I am thinking of switching from my current Thunder series 650cfm to a Rochester Quadrajet. I have been doing some reading and I found out that ALL Quadrajets made after 1972-1973 are 800cfm units. I think that with the smaller primaries it will give me better streetability and with the large secondaries, once I open it up the performance will be there as well. Jet Performance makes 3 different stage Quadrajets. Stage 1(750cfm-$357.99) is a simple rebuild, stage 2(750cfm-$361.99) is everything of stage 1, plus a fully recalibrated idle, off idle, main accelerator pump and high speed circuits. This provides improved idle quality, instant throttle response and maximum torque and H.P.. Stage 3(750cfm-$397.99) is for serious street/strip & race applications such as circle track or NHRA stock or super stock. Fully modified with oversized .149 needle & seat assembly, Main well size is increased & balanced to assure proper top end, high RPM fuel flow. Extensive circuit modifications are performed to keep the fuel curve correct throughout the RPM range, clean idle quality, & instant off the line response.

Seeing as how I don't do much racing, I feel like the stage 2 would be better for my application.
Stage 1(800cfm-$346.99)
Stage 2(800cfm-$367.99)

I know I will have to change my manifold, but that is fine. Right now I have a RPM Air Gap. They don't make the RPM Air Gap, but they make a Performer Air Gap(Idle-5500rpm). My only concern is my cam. Performer RPM(1500-6500rpm). I don't think I will have to concern myself too much with the cam because the car has such a low rear(4.10) that it rarely, if ever, sees under 2000rpm on the street anyway. With the Performer Air Gap, Spread bore manifold, stage 2 Quadrajet(800cfm) carb, and the Performer RPM cam I should have a pretty good combo. What do you guys think? Thanks for the advice.

Changing from the rpm to the performer intake will cost you a LOT of upper end power.
The rpm intake is well matched to your cam and gearing. You have a good combo now.
There may be room for some improvement with your carb calibrations, but that's only a tuning kit and some reading away.

Jim, I hear what you're saying. I have the tuning kit for my carb and I have been playing with the tuning. My theory behind the swap was "the best of both worlds". I could get the economy of the Quad's small primaries and the performance when the large secondaries opened, but I hear what you're saying about matched components. I thought that may lead to an issue since they don't offer an RPM Air Gap version of the spread bore(Quadrajet)manifold. They do make adapter plates that I can use for the spread bore, but I think that would probably be even worse due to the non flowing shapes from the spread bore of the carb and adapter plate to the mouth of the square bore manifold. I got what I was looking for when I posted this particular thread. I kinda new it wasn't gonna have any benefit, but I wanted to make sure with some outside input. Thanks. Next comes the stall converter, I suppose. I don't want anything too rediculous, just enough to get my launches into where they need to be to take FULL benefit of the RPM cam and intake. Maybe a 2000rpm stall, but then again, the car runs good for now so I may just save that money for the auto to manual swap that's in the future and also the heads.

I like heads, heads are good. Heck, good heads are everything, with no down side if they are sized right.

overdriveautomatic with 3000 rpm lockup converter will give you what you want.
A tko will too.

This site has a lot of info on the Q-Jet for you to research.


All the QuadraJet Information I have found on the Internet (http://www.florida4x4.com/tech/quadrajet/index.php)


TTFN
Mat

Get a Quadrajet from Sean Murphy Induction. I also would use a carb. adapter and keep your intake.

http://www.smicarburetor.com/

http://store.summitracing.com/partdetail.asp?autofilter=1&part=EDL%2D2692&N=700+%2D118707+115&autoview=sku

Tom, won't the carb adapter actually impede the flow? By going from spreadbore to the square bore shape, it's more sharp edges that can cause turbulence and slow down the flow.

I don't like adapters at all. I do like perf rpm's, and wish they came in a spreadbore configuration, but they don't.

