Intake Selection; Square Bore Vs Spread Bore

What is the difference between Square Bore and Spread Bore.

Small Block Chevy
Demon carburater
I am looking at an Edelbrock performer RPM Air Gap and it is offered with both openings.

Thanks
TZ

 

Square bore = both the primary, and the secondary throttle bores are the same size.

Spread bore = the secondaries are much larger compaired to the primaries.

Pick the one that matches you carb. spead bores are suppose to have a better part throttle feel/economy due to the smaller throttle plates

 

If you have a demon carb, you need the square bore.

 

spread bore is for Q-jets and the holley and motorcraft spread bore carbs. Square bore are for Carter and Edelbrock AVBS and all of the holleys with the same size primary and secondary throttle bores. Oh and the demons.

 

Thanks,
Any other suggestions besides the Edelbrock for a good street dual plane intake?

 

Spread bore above



Square bore above ...

 

ok, i've always wondered but never asked:

are the mounting bolt patterns on all spread bore carbs/intakes the same?

same question for square bore carbs/intakes?

 

I think? Q-Jets and Thermoquads have same patterns??...
But there are different square patterns.....a larger one for Holley Dominators...

 

I learned something today.

 

You can put a square bore carb on a spread bore manifold,with a flat plate adaptor(Edelbrock Performer). You can't put a spreadbore carb. on a square bore manifold(directly) but a thick adapter may work,,, BTW the Performer manifold is a good one for the street(1500- 5500rpm)

 

If you look at the Edelbrock carbs, you'll see that they are double drilled for the small (50s or so) & large bolt patterns.

 

My twin brother runs an AirGap intake and a 750 double pumper and he swears by it. Keith

 

I've bolted Edelbrock AFBs to aftermarket Q-jet spreadbore manifolds without adapters, you just have to use the right gasket to assure the seal at the big bore end.

 

Spreadbore secondary plates will not clear all adapter plates, they will hit about half open, don't ask me how I know this.

 

There are 3 common 4 brl carb bolt patterns. There is a small one used by early small 4 brl carbs, the standad large one that all modern carbs take, and the 4500 dominator flange and bolt pattern which is huge in comparison.

Most aftermarket carbs are double drilled so you can use either of the non dominator bolt patterns.

 

I already have the edelbrock performer rpm air gap #7501 but its square bore and i want to put my 69 rochester quadrajet on it, does any know what adapter i need? Thanks for any help.

 

They are the same pattern. I had a T-Quad on a Chevy 327 many years ago.

Bob

 

The intake you want is the performer r.p.m. Air Gap. If you can, top that with a wilson tapered 4 hole spacer. Then hold on.

Sent from my DROID device using the TJJ mobile app

 

Fellas, its been 5 years.

But this guy might need the help , he bumped it.

 

I have a Holley square bore carb that has 1-9/16" primaries and 1-11/16" vacuum secondaries...
It's off a '70 Chrysler car...
I think it's a 735 cfm carb that has the standard Holley bolt pattern...
That be a cool carb to tweak for mileage and performance...

 

wow this is an oldie.
I ended up with an Edelbrock air gap and a Demon carb. Have put 20-30k on the car since I got it going.