Vacuum or mechanical secondary

I don’t think how much you drive it on the street has any bearing on using a DP carb at all.

If you get a reasonable DP carb for the application and tune it, it will be no different than a similar sized, properly tuned VS carb.

Carbs don’t push fuel. The vacuum signal created by the engine draws fuel from the carb. The DP carb drives on the on the front barrels just like a VS carb, until you push the pedal pretty far down.

Some DP carbs, because of there design parameters and intended use will not be well suited on some mild street cars. This is because the starting point is too far off the mark and many hobbyists simply don’t have the tools and know-how to bring the tune around. That said, a 430 hp 440 will be able to handle pretty well many DP carbs.

BSB67 said:

I don’t think how much you drive it on the street has any bearing on using a DP carb

Carbs don’t push fuel.

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Uhh, the accelerator pumps do.
Normally I take your comments as gospel but a strong pump shot is not going to lead to long cylinder wall life for guys that drive 100% on the street.

Kern Dog said:

Uhh, the accelerator pumps do.
Normally I take your comments as gospel but a strong pump shot is not going to lead to long cylinder wall life for guys that drive 100% on the street.

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Same as every port fuel injected engine on the road except they shoot straight into the cylinder.
The pump shots ( yes primary as well, the one youre shooting in all the time) are designed to break up on the boosters and atomize for the correct AF ratio. The carb designers saw this internet arguement coming long before the internet.
A bunch of nostradamas's , really.

Kern Dog said:

Uhh, the accelerator pumps do.
Normally I take your comments as gospel but a strong pump shot is not going to lead to long cylinder wall life for guys that drive 100% on the street.

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I can change a pump shot rate and volume in 2 minutes. It doesn’t define the carb. You can drive all day and never use the rear pump.

And if it is tuned properly, you’re still targeting the ideal air to fuel ratio when using the pump. I.e giving the motor only what it needs. If you want to slow a car down make the pump shot too rich.

RemCharger said:

Same as every port fuel injected engine on the road except they shoot straight into the cylinder.
The pump shots ( yes primary as well, the one youre shooting in all the time) are designed to break up on the boosters and atomize for the correct AF ratio. The carb designers saw this internet arguement coming long before the internet.
A bunch of nostradamas's , really.

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Just about all the newer engine designs use the GDI (Gasoline Direct Injection) methodology......gas is injected directly in the combustion chamber, just like a diesel engine, immediately after the intake valve closes, milliseconds ahead of the spark. The injection pressure is high...1500-1800 psi. The engine has a high pressure pump fed by a lower pressure pump, all controlled by the on board ECM.......FYI.....just my thoughts......
BOB RENTON

Correct Bob

As a lot of members have already stated , it comes down to how well you have the carb tuned. People that have had a VS and then throw on a DP and say it was like a hole new car... chances are that the VS carb just wasn't tuned correctly. They threw on the DP and it was probably set up closer to what their engine needed , not solely because of the DP design.

Personally I run a modified holley 3310 with a proform main body that incorporates the down legged booster and a few other design changes that were not in the stock 3310 body . I actually made a thread about it I'm sure you could look it up if wanted . It's a street car but I do race the car regularly, nice crisp throttle , cruise smooth and clean , and when I want to try and put my foot threw the floor board, it does what it should. But I didn't just throw it on the motor and it ran like that. Everything has been tuned.

IT all comes down to tuning which ever carb you go with. I run a manifold vacuum gauge, and AEM wideband . These two gaugues alone will REALLY help you dial in your carb , the A/F gauge gives you so much real time info and takes a lot of guess work out of which direction to go with tuning ( rich or lean ) . Checking and learning to read your plugs will also help confirm your tuning.

Finally... engine masters did an episode on this exact subject. The results.... a DP out performed the VS ( peak power ) .

If you don’t want to mess around, go with VS. Smoother & easier on the street.

I've got 4 carbs: a 600 V/S Holley, a 650 AVS2, a 700 Holley D/P, and my new favourite, a Holley 650 D/P.

The Double Pumpers feel better to me on the street. I've run them when the car was auto and now as a 5 spd manual.

The car feels more responsive with the D/P carbs. Gas mileage has been pretty much the same. You drive on the front 2 barrels most of the time, same as the V/S. I don't buy into the extra engine wear theory depending on your driving style. This is important I think. If you jump in the car and drive with a heavy, numb right foot then the D/P on the street will use more fuel and be washing down the cylinder walls as KD said. But I don't drive like that. I meter out the throttle carefully,. I still hit it hard though, but only when it's time to let the horses run.

Try them both, what have you got to lose? It's only a few hundred bucks. Unless you're struggling to pay the bills, then you have to spend your money on something.

When my engine guy had my engine on the dyno, they both commented about how well my carb was tuned. I had this this carb on my old engine. I was just wondering because I bought this Edelbrock for $200 new in the box. I could buy a Fitech system that he has for 1/2 of new.

I think anyone that tells you that having a vacuum secondary carb is the same as a mechanical secondary, hasn't driven both. It's just theory in their heads. Vacuum secondaries just suck on a performance car. No way they can match the response of a double pumper. That said I have vacuum secondaries on my car, two of them, but the blower fixes the response problem.

Moparfever65 said:

I think anyone that tells you that having a vacuum secondary carb is the same as a mechanical secondary, hasn't driven both. It's just theory in their heads. Vacuum secondaries just suck on a performance car. No way they can match the response of a double pumper. That said I have vacuum secondaries on my car, two of them, but the blower fixes the response problem.

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I haven't driven a car with a double-pumper. Does the DP throttle response also hold true for the primaries when the secondaries are not engaged, or only when you mash the throttle?

68 Sport Satellite said:

I haven't driven a car with a double-pumper. Does the DP throttle response also hold true for the primaries when the secondaries are not engaged, or only when you mash the throttle?

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No, only when you open the secondaries. You do have more options on the primary pump shot on Holleys than other vacuum secondary carbs. For instance on an Edelbrock (or Carter) you have three positions of adjustment on the pump lever arm and that's all. It adjusts stroke distance and velocity. On Holleys you can change cams that affect stroke and velocity as well as changing to different sizes of pump housings. That goes for the Holley clones as well. Don't get me wrong, I started my hotrod life as a Carter AVS fan, and love the engineering of the fuel bowls that don't pour out when you change jets, and metering rods that give you fast and simple adjustments, but Holleys bring the punch.