#21  
Old 06-02-2022, 04:00 PM
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Cliff R Cliff R is offline
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"the early Pontiac SD455 and Buick 455 1970-74 carbs were always listed as 800cfm rating"

The 71-74 Buick 455, 73-74 Pontiac Super Duty carbs are the only large castings made before 1975.

All front inlet 1975-up automobile carbs will also be the larger castings.

Some side-inlet Chevy units were stil the smaller castings thru 1980.

All of them would flow 850cfm if the stop were ground a bit to mimic the open angle as used on the Edelbrock 850 cfm Q-jets.

The 1971 Pontiac carburetos missing the outer booster rings flow 827cfm.

Over the years folks just got in a habit of calling any large primary bore q-jet with a "bump" in the castings "800 cfm". Most were less than 800 cfm as the stop for the air doors limited the full open angle some.

As far as making power once you dial in any carburetor for the application and it flows adequate CFM nothing else is going to outrun it. I've proved this over and over again on the dyno and at the track back to back testing. IF you are using a dual plane intake designed for a spread bore carburetor it actually has an advantage over a square flange carburetor on the same intake as the plenum areas are wider, the intake has more volume, and the carburetor bores are lined up right in the center of the plenum areas.

This is how I continued to outrun every other type of carb tested against my 1977 Pontiac Q-jet for so many years. We ALWAYS made more power on a dual plane intake than anything else bolted on right after it for the next run.

My Q-jet didn't fair so well on single plane intakes and power was typically WAY down unless we used at least a 1" spacer to get the huge secondary throttle plates up and out of the plenum area.

As far as running w/o a primary power piston and metering rods it works quite well and eliminates any moving parts plus more precise fuel flow thru a single hole vs a hole with a metering rod hanging in it........

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  #22  
Old 06-02-2022, 04:20 PM
GTO-relic GTO-relic is offline
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this has 89 primary jets, 116 secondary jets.
that's in thousandths, actual size.
want to jet it up ? drill out the secondary jets with an 1/8" drill bit
not kidding .... that's what the Buick guys did for 30 years....
Buick intake manifolds are flat like a workbench,
the big 1000cfm carb would slow down velocity,
so the fuel could make the sharp turns, resulting in higher flow,
and make more HP
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  #23  
Old 06-02-2022, 04:21 PM
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accelerator pump diameter is huge, connected to squirters with a small hose, slips on between those 2 fittings.
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  #24  
Old 06-02-2022, 04:26 PM
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the primary and secondary main wells, also contain the air bleeds pressed into them from the top, the jets cap them off. the main well is drilled from the top to accept the primary metering rods. there's also additional air bleeds in front of the primary bores in the corners- they do both pull air in during idle/off idle, and discharge fuel at high speed as well, thereby shutting idle fuel if memory serves...
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  #25  
Old 06-02-2022, 04:49 PM
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Carter Comp on left, QJet on right
Carter has 1.5" primaries, 2.250" secondaries
Qjet has 1.375" primaries, 2.25" secondaries.

1/8" bigger primaries on the Carter
the phenolic center section of the Carter doesn't have a shelf in the front of the primary bores, it's more open than the Qjet
it's a Super Qjet on steroids...
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  #26  
Old 06-02-2022, 09:01 PM
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Cliff,
It would be great if those Qjets flowed 850 cfm, but they've been called 800 cfm by some very reputable sources for a very long time- such as Doug Roe, back in the day when they were still being put on cars new, i.e. 1973.
If we're going to now second guess these sources, then we can second guess all the carburetors made out there, and add 50 cfm at will. You know as well as I do, the people who started using the 850cfm number, were Edelbrock, when they copied a truck carburetor exactly. Opening the air valve or throttle a hair more, doesn't add 50 cfm. I'd have to see it on a flow bench at 20" to believe it.
I remember specifically seeing from several sources, information stating the throttles should NOT be opened into a vertical position, they should be in line with the baffle- and it was designed that way for a very good reason- not to open all the way.
If you have enough motor and rpm, yes indeed it could flow 850 cfm. But that's not what it would flow at 1.5" HG, or 20" water, the traditional testing standard. The guy at Stealth Carburetors explained it this way, when I asked him what their carbs flowed:

