Proper Q-jet for 400cui? - Pontiac GTO Forum
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post #1 of 13 (permalink) Old 02-22-2013, 10:58 AM Thread Starter
 
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Proper Q-jet for 400cui?

I'd like to put a factory q-jet on my lemans. It's not the original engine so I'm not worried about that kind of proper. From what I understand Q-jets came in 750 or 800 cfm. So armed with that little knowledge, which is best for a 400 with a mild lunati cam, and 142 heads shaved down to 85cc, .030 over pistons creating a 9.3cr.


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post #2 of 13 (permalink) Old 02-22-2013, 01:28 PM
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Honestly since you are not worried about originallity 90% of Q-jets are the "same". I would obviously avoid the electronic Q-jets of the early 80's.



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post #3 of 13 (permalink) Old 02-22-2013, 01:55 PM Thread Starter
 
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Proper Q-jet for 400cui?

By same do you mean that 750cfm and 800cfm types are about the same?


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post #4 of 13 (permalink) Old 02-22-2013, 02:10 PM
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post #5 of 13 (permalink) Old 02-22-2013, 09:57 PM
 
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Originally Posted by SPCMarty View Post
By same do you mean that 750cfm and 800cfm types are about the same?


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Most 750 Q-jets are basically the same design, but the CFM ratings were determined by the degrees that the secondary butterflies opened. IOW, a Q jet on a 350 Chevy might only open 75% on the secondary butterflies reducing the cfm that it will actually flow. There are different secondary shafts and adjustments that can be made in the linkage.

If you have the means, a modified Q-jet is definitely worth the money, but you can hop them up with the proper parts and plenty of research. I'm by no means an expert on them, but I've done a few tricks to a couple of them and they'd flat out scream.

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post #6 of 13 (permalink) Old 02-23-2013, 09:03 AM
 
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Marty,

Get a copy of Cliff Ruggles qjet book. Will be the best money you ever spent for carb info. The late 70's qjets are preferred, and are mostly 800cfm. As noted above, the restrictions were made by flap openings, jets, etc. My latest qjet is from a 301 firebird, and has a larger primary bore than my 1972/400 qjet! Secondary bore size is the same on these carbs, it is the primary bore size that changed the CFM ratings.

Here is a quick & easy way to tell bore size, which is how we used to find the "right" 2bbl carb in a stock racing class when combing the salvage yard... find a drill bit that "just fits" inbetween the primay bore and the venturri. if that drill bit is sloppy (or wont fit) on the next carb, you will have an idea of where you are at. I will look at the drill bit sizes when I get out to the shop.

But do not worry much about size (i have heard size does not matter)... I just ran my iron head 455 on the dyno with a 750cfm "stock" qjet, and got over 400hp and 500 tq. Would have liked to run my 800cfm qjet, but was having a couple of issues.... For comparrison, we lost just a couple of hp running a 750cfm holley vac secondary and adapter plate to the stock manifold on that same dyno series.

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post #7 of 13 (permalink) Old 02-23-2013, 09:32 AM
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^Adapter plates kill airflow/power.



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post #8 of 13 (permalink) Old 02-23-2013, 10:26 AM
 
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^Adapter plates kill airflow/power.
A plate that came with my car is the "open" style, and I can see where that would really hurt, especially taking a square bore to a spread bore manifold. The dyno guy had a fancier looking adapter plate that was not open, but rather 4 holes that had a smooth transition to the spread bore intake. Not optimum, but probably much better than the open adapter that I have!

Or better yet, just keep a qjet on the spreadbore manifold!

I have a performer RPM that I would like to try with the holley, just for kicks. But since I am floating values at 5,200 rpm, probably do not need a higher RPM engine combo. In addition, previous owner shaved 15mil off the maniflold to seal on shaved heads, so I am not sure if the shaved manifold will seal on my stock heads...
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post #9 of 13 (permalink) Old 02-23-2013, 11:38 AM
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Hey Marty, on a mild 400 even a 750 cfm QJet ought to be more than enough to keep it happy so don't obsess over finding an 800. Getting the carb set up correctly for your engine is much more critical than sheer size. Add me to the list of people who recommend Cliff's book. It goes into pretty good detail about how to rebuild one stock and make sure it's right, and also what modifications make sense AFTER you get it right in stock form. In the book he emphasizes making changes in very small increments and let me reinforce that here too. Especially when it comes to enlarging the various fuel and air passages in the carb body. Go slow and make small changes, testing after every one, even if it means removing and disassembling the carb multiple times. It's very possible to open a passage too much and make the carb run worse, and once you do that - the carb is ruined. Then you get to go find another one and start over. Cliff can build carbs, but he's usually backlogged so far that you're looking at more than a year's wait time. Lars has some very good information too that you should read, and he also can build you one. I find it more satisfying to do my own, personally, but if you're not comfortable with that then by all means take advantage of the experts.

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post #10 of 13 (permalink) Old 02-23-2013, 09:01 PM
 
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Here are a couple of pics, using a #5 drill bit, which is about .202. You can see where it fits into the larger bore, but not the smaller primary bore. Also, at the bottom of the larger bore, you can see a "nub" protruding from the side, which is also an indicator of the larger bore. But as Bear said, I would not go too crazy trying to find the larger size.
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