# Dual Quad on 64 389



## tguggino (Aug 31, 2013)

Any advice on the idea if running a dual quad set up on a 64 gto 389?

Does it work? What is the best manifold, carb, cam header, set up? (Stock heads which can be worked if needed) 

What are the positives vs negatives? (Outside of cost). 


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## 666bbl (Apr 13, 2014)

I would think you'd take the same approach as any other hipo motor getting fitted with 4X2s. Is the idea to use as many OEM parts as possible or do an aftermarket Edelbrock'ish get up? I think an older SD rig would look bitchin in there. I can't imagine how many casual observers would question how "special" the car is because it got those vs trips.


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## BearGFR (Aug 25, 2008)

I think it'd be cooler'n snot.  

The trick, as with any carburetion setup is to size it for what the engine needs. There is most certainly such a thing as too big. Carburetors need a certain range of air flow velocity in order to work properly and meter fuel correctly, so if you (for example) were to put a pair of 1050 Dominators on top of a stock 389 it's doubtful that the engine would be able to generate enough flow velocity for it to even start and run, much less have any semblance of drivability. So, your questions about which cam, etc. would be right for the carbs are actually backwards. Build the engine with the cam, heads, valve train, exhaust, etc. that both meet your budget constraints and also how you plan to use the car (street, strip, both, in-town, highway cruiser, etc.) and THEN pick an intake/carb setup that can supply the correct amount of air at the correct velocity for that setup. Remember that a car is a system, not just an engine, so vehicle weight, transmission type, and rear gear all need to be considered too.

Bear


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## geeteeohguy (Feb 2, 2008)

OK....I have some experience in this area. The old fashioned Offenhauser 360 intake works well with a couple of Carter AFB's....a friend's car had a hot 455, 4.56 rear gear and a 4 speed, and he ran two 600 CFM AFB's. Tons of power, great drivability, and a total gas hog. The carbs _just_ cleared the hood scoop on his '66 GTO, so it was a sleeper. Drove a 4 speed/4.11 geared '66 GTO 389 with an Edelbrock cross ram and two AFB 500's, and same thing: a lot of power (especially mid range and top end), good drivability, and used a lot of gas. The last dual quad car I did not drive, but rode in. '74 Firebird with a mild 455 and two 800CFM Quadrajets on a custom intake. The car had lazy 2.41 gears and was a 4 speed some of the time, and an automatic the other part of the time. Car went like the blazes, a little soft on the bottom end due to the gearing and the low velocity of the intake, but the top end was unreal. Also, a fuel hog, but not as bad.....the primaries were pretty small. What the previous posters said, though: when upgrading, look at the entire car. Very easy to over-fuel and get LESS performance than with a stock set-up. My experience? Dual quad cars, done right, are very hard to beat on top-end and mid-range acceleration. They are visually very cool. They are expensive, and can be difficult to set up. (I had one stick WOT on me....not good). They will have poorer driving quality on a street driven car and use a LOT more fuel, even when you're cruising. For this reason alone, I never ran one on a personal 'driver' car. Think about what you want out of your car before you pull the trigger on this one.


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## PontiacJim (Dec 29, 2012)

Dual quads are always eye candy. Dual quads on a tunnel ram are even better. They do suck up the gas. Most applications use standard carbs which have idle mixtures, unlike some of the early '60's cars which did not use this on the second carb, sort of like the outboard carbs on a tri-power set-up. You can turn the idle screws all the way in, but I don't think there is any real gain here. Had a pair on a Chevy and got 8-10 MPG, but had a big cam to go with it. Mid to upper RPM's would rip your head off because of the additional air flow and gas.

As another option, if you want to keep your engine stock and not build it up, run off of one carb. Make a form fitted blocker plate to insert under the other carb to close it off from the manifold. Install a dummy throttle rod that simply slips through the other carbs linkage. It'll look like functional dual fours and none will be the wiser to know any different. Did this on a '57 Pontiac I had. Impressed the heck out of everybody, but they never knew the truth.:smile2:


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## tguggino (Aug 31, 2013)

Cool. Good advice. I ended up buying a rather plain 64 post coupe. Very clean, original with about 68k miles. Since early big pontiacs had some cool Dual Quad I feel it is " in the linage" to use them to spice up this car. I will be search for a nos 60 to 64 dual quad once I get the car to my house and really assess what needs to be addressed. 


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## extinctmake (Feb 8, 2011)

I find this thread very interesting and the participants are among those I respect a great deal on this forum.

