900 CFM Q jets

Out of curiosity -- I googled it -- Found in a post on another forum the author claimed that just by removing the primary choke flap assembly from the carb. It goes from 750 up to approx. 790 cfm... good enough answer for me... Don't know if it really makes that big of a difference in the big scheme of things, is correct or need to jet up one for it... But in theory it makes sense... A restriction is a restriction..

 

Yeah, I know it's an old thread, but I'm digging it up anyhow.

So approximately 750-790 by removing the choke plate and shaft. Perhaps bump that up from 790 to 800 by filing down the throttle blade screws
and thining the shaft?

Anybody know how to effectively and SAFELY take downt the screws and thin the shaft. Seems straight forward, but I can see that biting you in the ass if
you take them down too much and the throttle blades fall down your intake at WOT. Loctite should do it or does gasoline deteriorate it?

Not that my car warrents it. Just curious.

Well, I may remove the choke plate. Who needs a choke AZ?

 

I just bought a Edlebrock RPM Q jet that is 850 CFM and the 73 Vette HAULS!!!! We have it on a 454 with AFR heads and the car is a monster!!!

He gets 17 MPG compared to the 12 MPG with a new out of the box edelbrock square bore..... Idle is better, part throttle is better, and WOT is WAY better.

Now we just need to get the nitrous worked out and the car will be killer fast.....

 

old thread, like to comment
there's a lot of good info responses heere, but on also must read between the lines.
doing a comparison between 8 carbs on a 12.2 e.t. car, is only good for THAT specific engine and car - not across the board.
12.2 e.t. is a relatively mild combo. we went 11.30 with a 3900 lb. 455 car, and a Edelbrock Perf. 800 carb, with only a metering rod change out of box
does that mean Edel is the only way to fly at 11.30 ?
NO - we'd have gone even faster with a Holley- but it was a street car
that's the qualifier- a spreadbore is a street carb, not a race carb. the small primaries give better part throttle and mileage
has anyone ever used a QJ in NASCAR back in the 1960's ? hell no- that would be laughable- and the engines back then only made 550-585 HP with iron heads
when the combo goes past the 500 HP portal and the heads flow 300 cfm and more, a Qjet can't feed the engine as well as a Holley
many "superstockers in the 9's" have stock iron heads that flow only 260 cfm, and must run a Qjet due to class rules
if they could run an 850 DP or 950 Holley, they would- gladly- the cars would go faster at wot
you can't judge a carb by e.t. in CLASS RACING where rules require that carb, and use that finding across the board.
there are heads out there for Buick 455 that flow 400 cfm now
a Qjet won't feed them as well as a Holley can at wot
if you want to do a comparison, look at the OPEN CLASSES where any carb is legal
there are no Qjets in those classes, cuz they are somewhat inferior and slower to a true race carb
keep in mind many Qjet builders got into the biz, because the cores were $10 each and yield a high profit margin when "built" for buyers who know little about carbs
there's really not much to them if you study the QJ carb-an old 4GC 4bbl is more complex
it's easy money once you tool up to tune them- ask me how I know- I did it for years
in reality anything over 400HP should really get a Holley for max HP. that's what GM and Ford did from the factory
a lot of what you read on the net, is biased to benefit a small business, or sell something
if Qjets are so good, how come top drag racers in open classes never run them ?
all I'm saying is keep an open mind, it's hard to beat a Holley 3310 750 vac sec, or 4781 DP for drag racing. used cores are quite cheap on Ebay now

ps- for the record, run the early 800cfm qjet, it is definitely better than the 750cfm back to back. the QJ will respond to milling the choke horn off- at high rpm, more power and flow

 

Appreciate your post, you like Holleys. Got it.

And Buick themselves used a Holley for their Stage 2 builds, Kenne Bell used to love Thermoquads. Everybody's got a horse in this race.

But I think that Cliff has been trying to point out, in the engines and levels of HP that probably 90% of us are playing with the Quadrajet is one of the best, if not the best choice of all the carbs out there. They are flexible, tunable economical and (yes) cheap. They can support a 500 hp car easy,and on street/track cars like mine with between 260 to 325 hp at the wheels they are hands down the best choice. I guess Cliff's message is: don't believe everything you see or hear - test for yourself. Magazines have advertisers and race teams have sponsors, and they like to see their stuff in action whether it's the best choice or not.

Test, analyze and choose. So far, the Q-Jet works for me.

