People running dual carb setups, have any of you ever ran two different sized carbs on your manifold at the same time ex: 650 primary and a 750 secondary? Is this possible and what are the added benefits?
I doubt you'll get much of a response. Generally speaking, it creates a world of unknowns. It shouldn't matter in driveability any more than running a progressive linkage and never getting all in on the second carb. It is likely to create an issue when you are all in and using the second carb. The WOT mixture distribution would probably be uneven enough to happer power output, perhaps even create stresses on the crankshaft. Can it be done? sure. Why? Using what you've got? If you can't afford to do it right the first time, what makes you think you can afford to fix it later?
Well, Wicked...I can't believe you didn't get any responses 'til now. I saw your thread the other day, but didn't post 'cause I'm no carb expert, and I figured there'd be more knowledgeable people responding. But...maybe you'll get some feedback now that it's back to the top. As I said, I'm no expert, but I know a little...so I'm gonna have to disagree a little with Scotty. I know for a fact, that I've heard of guys running bigger end carbs on a 3X2 setup. This scenario you present wouldn't be much different. All dual quad setups use a common plenum. That means that the mixture is gonna get distributed reasonably well enough. So I don't see any "mixture" issues by simply using a bigger cfm carb as the secondary. ***uming all other aspects are attended to and correct, such as vacuum advance, I'm sure it's not going to "hamper" performance, unless that extra 100 cfm puts the fuel-deliver over the top in relation to how much cfm the engine can handle. And I don't see it hurting the engine, or "creating stresses on the crankshaft". I think that the most important issue hear is the total cfm that the engine needs to run efficiently. It's a fact that most street engines run best with a single four-barrel carb. Most in fact run better with one 650 cfm carb as opposed to a 750. Many people use way more carb than what's needed or desired. What size engine are you using? If it's a typical smallblock, two carbs adding up to a total of 1400 cfm is a lot! I believe that even a Holley Dominator has less than that. That being said, there are a lot of dual quads being run, but I think the smarter guys use smaller carbs for the street...something on the order of 500 cfm each. So bottom line as far as I see it, you're not gonna hurt the engine, but it might not run as well as it could. Sorry I can't be more specific...maybe some others smarter than me can help. Good luck.
it WORKS, IF!!!! you do it right , and yea, it makes it diffrent.. whitch is a GREAT reason to do it all by it's self, and if you do it right you can have a great road and strip car. i did it with a sand rail back in 98 , powered it with a 460 and fed it with a 550 and a 850 holly, i had it so the 550 and the 850 meshed intakes, so there would be no uneaven results, meaning i made my own manifold , worked well.. very well, sold the intake to a sweedish guy that build hillclimbers.
I believe chevy used to do this , I think 409 dual quads were 2 different sizes.My buddy had a 1969 dart with a 340 sixpack and the center carb was changed to a 500 cfm ran fine
Rickybop, this is not a true statement. Not all dual quad intakes share common plenums. Also the low rise stock style dual quad intakes have a very very limited sharing of the plenum because the plenum area is very small. The mixture would be much better is the plenum was big like that of a tunnel ram, but the lower regular dual quad intakes some are dual plane. This really gets into distribution issues between primary and secondary carbs. To answer the questions, YES it can and has be done. I spent most of a summer playing around with an offy dual quad intake on a sbc in my 55 pick up. I drove it daily and also played at the dragstrip on weekends. I played around with carbs, linkages etc. I found progressive linkage was by far the best to have. With the progressive set-up, it didn't matter what carbs were on the intake, same cfm or not it was quite driveable and fairly good mannered. I felt like the performance was ok, but when you would pull the plugs and inspect them there was a lot of difference in the plug coloring, meaning to me that the fuel distribution was night the same. Using direct linkage, it made a huge difference in having two of the same cfm carbs versus two different ones. The performance was much lousier, and the drivability/manners ****ed. When I had two of the same cfm carbs on it, it was a little better, but overall I think the biggest problem with dual quad set-up on low rise on the street is not having the carbs set up correctly and not having progressive linkage. The best results from my messing around with my set-up was with two matched carbs and progressive linkage. If you are wondering, because you have two carbs on hand and want to use them, you could and it would be ok. Make sure to set the linkage up progressively and be patient as it will take some tuning and adjusting to get it liveable. If you are wondering because you do not have carbs yet, but want to get the correct total cfm, them I'd say do what you have to do but try and keep the the same cfm each. If you are uncertain as to what carbs to use because all the responses have you confused...well you aasked for it!! JK. I say, look at what the factory had the 401 dual quads...they were not 400cfm. The factory cars ran fine...and with some added performance parts(headers, better breathing air cleaners, etc) you will be more than fine. By the way, the small block I had was not a race sbc, it was 357ci, with a decent hyd. flat tappet cam, "double hump" heads with only pocket porting and gasket matched to the intake, 11-1 compression. It was built to the factory 350/350hp specs. with the exception of 1.6 ratio rocker arms. It had a turbo 400 with a Coan 3500 rpm stall convertor, 4:11 gears. The truck ran a best of low 13's 105mph with the dual quad intake. I changed to a factory 1970 Z-28 aluminum intake and holley 780 vacuum secondary(what factory had on this intake) and ran 12.50's at 110-112mph. The last intake I ran on this motor was an edlebrock dual quad tunnel ram. I had put the same carter afb's on it from my offy dual quad intake and ran 12.70-12.80's 112-115mph. The tunnel ram lost et due to slower 60's times...it just didn't have the bottm end of the other two intakes, but pulled like hell after about 5,000 rpm. I ****ed the truck when I was racing at 6500 rpm. I changed out the afb's to a pair of carbs everyone said would never work, and ran 12.0-12.10's at 117-119mph. I still drove this truck every day with the tunnel ram on it and never once had any problems nor was I unhappy with the results.
