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 Q Jet Secondary Air Valve 
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Joined: Sat Mar 29, 2014 10:27 am
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Deleted post.


Last edited by Papastoy on Wed Sep 09, 2015 6:06 pm, edited 1 time in total.



Tue Sep 08, 2015 5:24 pm
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Joined: Sat Jun 21, 2014 5:38 pm
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Your engine might not pull enough vacuum to open the secondary air flaps all the way. Remember, a Quadrajet operates on vacuum and the secondary air valve only opens has far has the engine can draw it open. Just because it is rated a 800 cfm doesn't mean your engine will see the full 800 cfm.If that makes sense.


Wed Sep 09, 2015 1:40 am
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Joined: Sat Mar 29, 2014 10:27 am
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Deleted post.


Last edited by Papastoy on Wed Sep 09, 2015 6:07 pm, edited 1 time in total.



Wed Sep 09, 2015 7:50 am
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Joined: Sat Jun 21, 2014 5:38 pm
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Well that is the question. I believe most engines under 400 cubic inches won't pull enough vacuum to open the air door all the way. I've built quite a few Qjets for circle track guys that insist on using a 800 cfm Qjet over the 750. Most only have a 355 cubic engine I pretty sure they don't get the secondary air flaps all the way open on a 750 cfm. Several times I've rebuilt 750 cfm carbs for them and later they find a 800 cfm and have me rebuild it. They never notice a "seat of the pants" difference either. Not trying to argue, just stating a point that a lot of people get all worried about cfm rating when proper carb tuning is more importatnt. Instead of setting your air valve to open more. Try setting it to open less till you notice a performance loss.


Wed Sep 09, 2015 9:42 am
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Joined: Fri Feb 08, 2013 2:37 pm
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Papastoy, why did you delete your question?


Wed Sep 16, 2015 12:40 pm
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Joined: Sat Mar 29, 2014 10:27 am
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I guess I didn't frame my questions clearly. So I'll start over.
Lets assume you have an engine that CAN open the secondary air valve all the way to the factory stop. Is there any benefit to modifying the factory stop to allow the valve to open further? I'm curious as to why some folks give a specific opening distance. I didn't see that mentioned in Roe's original book, but one of the more "popular" Qjet authors specifies several different values for his recipes. Opening the valve further was given as reason that the Edelbrock 1910 Qjet has a "flow" rating of 850cfm vs the factory rating measured by a mag several years ago of about 812cfm. I realize that if you open too far, you will adversely affect fuel flow from the nozzles. Just wondering if there is really anything to it thats all.


Wed Sep 16, 2015 4:43 pm
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Joined: Wed Oct 23, 2013 8:01 am
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There must be an optimum angle for the air valve, I don't know what it is. I know some measure from the choke horn to the trailing edge.
Maybe Tuner can shed some light?


Thu Sep 17, 2015 8:27 am
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Joined: Sat Mar 29, 2014 10:27 am
Posts: 42
For future reference, I found one of Tuner's replies to a Qjet ? on another forum that covers my ?:

Quote:
The secondary air door is another deal entirely. The best WOT location is definitely salt-to-taste and individual engines will want what they want. As you see, factory carbs differ and some open further than others. The earliest ’65 & ’66 BBC carbs opened 90° but they also had a baffle with mixture distribution ducts and didn’t seem to work right without them as far as I could tell with the butt dyno.

Another air door example is the QJet on the Turbo Buick, it only opens about 45° and as such it doesn’t reach the WOT tip of the metering rod. If you modify it to open further it kills the power no matter what metering rod you use. On stock and modified engines I have jacked around with them several times and the factory setting is the best even though it makes a much ‘smaller’ carb out of it.

A flow bench says 90° on both throttle and air door flows the most air but the dynos and time slips say less than 90° is what makes the best power.
:D


Sat Oct 10, 2015 7:38 pm
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