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Old 09-15-2012, 06:33 PM
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cobalt327 cobalt327 is offline
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Quote:
Originally Posted by LEROYDOZOIS View Post
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
The small Q-jet primaries that work so well to give good throttle response and good economy in many applications- when used in some other applications- can give less than ideal results.

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.

Last edited by cobalt327; 09-15-2012 at 06:41 PM.
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