The Stovebolt Site Forums The Shop Area Engine Shop offy dual intake versus triple intake?

whats the pros and cons off each? i have a dual offy intake for single carbs....looking at a triple carb intake for my 235. its not installed yet.

Will the rear carb (and/or air cleaner) clear the firewall, without using/making an "offset adapter" for the third carb?

hmmm not sure....havent paid that close attention to ones ive seen set up on other trucks...dont know if they had to mod any thing. maybe i should just stick to my dual intake

Carburetors are not a fashion accessory. If you don't know what they do, you should leave it alone.

"Carburetors are not a fashion accessory."

Carburetors most definetly are a fashion accessory! Nothing says wow like a row of carburetors. If one is good, and 2 is better than 3 should be even more better. From a practical standpoint 3 ballanced carbs should distribute the air and fuel better than 2. This would assume a nonprogressive linkage between the carbs. In other words, all 3 carbs would open and close at the same time. If the carbs are selected correctly, fuel economy and performance increases can be seen over stock systems. There are a lot of challenges to making this work, and low end torque will likely suffer for some moderate increases in performance above 4000 rpm. 3 carbs might not be very practical for street use... but then again I'm not a very practical guy.

For the record... I have 2 1bbls on a brittish 4 banger, 3 deuces on a pontiac V8, and 5 deuces on a gmc 302 6 cylinder. Pictures of the GMC 302 can be seen on Jim Carter's site along with a write up. http://www.oldchevytrucks.com/Showcase/the_king.php

And nothing says "I'm screwed" like the owner who"just bolted them on".

Don't Panic... I'll let you drive it. Just come to SF.

Can you give some definitave reasoning for your above statement?

3 intake ports, 3 small carbs above the ports equal better air fuel mixture to all 6 cylinders. Have you seen the dyno reports by Bill Fisher or Leo?

The only issue I see in using 3 carbs is a possible loss of low end torque do to lower intake velocity. Again with a perfectly setup system, it could be more effecient and crank out more HP at the same time. I agree it could be challenging to setup a perfect system, but not impossible.

They definitely are a fashion accessory and then some.
Saw an inline six with six motorcycle carbs on it once, all polished and it ran like a swiss watch, just a beautiful work of art.
But admmitedly not for a novice or the faint of heart.

Pretty negative comment dude....

"Negative" or "positive" would be in the eyes of the reader, but Panic's comment is SO TRUE!

If I had a dollar for every British sports car with FACTORY multiple carburetion that came in barely running because the owner had purchased a popular book on S.U. carbs that had erroneous information and tried to adjust the carbs themselves; I could buy Hawaii and retire!!!

Multi-carbs are great! BUT one's first experience SHOULD be with the old "master mechanic - apprentice" system. One really needs to learn hands-on from a friend who knows.

Not that it absolute cannot be learned by oneself, but easier with a mentor.

Jon.

The man asked what the pros and cons were, but those comments weren't constructive to helping him out at all.
True, if you don't know what your doing with carb setups, then you really shouldn't mess with something you have already working fine.
Personally I would stick with two carbs max, because thats what I feel comfortable working with, and yes I've done plenty of Brit setups as well, and even for an experienced carb guy, those are a pain !
Multiple carbs can absolutely mean multiple headaches !

The following articles have been posted before, but will post them again. They do address some of the pros, cons, and issues, particularly the issue that many three carb set-ups will NOT fit in all vehicles:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

http://www.thecarburetorshop.com/Triple1barrels.htm

The way I personally learned about adjusting multiples AND the "book" to which I earlier referred was on my own TR-3 in about 1966. I ordered the *&^%$#@ book and proceded to attempt to adjust the carbs. I got the carbs so fouled up the engine wouldn't even run! Fortunately, I had the good sense to back out all the "adjustments" and readjusted everything to factory specs. SURPRISE! It started and ran pretty good! I grabbed the book, and drove the car to a dealership; and paid good money to get laughed at! But the money paid was worth it; as the old mechanic who set the carbs showed me how, and explained that the book was just exactly backwards; and by using the instructions in the book you always got the front carb too lean and the rear carb too rich, which is exactly what I had accomplished.

Reading is good; but sometimes it gets a novice in trouble. Having a mentor is MUCH better.

Jon.

And just for the record, if the three carbs WILL FIT, and the owner DOES HIS/HER HOMEWORK, and SELECTS THE PROPER SIZED CARBS; then (my opinion) the three carbs on the 235 are superior to two.

Jon.

Some people get a lot of personal satisfaction out of being rude, for some unknown reason. Even if somebody doesn't understand the physics of multiple carburetion, that's not an excuse for making arrogant comments. There is a physical fit problem to a 3-carb setup on a stovebolt truck due to firewall and steering column clearance issues, and unless very low-volume carburetors are installed, the venturi velocity in each carb will be so low that low speed operation will be virtually impossible. While there might be some advantage in the way of more even fuel distribution, I doubt if there would be enough gain to justify the hassle of keeping three carbs adjusted and synchronized. A better approach might be to install dummy carbs on the ends for visual effect, and stay with the original carb in the center for good all-around performance.
Jerry

Pro: Wow Factor
Con: Difficult to setup
Pro: Modest gains in performance
Con: Cost of 3 carbs is usually more than 2
Pro: Better air fuel mixture is delivered to all cylinders
Con: Reduced intake charge velocity inhibits low end torque
Pro: Something different
Con: Something different

What else is rude?
Deciding when someone is rude.

