Basic Q-Jet question

[TrcksR4ME]

Is there a difference between a passenger car model and a truck model? Looking at Jet Performance q-jets and they sell the 750 cfm as a car model and the 795 for trucks...just curious what the difference might be?
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'77 K5, Warn M8000, 402/350/203, 44/14bff, 4"/36" Irok's

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[pauly383]

They were all either 750 cfm commonly , or 850 cfm rarely . Most of the differences were in the tuning for the original application , but even the smaller motors Quadrajets were able to flow 750 cfm if you changed internals and put it on a motor that could use it .

You may want to talk to someone at Jet , or wherever you choose to get one and see what they recommend for your application

 

[Snyiper]

The 750 were the most common and suitable for most stock to mild applications. You'll find an array of differences in applications from Jetting to metering rods to the type of rod hangers...Good advice is to get ahold of a good Qjet guy and tell him what you have he can tell you what you need. There is a excellent Qjet book out there that goes into great detail but The name escapes me at the moment (and I have 2 copies)!!!! Don't let the Jet intimidate you it is a good carb with lots of tuning ability and most if not all work can be done while it is on the truck!! I prefer a Qjet over Holly, why cause I am more comfortable with the QJet not thats its better but I understand it better thats all.
Glenn

 

[eds77k5]

i replaced my old qjet because the PO had removed one of the butterflies and the choke was disconnected, my kids friend had some qjets layin around so he got me one, i checked the numbers and it came from a 79 pontiac, had electric choke, i bolted it on and it is running a little rich but works better than the old one until my new one gets here. maybe it was on a firebird? the kickdown is different.

 

[dyeager535]

No there isn't.

The external differences (not including CCC Q-jets in this comparison) are going to be choke setups (sometimes easily changeable, sometimes not) the front "face" and top changed based on emissions, you either have a "squared off" front for "pre-emissions" (not really true statement, but how its usually described) and then the front face with a piece jutting out of it. (not flat, thats for sure) The top part is different as well on pre/post emissions...the post emissions one has a dome on the passenger side top front of the carb. Vacuum line fittings varied widely as well. I'm sure some other stuff changed, but those should cover the major variances.

750 vs. 800 applications *seemed* to be totally random, but I suspect it has something to do with torque production. From recent(?) testing, it appears that higher CFM carbs produce more torque. GM applications seem to support this theory...the only 800CFM carbs I *know* GM used on trucks, from pulling them myself, are from the early 80's 305 motors. If you look to cars, a 307 Olds motor used 800CFM exclusively, while the 305 Chev (car) from the same vintages used a 750CFM.

In all cases I've seen, the CFM is is mechanically restricted on the secondary side by how far the butterflies could open. More restriction the later in production you get, but that is easily "correctable".