Holley vs Quadrajet
Thread Starter
69 Ol' Blue
Joined: Jan 2020
Posts: 205
From: Southwest Virginia
Holley vs Quadrajet
Let's get the conversation going. I want to hear from experience which set up is best. I have a couple different setups that I am considering running on my freshly built 1970 455. One is an Offenhauser dual plane 360 aluminum intake with a Holley 750 double pumper. The other is the factory cast iron intake with a 750 cm Rochester Q-jet. I've read a little about the higher velocity at lower rpm's related to the Rochester. I'm thinking that with a bigger cam (314 duration) the Rochester may help responsiveness in town driving and will still flow well at higher rpm's (6,000-ish). I may see the occasional trip to the track too. Thoughts?
Registered User
Joined: Jun 2010
Posts: 3,613
From: Rogues Island, USA
My thoughts are:
Not too many people can tune a Rochester properly especially with the cam you are proposing. Your numbers at .050 would be more helpful.
No need for a double pumper, vacuum secondaries work just fine.
750 seems a bit small for a healthy 455.
Not too many people can tune a Rochester properly especially with the cam you are proposing. Your numbers at .050 would be more helpful.
No need for a double pumper, vacuum secondaries work just fine.
750 seems a bit small for a healthy 455.
Registered User
Joined: Sep 2017
Posts: 5,171
From: St. Paul Minnesota
Registered User
Joined: Sep 2018
Posts: 632
From: Newburyport MA Area
Here's what some here shared with me ...
(and so much more on the internet / google)Power Tuning a Quadrajet Carb
AND
The small primary bores gave good fuel economy, throttle response, and emissions control, while the huge secondaries when called upon would boost air flow to 750 CFM for most versions and 800CFM for a few others. Many 600+ hp Super Stock drag race cars utilize them (stock carb as required by rule) and go deep into the 9 second range in the quarter mile.
Usually the biggest problem in getting a Qjet to perform properly is getting the secondary air valve to open at the proper rate. Secondary jetting is easily changed by changing metering rods, which doesn't require any carb disassembly. There are no replaceable secondary jets, just rods. The primaries have both replaceable jets and metering rods but the top of the carb must be removed to get to them. Due to the small size of the primaries and effort required to change primary jetting, and the relatively small effect it would have on performance, it's usually not worth fooling with unless there is a severe lean condition. The Qjet is a somewhat complex piece as carbs go, but once one is right, it usually stays right for a long time. "Set and forget".
To set up a Qjet for good street performance, first a basic rebuild is recommended if the carb is old and hasn't had one lately. You can do it yourself with an approx $15 rebuild kit, basic tools, some carb cleaner, a tooth brush (not your wife's), and some patience. Blow out all the tiny passages with compressed air if you can. Before reassembling, seal the bottom of the secondary fuel wells with epoxy or JB Weld. They are notorious for leaking and should be pointed out in the rebuild instructions. When the carb is assembled and adjustments made to spec, turn your attention to the secondary air valve rate of opening. This is the big flapper on the very top of the secondary throttle bores. There is an adjustable tension spring and linkage rod connecting it to a vacuum diaphragm on the passenger side. Most stock Qjets open the air valve too slow. To correct this you can loosen the spring tension by loosening the small allen-head set screw (on the bottom of the air valve pivot) and use a small screwdriver to adjust the screw (in-line with the pivot) controlling the tension. Then tighten the set screw. If you have a bog caused by the secondaries opening too fast, tighten the spring tension in small steps til the bog goes away.
If loosening the spring tension can't produce even a slight bog, you'll have to do more. You can drill and enlarge the tiny vacuum orifice that's down inside the vacuum hose connection of the plastic diaphragm housing to allow the linkage rod to move faster. The orifice is about .010 diameter and should be drilled to .015-.020. But this requires tiny bits and a pin vise. The simplest and easiest thing to do is remove the air valve linkage rod completely and tighten up the spring tension til the bog almost goes away. I say almost because a slight bog is usually unavoidable with this modification. But boy will it wake an engine up! A caution is that on some models the linkage rod also doubles as the choke unloader so a choke adjustment may be required.
(and so much more on the internet / google)
Power Tuning a Quadrajet Carb
http://www.route66hotrodhigh.com/PowerTuneQjet.html
ANDThe small primary bores gave good fuel economy, throttle response, and emissions control, while the huge secondaries when called upon would boost air flow to 750 CFM for most versions and 800CFM for a few others. Many 600+ hp Super Stock drag race cars utilize them (stock carb as required by rule) and go deep into the 9 second range in the quarter mile.
