Vaccuum Vs. Mechanical Secondaries???

[72step]

I've been Carb shopping for a while now, and it's between the Holley Street Avenger, or the Barry Grant SPEED DEMON. I'm wondering what's more preferable Mechanical or Vacuum Secondaries on a Carb? Just by the sound of it, I would think Mechanical would be more reliable, but I'm not sure. I'd appreciate any help you all could provide. Thanks....

[Mike C]

http://www.hybridz.org/ubb/ultimateb...;f=21;t=000054

This should help you out.

[tom hand]

Manual transmission=Mechanical Secondaries
Automactic trans=Vacuum Secondaries

My 2 cents

[crazy longhorn]

For primarily street driven, I prefer the vac secondaries. They are also pretty forgiving on an over carbed situation. I have a 3310 Holley (750 cfm ) that has been run on anything from a mild 355 to a warm 455 with a little tuning.

[72step]

Thanks, very helpful. Now, what about CFM's? Everybody tells me a 750 has no business on a small block. I have a 327, and I would'nt consider it anywhere near mild. It's on the margins of being non-streetable. I was thinking of a 670 Street Avenger, what about a 750 Holley DP? Is 750 too much? Right now I have a tired 750 Carter AFB, and it has run rich since new, you can smell it in the exhaust, however, when you try and adjust the mixture, the carter doesn't seem to respond when you turn the screws and the engine is supposed to like want to die before you turn it back you 3/4 turn or whatever.? Thanks

[busterwivell]

In my humble opinion, you are overcarbed with your 750 AFB and you will still be with another 750.

[Fred T]

Get a carb in the 650-670 range. 750 is way over for a streetable 327. You already know that from the one you have.

[72step]

Thanks, now I know I need a 650 or 670 double pumper with vacuum secondaries. Now should I go with the Holley or Demon? I don't know, there's a 40 dollar difference. The holley 670 Street Avenger is $359, and the Demon with Vacuum Secondaries is $399. I'm thinking the Demon. Help me guys!!!!!

[slam33]

Are you looking for a streetable, good running motor or are you trying to run a race motor on the street ? The Holley is a street carb, jetted for low end torque. Holley doesn't expect people to run the thing past 7 grand all day long. It's ment to go up against the Edel 1406, which for street trucks is the best carb out there. The Demon is aimed at the guy who wants to run a 1/4 mile on weekends. The trick to picking parts is to identify what you want to drive then build that driver. If you want a street truck, build a nice street truck. If you want to race, then build a race truck but be prepared to work on it all the time. There is no such thing as a race car that never needs work. As for your carb look at a 600 to 650 for a 327 in a truck.

[bigjimzlll]

I'm not so sure about the 750 being to big...I had a 600 on my engine..thought it was under carbed..went with a 750...a vast improvement in power, all the way through the rpm range, but the gas milage has suffered, so I will run the 600 on the street, and the 750 at the track.. Im sure if I jetted the 750 a little leaner it would be ok on milage, but since I have both carbs......

[70shortnphat]

750 cfm and 795 cfm carbs were factory on engines even as small as a 305.

[slam33]

Q-jets have 1 inch primarys. In all acuallity if you want to talk volumetric efficancy, a 350 needs 495 cfm to run right. I ran a 750 on a tunnel on a 327 but I had the damm thing build to the hilt. It had a 1400 rpm idle when warm and you were either on it or off it. It was a great hot rod and the worst street truck I ever built. At the time I had my own parts house so I could keep ahead of it, but my counter guys, machine shop guys and even the stock boys and delivery guys gave me hell about it every time I towed it in with a melted cluch or a L shaped drive shaft. The 750 wasn't the problem it was just part of the problem.

[Ackattack]

I was looking at carbs the other day and saw that summit had a road demon carb (~ 670 cfm ~ $320) that was a little cheaper than the holley. I decided to do a little work on my edelbrock and keep it though.

There is some formula out there to give you a rough estimate of carb size. Has something to do with engine size and max RPM.

[bigjimzlll]

495 cfm for a 350 is ideal for VE???? I know the formulas too..thats to blanket a statement..it depends on too many variables. mainly what max rpm

[dion]

I know plenty of people with 750's on their small blocks and with a little fine tuning run great, I have a 700 Q-Jet on mine. If you have the right engine and know how to work a carb you shoudln't have much trouble.

[67shortie]

What is anything near mild? A 327 is a 327 in terms of cfm and air reqirements at max rpm's. The only thing you would need in terms of being bigger for cfm on the carb is running a very large plenum like a tunnel ram application or blower even more. If you are planning on running this on the street with a heavy truck, 3600+lbs, that engine will want velocity to build torque in an effective rpm range. My vote is for a smaller carb, the Holley street avenger 670 cfm in a vacuum secondary. Run it out of the box, see how it likes your fuel need, and jet from there for maximum power.

[Pont]

Waaaaay too many variables to say that a certain size carb would be best with a paticular engine displacement. A 283 or a 327 could easily handle more carb than a 350, or larger, if it had the right cam (flow), heads (flow), intake (flow) and exhaust (flow). See what I'm getting at? However, you can just as easily run out of cubic inches with too much carb. What compression are we talking about? How far from mild is the 327? Do you run it on pump gas? 87 octane? 93 octane? Do lots of research and listen, listen, listen to guys that have been there, done that. I've learned more from talking to people than I probably ever will by reading a "hotrod magazine", or "How To" book. It is easy (in my opinion) to over carb an engine. I've seen it time and time again. Someone will throw on a set of headers, slap in a bumpy stick and install a 750DP or 800DP and wonder why it runs like crap. The only advise I can give is to make sure all your components are designed to work together (not too much of this and too little of that) and that horse power is built on the top end. A carb is an important decission in making it all work together. Knowing how to fine tune an engine always helps too! Just remember, the more efficient it runs, the better it will run.

[72step]

It is a 327 block-HN-code-65 Vette, originally came with a FI and put out 375 hp. It's got a strong bottom end of course, forged steel crank, forged TRW pistons, .030 over, balanced assembly, Erson geardrive, 10:1 Compression ratio, Comp Cams 280H, 2.02 heads, stainless valves, Crane roller rockers, chrome moly pushrods, Edelbrock Performer RPM, Carter AFB 750 CFM. Holley Mech fuel pump soon to be replaced with elec. Headers, 2.5" exhaust. I might be over carbed, but the Carter sucks, I've never liked it since new, but now it's been acting up because of sitting. I don't want to over carb, but I sure as hell don't want to under carb and lose power. Currently the carb has a bad flat spot at the bottom end. Oh, and I'm burning super unleaded, 101 fuel once in a while, or I add an additive.

[wnc mountain]

I agree that there are more factors that effect cfm carbs. it depends on the build. I ran a holly 750 dp on a 304ci jeep. had a high rise, cam, .30 over, flat tops, headers, electric fuel pump. snapper rear axles like twigs though. Didn't run rich at all. So in a nutshell, it depends on the build and how your going to use it. mine screemed through the mud most of the time so I needed cfms for FLOW.
Thank God I grew out of that.

[6869704X4]

I would have to get out the Q-jet book to be sure but I dont think a Q-jet will always put out its max cfm. If I remember right the top plate for the secondaries regulates itself based on vacuum or airflow or something. This is if its set up right.