[i82much]

I keep reading in various places that if you granny shift a manual transmission, i.e., let off of the throttle for each gear change, that a vacuum secondaries carburetor won't reopen the secondaries fast enough when you slam the throttle back to the floor.

Is there any truth to this?

[stickboy]

Yes there is truth to it. A lot more with manual trans than people talk about. Qjet forced me to learn how to powershift. 750 DP, 850 DP much more responsive on re-entry.

[Cliff R]

A Q-jet is not a vacuum secondaries carburetor, they are mechanical. For stick shift applications the choke pull-off release time must be shortened up until it is quick enough that it doesn't keep the secondary airflaps from being dampened between shifts. Combined with light spring tension, and a custom quick release pull-off, they can be tuned for flawless transition back to full throttle between shifts.

Q-jets are also becoming very popular for circle track racing, where there is a LOT of on and off the throttle going on. Some builders don't even use a pull-off at all for these set-ups. We install one here, but it will have a really quick release time, so there is no lag in the power curve getting on and off the throttle quickly. If a pull-off is not used, it takes too much spring tension on the airflaps to prevent a stumble, hesitation, or some noticeable transition.......Cliff

[stickboy]

I wouldn't use a 3310 or a Carter on a stickshift either. No way. Anytime you "delay" the secondary opening with an air valve, IMO just plain sucks throttle response and ET compared to a true mech secondary WITH secondary accelerator pump on a stick car....but go ahead and try one out, maybe Cliff will give you some tuning tips.

[Cliff R]

Stickboy is correct, the air valve presents a problem unless the opening rate is perfect for the application. This requires several things to be going on at the same time so there is no lag in throttle response or engine power going quickly off the throttle, then back on it again (shifting).

I woln't go into the way we get this done here in great detail (no need to educate the competition), but with the correct settings, and calibration, the q-jet will work equally as good as anything else, if not a tad better. The better part would be that we can completely eliminate any lag, and power loss by using the airdoors opening rate, and fuel from the POE system to our advantage. It becomes next to impossible to induce any sort of stumble into that deal as the airdoor opens by demand, adding the needed amount of fuel to the engine as airflow increases in direct proportions to the flap angle increase.

I also agree that the vacuum secondary Holley's are a HORRIBLE choice to do the same thing. I used to use them back when I first got into this hobby, and have had those vacuum units apart so many times I'd wear out the threads for the screws, and rip the diaphrams in persuit of perfection. I bought the entire Holley spring pack, testing each spring at the track one at a time, including clipping a little from each spring coil and testing, doing it over, and over again, hundreds of times, and never could get one flawless.

I could bolt on a Holley double pumper carb, play with the cams, larger pump(s), and squirters a few times, and get them dead on the money in all areas. For the topic in question, the DP carbs are the best choice if you don't have the time/patience for these things. They have PLENTY of pump capacity, adjustments, and flexibility to EASILY cover up any lean spot going quickly on and off the throttle between shifts, or road racing where you may be in and out of the throttle a lot........FWIW.....Cliff

[leeklm]

I have not performed a side x side test, but Have not really noticed any issues with my 800 cfm qjet prepped with cliff's recommendations. Since mine is mostly street, I don't mind giving up a couple of 10'ths on the strip for the other benefits of a qjet.

[Cliff R]

Couple of tenths? Holley's are usually closer than that to the q-jet, once you get them dialed in correctly!.....LOL....Cliff

[Nitebird]

Every track that I have ran at I have not seen anybody running a Q jet.....but rather the whole field of cars usually is running some type of Holley based type of carb or fuel injection.

If the Q jet is that great why don't we see more of them being run?

[leeklm]

Originally Posted by Nitebird

Every track that I have ran at I have not seen anybody running a Q jet.....but rather the whole field of cars usually is running some type of Holley based type of carb or fuel injection.

If the Q jet is that great why don't we see more of them being run?

They don't know what they don't know... Just kidding. I am certainly no expert, but if your primary goal was drag racing, I would probably stick with the Holley for the simplicity of tuning to different conditions. Now if you are primarily a street car with an occasional trip to the strip, hard to beat the road manners and good performance of a qjet.

I had an old Chrysler 383 back in the day and was always jacking around with your typical holley (while not really knowing what I was doing). A GM buddy handed me a stock qjet and said, here, bolt this on. The car (street only) never ran better!

[Cliff R]

We just can't get enough of them done fast enough!