"it is dependant on the engine

a 1.250 venturi carb (650) on a 350 engine at 4000 rpm flows 400cfm
a 1.250 venturi carb (650) on a 430 engine at 8000 rpm flows 880 cfm

the actual "flow" is dictated by engine size and rpm not carburetor size

the true CFM is not determined by the carburetor ---it is determined by the engine, but in the retail world this is how people determine CFM with a carburetor:

1.6875 throttle blade--1.250 venturi =650
1.6875 throttle blade---1.300 venturi=700
1.6875 throttle blade--1.345 venturi = 725
1.6875 throttle blade--1.375 venturi =750"

so...can you add 50 cfm to a Qjet ? sure, if you have a monster motor, and rev the piss out of it, until it pulls that much through the venturis by sheer force.
but the standard of the industry still is 1.5" HG/20" water. not what the biggest engine made can pull through it.

  #27  
Old 06-03-2022, 12:06 AM
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@'relic;
I don't really know what you're trying to do here.

The OP was asking for advice on modifying their Quadrajet's metering, and you started up talking about these other carbs;
I was following, because frankly I thought what you were showing looked neat - even if it had no bearing what so ever on what the OP (tremo) wanted to glean from the membership here.

But now you're trying to argue with what Cliff has shared?

Please don't try to antagonize the man;
I for one appreciate what he and a few other Quadrajet experts share.
I am grateful he still comes to this corner of the internet.

Doug Roe's book was good for it's time, and still offers some great information if you can find the original printing (HP-35; 1973);
The original printing was so much better than the latter condensed version, but even some of his tips have been proven over time to not offer the gains suggested;
The thing that immediately come to mind from Roe's book was the drilled out idle tubes;
I sold his latter book probably over a decade ago, but I seem to recall that even that printing was still touting the drilled out idle tubes shown on pg 256 of
Roe's original book - Cliffs book offers why this is a bad idea on page 104 of his book.

Beside's unless I am mistaken flow measuring is done on a bench designed to measure airflow, not a motor.

Oh wait... I just noticed this post:
Quote:
Originally Posted by GTO-relic
let's be honest here. the first thing an honest Qjet builder will tell you is,
you will always make more HP with a Holley, inch for inch.
Ok, you got me.


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  #28  
Old 06-03-2022, 04:47 AM
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Doug Roe published all the Rochester carb specs for 1-bbl, 2-bbl, 4-bbl carbs,
back in 1973. Qjet with air valve wide open 90 degrees, is 800cfm
not 850cfm.
the logic oh, Edelbrock said a truck carb repop flows 850,
therefore ALL 800 Qjets in the past 50 years also flow 850,
is a stretch.
what's the flow bench say at 1.5" HG ?
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  #29  
Old 06-03-2022, 07:28 AM
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Quote:
Originally Posted by unruhjonny View Post
The thing that immediately come to mind from Roe's book was the drilled out idle tubes;
I sold his latter book probably over a decade ago, but I seem to recall that even that printing was still touting the drilled out idle tubes shown on pg 256 of
Roe's original book - Cliffs book offers why this is a bad idea on page 104 of his book.
Those are the fuel pick-up tubes for the secondary POE-system.

FWIW

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Old 06-03-2022, 08:09 AM
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"CE metering rod is the richest long power tip rod originally made by GM for any quadrajet"

Whatever published source you used was in error, I suggest that you get your micrometer out and do your own testing.

"the early Pontiac SD455 and Buick 455 1970-74 carbs were always listed as 800cfm rating"

Incorrect again, the 1970 Buick 455 carburetors are the smaller castings. Two errors in your information. Not trying to be critical but IF you are using research as the foundation for your the information you are posting it is inaccurate. How accurate was the Doug Roe testing from 1973? Did he actually do the testing, and did they use an adjustable stop or vary the open angle of the secondary air flaps during that testing?