Most adapters are either open or smoothed for a nice transition. It should act just as a carb. spacer does. Jim, Edelbrock does make an RPM intake for Quadrajets... or is that not what you were saying. This manifold can also be switched to a square bore just as you can with the performer.

http://www.edelbrock.com/automotive_new/mc/manifolds/chevy/perf_rpm-sb.shtml

Jim, Edelbrock does make an RPM intake for Quadrajets... http://www.edelbrock.com/automotive_new/mc/manifolds/chevy/perf_rpm-sb.shtml
Yes indeed they do, # 7104

I guess it's the air gap they don't make for q-jets?

I am thinking of switching from my current Thunder series 650cfm to a Rochester Quadrajet. I have been doing some reading and I found out that ALL Quadrajets made after 1972-1973 are 800cfm units ...

You must have read that on the web ... :sad:

:noway: were 'ALL Rochester's' after 197(whatever) "800cfm" :noway:
Only on the higher flow units was CFM increased from 750cfm to 800cfm :yes:
And the majority of those were Buicks, Olds and Pontiacs in the earlier 70's.
(Cads already used the 'bigger' units)
Chevy BB's used several different CFM ratings depending on their application.

The Bowl venturi openings are 1 3/32" for 600/750 and 1 7/32" for the 800(+) CFM units.
The Secondary Throttle Blades open at different angles on the lower flow units than the higher flow ones - this is GM's clever way of utilizing the same parts on numerous applications.

There were plenty of 'smaller' 600cfm applications up until the end of "carb'd" engines in the early 80's.
Loads of 305's/350's through the 70's into the 80's had 600cfm Rochester's.

Jim, the Air Gap is also drilled for both spreadbore (quadrajet) or square bores.

http://www.edelbrock.com/automotive_new/mc/manifolds/chevy/perf_air_gap-sb.shtml

Tom, Edelbrock makes a Performer Air Gap for spread bore, not a Performer RPM Air Gap. It's the RPM that makes the difference. The Performer is for idle-5500, the RPM Performer is for 1500-6500. I have the Performer RPM Air Gap manifold and cam, not the Performer Air Gap intake and cam.

You must have read that on the web ... :sad:

:noway: were 'ALL Rochester's' after 197(whatever) "800cfm" :noway:
Only on the higher flow units was CFM increased from 750cfm to 800cfm :yes:
And the majority of those were Buicks, Olds and Pontiacs in the earlier 70's.
(Cads already used the 'bigger' units)
Chevy BB's used several different CFM ratings depending on their application.

The Bowl venturi openings are 1 3/32" for 600/750 and 1 7/32" for the 800(+) CFM units.
The Secondary Throttle Blades open at different angles on the lower flow units than the higher flow ones - this is GM's clever way of utilizing the same parts on numerous applications.

There were plenty of 'smaller' 600cfm applications up until the end of "carb'd" engines in the early 80's.
Loads of 305's/350's through the 70's into the 80's had 600cfm Rochester's.

Yes I did read that off the net, but I don't recall it saying that it only applied to the Buick's, Pontiac's, and Old's. You may be right. I am going to go back and read it again. Thanks for the heads up.

The article said that Quadrajet modles 4M, 4MC, and the 4MV built before 1972 were 750cfm. After 1972, those models were upped to 800cfm, so you were right, but they didn't apply only to Buick, Pontiac, and Olds. It was ALL Quadrajet's with those classifications. The article also gave some interesting breakdowns of the OEM numbers. It tells you what each number is for, like year, model of GM made for, etc. Here is a link to the article if anyone is interested:
http://www.hemmings.com/mus/stories/2005/01/01/hmn_feature21.html

Interesting... must be a marketing thing. Thanks, Tom

... Here is a link to the article if anyone is interested:
http://www.hemmings.com/mus/stories/2005/01/01/hmn_feature21.html

Wow - I don't know quite what to say, except this article really fits that old saying "don't believe everything you read" (specially on the web ... ;) )
Maybe Hemming's should go back to writing captivating articles on Carbs like the Sisson's and Marvel-Schebler's they are used to :yes:
They certainly left a lot of wierd information in folks minds with that one :o

I'm not gonna try to correct everything in that article - don't have the time or the inclination.
But - here are a few of the major points to get folks straigtened out that might read this thread in the future.

