CarburetorsCarb discussion and questions. Upgrading your Third Gen's carburetor, swapping TBI to carburetor, or TPI to carburetor? Need LG4 or H.O. info? Post it here.
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I'm planning making a 1970 LT-1 clone, sorta, using an 850 Q-jet for now until I can get a kick-*** carb.
Anyone know anything about these, and do you think it'll be good to 7000 RPM, while still giving descent MPG if I stay out of it?
I don't expect more than 15 on the highway anyway...
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Shane McConnell macgyver@nethirdgen.org [*]Owner of a 1991 Camaro RS, 305 5-speed.[*]Owner of a 1979 Camaro Z28, 350/350.
<li>Webmaster of www.NEThirdgen.org
Mac,
850cfm is over kill for even a high performance 350. FYI, the biggest cfm q-jet put on any stock GM muscle car was a 840 cfm Q-jet which was put on the '73-'74 SD455 Trans Am/Formula. No bigger than a 750 cfm should be put on a sb 350, unless you will be turning your car into a 10 second 1/4 mile dragster.
Per Doug Roe (author of "Rochester Carburetors"), the Q-jet was only offered in 750 cfm & 800 cfm models. The 800 cfm was only used on large C.I. BOP cars.
A Q-jet is just fine for a 350. Despite it's 750 cfm rating, a Q-jet works on small motors just fine due to it's small, efficient primaries. Hell, they put 'em on 231 ci V6's!!!
If you are making a '70 LT1, you are gonna have a real hard time getting the Q-jet to give acceptable idle and part throttle characteristics with a long duration cam (LT1 cams are big). It probably can be done, but you have to drill passages, etc. Not something for the novice Q-jet tuner to get into, IMO. I'll bet there is a reason GM used Holleys on all of the big-cammed small blocks.
Eelbrock Performer RPM Q-jet is awesome carb for you. Adjustable in cruise mode, too, which others do not have. Help hwy ,ileage a lot... Used a couple and nobody complaining... Except for the price...
That is a good point... the Edelbrock RPM Q-jet is an 850, and will work with a big cam, but expensive. I assume that Mac is talking about a smog era OEM Q-jet though, right?
[This message has been edited by novadude (edited September 14, 2000).]
Actually the largest OEM Q-Jet was the 1-7/32" primary, which flowed 800 cfm. There were never any 850, Edelbrock (Weber) modified an 800 cfm design to flow 850 cfm.
MRZ28HO is right. 800 is as big as they went in production versions. But what's 50 CFM between friends?
It'll work beautiful on your high RPM 350. You might just find that you don't WANT to switch to a "kick a$$" carb later, if you spend a little time tuning it.
BTW- if you find you are having tuning problems with the lumpy cam and sorta "hit a brick wall" shoot me an email and I'll show you how to open up the idle circuits THE RIGHT WAY to work with long duration cams. An indicator of this would be if you find the idle mixture screws are 4-5 turns out and you STILL can't get the idle rich enough.
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94 Firebird Formula M6- No options but Z rated tires. No mods over $10. 13.5@105.
79 Malibu "beater" w/junkyard 400 SBC, tweaked QJet 4bbl, finally a decent set of heads, a few other tricks. A maddening 13.000001 @ 108 on the motor with little traction. No nitrous runs with the new heads yet.
"One of the last remaining QJet tuners on Planet Earth!"
[This message has been edited by Damon (edited September 16, 2000).]
Sorry Dudes,
You are all wrong the biggest cfm Rochester Q-jet was the 840 cfm available on the SD 455 cid motor on the '73 and '74 Trans Am/Firebird Formula.
If you don't believe me check out the July 2000 issue of Hot Rod Magazine the article is "QuadraJet Jam" which on page 53 said the following:
"The largest production Q-Jet (840-cfm) was installed on the '73-'74 Pontiac Super Duty 455......"
novadude,
The Doug Roe "Rochester Carburetors" books is a good source of information however it does have some mistakes that I have caught in it (and I am no expert). Just as I pointed up above that the largest oem q-jet was 840 cfm not 800 as Doug stated. Don't get me wrong I think it is a good book but it does have its share of wrong information that can be misleading for the beginners who use it as their only source for carb work.
Actually Disco, and I don't mean to sound like I'm arguing, but I have found a but load of mistakes in magazines articles as well. The info I have (somewhere in my Q-Jet mess) is that the RPD codes only show a maximum 800 cfm (based on the primary and secondary diameter), there are no codes for 840 cfm. I'll have to dig it up when I have time, which at the moment I don't.
George,
In this case Hot Rod is right, I quoted them since they wrote an article discussing this recently. I know from experience that the q-jet that came with the SD 455 motor was a 840 cfm. I knew a guy restoring one a few years ago, and he went through quite a lot of trouble to find the correct 840 cfm oem q-jet for his '73 SD455 Trans Am. Since there were less than 1,000 SD 455 Trans Ams/Formulas made for '73 and '74, the 840 cfm q-jets are almost impossible to find. Here are the numbers of the 840 cfm q-jets:
#7043273 (1973-74 455 SD - with 4 speed manual tranny)
#7043270 (1973-74 455 SD - with TH400 automatic tranny)
Believe it or not the Rochester Q-jet came in following airflow ratings: 630 cfm, 735 cfm, 750 cfm, 800 cfm, and 840 cfm. However the 750 cfm and 800 cfm were the most popular q-jet applications.