As for the larger castings being "800" vs "850" cfm, I had the smaller primary bore castings tested, the 1971 455 HO castings tested, and the larger castings as well. My source came up with the same numbers that Edelbrock did for the larger castings when the air door was opened to the angle listed in my book in Chapter 6 about Edelbrock Q-jets.

I didn't make a big deal about it in the book because it is NOT easy to do that sort of testing and we also noted that companies who do "rate" their carburetors for CFM don't all use the same standards/test methods. Even so I mentioned it briefly a couple of times in the book and in this thread and already regretting it seeing how one person got their panties all wadded up about it.......FWIW.

I really think that the dyno and drag strip testing I did is more important and shows us the potential of the different Q-jets. Back to back to back at several private track rentals I tested a 1970 Pontiac RAIV carb, 1971 HO 455 carb, 1977 Pontiac 17057274 carb with the air door allowed to open the same as the Edelbrock 1910, and a Holley 4781-2 850DP carb with downleg boosters.

The dyno showed that all the carburetors were dead even to 4500rpm's before any differences started to show up. At the end of all the pulls the 1977 Pontiac Q-jet made the most power, followed very closely by the Holley 4781-2 carb, then the 455 HO with the Ram Air carb finishing last.

I backed up the testing at the track and the 1977 Q-jet ran .02 seconds and .3 MPH quicker than the Holley 850 which backed up the dyno differences to a "T". Remarkably the 455 HO carb was almost the same as the 850 Holley and for some reason showed slightly more MPH. To this day I attest that to the fact that we were using a dual plane intake with wide plenum areas designed for spread bore carburetors (They are lined up directly in the center of the plenum areas) . The 1970 400 RAIV carb lagged behind close to where it should have by looking at the dyno results. Nearly as I can remember is was down almost 2mph on top end and almost a tenth in ET. Sadly I lost those test results when my last hard drive crashed so don't have the details to put up here for comparisons.

Hope this helps some instead of continuing to fuel the "witch hunt"......FWIW........Cliff

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  #31  
Old 06-03-2022, 01:44 PM
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at the time of publication that was the richest metering rod with long taper made by GM,
Doug Roe was not in error, he was building these carbs and writing publications back in 1973 when they were new,
and he listed all the rods from leanest to richest in the metering/capacity flow section.
I did notice there may be some typos or transcription errors, because rods listed as medium taper that I own, have fairly long tapers.

speaking of errors...hey we all make mistakes...opening the air valve further than the stock setting, will cause the bottom of the air valve to progressively block the secondary main fuel nozzles. that's why they are offset the way they are from the factory, take a look sometime. if they are opened completely 90 degrees on the bottom, it will block the fuel nozzle stream severely and cause a lot of turbulence. it will also cause the top side of the air valve to progressively block incoming air, because of the way it's designed.

take a look, I happen to be building a carb for a guy with a '76 Firebird, 400 block, 421 crank, 60 over, 434 cid today, using a Buick 455 800cfm center section, and '73 Pontiac 455 Qjet top.
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  #32  
Old 06-03-2022, 01:51 PM
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and one more thing...the throttle blades should be in line with the steel plate baffle, so the air flows staight over both smoothly, not offset from each other. otherwise that too creates turbulence. the correct setting should look something like this, and beyond that, it's not gaining anything..