Seriously, I've been in the hobby long enough to see the carb choice deal go full circle.

Back in the 1970's the FIRST thing to go in the dumpster was the stock carb and intake. EVERYONE pulled them off, and went to a fancy aluminum intake and 600 Holley.

The kids who lived on Club drive with the rich parents usually got a "double pumper" Holley, and a Mallory dual point distributor without a vacuum advance!

I was lucky, and so poor that I had to use stock intakes and tweak stock carburetors and distributors. Even WAY back then my cars ran as quick as, or quicker than folks doing about the same thing with all sorts of aftermarket parts. I can't take all the credit, it really helped to see local folks like Ray Stover campaign his 1970 GTO in the Stock Class. He used to bring it to our local 1/8th mile track for test and tune. Amazing how hard that car left using the factory carb and intake. It made me even more committed to get my junk working that good!

Even today nothing has changed in that arena, we still use LOTS of stock parts, intakes, distributors, carburetors, etc, and make PLENTY of power with them, not to mention being very competitive on race days as well.

My small part in the whole deal is to help folks go fast with stock carbs, reworked exactly for their application. Keeps us PLENTY busy around here, and a high percentage of our customers are replacing some sort of aftermarket carburetor, realizing that if one really wants to, you can go just as fast with the stock stuff....FWIW.....Cliff

[i82much]

I really don't have any strong preferences. I just got a Quick Fuel 850 double pumper to replace my AED because I needed a choke. I also got an 870 Street Avenger that I am not quite sure what to do with it. I guess I was thinking about getting an RPM and trying the Street Avenger out on it and comparing it to the 850 with the Torker II and spacer, just for fun. Not track testing, just street driving.

I also have a Carter AFB on my truck. I will pick up a Quadrajet one of these days. For whatever reason, I kind of want to play with them all and fuss around, try them out, learn how they work.

[67GTO4SPEED]

If were building a race car, I'd have nothing but a Holley ( or Holley based) double pumper. No question about it. They are easily tuneable, have huge fuel bowls, and are quick and easy to make jet changes on. They work great for the narrow power band of a race car. However, on a street car, they use a lot of fuel, and are easily to get too rich if you car with little to no stall than still needs enough cfm to make power at the top of the tpm range. That leaves a Holley vacuum secondary carb, a Q- jet, or a Carter/Edelbrock AFB. Of those 3 choices, the Q-jet is vastly superior to the rest. Assuming you have all 3 versions of these carbs properly tuned, the Q- jet will make more overall power and the get better fuel mileage than either. The absolute worst of the 3, in terms of power production, is the Holley vac. It's not uncommon for vac sec Holley (or Holley based vac) to be down 40 - 50 average HP on a dyno compared to a comparably sized mechanical secondary carb. The vacuum secondary carbs are way weaker in the low RPM ranges. We used to have good running 750 Q-jet we kept laying around to show guys that came by the shop with 750 vacs how much power they were leaving on the table using them. It's just really hard to beat a Q-jet on a true street car in terms of power, economy, and driveability.

[i82much]

Originally Posted by 67GTO4SPEED

If were building a race car, I'd have nothing but a Holley ( or Holley based) double pumper. No question about it. They are easily tuneable, have huge fuel bowls, and are quick and easy to make jet changes on. They work great for the narrow power band of a race car. However, on a street car, they use a lot of fuel, and are easily to get too rich if you car with little to no stall than still needs enough cfm to make power at the top of the tpm range. That leaves a Holley vacuum secondary carb, a Q- jet, or a Carter/Edelbrock AFB. Of those 3 choices, the Q-jet is vastly superior to the rest. Assuming you have all 3 versions of these carbs properly tuned, the Q- jet will make more overall power and the get better fuel mileage than either. The absolute worst of the 3, in terms of power production, is the Holley vac. It's not uncommon for vac sec Holley (or Holley based vac) to be down 40 - 50 average HP on a dyno compared to a comparably sized mechanical secondary carb. The vacuum secondary carbs are way weaker in the low RPM ranges. We used to have good running 750 Q-jet we kept laying around to show guys that came by the shop with 750 vacs how much power they were leaving on the table using them. It's just really hard to beat a Q-jet on a true street car in terms of power, economy, and driveability.

The Quick Fuel has adjustable PVCR's, IFR's, and air bleeds. I certainly don't expect a Prius, but I think I can see 13 or so mpg out of my car with the double pumper on those rare days where I keep my foot out of it. As long as it isn't fouling plus and runs nice and crisp, I am ok with it.

