What are these? What do they mean? I'm looking to buy a Holley Carb, about a 750 CFM for the 455 swap. Which should I get?

 

Square bore (or "Holley pattern") carbs have all 4 bores of about the same size in a square pattern on the intake. Spread bore (or "Quadrajet" or "Thermoquad" patterns) have smaller primary bores and huge secondary bores, with the centers of the primaries farther apart than the centers of the secondaries.

 

Normaly Holley's & Edelbrock are square bore, and Q-Jet are Spread. Don't forget that there's also the Dominator style. So really there's three to deal with.
It really come down to the position of the thottle plates on the base. Spread bores will have the secondies more spread out then there primaries.

Just to cover my but, there's some Holley models that have spread bores, like the 4010 & 4011...

Ron

 

hmmm cool, I didn't know thermoquad where spread.

So most Edelbrock (ie AFB's) have square bores

Ron

 

I'm using an Edelbrock intake, so would it matter if I used an Holley? Would it matter if I used a Q-jet?

 

Actually the primaries are farther apart, but the secondary bores are so much larger they extend farther out at the sides.

Edelbrock makes intakes in both styles. Most Holley carbs are square bore, but you can put a square bore carb on an Edelbrock spread bore intake (I'm doing that on my car). Spread bore intakes are usually a little more expensive though.

 

If it's like a performer it's ready to go with a AFB or Holley. If you want to run a Q-Jet you'll need an adapter, and those things add a good 1" to your height - just so you know.

If there's a well theres a way, I mean you could run a Holley Dominator with the right adapter.

Ron

 

I think are posting is out of sync

Yea, it's been awile since I had a Q-Jet in my hands.
Anyhow you'll notice a spread bore intake, cause normaly (covering my but) they'll have a clover looking cut where the carb mounts.

Ron

 

Since we're on the subject of raising the height.... what use are carb spacers for? I've read articles of them actually adding power, but how?

 

You take that one 'Apeiron' hahahaha

 

Ok hmmm - a spacer adds intake volume, and in most cases will give you a gain on the top end, but you'll give up some low end.

It also can give you a good insulation from heat. So they will give you, at the least that benefit. With the right spacer of course.

Spacer height is a trial & error thing, so there's no telling if 1/2 is better than 1" with your setup.

Just don't give up air cleaner height. If you don't have hood clearance your better off dropping in a 1/4" - 1/2" insulating spacer.
Edelbrock make some pretty good ones. Also if you have a dual plane intake, get a dual plane spacer. Same with a single plane, get a single plane spacer

Ron

 

Sorry I'm late.

A 4 hole spacer will help increase the vacuum signal through the boosters and can help low-end performance, especially on a single-plane intake. Some have grooves in the holes, supposedly to improve mixing but I don't know how effective they'd be.

An open spacer increases plenum volume and can help improve high-end performance, especially on a dual plane intake.

Plastic or wood spacers also help insulate the carb from heat, as mentioned.