Speaking of using smaller carbs, this was my thinking when building my solid lifter 396. I was leaning toward two 500's, but the tech folks at Edelbrock, after being given all the particulars, recommended twin 600's. Another thought; those rules of thumb about cfm usually lean toward a smaller carb, but there have been a few dyno tests that blow that **** out of the water. Look at the factory 302 in the early Z28, a 775 cfm Holley on a 302! And believe me, they ran good!!!!
If you use progressive linkage it's not a problem. One carb is primary, and the other is secondary. Block the idle circuits in the secondary carb by bottoming the mixture adjustment screws, and back off the idle set screw untill the throttle is completely closed, then screw it back in to crack it open enough to keep throttle blades from binding in the bores. Done. This works with tri-powers or anything else that uses secondary carbs with progressive linkage. Eliminates most of the tunning h***el that everybody talks about when running multiple carbs. If it doesn't handle all that CFM well, adjust your linkage to limit the the open travel of the secondary carb. -Mike
Look at the factory 302 in the early Z28, a 775 cfm Holley on a 302! And believe me, they ran good!!!![/QUOTE] Sure the Z-28 302 ran good, but it was designed for road racing. CFMs are dependent on RPMs and the 302 was a high revving engine. I'd like to know how the low end torque was affected. Most guys here are driving on the street, not dragstrips or road courses.
Yes factory three twos ran a smaller carb in the center for primary and larger secondary outboard carbs. Now look at a quad jet. Small primarys and giant secondary. I see no reason your idea wouldn't work. It may be a tuning challange but maybe that's the fun.
I theory it wouldn't be much different than running a quadrajet the secondaries are bigger then the primaries. That said it could be difficult to dial in and it would probably like a larger plenum than a smaller one as they have a tendency to be a little more forgiving. Give it a whirl and let us know what ya come up with.
Will it work? Probably. Will it work well? Probably not. Gasoline engines are quite flexible. If you have an old enough vehicle to have running boards, then you can hire a 15 year old to stand on the running board and pour gas into the intake out of a leaky boot. Will it work? Yes. Will it work well? Probably not. Being more serious: there are enough eddy currents that impede tuning in an intake manifold without creating additional ones. Chrysler probably did the best job (opinion) of tuning their factory dual quads than anyone else. Sometime, take a look at the factory calibration. ON SEVERAL CHRYSLER DUAL QUADS, EVERY THOTTLE BORE IS CALIBRATED DIFFERENTLY! Why deliberately add complexity to your system, and take a chance on getting to redo the engine? Jon.
didnt stu hilborn do it on a his car in the 40's, before he injected it?? wasnt it like 4 carbs and one of them was an oddball bigger carb??
I'm running 2 differant size Edelbrocks on the sbc on top of the tunnelram. Was fairly easy to set up but it did take time to get an even burn going. Been on the car 2 years now with no issues
WICKED...i've been there, and there are NO benifits. just something us poor folks did in 1963. WHY? that's the only carbs. we had. it didn't hurt anythig...POP.
Yes he did just before making his first FI for the flathead. Stu felt the two cylinders on the right front were getting a short deal by firing one behind the other. The trailing hole wouldn't get a full charge. So he put three one barrells over the other port pairs and a two BBl. over that pair. It worked very well for the time. Engines don't know much about what is on the manifold. They see how much fuel and air is comming in and how evenly it's distributed along with how well the fuel is atomized. CFM ratings are just a way to tell one carb from another. Not a real limit to what a carb will flow. It;s up to you to make what ever you have run.
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