900 CFM Q jets

Just curious if anyone has been using these oversized Q jets? I am guessing they use 0.062" larger primary bores to get the CFM?

http://www.smicarburetor.com/products/sfID1/28/sfID2/9/sfID3/43

 

Well, what they do is cut out the outer primary booster ring, trim and smooth it out. And remove the primary flap. that allows more air flow through the carb. I wasn't sure it was that much additional flow.

Thing is, you really need a large enough cfm need to want to do these mods to a carb. For a big cube engine, with good mods, this could be fun. I might have to get my grinder out and give this a try. I got a few thirsty BBB's.

 

I have an old magazine article that shows how to make a qjet flow 980cfm. It has the increase of cfm for each mod but there is alot of work involved.
Ray

 

Can you please scan and post the article or tell me what article to buy?

 

The carburetors must also be completely recalibrated when the outer booster rings are removed. This is the part of the deal where experience, dyno and track testing comes in. I haven't had a single unit sent here to date that was even close for calibration after the rings were cut out of it, and most were from shops specializing in building them!

The outer rings in the q-jet are the single most important reason they work so well. They increase signal to the main system, and make the carburetor very sensitive to throttle angle, engine load, and vacuum changes. When the rings are removed, the q-jet becomes no better, if as good as a Holley spread bore carburetor in primary efficiency. At this point, the only advantage you have with one is the basic design of the unit, few places to leak fuel below the fuel level, and a center hung float with the jets on the bottom of the carburetor.

I do NOT recomend this modification for 99.9 percent of the applications it's done for. We have completely untouched stock 750cfm castings powering Super Stock cars into the 9's.....and we still see folks hacking the rings out of the late model large cfm q-jets to use it on a 350 engine with 8 to 1 compression and a Summit 204/214/112 camshaft.....WTF are they thinking there?

I wouldn't even consider this modification, unless your engine is at least 455 cid, and making over 600hp. At the track on my engine, we saw apprx .02 seconds and less than .30mph improvement using one over my stock casting (1977 Pontiac large cfm model). That tiny difference could have been the fuel curve as much as anything else, the added cfm brought nothing to the table for my engine making over 550hp/600ft lbs torque.......Cliff

 

Good point Cliff. I'll put my saw up.

When we engine dyno'd the 464, we gain 4 hp going from the 800 cfm qjet I built to a 950cfm Holley. I think it was 520hp at the time.

 

Did the pontiac carbs that were cast without the booster have driveable difficulties? Were they rated at that high cfm number 900? Why was pontiac the only one to do this?

 

"Did the pontiac carbs that were cast without the booster have driveable difficulties? Were they rated at that high cfm number 900? Why was pontiac the only one to do this?"


Very good question. I'll try to keep the answer brief and not write a book here.


In 1971 Pontiac decided to put big flowing "round port" heads on the 455 engines, even though the EPA was stomping all over "high performance" engines, and requiring much tigher emissions. The compression ratios were lowered considerbly for all engines in 1971, and the big heads on the 455 was Pontiac's answer to building a strong running "musclecar when things put in place at that time were against doing so.


To help feed the big 455 "HO" engines, Rochester produced "HO" carburetors. They were special castings lacking the outer booster rings, and even more important, redesigning/relocating the boosters which placed the bottom of the rings much lower in the casting. These new "HO" carburetors were still the smaller castings, but flowed 827cfm not having the extra rings in them.


The engineers quickly found that by removing the rings, they also killed the signal to the jets. This loss of sensitivity made these carbs much less efficient on the primary side. They were difficult to get thru emission testing, and dropped after a one year model run.

They came back in 1973 with a larger bore model using the rings, pretty much the same as the 71-74 Buick 455 carbs used at that time, but the Pontiac units had a hot air choke set-up instead of divorced.

These "Super Duty" q-jets made in 73 and 74 were basically equal in potential to the 71-74 Buick units. The rare 1971 HO castings have a very slight advantage in total airflow, but they do not work as well off idle/low rpm's as other models.

I've drag raced the Super Duty carbs, 71-74 Buick carbs, and the HO carbs back to back at one time or another against my 1977 Pontiac carburetor (larger casting).

They offer next to NOTHING in terms of ET or MPH compared to the 1977 unit. This testing was done on my old engine making 514hp/587 torque, and pushing my car to 11.50's at 118mph.

On the street, they single booster "HO" carburetors offer some advantage in "spirited" primary side only driving, but do NOT drive as well right off idle/low throttle angle/low vehicle speeds (they are not as efficient and use more fuel). They also require a much different calibration to even work well at all. The factory had them pretty lean to get thru emissions, they work much better with a slight jet/metering rod change......Cliff

PS: I would add here that NHRA thinks they work a lot better, and has outlawed them for use on anything other than a 1971 Pontiac vehicle and racing it as an 455 "HO" or 400 4-speed entry.

 

Thanks cliff, can't get enough q-jet info, pontiac was pretty slick back then. Ya gotta hand it to them for trying.