 

For the record the q-jet that I use still has the booster rings in place and a grinder or sanding roll has never touched it.
It is custom calibrated exactly for the application, and has been fined tuned for years on the dyno and at the track. For this reason its going to be difficult for anything else to outrun it either place.
I also own a custom built HP950, and a 4781-2 DP carb. Neither one of those will outrun the q-jet anyplace, dyno or at the track. Since they are all custom tuned for my engine, they run about the same at the track, but both Holleys use a LOT more fuel for normal driving than the q-jet, so they just sit on the shelf and collect dust.
I also run a stock intake, CNC ported and port matched, with the plenum areas opened up to the same apprx size/shape as the Edelbrock RPM. That intake also outran an RPM on back to back dyno runs by 6hp.
The truth here is that stock parts are better than we are led to believe. Ive learned a lot about that deal in recent years.
Listed below are 4 examples of parts we tested where the factory parts ran equally as good or better.
Ported factory iron intake vs a port matched Edelbrock RPM. The iron intake was 6hp better.
1977 Pontiac Q-jet vs HP950 and 4781-2 Holley. The q-jet made 2 more hp on the dyno than both Holley carbs, and ran .02-.03 seconds quicker in ET and .30-.40MPH faster at the track.
HEI vs MSD billet with 6AL box. A factory HEI with stock 990 module made the same exact power on the dyno as the MSD billet back to back testing.
Factory Pontiac RAIV cam (from Crower) with 308/320, 231/240@ .050 and .470 lift. 113LSA (.520 with the rockers we were using). The factory cam outran a custom ground Comp XTQ lobed cam with 240/242 @ .050 on a 112LSA with .580 lift. The factory cam made 494hp/567tq, peak HP at 5600rpms We swapped in the custom ground Comp cam and the engine LOST 10hp/22ft lbs torque and peak power dropped down to 5200rpms.
I could go on for days here, just throwing out a few real World examples of some of the testing we have done here with factory parts vs aftermarket parts.
This is where someone ALWAYS cries foul, a in most cases we modified the factory parts to compete. Well, sorry about all of that, we are racers here, and like to modify EVERYTHING!.....LOL.Cliff

 

400 HP?

Wow, I don't think so.

I have personally tested over 100 Performance Buick engines, from 360 HP to over 1000, with the majority of them in the 450-550 HP range, and have done dozens of carb swaps. I can tell you for sure that significant dyno indicated power does not start to show up until beyond 550 HP with big Holley's, and you have to get beyond 650 HP before it becomes a "must have". This is on a mid 2000's vintage Superflow 902 engine dyno, fully instrumented, with an expert operator who has dynoed over 8500 engines since the late 80's. He would agree with that statement.

As a drag racer, the reason most guys run Holleys is because they are simple, parts are plentiful, and everyone else has one. There are other reasons to run one, we will get to that in a second here..

As far as outright power on the dyno, and performance in the car, I have a Competition Series 1000TQ, that on motors up to 700 HP, no other carb has ever been able to outdo.. Not even a dominator.

I drag raced with it for years, and no Holley, no matter what we did with the pumps, cams and squirters, would ever stand my 4100 lbs GS on the bumper like that TQ would. 60' times, due to the engine response off the Trans Brake was a 4-5 Hundredths better than any Holley could ever do on that car, I worked really hard with them, as I have no "carburetor bias". This was a car that ran high 1.4 to low 1.5 60 foots.

That being said, for many, many years, I drag raced with a Barry Grant "Sportsman 750".. nothing more than a fluffed up 3310.. because it's throttle response off the throttle stop was the most consistent, and the car never spun the tires coming off the stop. I would still be running it, if some low life had not stolen it out of my trailer at the track, when I was doing some testing with a 950.

Saying that "drag racers use them, so that must mean they are the most powerful" is a gross over-simplification, there are many factors that go into carb selection, some having nothing to do with making the most power.

While an electric choke 4781 850 Double pumper is a wonderful street performance carb, it has been my experience that for up to 550HP, you simply can't beat at Q-jet, for all around performance. As Larry mentioned, the driveably is much crisper with the small primary carb.

JW

 

Very well said Jim. I'd also add here that my car is DEADLY consistent with the q-jet. I actually won a box no-box Gambler's race at our local track on test and tune night. For the last 5 runs R/T's were three .505's, one .514 and .530. Ran deadly consistent run to run, picking up a thousanth or so every run as the temps dropped going into the evening.


