defiant1

Need to know the pros and cons of each type of carburetor (Mechanical or Vacuum operated secondaries):

Thank you

d1

TripDeuces

I think vacuum secondaries are more forgiving on the street as far as set it and forget it. They'll open in a certain rpm range and be just fine for any street car.

Mechanical ones need to have the shooter size and rate of fuel volume (pump cam) set precisely for the engine in question. If you're not familiar with doing that then it's best to leave them to someone else. They also work best on a lighter car. By light I mean 3000#'s and under. They offer quicker response rate over a vacuum secondary as the two rear barrels will open instantly under WOT but are finicky if you're not familiar with them. If the one you get doesn't have the correct parts initially be sure to add in the added expense of picking up a secondary pump cam kit along with a selection of shooters.

As far as performance you're not going to see a huge difference here. Because a vacuum secondary carb doesn't have a fuel shooter it opens up the secondary air flap in a controlled manner so as not to cause a flat spot by having a huge gulp of air. That spring tension needs to give atmospheric pressure time to push the fuel into the secondary air stream. Mechanical secondaries fill the air with fuel at the same time so that doesn't happen but it still has to be precisely metered. That takes a little skill to do it right and possibly extra parts depending on your set up.

Maybe more than you wanted to hear but it sums it up pretty good I think. I've been around carbs as long as I've been around cars and I have to say I've never liked mechanical secondaries. A well set up vacuum secondary or variable secondary (Qjet, Carter AFB/AVS) carb will perform just as well 99% of the time and with a whole lot less tinkering.

defiant1

Thanks George. I have access to a Barry Grant Speed Demon carb for cheap ($200/relative to how much they go for retail). It is a 750 CFM w/mech secondaries.

The 425 I am in the process of building is going to be more performance driven, so I have been researching the double pumper type carbs to see if they fit the bill. Still looking to see if they will work well with the Torker I am going to use.

I already have a daily driver Oldsmobile, so I think my next car will focus a little more on raw performance without being an all out race engine.

Good info, and I don't mind tinkering on carbs. God knows am still recovering from my 2 months plus quadrajet debacle.

Thanks again.
Shaun

TripDeuces

$200 is a good price for a BG carb. BG is out of business though but I think regular Holley parts will interchange. On his web page he states that they are reopening soon but we'll see. I have the BG Six Shooter set up myself. (My only mechanically secondaries I like, lol) Just be aware that the Torker is a spread bore design so you'll need an adapter although the manifold is made with both bolt patterns. I used a simple square bore plate on mine. With the large open plenum of the Torker you'll definitely need more shooter in the rear of that BG. Good luck with it.

cutlassefi

That 750 will work fine, it's small enough and that 425 should handle it. But I'd use the new RPM instead of a Torker, no comparison for the street imo.

defiant1

Well, I just picked up the Speed Demon 750. If I find a helluva deal on a RPM Air Gap I will take your advice. The Torker was readily available and for $100. But I know where you are coming from.

I figured I would have to get a higher stall torque converter to use the Torker, but that will be a little later down the road.

cutlassefi

Not necessarily, that'll depend on the rest of your combo. And given that, a regular readily available Performer may end up being a better fit.

ah64pilot

I do not believe the Holley parts interchange with the Demon carbs. I just had www.davinciperformance.com build a carburetor for me. I was going to buy a second hand Demon but DaVinci urged me not to because the parts were unique and that since BG had gone out of business (for now) I might not be able to find components. I have run that 750 Demon before and liked it. This time around I opted to have one made specifically for my engine.

Dan DaVinci built me a Holley HP 950 flowing 930 cfm

PS...Dan didn't dissuade me from the Demon to sell me a carb. I got an HP950 for less than you can get it from Summit, and mine was specially tweaked (light bowls, anodized base plate, etc) and tuned for my engine

defiant1

Yeah, I had read that on your build thread that DaVinci was customizing a Holley for you. That is definitely cool. You will have to let us know how it performs with your setup.

I am trying to build this motor on the cheap (I know, talk about an oxymoron) but I try as I might. So, any deals I can take advantage of I do. And the Demon has been proven in the past, so I am optimistic in that regard.

Good luck on your build, it seems to be coming along nicely.

d1

ah64pilot

You will like that Demon...I liked mine years ago. It's a little tricky at the track because the transition from 2-4 barrels is so smooth you won't know how much throttle to give it. If you run slicks it won't be an issue but I never had money for them back then so I "feathered" the throttle to get the 295/50's to hook up.