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 "750" vs "800" CFM Q-jets 
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Joined: Fri Oct 23, 2015 8:43 am
Posts: 33
Is there any reason to use or avoid a large primary venturi version of the Q-jet on a mild street 350 ci engine? Is there any fuel mileage advantage to the smaller primary venturi, or is the difference insignificant? I've got a few cores to play with and I was wondering if it really matters whether I build a "small" or "large" Q-jet for a 350-375 hp engine.


Tue Jan 12, 2016 12:43 pm
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1702
I have about the same luck with either. Lately a large one has been tormenting me with wanting to be jetted outside the "normal range", whatever that is. I keep reminding myself this is why the jets are threaded. What I don't care for in the large venturi QJets is the T-slot is usually long and 1/3 or more of it is below the butterfly, and I think this makes the idle circuit trickier to get right..... or maybe I just haven't hit the zone yet. I have used brass tubing to sleeve the passage in the base behind the slot to block off some of the flow path to the bore but that still leaves the long slot in the bore, just restricts the exit from the idle down channel to the slot, somewhat like a T-slot restrictor in a Holley. This has seemed to have a positive effect but I haven't done enough of them yet to know if this is something that works in general or is simply suitable for the unique individual engines they are on so far. (every engine is unique, different cam, intake, headers, etc.) In general, I don't think a QJet needs much from the T-slot because the main starts so early due to the double booster, and for that reason the old mid-60s carbs with short T-slots are my favorite examples to clone.


Wed Jan 13, 2016 12:30 am
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Joined: Wed Oct 23, 2013 8:01 am
Posts: 108
Do they need the slots longer because the mains are later coming in being they are bigger?


Wed Jan 13, 2016 9:04 am
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 42
Went thru my stash and found that my mid 70's Pontiac,Olds, and Cadillac 800's had the short t-slot. Both 750 and 800 Chevys from mid 70's to 80's truck 800 had the long slot. 71 Chevy truck 750 had the short slot. On the Chevy's maybe it had to do with EGR that started in 73?


Wed Jan 13, 2016 11:54 am
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Joined: Fri Feb 08, 2013 2:37 pm
Posts: 1702
Papastoy wrote:
Went thru my stash and found that my mid 70's Pontiac,Olds, and Cadillac 800's had the short t-slot. Both 750 and 800 Chevys from mid 70's to 80's truck 800 had the long slot. 71 Chevy truck 750 had the short slot. On the Chevy's maybe it had to do with EGR that started in 73?
77cruiser wrote:
Do they need the slots longer because the mains are later coming in being they are bigger?

The logic of your reasoning is obvious and valid but I think the motivation was more inclined toward the emissions requirement to avoid the production of NOx demanded that the A/F could not be allowed swing into the lean region in mid-load operation, which is the target load range to be as lean as possible for economy. Another factor is the vertical location of the butterfly on the slot, some at the bottom and some more in the middle. Trying to understand all this by reverse engineering is like assembling a 10,000 piece jigsaw puzzle.


Wed Jan 13, 2016 12:42 pm
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Joined: Fri Oct 23, 2015 8:43 am
Posts: 33
Thanks guys.


Fri Jan 15, 2016 9:49 am
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