Thread: Square bore vs Spread bore?

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Can anyone tell me exactly what the difference is between a Spread bore and a Square bore carburator?

Thanks...Mike

Square bores have the same size throttle plates for primaries and secondaries. Spread bores have bigger throttle plates for the secondaries.

Originally Posted by Gerald Streeks

Square bores have the same size throttle plates for primaries and secondaries. Spread bores have bigger throttle plates for the secondaries.

And because of that the bolts position on the carburettor throttle plate will be in defferent positions.

Weren't spread bores made to replace quadrajets with the bigger secondaries?


Don

Spreadbores are just Edelbrock and Holley version of Q-jet.

I think they have a cooler sound when you hit the gas. When those big secondaries open it sounds like a big vacuum cleaner...whoooosh.

Originally Posted by southerner

And because of that the bolts position on the carburettor throttle plate will be in defferent positions.

I'm bidding on a Holley 4779-7 Square bore on E-Bay. Will the bolt pattern fit on my new Edelbrock Air Gap (for SB) ?

Looking at the picture of the bottom of the throttle plate, the bolt pattern isn't square.

Mike M.

Is it a performer rpm air-gap intake? b/c I believe they are made for square bore.

No, it's an Edelbrock Air gap but the bolt pattern is the same as my old Performer RPM.

If the bottom of the carb looks like this it should fit. I have the 4777 on a Performer RPM, which i believe is just the 650 opposed to the 750 you are looking at.

it will fit the air gap is made to accept square and spreadbore carbs... so no worries

Thanks guys for your replys and the nice picture. Looks like it should fit.

Camino, Yeah I have a 4777-7 (650 CFM) on an Edelbrock RPM on the Nomad now.

Doing an upgrade on the 355 cu:

New Trick Flow 195 aluminum Heads
Edelbrock Air gap manifold (Durashine)
Hydrolic roller cam (not purchased yet) looking at the Comp 276HR
Larger 750 carb (subject of this post)
RPM stainless Roller rockers

This is mostly a show car (56 nomad) but being an old racer from the 60's,
this thing has got to run pretty strong.

No problem, would that 4777-7 be up for sale??? I'm having some issues with mine at the moment, its a good 10 years old, probably just in need of a good rebuild.

Originally Posted by HellCaminoKid

No problem, would that 4777-7 be up for sale??? I'm having some issues with mine at the moment, its a good 10 years old, probably just in need of a good rebuild.

After I finish the upgrade and if the 750 works out it probably will be up for sale.
I have to win the one on EBay first.
I've added the choke to it and it has an 8.5 Power valve in it right now. Jets are .069 and .072