Least the novice should read something confusing from this thread. Rochester built 3 sizes of their spread-bore. ALL of the above have the same secondary side; the differences are on the primary side. Carter made several different versions of the TQ; but up until the late 's and very highly smog controlled engines, the two common versions were and CFM.
A CFM was also available as an over the counter race carburetor. The early Rochesters introduced in were all 's. Carter made many of these for Rochester and saw the issue with the larger engines and the early production TQ was , quickly followed by an Rochester experimented with an in only Pontiac. Rochester pretty well standardized on the around So basically, the same size carbs were NOT used on all sized engines. You still have to do some homework when picking the best carb to use.
Thanks for the info. If they only used the in the early years, then the Pontiac six and the cad used the same size carb. If they only used the in later years, then the V6 and cad used the same size carb. Joined: Sep 12, Posts: 42, Profile Page. Joined: Apr 6, Posts: 10, Profile Page. I had a Holley dp spreadbore on the t-bucket when it first hit the streets. It was by far the best seat of the pants feel when those huge rears opened up.
I switched to a Holley dp squarebore which was a tad faster over all but lost the whole loose your stomach feeling when opened up. Joined: Sep 3, Posts: Profile Page. Does anyone know of any quadrajet gurus that do rebuilds? Gasr57 , Feb 25, Joined: May 29, Posts: 2, Profile Page. Yep and right in your area. From stockers to 9 second cars-yes with a q-jet.
He did mine and it runs like fuel injection. He has a long waiting list but well worth the wait. He even has a book out how to work on q-jets. Joined: Jul 27, Posts: 11, Profile Page.
AHotRod , Mar 17, Joined: Oct 26, Posts: 5, Profile Page. Wayne, Indiana. ClayMart , Mar 17, Joined: Oct 24, Posts: Profile Page. Rebuilding a Q-jet isn't really that tough of a job. A good carb kit and instruction sheet, a clean well lit work area, some basic tools and a shop manual might be handy. It does take a bit of patience and concentration though, and those may be the two biggest challenges. It's not a job to try while drinking, eating, talking on the phone, surfing the net, babysitting, texting, watching the TV and surfing the 'net.
But after it's all back together and working, feel free to have a couple beers and yell at the kids! You must log in or sign up to reply here. Show Ignored Content. Share This Page Tweet. Your name or email address: Do you already have an account? No, create an account now.
Yes, my password is: Forgot your password? Atomic Industry www. My Cookies. Frequently Asked Questions. Forum Rules. Re: Square bore Carb VS. Spread bore carbs typically have smaller primaries than secondaries.
There are advantages to both designs. Hopefully some else can shed more light on the subject. The Bandit - Nova SB, 9. Holley made a replacement carb for that. The spread bore carbs have the centers in a trapazoid shape. The primaries are tiny and the secondarys are huge. Holley also made a carb for the spread bore called an economizer. Can't remember the number, was it a or a ? It was cfm. The primary was about the same size as the Q' Jet but the secondary was about half the size of the Q' Jet.
As far as I am concerned, the Q' Jet is the best carb for the street. Just thought I'd through that in there. My idles perfectly, acceleratres smoothly and has plenty of top end power all from a relatively small displacement V8 in a heavy car. The is a Chevette carb - progressive 2 barrel. The C is the computer controlled version of the Wish I could find a for my 'vette so it's a CHEvette, but it's still a 'vette Otherwise, the description of the difference between squarebore and spread bore has been pretty well covered.
Tim D. Considering that the primaries are smaller in a Q jet, and when your just cruising around you are normally running off just the primaries, you will get better street manners. Because the primary is smaller, the velocity of air traveling through them is quicker so you get snappier response and better part throttle charactoristics, which is why the Q jet is the king of street minded carbs, but because of the big secondaries, it still evens out and ends up being a carb that can still flow a lot which means that it will support a performance motor as well.
Another thing about Q jets is that they are much more forgiving than any other carb because they work off of engine load. Sure a vacuum secondary holley does too, but the Q jet really does a better job of this, so its harder to overcarb your motor when your using a Q jet cause the Q jet will only deliver as much air as your motor wants for the most part as opposed to a holley which will give the motor as much fuel and air as it can when you mash the throttle.
And if the holley is too big, you will know cause you will have crappy low end power and and part throttle response and it will take more tuning to keep the oversized holley from stumbling when you first mat the loud pedal. But the tuning options for a holley or any other aftermarket square bore carb will provide for a better high performance carb in the long run if your main focus is high performance.
Also, about a square bore on a spread bore manifold, it can be done but isnt as good as it could be if the manifold was properly matched. With the two different sizes, it will change up airflow charactoristics and can distort the vacuum signal to the carb making it a pain to get running as well as it should.
Ive done it before with a holley on an edelbrock spread bore manifold and it wasnt too bad better than I thought , but I did eventually swap around manifolds with the same edelbrock manifold, one only in a square bore design and noticed it to be a little smoother operating.
You can get an adaptor plate thats tapered to compensate for the size change of the inlet of the manifold and the bore size in the carb, and this will clean things up pretty good though, but I would still rather run the right manifold with the right carb. Good luck. Spread bore AM - Post In response to Black99CobraMofo If its a daily driver which fuel use is a significant concern, a spreadbore design will be much easier to live with.
If performance is the primary objective, four equal venturi will provide much better fuel distribution and hp.
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