I am inspired by this thread to ask about the carburetor size calculator and how it factors in deciding how much carburetion to run.

I have a '65 GTO that is very close to restored original and it has the 500 cfm Carter AFB, stock grind cam, 077 heads, a too high compression ratio, stock exhaust manifolds, and the OEM points distributor slightly re-curved. 

The calculator which I attached the link so it can be accessed, determines carb size by engine displacement, maximum rpm, volumetric efficiency, and if your carburetor type is stock or modified. The volumetric efficiency metric is determined over whether the carburetion is stock or modified.

Carburetor Size Calculator | GTSparkplugs

Here is what I found regarding my car: 389 CID, 5,000 max rpm, volumetric efficiency .75and carburetion stock. It computs to my engine needing 422 cfm. Not much. 

Even when I increase the volumetric efficiency to .85 which falls under a mildly built to high end stock engine, the cfm requirements jump to 478 cfm. This is still within the range of the 500 cfm AFB on my GTO.

Does this mean if I added a larger carburetor to my 389 I would be over carbureting my engine? 

Is this calculator even legitimate?

I anticipate an interesting discussion.


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## PontiacJim (Dec 29, 2012)

Here is a little something I found on the web:

"Sometime during the 1950’s, engineers found that a passenger engine with a four-barrel carburetor would not maintain a vacuum of 3 inches of mercury at wide-open throttle; and by some convention 1 ½ inches of mercury was chosen for rating 4-barrel carburetors. The ratings for 1-barrel and 2-barrel carburetors were left unchanged.

To convert from one system to another (with a very small percentage of error) is relatively simple. Simply use the square root of 2 (1.414). Thus to convert a two-barrel rating into a four-barrel rating, divide the two-barrel rating by 1.414. To convert the four-barrel rating to a two-barrel rating, multiply the four-barrel rating by 1.414.

This worked very well up through the mid-1960’s, when carburetor comparison tests became popular in car magazines. One carburetor company determined that the results could be skewed by rating their carburetors “dry” (air only), instead of the conventional “wet” (a non-flamable liquid with the density property of gasoline and air mixed). Rating the carburetor dry would add approximately 8 percent to the rating (example – a carburetor rated on the four-barrel rating scale at 500 CFM would now amazingly flow 540 CFM).

So, to give an example:

Using a Carter 4-barrel AFB carburetor that is rated wet and rated 500 CFM for an example.
To convert this rating to the 4-barrel dry scale, add 8 percent (multiply by 1.08). 500 CFM multiplied by 1.08 is 540 CFM."


From what I have found, the smaller 2bbl carbs as used on the '65 and earlier tripowers flowed 278CFM's rated at 3.0" of mercury, so they flow 196CFM's using the 4bbl measured 1.5" of mercury. So, all three combined would flow close to 600CFM's at 4bbl measurements. 

Choosing a carb takes into many considerations, ie engine internals. The carb calculators in my opinion are just a good starting point. They probably represent a basic street driven, small cam, low RPM engine. They don't take into account big cams, 10,000 RPM engines, big flowing heads, tunnel rams, etc.. 

The 500 CFM is small, in my opinion. But, it gives good throttle response, and takes into account the max RPM's the engine will turn, and cam rating. Modifications, more RPM's require more air flow, thus bigger carb. The 400CI (even the HP 350CI) went to the 750CFM Q-jet and had no problems. The smaller primary bores and vacuum operated secondaries were key, but they also got bigger cams and better flowing heads.

If you wanted to experiment and try a seat of the pants comparison, I would think the 625CFM AFB carb would be better if there were no clearance issues between the base of the carb/throttle blades and the factory manifold.


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## BearGFR (Aug 25, 2008)

Yes, be very careful when trying to size a carburetor based on one of the calculators and using the manufacturers' CFM ratings. You have to know what the numbers actually represent. Does the calculator give you raw theoretical CFM airflow that the engine can actually use, or does it take into account the difference between how a carb is "rated" and what it can actually deliver in use? A carb can't function without SOME amount of manifold vacuum present, but the fact that there's vacuum present already means that the engine is "wanting" more air than it can get. To get the very best out of a carbureted engine system, you've got to hit the sweet spot where there's the absolute minimum amount of vacuum present that the carb needs in order to operate properly (and idle) but not one bit more than that. I've not yet seen an objective carburetor comparison that has hard data showing what various carbs flow at equal vacuum depressions and also how much vacuum each design needs in order to operate efficiently. For that reason, selecting the "best" carb for any particular engine combination is still as much art as it is science.

