Help chosing a carb size 650 or 750

[milner351]

As usual - I concur with Scott. For a primarily street driven vehicle with an automatic and mild stall converter - a vacuum secondary is going to make more people happy more of the time.

[91hrdbdy]

Well, seems there only 2 views here the 650 is fine or the 750 is needed.. i have a 2500 stall converter. maybe ill attempt rebuilding the carb. its just runs so rich right now it kills every one near it with the fumes.

[malc]

A lot of folks donīt like them but Iīm more than happy with my 750 Speed Demon, vacuum secondaries, 355 Chevy motor.

[greenjunk]

No matter what you're going to have to tune it.

[jcharliem]

Take these numbers and plug them into that Carb Calculator, 355ci, 7000rpm, 112.8% VE.
If you use the Wallace Racing Calculator, they say anywhere between 813-1056 cfm. I put a 830 cfm Holley on my 355.

http://www.wallaceracing.com/intakecfm.php

IMO without knowing the VE of the motor proper carb sizing is just a stab in the dark. And when stabbing in the dark bigger is always better, you can't go wrong with the 750.

BigAl / TC... whatever. You haven't been within 100' of a mill producing 112.8% VE. Get real.

To the OP... go with the 750cfm vac sec.

[Scott Liggett]

Well, seems there only 2 views here the 650 is fine or the 750 is needed.. i have a 2500 stall converter. maybe ill attempt rebuilding the carb. its just runs so rich right now it kills every one near it with the fumes.

Nearly all the time a rich condition like you are describing is either the floats set too high, or the power valve blown. Blow out the air bleeds with carb cleaner too. They get gummed up over time, reducing the air flow, which causes a rich condition. $35 for a rebuild kit is much cheaper than a new carb.

[BigAL]

BigAl / TC... whatever. You haven't been within 100' of a mill producing 112.8% VE. Get real.

Oh Really??, I own the motor dumbass, here's a pic of my dyno results the last column is VE, tell me that doesn't say 112.8%.


Here's the bad boy on the dyno.


You ever think I might actually know a little something about designing an engine combo, by the way this combo made almost 500hp with a weepy 280 duration .525 lift cam. So go back to that little hole you crawled out of and STFU.

[yellomalibu]

C'mon guys, these arguments are doing nothing to help the OP decide which carb is best for his mild 400 in a pickup truck.


Carry this hijack over to TC's "volumetric effeciency" thread and leave this thread to help the OP decide which carb is best for his project.

[Beagle]

Well, seems there only 2 views here the 650 is fine or the 750 is needed.. i have a 2500 stall converter. maybe ill attempt rebuilding the carb. its just runs so rich right now it kills every one near it with the fumes.

Winner winner Chicken Dinner! Like Greenjunk said, they all want some tuning to be right. Like Ron Ward said, you can probably buy all the stuff to rebuild and tune your current carb multiple times as cheap or cheaper than the Summit carb... Chinesium quality control is all over the map. You might get a great one, or it may be crap straight out of the box and never be right.

World says they only offer one size intake runner for that head, and that's a big damn head for a Mild 400.... 220cc intake runner just seems big for a mild build, but Mild to one person is not necessarily Mild to another person. The cam specs would be nice to know. With a rumpity cam and a big ol head I'd lean towards a 750 personally. If it doesn't make a bunch of vacuum, I'd also lean towards a mechanical secondary, but that's just me.

[redneckjoe69]

i agree,...rebuilding your current carb seems to make the most sense. i'm a holley fan but i rebuilt an eddy carb a few years back for a buddy. it was a piece of cake. ran great.

[68scott385]

http://www.edelbrock.com/automotive/mc/carburetors/pdf/carb_owners_manual.pdf

This is the Edelbrock Owner's manual for their AFB carbs. IF you need to know how they work you can read up but there is a tuning graph for each cfm / model in the back. After many years tinkering with their carbs I don't care for the graphs. I plotted my own points using a spreadsheet and flow area.

fwiw I think the 650 will suffice if you don't regularly turn over 5000rpm.

[STINEY]

I'm in for 750 vacuum secondaries too.

A larger (or more airflow) engine will pull harder on the venturies of a smaller carb, in effect "fooling" that carb into delivering more fuel than is actually needed. Simply putting the correct size cfm carb can cure many ills.

In other words, it can be very difficult to tune around the issues a too-small cfm carb can cause. Not impossible, but the result will be a rather large compromise as well, not what you want for a driver.

[BigAL]

Edelbrock Carbs are Junk, you'll be much happier with a holley. Can't go wrong with a 3310.

[68scott385]

Edelbrock Carbs are Junk, you'll be much happier with a holley. Can't go wrong with a 3310.

Yep, the AFBs are just as much a junk pile as the Q-Jet, that's why both styles were OEM products for so many years. We all now know that it was a conspiracy between the OEMs and Holley to generate business for the aftermarket. Yep, you hit the nail on the head.

It is SO very easy to spend other people's money, why not have the OP put an overdriven blower on it with two Holleys, that'll fix all his problems, right? If the OP already has an AFB, why not spend a few bucks tuning it instead of spending $300 -$500 or more for a new carb that he still has to tune?

[BigAL]

Yep, the AFBs are just as much a junk pile as the Q-Jet, that's why both styles were OEM products for so many years. We all now know that it was a conspiracy between the OEMs and Holley to generate business for the aftermarket. Yep, you hit the nail on the head.

It is SO very easy to spend other people's money, why not have the OP put an overdriven blower on it with two Holleys, that'll fix all his problems, right? If the OP already has an AFB, why not spend a few bucks tuning it instead of spending $300 -$500 or more for a new carb that he still has to tune?

Given the fact that he is running a 1406 Edelbrock which is a 600 cfm carb, I don't care who you are, you don't know what your taking about if you think that is the right size carb for a 400 cubic motor. He's going to have to buy a carb anyways, save the money on the rebuild kit and just get a Holley 3310, it is a 750 vacuum secondary carb that will only meter what the engine needs for airflow do to the vacuum secondaries. It is the most logical way to go and he can get a new one on Ebay for $315.