I need help choosing a 4 barrel carburetor and spacer as my current 2 barrel carb is shot. I get more confused the more I research.

I have a 292 that I plan on building with ported g heads, modified b intake, ram horns, and either a Mummert y-270 or one of his y-280 cams. The 292 will go in a f100 with 3.89 rear end and a t5.

I'm leaning towards an Edelbrock thunder 500 cfm, a Holley street avenger 570 cfm, or a Holley 4160 600 cfm. I'm not sure if the 600 cfm would be too big. I have read some post saying that 600 cfm would be too big and some say that it would be fine. Cfm calculators indicate 500 cfm is the desired cfm.

Which carb would you recommend for my build? Also, which carburetor spacer would you recommend? There are just as many options with respect to the spacer as there are for carbs.

Thanks.

Either the Edelbrock 500 or Holley 600 will work well. The Holley won't be too big, because the rear barrels open by engine demand. You will never see the carb opened to 600 CFM unless you plan on running 7000 RPM.
As far as a spacer, I can't speak from experience, but maybe a 1" spacer with four holes. Need to make sure the holes are close to the size of the holes in the manifold.

   Sal

Good point, I know it won't get near 7k rpm. 6k rpm, if that, is more realistic.

The Street Avenger carbs are pretty nice.  With ported heads and either of those cams, go with a 670.  The 570 is for more tame, closer to stock builds.  Factory refurbs are sold on eBay, direct from Holley, for much less than a brand new carb.  The 570 should work, but it will be super lean out of the box, for that motor and may not meter fuel as accurately as the 670 - it's much more complex than raw cfm.

With spacers, the limit is hood clearance.  Get a few 4-hole spacers, phenolic or wood, of different thickness and try them.

The modifications to the B intake should include opening the bores to match the spacer.  If you have the money, a Mummert or Blue Thunder intake would be a good compliment to the other parts you will be using.



Lawrenceville, GA

As a general rule, always over-size the carburetor when dealing with vacuum secondary carbs.  Vacuum secondary carbs when operating as originally designed will only open the secondaries the amount actually required by the engine.  This makes them more suitable in most driving scenarios than a double pumper or mechanical opening secondary carb which is only the correct cfm size in the rarest of situations.
 
With that said, modified Y engines can handle up to 750cfm vacuum secondary carbs just fine.  500-600 cfm carb selections seem to be the norm for many mild Y builds but superior fuel metering in the newer carb designs will allow much larger carbs to be used without a sacrificing drivability or low end torque production.  Here are links to articles previously published in the Y-Block Magazine covering some of the carb and spacer selections.
http://www.eatonbalancing.com/blog/2015/05/01/unported-iron-heads-can-still-make-over-a-hp-to-the-cubic-inch/
http://www.eatonbalancing.com/blog/2014/11/23/intake-manifold-plenum-slots/
http://www.eatonbalancing.com/blog/2012/09/29/carburetor-spacer-testing/

Lorena, Texas (South of Waco)

Awesome articles about spacers and the slots.  How is the low end with the tapered spacer?  Is it best for a race motor or might it work well on a lower powered street motor?

I wonder how tapering a 4 hole spacer in a similar fashion for use with a dual plane intake, so that the top is 4 holes and the bottom is 2 slots,would work.



Lawrenceville, GA

I think the work that Ted did with the spacers seems to illustrate that the two sides can be independent (two parallel slots) and the 4 hole spacer above can just be tapered together fore and aft (primary to secondary as it were) - and the thing will work pretty well. The key being the axis on which the butterfly is turning.... how it guides flow out the bottom of the carb.
When the butterfly of the primary is tipping down so that the trailing edge is dropping below the base plane of the carb - having that tapered flow guide between the primary and secondary bores does the job.  

I remember having checked out what happens when you just stack up some gaskets on a "B" with the smaller bores - to make a Holley 600 butterfly swing. And somewhere down there the trailing edge is trying to "re-seat" on the lower, smaller bore. A disaster for primary flow. 

 
 Steve Metzger       Tucson, Arizona

The angle of the butterflies is an interesting observation..  They open in opposite directions, so at part throttle or with the secondaries slightly open, some of the airflow would be directed toward the center of the front and rear holes.



Lawrenceville, GA

Just to support some of Ted's articles, I am running a 570 CFM Street Avenger and I was using a 2"  open aluminum spacer. When we had my engine on the dyno, we switched from the 2" open spacer to a 2"   4 hole phenolic spacer and gained 12 ft / lbs of torque (HP was unaffected)! 

I love the smell of burning rubber in the morning!

Thanks, guys, I'm leaning towards a street avenger 670 or holley 4160 750 cfm and a 2 inch tapered spacer.