Bear


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## geeteeohguy (Feb 2, 2008)

Extinct, what these guys said. THAT said, and going non-technical, I had a '66 GTO with a bone stock, '67 400 out of a Catalina. It had the 066 2bbl carb cam and 061 heads. It ran very strong with the 500CFM Carter AFB on it. I bolted on a '66 tripower, and it became a completely different car. It was soooo much faster and would rip your head off. No other changes were made: intake only. The Carter AFB for these cars was a practical, dependable system for a car used to drive every day. Remember, these were everyday cars back then, not special or garage queens. The Carter was a good compromise between performance and economy. But, truth to tell, I've seen a LOT of carburetion added to these old engines, and it only seems to make them faster. Now small block Chevys, different story. Very easy to 'over-carburate".


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## tguggino (Aug 31, 2013)

This is a great thread. Happy I finally have a GTO so I can stop asking all the theory questions. You guys have been awesome and you really helped my Dad and I pick the right car for us. Thank for that. Talked with the old man today. He is feeling we need to refresh the motor and is enthusiastic about the dual quad usingNOS pontiac parts. 


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## extinctmake (Feb 8, 2011)

I see, so it seems my GTO has the minimum sized carb and is capable of more without driveability issues. The car does run fine and it is not used on a drag strip and it could be a decent daily driver.

The reason I am asking, and I hope I'm not hijacking tguggino's thread, is I have consulted with you all a couple of times in the past about my long range plan to rebuild my numbers-matching 389 to run on today's octane. And in those conversations I have received advice to run Ross reverse dome pistons, after market steel rods, stainless valves, and use higher lift Comp Cams grinds like XE 262 or XE 256. 

I have taken your advice towards my plan and the challenge of my build will be to maintain all the originality of the engine as in the 077 heads, numbers matching distributor, manifolds, and fuel system. It will look stone stock while having the internals you all have recommended. I will even run the Comp Cams roller rocker arms if they will fit under my stock valve covers.

This is the root of my concern. If I build a 389 that differs internally from original and has a much different valve train with much higher lift and at least the same duration, will the 500 cfm Carter AFB still be sufficient enough to feed this engine? This is why I question the validity of the carburetor calculator I mentioned in my earlier post. 

I guess I could try the AFB and see what happens and change to a Tri Power $$$$$ if the AFB doesn't cut it. Luckily I can continue running the correct engine in my car while collecting pieces for the numbers-matching engine.

I really appreciate the input all of you provide.


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## 666bbl (Apr 13, 2014)

I used one of those whiz-bang calculator programs just for fun when I built my racer. I set the V.E. at 110%. I can't remember where I found the reason to do that, but the roller cam, high compression, intake volume, etc all came into play. It said I needed 950 CFM to do the job. There's another little 'spoken rule' that applies to this. ADD 100 CFM TO THE FINAL NUMBER. Why? Sheets fellas, I don't remember the basis or where I learned that but it was in print. Lo and behold the choice of a 1050 Domniator worked extremely well. I had a situation that almost nobody here in these pages have. WFO all the time. Drivability? Who cares! Throttle response? That's the driver's job! From the staging lights to the win lights, all wide open. There were some interesting side effects. Of course idle was relatively high at 13-1400 but the throttle response was very small blockish in character. Vacuum? I could have run power brakes! Then just for fun I put a super-double throw down-custom-modified 1250 CFM Dominator on it, borrowed for 1 night. The car did pick up almost .2 in ET and 3-4 MPH. I could have bought it, used (!) for $1000. Now this was back in 96 when all those custom built carbs were becoming the fashion statement of choice on the top of a BBC. Not for bracket racing. I could win just as much at 10.32 as I could at 10.18. 

That's my story so I've told it my way  Reading some of the the other inputs and past experiences it seems the "...add 100..." thing is worth doing, no? I can't wait to see the engine pics when the OP gets this going. If I stumble onto any Indian dual quads I'll let you know.


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## tguggino (Aug 31, 2013)

666bbl. Thanks for the input. All cool stuff.

All.
I see a bunch of Offy dual quad manifolds on ebay for the 60 to 64 Pontiac engines. They range from $200 to $400. Any advice on how to shop these beasts? I am pretty sure I am going to need a low profile. The car has power brakes so that is a must as well. Are there specific items/issues I should be looking at?