RUDE:

Telling a lady her face would stop a clock!

NOT RUDE:

Telling the same lady that when you look at her, time stands still!

How about we all lighten up, and see if in all our drivel, we have helped the OP with his/her question. How about it, original poster?

Jon.

Jon, I just read your 3 carb 235 article. It was very informative, and easy to read and understand.

Thank you!

well think i need to go dig thru my shed and see what type and how many carbs i got laying around before i get to far about getting a tri power intake.....

I must say, the quality that sets this forum from the rest, is the lack of negative comments, and I value that.

Carry on bolters.

fosters51deluxe5,

Have you seen the May 1955 HRM article about Frank McGurk's series of dyno tests on a 261? Going from a stock intake to dual Stromberg BXOV-2s increased HP by about 10 to 15% from 2000 rpm to 4500 rpm. Going to a triple BXOX-2 intake did not provide a significant improvement until about 4000 rpm. At that time, the 261 had been bored to 278, so the benefits on a 235 would be less. I ran a similar dual carb manifold on a 235 and a 261 back in 50s. there was a noticeable performance benefit and the drivability was as good as stock. I did know of the Carbking at the time, but McGurk's tuning instructions were about the same.

I also ran a Harper manifold, on a couple of 261s, with three Stromberg 48s in the summer, and with three 81s in the winter. This setup was not as drivable as the dual single barrels, but it was my daily driver for several years.

Hoyt

I find the Edelbrock catalog to be informative on this subject. Of the many "crate" engines in the catalog are two 350 cid SBC units that are identical except for the number of carburetors. The single 4 barrel unit makes more HP and torque than the dual quad unit.

Since they both come from the same source, we can assume that both are in optimum tune, an apples and apples comparison.

I have asked people why they run dual quads if a single quad is more powerful and the answer is always one about aesthetics.

So it appears that the answer to questions about multiple carburetors is "Do you want impressive power or impressive (to some people) looks?"

That's a very accurate observation. Over the past 50+ years, I can remember several factory multiple-carb setups, and virtually all of them shared one important feature- - - - -they were all progressive-throttle setups, where the extra carb(s) only opened up at or near full throttle, and they opened very quickly to allow extra air volume to the engine at wide open throttle. The early-50's Buick straight 8 engine had a twin-carb option, and so did the Hudson flathead six. Oldsmobile, Chevy and Pontiac V-8's in the late 50's had 3-2-BBL setups where the engine ran on the center carb except for full throttle, then the outers opened up fully with vacuum linkage. Corvair had a 4-carb setup with the second carb for each cylinder bank either wide open or off. None of the extra carbs had idle mixture circuits- - - -they didn't need them. Chevy, Chrysler, and Ford offered 2-4-BBL V-8's with synchronized throttle opening, but they all used much lower CFM carbs than their single carb engines. It was easy to spot an early Corvette with a 265 engine and two 4-barrels by the blistered paint on the hood from the carburetor fires.

I worked on a lot of marine engines in the 50's and 60's with multiple carburetors, but they only ran well at wide open throttle. Marine engines don't need low end torque or midrange flexibility in traffic- - - -they're either idling or running full throttle, for the most part. Installing an aftermarket dual-carb manifold, or more, and using two or three of the same carb that works in a single application doesn't make very good sense.
Jerry

Most (certainly not all) of the factory multiple carb setups on V-8's ran progressive linkage.

Most (certainly not all) of the factory multiple carb units on inline engines ran solid linkage.

Today, since for years the racing sanctioning bodies have been penalizing V-8's with 2 x 4 setups; most manifold development in the last 25 or so years has been with single carbs. Today, the single 4 is much closer to the dual four (V-8's) than it was 30 years ago because of better manifolding. However (opinion, but I haven't been proved incorrect yet), a properly selected and calibrated dual quad will still outperform a properly selected and calibrated single quad on a V-8 engine.

However, even after proper selection of the carburetors, tuning of dual quads for optimal power is MUCH more difficult than most realize.

Sometime, take a look at the calibrations used by Chrysler on their dual quad hemi. The Chrysler engineers really did their homework on these. Calibration for each of the 8 carburetor barrel is different.

And I run my dual quads for power and economy, the hood is rarely raised!

Jon.

Half the fun of having a old truck is messing with the engine, and what better way to kill a afternoon then tuning multiple carbs.

Joe

"However (opinion, but I haven't been proved incorrect yet), a properly selected and calibrated dual quad will still outperform a properly selected and calibrated single quad on a V-8 engine."

Carbking, why is the Edlebrock 350 cid SBC more powerful and more torquey in the single quad version than in the dual quad version? It is hard to imagine that an industry leading company that has been developing, manufacturing and selling performance parts for over 70 years is not capable of optimizing their own crate engines in their own dyno facility.