R/T: .505 .514
60': 1.6230 1.6289
ET: 7.3169 7.3188
MPH: 94.35 94.36

Two back to back runs that night 23 minutes apart. Car was being raced in full street trim, thru quiet mufflers on DOT's and driven to the track......Cliff

Below are three runs from a few month ago, drove to a local car show at the track and made a few blasts just for fun.

 

Cliff, have you ever monkeyed around with the old Carter Comp Series thermoquads?

 

Yes, we were given a new one about 15 years ago, it came from the Carburetor Shop in Eldon Mo. Jon is one of the "old timers" and very good with AFB's and TQ's.


I could not get the TQ to work better anyplace than my Q-jet, but it did work very well. Ran it on the dyno, street and track. It was sort of a PITA to use with my application being wide and rear fuel inlet, pan had to be shimmed up to clear the linkage, etc.



Couple of things I didnt like about it, most important not being able to change secondary jetting in seconds like the q-jet. With a little spring adjusting it dialed right in for transition onto the huge secondaries, so good to go there. I consider the adjustable secondary airdoor and primary APT system one of the very best features of the Q-jet and TQ designs. Complimented by smaller primary bores and triple venturi areas, NONE of the other designs can even compete with them for smooth off idle, drivability and fuel economy for normal driving.


Even with my custom built/tuned HP 950 (large annular boosters), and 4781-2, they are just not as smooth right off idle, and will consume more fuel for normal driving than the q-jet, all else being equal. I did quite a few test loops with all of those carburetors, filling up at one station, combining city/highway driving then back to the same station to re-fill. My q-jet went between 200-230 miles on 14-14.4 gallons of fuel. None of the other carbs got past 180 miles, the 4781-2 with its downleg boosters was clear down at 160 miles per tank.


What it really boils down to here is that all of the large bore high performance carburetors are somewhat of a compromise in design. To get the big airflow in a square flange package the large primary bores are required, and lack the sensitivity of the spread bore design. They need a LOT more fuel to the idle/off idle system so they can feed the engine thru the transfer slots to get the main boosters on line. The smaller and less sensitive the booster the more fuel they will need. My downleg booster 4781-2 850 DP carb was flawless everyplace, street, dyno and at the track, but a gas guzzler compared to my 1987 Pontiac q-jet simply because of how it needed to be set-up to drive well. Attempts to go into the metering blocks, PVRs, idle system, airbleeds, etc, and lean it up right off idle hurt throttle response at low throttle opens and light engine load. I got it as good as possible, and it still came up considerably short on fuel economy as my q-jet, but was absolutely FLAWLESS everyplace for performance.


For years I used that carb as a loaner and dyno test mule. I ALWAYS had trouble getting it back, and the dyno shop told me on more than one occasion to never touch it. The same dyno shop also hated the fact that I could come in there and bolt down a 455 cid engine with a stock intake and q-jet on it with no spacer, then commence to crank out the kind of power than we did. Attempts were made many times to outrun my combination, but always came up just a tad short, simply because my stuff was fine tuned EXACTLY for my engine, with decades of street, dyno and track testing.


This fact brings me to the most important part of the discussion. Im NOT selling anything here, we run a long backlog and not looking for any more work. I simply tell folks to run whatever they want to, and if they want to run good with factory parts, you arent really giving up anything, or at least nearly as much as the aftermarket and a lot of motorheads would like you to believe.


Ill leave you with a real cool story. Guy comes to us with a big block Chevelle project, wanting his 1970 454 to make BIG power but look completely stock everyplace. He hires us to do the carb and distributor. I also provided guidance for the engine build, telling him to put a stroker crank in it (close to 494cid nearly as I can remember), get the compression up to mid 10s, correct head porting, mods to the flat typically dubbed a POS iron intake, and specs for the hydraulic roller cam conversion.


Dyno day comes and hes unloading his new engine at a local dyno shop. The entire time hes taking all sorts of flack from the hired help, telling him to get that POS Quadrajunk and doorstop intake off of that engine so they can make some real power with it, etc.


They warm up the engine and set the timing, then make a pull on it. He said they were dialing 911 and running for the exit doors. It cranks out over 500hp and 600ft lbs torque!!!!


Ive got at least another dozen stories just like that one..Cliff

 

That is an awesome story Cliff!

I recently helped a friend with a mild rebuild of a Caddy 500 and it worked out great! It was only bored enough to clean up the bore, ran 9:1 for low octane fuel, ported iron heads, mild cam, stock intake, and I had the q jet custom made by Everyday performance... Bolted it on the dyno and it was making 575 foot pounds of tq and 490 hp... Really mild combo! They put it in a mud truck and it is dead reliable and good on fuel...