this one I drilled bypass air holes to .193" , 73 jets, no metering rods, because the cam is huge,
and he's already having an issue with another Qjet carb he paid good money for from a well know Qjet building shop, we've all heard of...
that one is not idling the big cam correctly. his existing carb has primary nozzle drizzle and smoking out the exhaust too rich.
this one I opened up idle tubes to .043" and the idle down channels are .055" stock form leaving them alone- on the Buick 800 center.
FWIW the idle tubes can be drilled wide open to .068" and the down channel will become the restriction, being it's smaller.
I've done it before and it idled at 1000rpm in park, and 800rpm in drive TH350, with a 250 @ .050" solid flat tappet cam, on my own car 20 years ago.
that also was a Buick 800 center section, with Pontiac top.
the Buick 800 centers have smaller air bleeds, so they are richer.
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Last edited by GTO-relic; 06-03-2022 at 02:00 PM.
  #33  
Old 06-03-2022, 02:04 PM
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A lot has happened since 1973 or whenever the Roe book came out.

The angle of the secondary airflaps is well covered in my book with recommendations plus the maximum open point as used on the Edelbrock 1910's.

I verified the maximum open angle by removing the stop and drilling the airhorn for an adjustment screw to find the point of no return or maximum effective angle where going further doesn't improves nothing or has diminishing returns. This testing was done on the dyno and backed up at the track with drag strip runs on several different outings.

When I wrote my book I didn't use any reference material, and have actually never owned the Roe book or have even looked at one since my high school days..........

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  #34  
Old 06-03-2022, 07:03 PM
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I am enjoying the information being presented here…..

I also enjoy a variety of publications on the Perfomace modifications of qjets and Pontiac’s in general ….

McCarthy, Roe, Ruggles, Hand….. they ALL offer something. Do they cover every “trick in the book”? No…. You still have to get out and try some things for yourself….

Thanks to those willing to offer their experience and wisdom….

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  #35  
Old 06-04-2022, 05:28 AM
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GM Qjet secondary metering rods available c. 1973, from richest on top, to leanest on bottom
and...how the rod tapers work
knowledge is power...Merry Christmas
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  #36  
Old 06-04-2022, 05:35 AM
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jets and primary metering rods
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  #37  
Old 06-04-2022, 05:36 AM
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metering area chart for jets, metering rods
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  #38  
Old 06-04-2022, 05:37 AM
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FWIW, secondary hanger specs by letter code.
yesterday I just bent an "N" hanger, to "B" hanger specs,
and put it the 800 with CR rods just to start the motor.
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  #39  
Old 06-04-2022, 05:57 AM
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flow rate capacity for Rochester carburetors, including monojet, 4G, 2G, Qjet.
the handwritten numbers for 2G are flow at 1.5" to compare to a 4-bbl at equal pressure drop. Unless you're using a '66 tripower intake with the big 2bbl carbs, the early tripowers with small center carb really aren't worth running, unless it's a numbers restoration or show car. A single 800 Qjet, or 850 Holley flows the same, or more- and is a lot less maintenance and complexity, more equal fuel distribution, and better intake runner layout. With the availability of 1000cfm Holley carbs based on the 850DP, and Dominator carbs, there's really no reason to run any tripower at all- except for nostalgia, looks, the sound, or the hell of it cuz it's there on your garage shelf.
regardless, if you run the '66 tripower intake with the 3 big carbs, it'll have 900-920cfm total capacity...measured at 1.5" HG like a 4-bbl is.
the downside is, eventually rebuilding 3 carbs instead of one, and the tangle of fuel lines, progressive linkage.
20+ years ago a friend from Detroit area obtained all the flow numbers for various factory and aftermarket intake manifolds, connected to a head on the flow bench.
the tripower intakes actually flowed very poorly. the vintage aftermarket intake manifolds flowed pretty lousy as well. the best flowing were the Victor, Nash/Warrior, Torker II, Performer, RPM, Street Dominator, Torker 1, and GM iron 4bbl 1967-74, more or less in that order.
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  #40  
Old 06-04-2022, 08:45 AM
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1973?

No wonder there are so many errors in your outdated information.......a LOT happened in the next 50 years, including J, K, L, M series primary metering rods, quite a few secondary metering rod offerings, and CFM testing using the later castings allowing the secondary flaps to open past 90 degrees.........FWIW.......

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