Thread: Carb question

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Everybody seems to recommend a vac secondary carb when you have an automatic transmission. Why is that? What's the difficulty in setting up a mechanical secondary carb with an automatic trans?

Vacuum from the engine determines when the secondaries begin to tip in on a vacuum secondary carb to eliminate engine bog. And yes you can adjust the mechanical linkage on a mechanical secondary carb to accomplish the same thing, but it's a major pain in the butt and you generally end up with the secondaries not completely open at WOT.
With a Holley vacuum secondary carb you can fine tune the secondaries with their spring package.

the beauty of a vacuum sec carb is it gives the engine no more than what it ask for .. but a big enuff engine can easily swallow a double pumper .. i run one on my 429 .. the windsor loves a 750 vac carb..

Originally Posted by NTFDAY

Vacuum from the engine determines when the secondaries begin to tip in on a vacuum secondary carb to eliminate engine bog.

The bog you mention, what's the cause?

Originally Posted by ceh383

The bog you mention, what's the cause?

A carburetor works off Bernoulli's Principle. Airflow velocity is increased at the venturi(s) because they are smaller than the carb inlet at the airhorn. This increase of velocity creates a vacuum that pulls fuel from the reservoir bowl to mix with the incoming air. If there is not enough air flow (thus insufficient vacuum) to pull fuel and atomize it properly, then you will experience a bog. That's what happens when all four barrels open at low rpm's. Too much carburetor or too little motor, however you want to look at it.

Most carburetor manufacturers will recommend a vacuum secondary carb if you are using a converter that stalls at less than 3000 rpm's. Over 3000, there's enough airflow to support all four barrels and get a good mix, assuming you have sized the carburetor properly for the size of the motor. For max power street and strip, a good rule of thumb is cubic inches times 2 = CFM.

Originally Posted by techinspector1

A carburetor works off Bernoulli's Principle. Airflow velocity is increased at the venturi(s) because they are smaller than the carb inlet at the airhorn. This increase of velocity creates a vacuum that pulls fuel from the reservoir bowl to mix with the incoming air. If there is not enough air flow (thus insufficient vacuum) to pull fuel and atomize it properly, then you will experience a bog. That's what happens when all four barrels open at low rpm's. Too much carburetor or too little motor, however you want to look at it.

Most carburetor manufacturers will recommend a vacuum secondary carb if you are using a converter that stalls at less than 3000 rpm's. Over 3000, there's enough airflow to support all four barrels and get a good mix, assuming you have sized the carburetor properly for the size of the motor. For max power street and strip, a good rule of thumb is cubic inches times 2 = CFM.

I knew someone would come on that could explain it a lot better than I could. Thanks, Richard

Alright, good answer. I have seen Cfm = (engine size/2) x (maximum rpm/1,728)] / VE % will give the correct cfm. Using this formula I would need a 546cfm carb. Using CID x 2 I would need a 710cfm carb....Big difference. A debate on correct sizing is not what I'm looking for here. Here's what I'm after... I have a 390cfm double pumper that came off a circle track(short track) car with a 355cid engine that lived in the 5500 - 8500 rpm range. I'm wondering if it will be worth the trouble to put it on my motor. It has a 355cid built for 2000 - 6000 rpm range, but I have an automatic. The carb is one of THESE.

Originally Posted by ceh383

Alirght, good answer. I have seen Cfm = (engine size/2) x (maximum rpm/1,728)] / VE % will give the correct cfm. Using this formula I would need a 546cfm carb. Using CID x 2 I would need a 710cfm carb....Big difference. A debate on correct sizing is not what I'm looking for here. Here's what I'm after... I have a 390cfm double pumper that came off a circle track(short track) car with a 355cid engine that lived in the 5500 - 8500 rpm range. I'm wondering if it will be worth the trouble to put it on my motor. It has a 355cid built for 2000 - 6000 rpm range, but I have an automatic. The carb is one of THESE.

My own personal opinion is that it will be more trouble than it is worth.

By either of the two formulas given to determine carb size this would be an under sized carb. Given that, would there be sufficient vacuum to pull fuel and atomize it properly?

Yes it will work just as a Stromberg 97 would work if you could mount it, but in my opinion drive ability will be adversely affected.

This is really a very special purpose carb. There's some tuning required to make it run well on a track and it appears to be very specifically designed for racing applications where the engine spends a lot of time at high RPM. See the tech advice on their web site.

http://www.vdlfuelsystems.com/80507_390_tech.htm

If you already have the carb, you can always try it and if it doesn't work, put a more conventional carb (600 cfm Edelbrock or Holley or a 750 cfm QJ).

Let us know what you end up with.

Regards,
Glenn

I've read the tech article and I have the carb, it was a freebe....It came off the race car because it failed tech inspection, the car ran great with it so I know it works. I currently have an Edelbrock 1405 and I don't like it at all. So if the VDL carb will work I'll use it....If not, It's time to buy something else. My big issue is time, I don't have a lot of it to mess around with something that is just not going to work.

I have had really good luck with the 1406 electric choke Edelbrock on 305-400 small blocks with mild performance enhancements. The 1405 works well in very warm climates. Also, the Edelbrocks, in general, tend toward the rich side out of the box and need to be tuned a bit. Fortunately, Edelbrock makes this very easy. Jets, metering rods and a gasket run about $20.

I'll be curious to hear how the 390 cfm works out for you. I think you may find it a bit small for low RPM but it will be fantastic on the freeway or at higher speeds.

Have Fun,
Glenn

I guess I'll try it out, nothing to lose but time I guess....
With the Edelbrock I have a flat spot off the line. I've tried everything I can think of jets, metering rods, squirters, springs. Went through the manual over and over, no matter what I do it has the same flat spot. It does seem to get worse when it's cold though. You get what you pay for, the Edelbrock was cheap....

It could be just the idle adjustment and proper positioning of the throttle plates which cause a stumble under light acceleration. Or your float could be to low causing a loss of fuel under hard acceleration from a dead stop.