 

...ground Comp cam and the engine LOST 10hp/22ft lbs torque and peak power dropped down to 5200rpms... <----- maybe this more aggressive profile needed more spring?

 

We had PLENTY of springs on it, and even went in and put another .030" of shims under them right on the dyno. I also moved that cam 2 degrees in both directions and power fell off even further both times. We still may have been seeing a bit of "lifter crash" over 5000rpms, but the cam was still down on power everyplace.

What really happens with these modern camshafts is that they reduce seat timing and really speed up the opening/closing events. They have LESS off seat timing, but improved .050 numbers and more total lift. This does NOT make up the ground when they are compared to bigger cams with slower events. The cam companies tell you that they do, but in all the actual testing weve done with them, they come up short on power compared to a longer duration cam doing the same thing.
If we move up to a roller cam this changes the deal completely. After we got our butts handed to us with the flat cam testing, I ordered a custom ground hydraulic roller cam with .360 lobes, 230/242 @ .050 and 282/287 @ .006. It cranked out 3hp/4ftlbs more power than the factory RAIV cam from Crower did. The roller lobes take the lifter diameter out of the equation and actually allow us to move more air thru the engine with shorter seat timing events (valves not open as long, less overlap, etc).
Not trying to start any sort of cam bashing thread here, but I will say that these new cam designs, at least the flat cams, are not nearly as good as we are led to believe, at least thats what Ive seen here on the dyno and at the track attempting to use them.
For our engines here, when a flat cam is used (most of our engines are roller cammed) we stick with slower ramps, longer duration, wider LSA, and add high ratio rocker arms and Rhoads lifters. This will mimic the power of a very well chosen roller cam at a fraction of the cost..FWIW..Cliff

 

Your experiences are super informational and useful. That is very valuable to everybody. Nothing beats real-world results!

JW has been saying the same thing about these profiles. Maybe one explanation of why these don't work so well is because the super fast lobe action causes inertia forces on the lifter check ball (or disc) to overwhelm the check ball spring, which results in the check ball not seating til late and losing plunger motion. (The lobe accelerates the lifter body upward, inertia keeps the ball down, but it has to move upward, with the help of the spring, to seat). I have read some where that the Cadillac (of all brands) racing program discovered this early on and then improvements to the roller lifters (lightweight ceramic check ball and higher tension check spring) made their way permanently to all of GM's production to solve the problem of fast profiles.

 

Q-jets can be sensitive to some air cleaner set-ups, especially how close the lid is to the carb, and type and shape of the lid. My carb absolutely HATES the K & N X-Treme lid, stumbles/hesitates/bogs etc when its in place. Runs fine with the factory lid or no lid at all, so something going on there?
As far as the lifter crash thing goes, Ive seen it enough with Comps XE flat lobes that I avoid them completely. It also makes no sense with factory type heads to have super quick seating velocity. The engineers designed those heads with a lot of low lift flow, so they could keep the lift down to reasonable numbers and not super aggressive on the lobe profiles as to kill the springs. Keep in mind they wanted the customer to have an engine that would make it well past the warranty period.
With Pontiac factory heads in particular, they have machined chambers with a flat floor in them, and almost all of them are 30 degree intake seats. They thrive on low lift and will make great power with lazy cams in them. Other designs may not be quite so sensitive to this deal, but at least when it comes to seating velocity and short seat timing events, Ive not been able to get those cams to work well. They also pull them down on tight LSAs, which only narrows up the power curve further. I completely avoid any small (short duration) aftermarket cams on tight LSAs from any vendor. About all they do from what Ive seen is make the engine sound aggressive, throw all its power at you very early, then down considerably on upper mid-range and top end power.
Ive also noticed, on the subject of camshafts, that the wider we go with the LSA on the big cube engines they more they respond by throwing MORE power at us over a broader rpm range. Its rare if we use a cam under 112LSA, and have been having more and more of them ground on 114LSA. These engine produce a more locomotive power curve, idle better, and pull harder on top end. The torque curves are pretty flat, no rush of power anyplace. This also spreads out dynamic compression, and lowers octane requirements at the same time. Some more food for thought when it comes to this topic..Cliff

 

Isn't this to some degree a vindication of the stock factory camshafts which were generally ground 114 and higher for LSA?

 

I'd say yes. Even today factory camshaft are ground on pretty wide LSA's. Maybe the engineers did know what they were doing with these things?......Cliff