|09-15-2012 06:33 PM|
What your buddy may have going on with the truck that he can't get "good" mileage with using a Q-jet, is likely he's using a carb w/the small primary casting (called the 750 cfm casting as opposed to the 800 cfm casting), along w/the truck being either heavy or geared low- or both.
There needs to be enough airflow to meet the engine's demands, obviously. In the case of the Q-jet (made worse if the carb is a small primary Q-jet), the secondaries on an unmodified Q-jet will begin to open long before the primaries are fully opened. The exact amount varies by application, but suffice to say some Q-jets begin to open their secondaries when the primaries are not open very far at all. So if the amount of airflow/throttle opening needed to drive the truck is more than the primaries can provide, the secondaries will open to fill the need.
If the truck was equipped w/such a carb, the engine will be running off the secondaries much of the time, regardless of how carefully the truck might be driven. And once the secondaries come into play, all bets are off. There will be no 'good' mileage in those cases.
This can be made better by tuning, but there are limits as to what you can expect from the Q-jet design (or ANY carb design for that matter). Regardless of how well it's tuned, if the application is wrong or expectations don't match reality, there can be disappointment at how the carb works as far as economy goes.
In the case of a Carter, etc., most will have larger primaries than a Q-jet. This gives a larger window where the primaries are able to provide enough flow to meet the engine's demands- without the secondaries being needed. In a scenario like that, the larger primary carb may give better economy as I'm sure you can understand.
I believe the Q-jet to be the best production carb readily available for use on a performance vehicle. It was used long after the Carter carbs were discontinued w/the exception of the Q-jet-like Carter Thermoquad, which it outlived as well- just not by as many years as the AFB-type carbs. The marine industry stayed interested in the Carter carbs into at least the '90s, and it was used side-by-side w/the Q-jet. But marine usage has many differences from automotive usage so they really can't be directly compared.
|09-15-2012 09:50 AM|
Being its 10:30am on a saturday I'm guessing hungover.
|09-15-2012 09:40 AM|
Qjets are one of the best designs. Avs and AFB are good too. The reason why drivability is better, because when the engine vacuum changes a rod opens or closes more of the jet hole depending on the condition. This maintains velocity. A Holley is really just holes and more throttle dependant. Tunning for some situations that is all you need. A carb responding to the engine vacuumin this manner should be better for more driven conditions.
They all can be tuned to work.
|09-15-2012 01:12 AM|
Back in the 70s there were a few people that used bigger carbs for better mileage.The thing is,there is a lot more to mileage than carb size.What makes a carb work well or accelerate quickly does not usually apply to better mileage. Holley double pumper carbs are "known" to have poor mileage,why?
Because the guys that use them yard the carbs wfo all the time,because they make power.Power burns fuel. Accelerator pumps toss in a lot of raw fuel to compensate for low intake velocity,double pumpers are self explanatory.
If your fuel/air ratio is correct,you do not change speed,do not go up hills,then your mileage will be about the same with all carbs.Drivers kill most of the mileage with aggressive advance of carb linkage,its all just math and the efficiency of the carb and its systems to circumvent our poor driving skills.Take the accelerator pump out of your carb and your mileage will increase,drive ability will be terrible
|09-14-2012 11:05 PM|
Qjet...Great Carb. (If you can read)
How much money you got?
|09-14-2012 10:54 PM|
Saying the Qjet "wasn't that great because they stopped using them in 1989" is like saying that the "Camaro wasn't that great because they stopped making them in the 2000's."
The Qjet is a great carb... and I agree with you that tuning is the biggest factor.
|09-14-2012 04:08 PM|
|ap72||AFB (edelbrock) carbs have been shown many times to produce better mileage than quadrajets, BUT it is all in the tuning. The bad thing about edelbrocks is that while the rods, jets, and springs are easy to swap going beyond that gets tricky.|
|09-14-2012 04:04 PM|
Tuned properly, a Qjet will get good mileage because of those tiny primaries....start stomping on it and it will suck gas with the best of them.
The motorhome gets 1/2 the mileage than the Monte Carlo, because it weighs much more and is much less aerodynamic......Lots of factors in mileage
|09-14-2012 03:49 PM|
thats what i figured.
i actually like the design of my afb more then the quadrajet. ive been out and about and noticed at cruise my afr was super lean, but i was going down hill at like 1000 rpm and although that scared the $h1t out of me, i realized the motor wasnt working at all...
ive been with my gf at the park and changed rods before and or adjusted the mixture screws.
my big ol 1980 suburban with 33 inch tires is getting 8.5 mpg when i was driving 22 miles a day 35 mph tops and a few 25-30 mph zones. at times when im putting at 25-30 mph my afr goes lean in the 15.5-16 afr but as soon as i start going up slight hills it drops to 14 ish.
i know the carb pretty well, and i dont feel the mpg im getting is bad whatsoever. and i bet if i went to a qjet id have the same mpg.
|09-14-2012 02:20 PM|
a Qjet isn't that great, hence they stopped using them on production cars in the 80's. That being said it can be tuned to perform much better or much worse for a given application.
Most carbs can be decent for mileage (none will touch a good EFI setup) but its all in TUNING.
A bigger carb can deliver better mileage than a small one IF the bigger carb is tuned better. If the smaller one is tuned better then it will perform better.
|09-14-2012 02:11 PM|
bigger carbs for better mpg ???!!!
i have a buddy here in town that has a old bronco with a huge small block in it, and he was saying that whenever he had a big block motor that he could never get a quadrajet to give him good mpg compared to a eddy or afb carb. he was then talking about how with the smaller primaries the qjets choked the motors and he always had to step in it harder to get up to cruising mode.
and also to add, he has a qjet on his 305 monte carlo and gets 20 mpg intown, and 25-27 highway.
and a qjet on his motorhome with a 350 and he gets 10 ish which he said is good ( i know nothing about motor homes )
is this just a tuning issue on his part or are there just certain motors that like certain carbs ? i know everyone says how great a qjet is so im slightly hesitant to lean to one side or another