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## PontiacJim (Dec 29, 2012)

tguggino said:


> 666bbl. Thanks for the input. All cool stuff.
> 
> All.
> I see a bunch of Offy dual quad manifolds on ebay for the 60 to 64 Pontiac engines. They range from $200 to $400. Any advice on how to shop these beasts? I am pretty sure I am going to need a low profile. The car has power brakes so that is a must as well. Are there specific items/issues I should be looking at?


You can get an Edelbrock or Offy. They both are basically a copy of the factory 2 x 4 intake. You could grind the names off and paint it to match your engine. No real advice on purchasing one. Prices range based on condition and what the seller feels its worth. I think the $200 range is more reasonable and $400 too pricey.

If I were to use two carbs, then I would think a pair of 500CFM's would work with a matching cam.

Your 500CFM carb should work with all the engine changes, but it is still probably small for a single 4bbl.

You may want to email any of the Pontiac engine builders and get their opinion. Let them know your compression ratio, that you would like to run an Offy 2 x 4 with a pair of 500CFM AFB's if it will work with your compression ratio, and if they could suggest a Comp Cams camshaft grind that would work with that combo. They will let you know if it won't work or what cam grind you should choose.


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## geeteeohguy (Feb 2, 2008)

If you upgrade the cam and valve timing, you will need to upgrade the fuel, distributor curve, and exhaust to utilize the extra fuel and air pumped through the engine. It's a combination of parts that increases overall performance. Just installing a cam and using the stock intake, exhaust, and ignition curve is a performance killer. Also, be advised that you will want a '65-up intake if you have a '65-'66 389. The '60-'64 intakes will not fit your heads.


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## tguggino (Aug 31, 2013)

Hi. I have a 64 GTO all stock now. As part of the engine refresh I plan on creating the correct flow for most engine efficiency (in terms of power not fuel consumption). Thinking about it in just in common sense terms, with more fuel/o2 volume, intake port flow needs to be increased, the fire needs to be "hotter" and the exhaust ports/system need to handle the increased waste produced. 

Questions
How does compression ratio, combustion chamber and piston type/shape effect the equation. 

Looking to make this a street motor built with quality and precision. Our objective is to drive the car to local meets and have some fun with it. Not looking to race or make a museum piece. 


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## 666bbl (Apr 13, 2014)

Keeping it simple, compression increases help to get more energy from the fuel. Cam timing is pretty obvious as in too much of anything there can kill low end or throttle response, require more fuel to make power, make driveability miserable. I have a philosophy about the basics of the mechanical combinations which is to never forget them. In fact, embrace them. The other is to pick 1 source to handle the things I don't like machine work, valve seats, etc. The worst thing you can do is listen to a 1/2 dozen "experts" that will tell you "...you're doing it all wrong because my cousin's next door neighbor's ex brother-in-law's car did blah, blah, with...". Usually well intended but still not worth it. Here on these pages you can easily find the perfect combination that will be thrilling to drive and special to see, but again, embrace the basics. They'll never let you down. 

On a side note, I've been musing about E85 in our high compression gennie engines. Similar to tuning for alcohol, some serious power is being made with it with little more than alky-safe soft parts in the fuel system and proper ignition tuning. "Ours" were born with the extra compression and a 2X4 get up could certainly deliver the extra 30% fuel requirement. Worth a night or 2 of 'net surfing, no?

quick edit: Hot Rod article on E85...

http://www.hotrod.com/how-to/engine/ccrp-0611-e85-ethanol-fuel-test/


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## 1965goat (Sep 8, 2014)

Id get the edelbrock dual quad manifold and run 2 600cfm carbs on top of that. Reliable fun setup, and is a great look for the early style goats.


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## tguggino (Aug 31, 2013)

Why Edlebrock? Do they offer a benefit over Offy? Agree on size. What did the 64 389gto come with. 500 or 625cfm?


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## Instg8ter (Sep 28, 2010)

As soon as i get a break from work gonna start fitting this on the 462. RA dual quads anyone?


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## rickm (Feb 8, 2012)

500.


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## tguggino (Aug 31, 2013)

Picked up an Offy dual quad manifold. Being shipped now. I guess I should start looking for another carb and a linkage kit. 


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## BearGFR (Aug 25, 2008)

Instg8ter said:


> As soon as i get a break from work gonna start fitting this on the 462. RA dual quads anyone?


So would you please hurry up and get that thing on and running? I *need* to hear it!!

Bear


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