Q-Jet carb tuning for performance engine
Thread Starter
Racer
Joined: Nov 2008
Posts: 469
Likes: 0
From: Tallinn (Estonia)
Q-Jet carb tuning for performance engine
Hi,
I am going to use my original 750 cfm Qjet carb with my 383 Stroker and I would like to know what kind of nozzles or needles (or how do you call them?) to order to make the more powerful engine work OK with the same factory carb? I know that I should get a 750 Holley but I would like to try with this one first since I kinda like the Q-Jet.
Thanks!
I am going to use my original 750 cfm Qjet carb with my 383 Stroker and I would like to know what kind of nozzles or needles (or how do you call them?) to order to make the more powerful engine work OK with the same factory carb? I know that I should get a 750 Holley but I would like to try with this one first since I kinda like the Q-Jet.
Thanks!
Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Hi,
I am going to use my original 750 cfm Qjet carb with my 383 Stroker and I would like to know what kind of nozzles or needles (or how do you call them?) to order to make the more powerful engine work OK with the same factory carb? I know that I should get a 750 Holley but I would like to try with this one first since I kinda like the Q-Jet.
Thanks!
I am going to use my original 750 cfm Qjet carb with my 383 Stroker and I would like to know what kind of nozzles or needles (or how do you call them?) to order to make the more powerful engine work OK with the same factory carb? I know that I should get a 750 Holley but I would like to try with this one first since I kinda like the Q-Jet.
Thanks!
Lars will have the best info.
power valves are springs that are color coded. I find them at Pontiac dealers.
needles and seats. needles are different tapers and dia of jets/seats are self explainatory
and the sec hangers, how quick they open-height/lift and needles/rods are diameter.
last time I was serious about it, very hard to find stuff and no fuel mix equip available.
I think now there are after market parts.
Lars should know more.
I will say it was tough but worth it.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
If you already have a Quadra-Jet and a 'spread-bore' intake manifold, you don't need a Holley carb. To know exactly what rods & jets you need for your carb, it is important to know the carb's part number (stamped into the left-rear corner post of the carb body casting) and whether it has been rebuilt previously, or not. Also, specific information about your engine's build, the manifold vacuum level at engine idle, and your drivetrain configuration.
Thread Starter
Racer
Joined: Nov 2008
Posts: 469
Likes: 0
From: Tallinn (Estonia)
Thanks! But are there any carb specialists from who I could order a carb and have it preconfigured for my 383? Or is it something which you can basically only do when the carb is on the particular engine?
Thread Starter
Racer
Joined: Nov 2008
Posts: 469
Likes: 0
From: Tallinn (Estonia)
If you already have a Quadra-Jet and a 'spread-bore' intake manifold, you don't need a Holley carb. To know exactly what rods & jets you need for your carb, it is important to know the carb's part number (stamped into the left-rear corner post of the carb body casting) and whether it has been rebuilt previously, or not. Also, specific information about your engine's build, the manifold vacuum level at engine idle, and your drivetrain configuration.
Le Mans Master
Joined: Oct 2010
Posts: 6,937
Likes: 848
From: melbourne florida
2023 C3 of the Year Finalist - Modified
email lars and let him set it up. it would be good to be close on the vacuum reading but the carb can affect that. i think with the cam specs and compression lars can get really close
Le Mans Master
Joined: Dec 2007
Posts: 6,422
Likes: 591
From: McHenry Illinois
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Team Owner
Joined: Jan 2007
Posts: 53,648
Likes: 878
From: Southern Cal Ca
St. Jude Donor '22
Oh, thanks for this info. I do have the Edelbrock Performer RPM Q-Jet so I guess it should fit. I will check the carb number and get back to you ASAP. We rebuilt it with a friend who knows a bit more about these but he doesn't know the performance mods. I put the whole rebuild kit into it. I guess I can't know the vacuum level before the engine is actually running, though... The transmission is Super T-10, I will check the rear end once again. What else is needed to know about the drivetrain?
edleb has a tune kit for 70 bucks.
I don't know if lars does edlebrocks
Race Director
Joined: Jan 2000
Posts: 12,399
Likes: 247
From: San Jose CA
Very good carbs.
Generally speaking, the idle circuit will have to be tweaked for large cams, and then main jetting/rods and secondary rods, air valve timing will have to be adjusted.
As mentioned, the power piston spring can be replaced to better tailor rod retraction to your engine's vacuum curve. A big cam will have less vacuum and thus require a lighter spring rate on the power piston spring to keep the metering rods seated at cruise.
Burning Brakes
Joined: Nov 2012
Posts: 788
Likes: 100
From: Indianapolis IN
You can order a book that will help your tuning, they have an overseas distributor in Cobham,Surry, England. www.brooklands-books.com
Look for the title ; "Rochester Quadrajet Carburetors" - by CarTech/ SA Design, authored by Mr. Cliff Ruggles.
