what size carb to run??????????Help!!!!

[mod58]

Need help with carb brand and size,so many to choose from!!!!!!! 82 SWB 4x4 TH350 with stock converter,410 gears. 350.ci,9.32 to 1 compression,Comp Cams hydraulic roller,210/214 duration @.050 lift,474/474 lift.Vortec Bowtie small port heads,Edelbrock RPM Air Gap intake,MSD HEI and long tube headers.I realy like the DEAMON 650 cfm vac sec cratemotor carb!!! What do you think!!!!!!!!!!!!!!!!!! Thanks>>>>>>

[Albatrossian]

Originally Posted by mod58

Need help with carb brand and size,so many to choose from!!!!!!! 82 SWB 4x4 TH350 with stock converter,410 gears. 350.ci,9.32 to 1 compression,Comp Cams hydraulic roller,210/214 duration @.050 lift,474/474 lift.Vortec Bowtie small port heads,Edelbrock RPM Air Gap intake,MSD HEI and long tube headers.I realy like the DEAMON 650 cfm vac sec cratemotor carb!!! What do you think!!!!!!!!!!!!!!!!!! Thanks>>>>>>

A 650 will be a nice carb with that setup.. I don't think I'd go any larger unless you were going to a QJet.

[swb85]

600cfm is gobs plenty for that motor, I wouldn't go any bigger. A truck avenger 570 would be perfect for your truck if you want a square bore carb.......a good q-jet would work well too.

[bad6772]

[ton*loc]

why are you all recomending such a small carb ??



in chevyhiperformance magazine like two months ago they did a carb test on a 305 with 268 high energy cam and vortec heads and rpm air gap. they tested like 4 different carbs ranging from 600 up to 950 and the one that ended up with the most power was the 850 it ended uop with 360ish hp or something along those lines

[Thumpin455]

Qjet.. ALways a fan of the Qjet. Of course it helps that I build them, but otherwise a 650-750 vacuum secondary Holley or Demon. I have had good luck with all the above, and a vacuum secondary 750 isnt too much for what you have.

[Pyrotechnic]

You can run a square bore 750 CFM carb on a 302 if you have the heads and cam and can turn high enough RPM. Sure, those peak HP numbers at redline will be impressive. But low end torque ? Not so impressive.

With this truck being a 4x4, having good flowing production heads, mild cam, and near stock powerband, and installed in an application where low end torque is most important, a square bore 600 to 650 CFM carb is sufficient.

And yes, a factory Q-Jet on a 305 will be able to flow 750 CFM. So shouldn't an aftermarket carburetor be sized the same way ? Not exactly. The Q-Jet has tiny primaries that produce great low end torque on any motor, but most importantly, the secondaries are variable in air and fuel flow based on how much air the motor can suck through the secondaries. Above the secondary throttle blades there is a spring loaded air door. Attached to this air door are metering rods that hang into the secondary jets. The more the door opens, the more fuel those jets can flow. Installed on a small motor, the air door may only open a little allowing a small amount of air, and the jets are only flowing a small amount of fuel because the fattest end of the metering rods are still in the jets. Installed on a big block, the air door gets pulled all the way open allowing maximum air flow, and the thinnest end of the metering rods are in the jets allowing maximum fuel flow. This is why a single Q-Jet could either be installed on a 305 small block or 454 big block and run fine. The Q-Jet will only flow as much as the engine needs.

Your typical square bore Holley or Edelbrock AFB has a secondary fuel metering system that is not variable, and the secondaries are either open or closed, so sizing the carb correctly is critical.

[Billla]

Originally Posted by Pyrotechnic

Q-Jet has tiny primaries that produce great low end torque on any motor

This is really the key to the Q-Jet, and one of the design criteria for any spread-bore carb that really makes a difference on a daily driver. Driveability is all about intake velocity - too large primaries/secondaries, too large intake runners and too large head intake runners all make power - but not at the RPMs we use in a daily driver. Smaller is always better for driveability and gives up very little power, which is also why we use vacuum-secondary carbs for DD's

For a square bore, the oft-stated formula works pretty well:

CFM = ((CI x RPM)/3456) x VE, with VE around 85% for a 1HP/CID engine. This means with a 600 CFM square-bore you're not into the secondaries until 3500 RPM, and you're not outbreathing the carb until 7000 RPM A little bigger is fine (650, 670) but a 750 would just be a dog.

My pushback, as always, on the Q-jet is just that it takes an experienced tuner to make it work if the engine is relatively warm - and parts are getting more and more difficult to come by. As a "drop on" carb, it's typically going to be closer to the right tune than anything aftermarket, but if it does require tuning you're going to need professional (or very experienced) help. Not starting a(nother) Q-Jet war - just my experience, take it for what it's worth

[72lb4x4]

According to "Holley Carburetors and Manifolds" page 8 graph, a 350 CU engine at 80% volumetric efficiency needs 600CFM at 6000 RPM. The book is from 1987, so it may be interesting to hear differing opinions, or ideas after 20 years.

Certainly head design has changed, but not everyone has newer heads...

[Thumpin455]

Pyro is right on with the Qjet, they are great economy carbs and they can still make great power on par with Holleys with the same tune. I loves my Qjets.

A mechanical secondary Holley, or double pumper, is the one that you need to size absolutely correctly. An 850 on a 305 with mechanical secondaries will be a bog monster in a truck.

Vacuum secondary Holleys will only open the secondary as much as it needs to, but the opening rate can be adjusted with springs in the dashpot. One of the things I do with them is add a Quick Change kit for the springs, you remove two screws and you can swap the spring to a lighter or heavier one in seconds so the secondaries open earlier and faster or later and slower. While it isnt as good as a Qjet IMHO, they do work quite well when tuned correctly.

The best one I have is a 780 with downleg boosters, two metering blocks with power valves, and quick change secondary springs. On smaller engines like 350-428, it runs great doesnt bog. I havent tried it on a 5.0L yet. Its the carb I plan to run on the 351C in my 67 Cougar.

With Holleys I prefer center hung floats with dual inlet bowls rather than a transfer tube. Dual metering blocks arent absolutely needed either since the vacuum secondary carbs will have a plate that acts like the jets for the secondary circuits. You can change the plate, but usually its pretty close to optimal.

Lastly its not so much the size of the carb that matters, what matters is the design of the carb. The shape of the venturi, boosters, and the efficiency of the carb make a much bigger impact than how much airflow it would theoretically flow.

Pick up Cliffs book, get the primary shaft bushing kit, a tap and die set, and rebuild a Qjet following the book. You will love it.

[Albatrossian]

Anyone got a ISBN number or a link to where I can purchase Cliffs book? I love me a QJet, but I'm not good at building them.

[Billla]

Originally Posted by Albatrossian

Anyone got a ISBN number or a link to where I can purchase Cliffs book? I love me a QJet, but I'm not good at building them.

0-89586-301-4 is the book I use.