(note - qoutes are from the above referenced Hemming's article)
"... On these pre-1976 units, the next number immediately after these three digits, will be a "2," if the carburetor is indeed a Quadrajet. Occasionally you will see some with a "5" but these are for California emission standards carburetors.
... The next digit following the "2" will indicate the type of GM car it was taken from originally (1) . A "4" would mean it came from a Buick, a "5" would indicate Oldsmobile. Pontiacs are usually a "6" or a "7," and Chevrolets would have its next digit as a "0," "1" or "2." The last two digits can indicate whether the carburetor came from an automatic or manual transmission-driven car. Usually an even number will indicate an automatic; odd numbers indicate a manual transmission. Cadillacs, however, do not follow this rule of thumb. They could be numbered with any of the above model designation numbers.
... The post-1976 units do not follow the above rules. Eight-digit casting numbers began in 1976. These carburetor identification numbers start with 1705 or 1708, depending on whether they were built in the '70s or '80s; 1705 series numbers would identify 1976 to 1979 carburetors."

This is far from correct. All Rochesters can be decoded for original Series, Year code, Emission tuning, Division use and probable/possible transmission application.
There are Four (4) numbers before the Year Code on pre-1975 units, not "three".
The numbers in the Year code also identify if the carb was originally installed on an A.I.R. Equiped vehicle - third digit starting in a "3" will indicate these carb.s, so a Year code of 38 = and 1968 A.I.R. carb v.s. a 28 which is a non-A.I.R. calibrated carb. (this holds true for 1962~1969 carbs.)
The Transmission Application codes were ignored for the most part on Cads (as stated in article), some Olds and most GMC/Chevy trucks - the linkage type is indicated by this designation, not necessary what transmission the vehicle may have originally had. Stick/Manual trans. vehicles are often found in the truck lines with 'Auto' carb. indicators.

Here is a '170' unit number decode;
# 17083222
(with Date Code 0183)
Is a 1983 model year, 49 state Q-jet, Chevrolet division with an automatic tranny.
Manufactured on the 18th day of the year, in a year ending in 3. For, January 18 1983
Originally installed on a 1983 1/2 ton Chevy Pickup with a 305 and automatic tranny.


"... the earliest units, made before mid-1968, had a round metal tag attached to the fuel bowl.
... In late 1968, the 7-digit part number was stamped vertically into the main housing, just behind the secondary throttle plate linkage."

Carter Carburetor Corp. manufactured units have the 'round' tag indented into the carb main body - Rochester manufactured units have the number stamped into the main body.
Rochester division had increased production of carbs by 1968 to the point they manufatured almost all the units, so the majority of units have the number stamped into the main body - but, Carter continued to do contract manufacturing for some specialized (low production/limited application) units into the early 70's.

The article ignores the major differences in a '70' Series carb v.s. a '170' Series carb.
A '170' Series is a completely different carb internally and the common parts (like metering rods) are not interchangable.
There are also changes to the Main Body to accomidate the APT (adjustable part throttle) system these carbs used.
The post-74 Rodchesters are good carbs for stock engines, but the they suck for performance application ...

Always choose a 1967~74 carb. as a stating point for performance modified carbs and you will get good results.

Here is a short Decode listing for future searches:
Example = 7028508 (you'll never guess where I got this number :D )
70 = Rochester Products Division of GM
28 = 1968 Year Model code
2 = California OR High-Altitude Emissions application
0 = Chevrolet Motor Division of GM
8 = Automatic (even number) original application - linkage has kick-down provision.

Hope some of this helps prevent problems decoding these carbs;
John


Notes:
1.) there are also other designations not given - an 8 will indicate a off-highway, industrial or Marine application not tuned to meet any emissions standards - a 9 will indicate a carb. manufactured for use on a Ford, Chrysler, Avanti and/or other Non-GM automotive application, tuned to meet necessary emissions applications.

John, thanks for the updates. I have been reading and have seen references to adjustable part throttle. What exactly is this? Thanks.