Look for the title ; "Rochester Quadrajet Carburetors" - by CarTech/ SA Design, authored by Mr. Cliff Ruggles.
Thread Starter
Racer
Joined: Nov 2008
Posts: 469
Likes: 0
From: Tallinn (Estonia)
Thanks for all the advice. I am contacting Lars. But what do you suggest - is it doable by myself to make the 383 run alright with the Q-Jet? Or am I really better off with a Holley or similar?
I am quite sure the carb is the factory Q-jet, not Edelbrock.
I am quite sure the carb is the factory Q-jet, not Edelbrock.
Advanced
Joined: Jan 2013
Posts: 99
Likes: 3
From: Salem Ohio
You won't gain anything buying a Holley, It will need the same things done to it to set it up. Stick with the Quad and listen to Lars I here he really knows what he is doing. Be patient this is one of the many last steps to making it run right.
Be patient this is one of the many last steps to making it run right.
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
If you set the carb up according to the "Quickie Performance Setup" instructions in my Q-Jet Setup paper, it will run very well. The paper has sources for all the parts needed. You can e-mail me for a copy of the paper. This assumes that the carb is a good, rebuildable carb, and that it's not a commercially rebuilt POS...
I can also set the carb up and test it for you. Again, you can e-mail me for my Services Info Sheet.
Lars
V8FastCars@msn.com
I can also set the carb up and test it for you. Again, you can e-mail me for my Services Info Sheet.
Lars
V8FastCars@msn.com
Thread Starter
Racer
Joined: Nov 2008
Posts: 469
Likes: 0
From: Tallinn (Estonia)
Thanks for all the help. Lars sent me his documentation which I should read and see if I am able to carry out the performance mods myself.
Just out of curiousity - if I put the carb onto the 383 in stock configuration, how is the engine going to run - is it just lacking power it has or is it going to hesitate etc?
Just out of curiousity - if I put the carb onto the 383 in stock configuration, how is the engine going to run - is it just lacking power it has or is it going to hesitate etc?
Tech Contributor
Joined: Aug 1999
Posts: 14,380
Likes: 6,391
From: At my Bar drinking and wrenching in Lafayette Colorado
The Edelbrock QJet is a California C.A.R.B-approved carb, so it's set up slightly lean for a performance application. But if it's in good condition without any alterations and "creative" jetting, it will likely run reasonably well.
Lars
Lars
Thread Starter
Racer
Joined: Nov 2008
Posts: 469
Likes: 0
From: Tallinn (Estonia)
I don't think that my Qjet is actually an Edelbrock Qjet since it has 1902 as I remember for the last numbers of the carb serial. However, I will double check it and give the full number.
Commander 47 thanks for the tip. I got some confidence from the AFR dyno tested engine combos where they use Edelbrock Performer RPM. I have that manifold already from earlier times and I'm just planning to use it this year with the engine. I will ofcourse upgrade next year with some other items if I feel necessary.
Commander 47 thanks for the tip. I got some confidence from the AFR dyno tested engine combos where they use Edelbrock Performer RPM. I have that manifold already from earlier times and I'm just planning to use it this year with the engine. I will ofcourse upgrade next year with some other items if I feel necessary.
Melting Slicks
Joined: Oct 2002
Posts: 2,317
Likes: 111
From: St Louis MO
Having used the quadrajet carb on an Edelbrock Performer RPM Q-Jet intake on my 406 SBC (mid 11s in the 1/4), start with its current configuration before making any changes. The extra cubic inches will result in a slightly higher running vacuum at any rpm - the power spring will be fully compressed at cruise keeping the mixture the same as before (intake vacuum will be 16" to 19" Hg). Fit a vacuum gauge in the cockpit where you can see it when driving and an oxygen sensor gauge (even a narrow band O2 sensor will work for the tuning).
Target stoichiometric for all vacuums above 10" Hg; target best linearity of performance from 10" to 3" Hg (slightly richer than stoichiometric); target best full throttle performance with secondary metering rods which will probably require thinner rods for best power - this will be definitely richer than stoich - a narrow band O2 sensor will be maxed out).
Other modifications for best performance such as secondary air valve tension and opening angles are available from Lars and many other literature sources.
Target stoichiometric for all vacuums above 10" Hg; target best linearity of performance from 10" to 3" Hg (slightly richer than stoichiometric); target best full throttle performance with secondary metering rods which will probably require thinner rods for best power - this will be definitely richer than stoich - a narrow band O2 sensor will be maxed out).
Other modifications for best performance such as secondary air valve tension and opening angles are available from Lars and many other literature sources.