Usually the biggest problem in getting a Qjet to perform properly is getting the secondary air valve to open at the proper rate. Secondary jetting is easily changed by changing metering rods, which doesn't require any carb disassembly. There are no replaceable secondary jets, just rods. The primaries have both replaceable jets and metering rods but the top of the carb must be removed to get to them. Due to the small size of the primaries and effort required to change primary jetting, and the relatively small effect it would have on performance, it's usually not worth fooling with unless there is a severe lean condition. The Qjet is a somewhat complex piece as carbs go, but once one is right, it usually stays right for a long time. "Set and forget".
To set up a Qjet for good street performance, first a basic rebuild is recommended if the carb is old and hasn't had one lately. You can do it yourself with an approx $15 rebuild kit, basic tools, some carb cleaner, a tooth brush (not your wife's), and some patience. Blow out all the tiny passages with compressed air if you can. Before reassembling, seal the bottom of the secondary fuel wells with epoxy or JB Weld. They are notorious for leaking and should be pointed out in the rebuild instructions. When the carb is assembled and adjustments made to spec, turn your attention to the secondary air valve rate of opening. This is the big flapper on the very top of the secondary throttle bores. There is an adjustable tension spring and linkage rod connecting it to a vacuum diaphragm on the passenger side. Most stock Qjets open the air valve too slow. To correct this you can loosen the spring tension by loosening the small allen-head set screw (on the bottom of the air valve pivot) and use a small screwdriver to adjust the screw (in-line with the pivot) controlling the tension. Then tighten the set screw. If you have a bog caused by the secondaries opening too fast, tighten the spring tension in small steps til the bog goes away.
If loosening the spring tension can't produce even a slight bog, you'll have to do more. You can drill and enlarge the tiny vacuum orifice that's down inside the vacuum hose connection of the plastic diaphragm housing to allow the linkage rod to move faster. The orifice is about .010 diameter and should be drilled to .015-.020. But this requires tiny bits and a pin vise. The simplest and easiest thing to do is remove the air valve linkage rod completely and tighten up the spring tension til the bog almost goes away. I say almost because a slight bog is usually unavoidable with this modification. But boy will it wake an engine up! A caution is that on some models the linkage rod also doubles as the choke unloader so a choke adjustment may be required.
Thread Starter
69 Ol' Blue
Joined: Jan 2020
Posts: 205
From: Southwest Virginia
Here's what some here shared with me ...
(and so much more on the internet / google)Power Tuning a Quadrajet Carb
AND
The small primary bores gave good fuel economy, throttle response, and emissions control, while the huge secondaries when called upon would boost air flow to 750 CFM for most versions and 800CFM for a few others. Many 600+ hp Super Stock drag race cars utilize them (stock carb as required by rule) and go deep into the 9 second range in the quarter mile.
Usually the biggest problem in getting a Qjet to perform properly is getting the secondary air valve to open at the proper rate. Secondary jetting is easily changed by changing metering rods, which doesn't require any carb disassembly. There are no replaceable secondary jets, just rods. The primaries have both replaceable jets and metering rods but the top of the carb must be removed to get to them. Due to the small size of the primaries and effort required to change primary jetting, and the relatively small effect it would have on performance, it's usually not worth fooling with unless there is a severe lean condition. The Qjet is a somewhat complex piece as carbs go, but once one is right, it usually stays right for a long time. "Set and forget".
To set up a Qjet for good street performance, first a basic rebuild is recommended if the carb is old and hasn't had one lately. You can do it yourself with an approx $15 rebuild kit, basic tools, some carb cleaner, a tooth brush (not your wife's), and some patience. Blow out all the tiny passages with compressed air if you can. Before reassembling, seal the bottom of the secondary fuel wells with epoxy or JB Weld. They are notorious for leaking and should be pointed out in the rebuild instructions. When the carb is assembled and adjustments made to spec, turn your attention to the secondary air valve rate of opening. This is the big flapper on the very top of the secondary throttle bores. There is an adjustable tension spring and linkage rod connecting it to a vacuum diaphragm on the passenger side. Most stock Qjets open the air valve too slow. To correct this you can loosen the spring tension by loosening the small allen-head set screw (on the bottom of the air valve pivot) and use a small screwdriver to adjust the screw (in-line with the pivot) controlling the tension. Then tighten the set screw. If you have a bog caused by the secondaries opening too fast, tighten the spring tension in small steps til the bog goes away.