 

Thanks. Some of the info we put up on the websites is not so well recieved. Especially the dyno runs we made a few years ago testing the new (at that time) KRE heads. Those runs ended up in HPP and Popular Hot Roddings Engine Masters edition.

Late in the testing, they asked us to include a 4781-2 Holley 850 DP carb, to "see how much power the engine would really make"?..........It LOST a couple of HP back to back against the 1977 Pontiac Q-jet.

A few days later, we re-installed the engine, did a private track rental, and ran the car all day long to see how the new heads worked? Late in the day, we installed the big Holley carb, and the car slowed down .02 seconds and .30MPH. You don't see much in print about that part of the testing, wonder why?........Cliff

 

It's a very old mag.. I'll try to find it this weekend.
Ray

 

I'm running a 750 CFM -- am wondering if that old article states how much more flow on the primary side with just the choke flap removed.. o No:

Seems to run pretty good without it, once warmed up... lol...

 

Cliff,

Same thing happened a long long long time ago with a well known Buick at a major carburetor manufacture's dyno facility. The manufacture's newest kick-a$$ carburetor couldn't match the Q-Jet numbers down in the RPM band where it counted. The magazine article did not mention that.......uzzled:

The article only posted the lower numbers for the newer kick-a$$ carburetor......ou:

Tommy

 

It was my first lesson to the "politics" and propoganda associated with aftermarket parts.

Sure, if the big Holley would have made more power, there would have been something said about Cliff's carb doing well, BUT the big Holley did better.

Basically, you are allowed to run against them, but not allowed to win, as nothing will be put in print about it.

I repeated this test a few years later against a well prepared Holley HP 950, and no one got to hear about that either!......Cliff

 

Well my upcoming book will tell readers about the Q jets vs Holleys... I also have some articles coming out in Canadian Hot Rod Magazine which also talk about this topic... I will get you a copy of the Mag when it prints, you might enjoy the article.

 

Cliff,

Is there anything that can be done to a qjet that would significantly lower ET's other than to make sure it is properly jetted, not worn out, and properly adjusted? I have never read anywhere where there is any one modification, or combination of mods, that result in a panacea of increased power on any particular Qjet. For example, on a typical 11 or 12 second 1972 GS, if an NOS Qjet existed and was installed, jetted properly, and had the secondaries adjusted for the conditions at the track, is there anything that can be done to it to significantly lower ETs? I am aware that as engines become more radical and make 550++ HP, idle circuits have to be recalibrated to accommodate larger cams, and the small bowl becomes a concern. It seems to me that we need to spend a lot of time in many cases making the carb "new" again and getting the jetting and adjustments right, but tricks like removing boosters for more power or even enlarging the baseplates make no real difference.

 

It doesn't matter what carburetor you are using, the same rules apply. It MUST be completely/correctly rebuilt, and tuned for the application. The more knowledge/experience that you have on the carb that you are working with, the easily and quicker this process will be.

Any carburetor chosen, MUST be kept full on hard runs. The size of the fuel bowl is NOT an issue with a Q-jet. It only has one needle/seat assembly, so all the fuel to feed the engine comes in thru one hole, not two, like most other carburetors, AFB, AVS, Thermoquad, Holley, etc.

Of the carbs listed, the TQ and Q-jet are the easiest to tune for transition onto the secondaries. The AVS is OK, but I hate the design (square flange). The spring loaded air door deal makes these carburetors very flexible, and they can be quickly dialed in for nearly any engine/drivetrain/vehicle combination.

The big advantage of the Q-jet over the other models, is that all tuning at the track can be done in seconds, by changing pull-off opening rate, metering rods, and secondary air flap tension. I would mention here that some models have several places for fuel to escape rear of the front vent on hard launches. If you can't tune the secondaries to get rid of a stumble/bog/hesitation, etc, it's most likely tossing some fuel out the top of the carb, and your tuning efforts will NOT remedy the problem.

In any case, as it relates to the topic, it really doesn't matter what carb you choose to run, any/all of them will need dialed in exactly for what you are doing, and once you find the ideal settings, there will be very little, if any performance difference between any of them......at least from what I've seen here.

Case in point. A few years ago we took 8 carburetors to a private track rental, and ran them all day long on two different vehicles. One 12.20 car, and my mid 11 second car.

We had 5 q-jets, a Holley 4781-2 850 DP, custom Holley HP950 style carb, and an out of the box Edelbrock "Thunder Series" 800 AVS.

For most runs on either vehicle, the spread in ET and MPH was only a few hundreths between any of the carbs tested, except for the AVS. Since it was "out of the box" and we carried no tuning parts for it, it was the slowest of the bunch. Still not much less than a tenth and 2mph off the others, but still slower on both vehicles.

All of the other carbs, from the big 850DP down to the 5 q-jets, which were all custom tuned, there was very, very little difference in ET or MPH on either car with any of them......Cliff

 

Not bad, what's that like $200 per horsepower? LOL

 

Just got an e-mail from Sean Murphy.... They are removing the booster rings as Cliff suggested, not something I would be eager to try even with a fuel hungry BBB.