[67GTO4SPEED]

Originally Posted by i82much

The Quick Fuel has adjustable PVCR's, IFR's, and air bleeds. I certainly don't expect a Prius, but I think I can see 13 or so mpg out of my car with the double pumper on those rare days where I keep my foot out of it. As long as it isn't fouling plus and runs nice and crisp, I am ok with it.

I've run DP's on cars milder than your's with no problems. In fact, due to my experience with vacs, I have double pulleys on a lot of street cars. I'm building my GTO with a Q- jet because it is going to driven a lot and I want to retain a stock appearing engine. My combination is going to be a lot milder than your's, and I know a Q-jet is capable of the HP I am shooting for and will result is better MPG. The thing about Holley vacs is that they aren't great on fuel nor do they make good power. I think you will be much happier with the DP than a vacuum secondary carb.

[Nitebird]

When you say good running Qjet...do you mean a stock Qjet right off the factory line....or one that has been tuned and parts added to perform to its possible best...because even Holleys can be tinkered with after they are purchased to perform even better than they do.

I would think a off the shelf Holley 850DP would outperform a stock Qjet.

[leeklm]

Originally Posted by Nitebird

When you say good running Qjet...do you mean a stock Qjet right off the factory line....or one that has been tuned and parts added to perform to its possible best...because even Holleys can be tinkered with after they are purchased to perform even better than they do.

I would think a off the shelf Holley 850DP would outperform a stock Qjet.

Any carb will need to be tuned for its intended purpose in order to run to its potential. If you are reasonably handy with a screw driver, I would purchase Cliffs book. After some reading, you could start with a $50 core qjet, and about a $100 or less in parts, and end up with a good running carb, while learning lots along the way. Or, after reading the book, you will say to hell with it and purchase a new carb... either way, you will learn a lot!

Another option is to spend a little more money to get an aftermarket TBI setup that runs off an o2 sensor. Some of those systems look pretty decent for a street car.

[67GTO4SPEED]

Originally Posted by Nitebird

When you say good running Qjet...do you mean a stock Qjet right off the factory line....or one that has been tuned and parts added to perform to its possible best...because even Holleys can be tinkered with after they are purchased to perform even better than they do.

I would think a off the shelf Holley 850DP would outperform a stock Qjet.

I wasn't comparing a Q-jet to a DP carb. As I said before, it is hard to beat a DP for all out performance. I was comparing the Q-jet we had to guys that came by with 750 vacuum secondary Holleys. A stock Q-jet will skull drag one of them.

As far as the carb we had, it was a late 60's big block Chevy carb that was rebuilt. As far as I know it was stock. It sure ran good though, and ran really well on high 7 and low 8 sec (1/8th) small blocks we were running at the time.

[Cliff R]

Holley and Holley clone double pumper carburetors aren't the hot ticket for set-ups with poor power to weight ratios, low stall converters, and low (numerically) gearing. It's ALWAYS been that way. A vacuum secondary carburetor will always be a better choice for those applications, as it simply takes too much fuel from the accl pump system to get the engine through transition.

I can't tell you how many times I've been at the track and watched some poor soul have to get out of his vehicle and put out the fire under the hood using a Holley double pumper carb on a "slow" running car with poorly optimized set-up.

Even on my 455 engine using a DP carb requires a 50cc accl pump and .037" squirters on both sides of it for "flawless" transition, and that's with a 3500 stall converter and 3.42 gears. It would still "pop" slightly if I tried to make a run before the engine and intake were warmed up and heat soaked some.

The q-jet operates differently, and only brings in air/fuel on demand from the huge secondaries. Quite a bit more forgiving that dumping the entire carb on the engine at idle speed, then hoping it will rev into the good power without "puking" all over itself.

This fact makes them much easier to tune than any sort of DP style carburetor, as the opening rate is controlled by the spring for the air valves and it uses a choke pull-off to keep the flaps from "whipping" open too quickly when the throttle is hit hard from idle or low rpm's.

The other thing that has never changed with this hobby, is that any carburetor that you select for your application will most likely need a bit of custom tuning to be dead on in all areas. It's pretty close to a lottery ticket to buy something out of a catalog (or on line), bolt it in place, and hit the street or track and find that engine and vehicle performance is flawless in all areas......FWIW......Cliff