Anyone have an opinion on the Rochester quadrajet compared to the Holley spread bore replacement. I picked one up for free (Holley) and it has the divorced choke and mechanical secondaries. Just looking quickly, it seems the original air cleaner may not fit over it. 

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The quad is better for a street application.  The Holly better for the track.

Tryed one on my 427, didn' t like it, after a while put the Q-J back on,  at the same time I also change the cam and intake, couldn't tell if there was any performance benifit with the spread bore, if you are looking for a little more performance,  pick up a good duel plane alum. square flanged intake, you can run both Edelbrock or Holley carbs, yes the air cleaner fits.

i dont know anything about hollys,but i do know afb carter carbs last a long time and they are real easy to rebuild,a monkey can do it..i been running my carter carb for 9-years on my vette and 5 years when it was on my camaro...i finaly rebuilt it 2-weeks ago after all them years..edelbrok carbs are the same as carter..

As far as I know the only aftermarket intake that will fit under the hood (big block) is an Edelbrock Torker which is a single plane.

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At the time Weiand had a dual  that fit under the hood don't know about now tho,  the spread bore tunes like a holley, easier to tune than a QJ, like float adj, jet change, like I said it didn't seem to give me more perf. over the QJ but it did use more gas, its a direct replacement, bolt it on see if you like it.  anips2009-09-04 08:10:36

My only addition to the discussion is ease of tuning.  You need to drain the gas and take apart a Holley to change jets, and you will probably do that several times before you get the best tune.  With a Q-jet, they lift out the top, and you can replace just the rods.  You also can tune cruise and open throttle by changing the step of the rods.

Larry

I have never been to impressed with a Holly, maybe on the dragstrip wide open but not on the streets. Quaddrasquat has always been a good carb. I still have the original on my 72 and it does great.

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thats what nice about the QJ is the metering rods, they are tapered so your you have a smooth transision from low to high speeds, and the secondary opening is adjustable, helps with fuel mileage, whats scarey is lifting the top while on the motor, the screws, clips, springs, etc while working over the manifold air intake, holley you dont have to do that, QJ or Holley either way you have to pull something. 

Thanks,

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I initially switched my Q-jet out for a Holley 4175 spread bore (vac secondaries).  Main reason was that the Q jet needed new primary shaft bushings in addition to the rebuild so I sent it out for work. I had rebuild the holley (took my best guess on the power valve and jets) and retro fitted an electric choke.

Bottom line is that I wish I had never bothered.  I got the Q-jet back right away and with a little fine tuning on the jets and primary rods, the part throttle response was far superior to the Holley (even though I think the holley's jet were close).  Maybe with some more tuning, the Holley would have worked out better, but the only advantage I could see was that fuel was always at the ready with those big float bowls (car would always start on the first crank of the starter).