That's interesting info. I had never heard of an 840 CFM QJet. I wonder how they got the extra 40 CFM out of it. I know the jump from 750 to 850 happened as a result of an increase in primary venturi diameter but where they pulled down an extra 40 CFM from there beats me. Any info on WHY it flows more?
Hi Guys, I must first state that the SD-455 Q-jet was 800cfm. They were the first Q-jet with the 1-7/32" primary bore. The standard carbs had 1-1/32" bore. Some big Buicks used the 1-7/32" carb as well from the 71-73 455's but these are rare. The only factory Q-jet to ever flow more 800cfm was the 1971 455HO Q-jet. These unique carbs are ultra-rare
and are numbered 7041267, 7041268, 7041270 and 7041273. These carbs are easiest identified by viewing the primary venturi. The lack of the outer booster ring improves flow along with reducing the chance for
nozzle drip. We ran these carbs on both of our Trans Ams with good results. They are highly sought after. They were never factory rated, but many enthusiasts have flowed these carbs to 838-839cfm without any modifcation. I'm not sure where the SD carb was hung with 840cfm, but the 71 HO carb would outflow it tremendously. If you would like to see
a picture of the missing ring, please use this url: http://albums.photopoint.com/j/View?...ence=0&res=hig
Rocky,
Take my word and Hot Rod's word on it (Hot Rod July 2000 issue), the two q-jet carbs (7043273 and 7043270) available on the SD 455 were 840 cfm. The SD 455 engine had a lot of high performance off the shelf parts and one of those was the two unique 840 cfm carbs. GM may have underrated them by only advertising them as 800 cfm. I remember reading years ago that a couple of engineers passed off these carbs as 800 cfm to GM management to get the carbs on the SD455 since if management found out they were 840 cfm carbs they would have never allowed them to be installed on a production engine. The truth is that the Rochester Q-Jet #7043273 and #7043270 carbs factory stock flowed 840 cfm. That is why so many 2nd generation Trans Am folks with 400s and 455s are dying to get their hands on these carbs since they were the highest flowing Q-jets ever available.
As for the q-jet on the '71 455 HO, I have not heard that it was more than 800 cfm nor seen any documented proof. However it would not suprise me if they were more than 800 cfm since the engineers at GM would sometimes try to slip one by the GM management and underrate a part's or engine's true capabilities. Afterall who believes a 1974 SD 455 Trans Am/Formula only produced 290 horsepower (as advertised by GM)? A fool of course.
Oh Rocky what brings you by this way? I know I have conversed with you on a few occasions on the 2nd generation T/A board. Well it was good talking to you again.
I'm not sure if Hot Rod knows everything, but when both Pete McCarthy and Nunzi Romano tell me something, I take their word for it. I don't want to argue, but that's what I was told. I purchased a 1990 Formula 350 from my dad so I checked into this site. I am trying to sell my 76 T/A because the new toy (2001 WS6) arrived today so I'll be on LS1.com as well. My dad will still have the 72 T/A so I'm not out of it completely.
Good info, Rocky. I have never seen one without the booster ring. So these were '71 HO only, huh?
Speaking of HO Pontiac Q-jets, my friend was showing me one this past weekend that was off of his '68 HO 400 Firebird. It had the letters "HO" stamped right beside the rochester part number, which I thought was odd. I didn't see anything else that would distinguish it from other '68-era Q-jets... probably just calibration changes.
The Hot Rod article was written by Steve Magnante who is one of Hot Rod's top technical editors. No offense he has more experience and credibility than Pete McCarthy and Nunzi Romano combined. Pete and Nunzi are knowledgeable but if they think the the two q-jet carbs available on the SD 455 were only 800 cfm well then they have as much credibility with me as Bruce Fulper (another overrated Pontiac expert).
Damon,
The two SD455 carbs are "devoid of the cast-in secondary booster rings common to all other q-jets" according to Hot Rod magazine. Which would account for the greater cfm rating.
I am sitting here, holding a 7043270 1973 SD-455 automatic Q-jet in my hands. Looking down the primary venturi, I see the normal dual booster ring design. It appears to be a normal casting carb. The primary bore measures 1-7/32" as does the 1977 17057264 800cfm Q-jet I am comparing it too. My 7041270 1971 455 HO Q-jet is devoid of those booster rings and measures 1-1/32" primary bore. Both the 71 HO and 73 SD carb have the notched secondary air valves where as the 1977 has full air valves. I understand that the HO and SD carb flow more fuel as both primary and secondary discharge nozzles are larger than the standard carb, but as for the SD carb flowing more than 800cfm of air, I don't believe it. I've spoken with the owner of The Carbuator Shop in Missouri and he verified the HO flow vs the SD flow. He HAS official documents from the Rochester Corp that shows the 73-74 SD carb rated at 800cfm. There is NO official rating of the 71 HO Q-jet, but he has flowed them as low as 838 and as high as 840 cfm. This guy is no stranger with Q-jets. He has factory equipment from the Rochester facility in which he can make one-off primary metering rods and jets for custom tailored applications.
[This message has been edited by Rocky Rotella (edited October 05, 2000).]
Don't you just LOVE old cars? Everything so well documented and easily researched!
Having NEVER personally seen a QJet with single booster venturi cluster I can beleive that any such carb would be a limited production piece. And that would definitely account for an extra 40 CFM or so. Too bad you can never count on factory specs when it comes to ultra-rare stuff like this.
