I am going to purchase the edelbrock Performer RPM AirGap intake for my big block, after all of your input. Thanx a bunch. Now..wats the difference between a spread bore, and square bore. Is there any differences in power or performance. THanx Josh
Spread, Square, Whats the difference?
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Unless I'm missing something (very possible 
) I think the only difference is in what type of carb you are going to put on top.
Spreadbore has the secondaries a lot bigger than the primaries (i.e, typical quadrajet), square bore has secondaries the same size as the primaries (i.e, typical holley).
) I think the only difference is in what type of carb you are going to put on top.Spreadbore has the secondaries a lot bigger than the primaries (i.e, typical quadrajet), square bore has secondaries the same size as the primaries (i.e, typical holley).
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I've never had any really great results with a spreadbore carb.
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As to results it all depends...
The idea behind a spreadbore is to use smaller, and thus higher velocity, primary throttle bores. These bores handle the general cruise mode operation and provide very good throttle modulation for street use. They also reduce the amount of change to vacuum with throttle change.
The much larger secondary throttle bores are meant to compensate for the relatively small primaries by being oversize. So with all bores open you have the equivalent of a very large CFM carburetor.
Note that some squarebore carbs are actually slightly spreadbore. For example the difference between the Edelbrock Performer 650 and 750 is solely the size of the primary throttle bores. I have run both versions of these carbs and see much more dramatic vacuum changes with the 750. It is also easier to modulate throttle in street traffic with the 650.
I think there is a lot of merit to the spreadbore concept for street use but I think the old Quads may have overdone it a bit. I would expect that as the bore sizes differ more that transitions become harder to make work.
So yeah as far as choosing an intake manifold goes, it just comes down to what carb you are bolting on top. But is you are also choosing a carb... then you now have just a little more info.
Steve
The idea behind a spreadbore is to use smaller, and thus higher velocity, primary throttle bores. These bores handle the general cruise mode operation and provide very good throttle modulation for street use. They also reduce the amount of change to vacuum with throttle change.
The much larger secondary throttle bores are meant to compensate for the relatively small primaries by being oversize. So with all bores open you have the equivalent of a very large CFM carburetor.
Note that some squarebore carbs are actually slightly spreadbore. For example the difference between the Edelbrock Performer 650 and 750 is solely the size of the primary throttle bores. I have run both versions of these carbs and see much more dramatic vacuum changes with the 750. It is also easier to modulate throttle in street traffic with the 650.
I think there is a lot of merit to the spreadbore concept for street use but I think the old Quads may have overdone it a bit. I would expect that as the bore sizes differ more that transitions become harder to make work.
So yeah as far as choosing an intake manifold goes, it just comes down to what carb you are bolting on top. But is you are also choosing a carb... then you now have just a little more info.
Steve
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It's easier to put a straightbore carb on a spreadbore manifold than the other way around. There are adaptors that will convert one to the other, but most spreadbore manifolds have dual bolt patterns--if you verify that the gasket will seal, you could just drop on a straightbore carb.
There has been some talk that the Eddy RPM Spreadbore has slightly higher performance potential because of a larger plenum. Personally, I don't think you could find the difference on a dyno or at the track. For me, I chose the spreadbore RPM because I wanted the option of using either style carb.
There has been some talk that the Eddy RPM Spreadbore has slightly higher performance potential because of a larger plenum. Personally, I don't think you could find the difference on a dyno or at the track. For me, I chose the spreadbore RPM because I wanted the option of using either style carb.
G
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·And fwiw, I honsetly think a good Q-Jet that is RIGHT for the engine combination is about as good as it gets for a Powerful STREET motor.
It's only until you go out of the powerful street motor range that a big Holley comes into it's own.
Just my 2 cents is all.
pdq67
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It's only until you go out of the powerful street motor range that a big Holley comes into it's own.
Just my 2 cents is all.
pdq67
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