If loosening the spring tension can't produce even a slight bog, you'll have to do more. You can drill and enlarge the tiny vacuum orifice that's down inside the vacuum hose connection of the plastic diaphragm housing to allow the linkage rod to move faster. The orifice is about .010 diameter and should be drilled to .015-.020. But this requires tiny bits and a pin vise. The simplest and easiest thing to do is remove the air valve linkage rod completely and tighten up the spring tension til the bog almost goes away. I say almost because a slight bog is usually unavoidable with this modification. But boy will it wake an engine up! A caution is that on some models the linkage rod also doubles as the choke unloader so a choke adjustment may be required.
(and so much more on the internet / google)
Power Tuning a Quadrajet Carb
http://www.route66hotrodhigh.com/PowerTuneQjet.html
ANDThe small primary bores gave good fuel economy, throttle response, and emissions control, while the huge secondaries when called upon would boost air flow to 750 CFM for most versions and 800CFM for a few others. Many 600+ hp Super Stock drag race cars utilize them (stock carb as required by rule) and go deep into the 9 second range in the quarter mile.
Usually the biggest problem in getting a Qjet to perform properly is getting the secondary air valve to open at the proper rate. Secondary jetting is easily changed by changing metering rods, which doesn't require any carb disassembly. There are no replaceable secondary jets, just rods. The primaries have both replaceable jets and metering rods but the top of the carb must be removed to get to them. Due to the small size of the primaries and effort required to change primary jetting, and the relatively small effect it would have on performance, it's usually not worth fooling with unless there is a severe lean condition. The Qjet is a somewhat complex piece as carbs go, but once one is right, it usually stays right for a long time. "Set and forget".
To set up a Qjet for good street performance, first a basic rebuild is recommended if the carb is old and hasn't had one lately. You can do it yourself with an approx $15 rebuild kit, basic tools, some carb cleaner, a tooth brush (not your wife's), and some patience. Blow out all the tiny passages with compressed air if you can. Before reassembling, seal the bottom of the secondary fuel wells with epoxy or JB Weld. They are notorious for leaking and should be pointed out in the rebuild instructions. When the carb is assembled and adjustments made to spec, turn your attention to the secondary air valve rate of opening. This is the big flapper on the very top of the secondary throttle bores. There is an adjustable tension spring and linkage rod connecting it to a vacuum diaphragm on the passenger side. Most stock Qjets open the air valve too slow. To correct this you can loosen the spring tension by loosening the small allen-head set screw (on the bottom of the air valve pivot) and use a small screwdriver to adjust the screw (in-line with the pivot) controlling the tension. Then tighten the set screw. If you have a bog caused by the secondaries opening too fast, tighten the spring tension in small steps til the bog goes away.
If loosening the spring tension can't produce even a slight bog, you'll have to do more. You can drill and enlarge the tiny vacuum orifice that's down inside the vacuum hose connection of the plastic diaphragm housing to allow the linkage rod to move faster. The orifice is about .010 diameter and should be drilled to .015-.020. But this requires tiny bits and a pin vise. The simplest and easiest thing to do is remove the air valve linkage rod completely and tighten up the spring tension til the bog almost goes away. I say almost because a slight bog is usually unavoidable with this modification. But boy will it wake an engine up! A caution is that on some models the linkage rod also doubles as the choke unloader so a choke adjustment may be required.
Registered User
Joined: Jul 2009
Posts: 3,182
From: St Augustine
Lot a paste and copy here, and the info requires the reader to be an expert modifying a Q-Jet. For the Olds guys wanting to modify their Q-Jets, find Doug Roe’s Rochester Carburetors book, and Cliff Ruggles book, a few paragraphs is almost worthless for the guy who is interested in becoming knowledgeable servicing their Quadrajet..
Registered User
Joined: Jan 2010
Posts: 6,061
From: central Indiana
I don’t think cliff is building carbs anymore, but he still sells the kits and part to properly rebuild and calibrate the carb for your application. Call him with all your engine specifics, the more you can tell him, the better the tune will be.
Administrator
Joined: Oct 2009
Posts: 42,385
From: Poteau, Ok
I've used both Holley and Rochester carbs among others. All carbs need to be set up correctly and fit the application to run correctly. The Rochester may get a bit better fuel economy during cruise.
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