Demons, open the secondary flow using a "need" criteria. In order to make sure the fuel is flowing into the secondary metering circuit, these type of carbs gradually bring open the secondary flow to avoid "bog". Mechanical secondary carbs, on the other hand, avoid bog by means of a secondary acclelerator pump, which gives the air flow a shot of fuel to make up for the lag in getting fuel flowing through the secondary metering system.

The reason you'll typically pick up in the 1/4 mile with a double pumper is you immediately have full flow through the carb upon WOT. The demand-type secondaries take a little time to develop full flow. Other factors typically come into play, such as improperly tuned demand-type secondaries, or mismatched carb size to the application, but spend some time around the pits at a drag strip, and you'll see almost 100% mechanical secondary carbs on race-only cars.

F-BIRD'88

And you'll see a lot of vacuum sec (holley) and airdoor/velocity valve type carbs (AFB,Qjet) being sucessfully used on many, many street and dual purpose street strip cars.
Discribing the secondary action as "gradual" is misleading.
When tuned right for your car the secondary throttles or air door open as quicky as the motor needs to have the extra air, as it accelerates up in RPM.
Much more flexable over a wide range of driving conditions then a double pumper carb.
Many people don;t understand a carb like a holley vac sec or a edelbrock or a QJET. Or how to tune them. for best performance.
There are tons 9sec superstock cars that go much faster than your or my car that use a stock type carb like a Qjet and a holley vac sec carb. these cars have to use the "stock carb" There is nothing gradual about the way these carbs perform.

Over all a Double pumper carb is usually not the best choice for a daily driven car 95% of the time and hold no real performance advantage. A lot of the time they do not perform as well. Unless you're willing to fine tune the primary side of a double pumper for your street driven car the typical DP gas mileage will probabily leave a little to be desired, the way the come calibrated from holley.

A car with a fairly built hi performance mtor with a 4 speed or with a matched stall speed converter is a good candidate for a DP. A car with an automatic with a stock converter (liker a th700r4) and/or less than drag strip gearing that is driven every day is better off with a vac sec or air/velocity controled sec carb most of the time.
its a matter of determining your real world driving needs and picking the best carb for the job.

7plagues

iTrader: (1)

would i be ok for a dp with my t56?

Air_Adam

Yes... I think it would have been better for F-BIRD to say "manual trans" instead of "4-speed". I'm sure thats what he was trying to say anyway.

F-BIRD'88

Is your engine stock, or close to it? Do you have low ratio rear gearing like 2.73 Does gas mileage matter?
If this is the case you'd probabily be happier with a demand sec type carb.

Air_Adam

My Camaro has a manual trans, street tires and 3.73 gears. I've tried both style carbs (750 VS and 750 DP) and IMO, they are both just as driveable on the street if setup properly, and the gas mileage wasn't much different if you drive like you are supposed to on the street. The double pumper did have noticeably more 'giddyup' when you romp on it from a bit of a roll, but takes more finesse to launch hard from a standstill than the VS carb. I'm using the vac sec carb right now though, because the DP was borrowed. Of the two, I'd go for the double pumper on the street and the track, and will be getting one over the winter.

Just my 2 cents.

7plagues

iTrader: (1)

lol far from stock look at my sig. just wanting more than this edelbrock... 3.42s

five7kid

iTrader: (14)

True, but those cars are nothing like a dual-purpose street/strip car, and the typical person on this Board spends nothing like what those guys spend to get those cars to run the way they do.

No argument there. That wasn't really the question here, but before recommendations are made, it needs to be completely understood.

Blackroc86

A Stock or SS Quadrajet can run 1500 to 2000 bucks on those cars.

five7kid

iTrader: (14)

I was talking to a guy out at the track with his H/SA '86 earlier this year. His LG4-based "stocker" had a 5400 RPM stall converter. My LG4 wouldn't even rev past 5200, let alone be able to do anything with that much stall.

When you're allowing the engine to rev that high at launch, of course a demand-based carb, especially one tricked-out for the track, is going to be wide open from the get-go.