In science, and engine development is science, the simplest explanation that is consistent with the data is usually the correct explanation. The simplest explanation appears to be that an over carbureted engine is not optimum in performance.

To me, this is just an item of interest and no minds will be changed. Some people think that an engine with all kind of shiny gadgets bolted on top of it looks cool, an engine appearance my wife calls a "wedding cake". Dual quads definitely contribute to that wedding cake appearance, especially on a tall, polished manifold like a tunnel ram.

Others want to use whatever is most effective, and ease of maintenance is also a plus to them. Thus the Holley (a little more powerful, leaky) versus Carter AFB style carb (stays in tune, no leaks)endless debate.

The thing that makes this forum unique is the endless flow of interesting topics like this. VCCA guys concentrate on whom to pay to do everything on their vehicles and 1936 Chevy owners concentrate on where to find repop parts. Stovebolt is about guys doing stuff in their own garages. Very cool!

yar - due to some horrible experiences with edelbrock products a few years ago; we no longer will use them. So I can make absolutely no comment on the examples you mention.

However, I have been working on carburetors since 1959. I have yet to see an engine with a single quad that could not be made to have more horsepower with the dual quads. As I posted earlier, the difference generally is not so great today, due to the advances in single four manifolding over the last 25 or so years.

The big issue with dual quads (no offense meant to anyone) is that most are too lazy to properly tune them. A single four is much easier to tune. And a well-tuned single four WILL outperform wrong or mal-tuned dual quads.

However, my 1968 shop truck is a F-100 with a 390/450 engine. And I converted it from a single four to a dual four. Mileage and power went up significantly. Mileage at 70 MPH is 22 MPG with the dual quads.

Jon.

As an example of tuning dual quads:

I figured the effective metering area at WOT for Carter 4345s and 4346s used on the Chrysler hemi. Remember for Carter carbs, the effective metering area is the area of the jet less the dynamic area of the rod. At WOT, this would be the power tip. These are FACTORY calibrations.

4345s (front carb)

Driver side primary 4.18E-3 sq in.
Passenger side primary 4.43E-3 sq in.
Driver side secondary 4.66E-3 sq in.
Passenger side secondary 5.41E-3 sq in.

4346s (rear carb)

Driver side primary 6.42E-3 sq in.
Passenger side secondary (also) 6.42E-3 sq in.
Driver side secondary 4.30E-3 sq in.
Passenger side secondary 3.63E-3 sq in.

3.63E-3 is "shorthand" for 0.00363

Is it necessary to go to this extent to make the dual quads work? No, but if you want the maximum effect from the dual quads, then yes.

Using "normal" jetting where the step-up rods (a.k.a. metering rods) are the same, all four primary jets the same, and all four secondary jets the same would probably cost in the vacinity of 10~12 percent HP. 10 percent on a 400 HP engine would be 40 HP.

For the enthusiast that is a "driver only" the single quad is probably the best way to go; but for the enthusiast that enjoys tuning, there IS more HP in dual quads than in a single.

Jon.

Jon, thank you very much for taking the time to answer my questions. I also have had a quality problem with an Edelbrock item for the engine in my street roadster. I bought a pair of "Performer RPM" heads and one had a lot of aluminum chips in the intake ports. When I contacted Edelbrock directly the rep I spoke with was quite hostile and I didn't pursue it with him. I then called the retailer, Summit Racing, and they sent another head to me immediately. I then totally disassembled both heads to make sure they were OK.

Because I check everything when assembling an engine, I also found that the Hastings rings had an excessive end gap. Hastings themselves took care of that immediately by sending me another set of rings that checked out OK. From this, and other experiences over the decades, I assume that quality control everywhere is lax and must be done at the consumer level.

For a street only vehicle is a dual quad setup of value if it is optimized? I'm thinking of my roadster. It is a 1932 Ford replica with a 350 Chevy engine (mild performance, Crane camshaft 218*/230* @ .050, Performer manifold and carb), 2004R trans, 3.80 rear gear ratio that yields an engine RPM of about 1500 at 65 MPH in OD.

Does correctly calibrating dual quads require a dyno? I got the single Edelbrock AFB style carb on my roadster in the ballpark using a long grade as my "dyno" and reading plugs at the top while experimenting with carburetor and distributor calibrations and trying different spark plug heat ranges. Would this same backyard mechanic method work with dual quads?

On your shop truck, what carbs and manifold do you use? How is a strong vacuum signal maintained at the low RPMs that a street engine runs at?

I also have made the effective metering area calculations that you refer to when I adapted a Thermoquad onto my '71 Dodge van that I bought new and still have. Again, a long grade was used as a "dyno" when trying many combinations of primary jets and metering rods and secondary jets.

You are probably right that most amateur mechanics would not be willing to put in the time to optimize carburetor calibrations, but I, for one, really enjoy the results that come from doing the work.

Ray - give me a call at 573-392-7378 (9-4 Mon-Wed central time) and we can discuss.

I really do like the dual quads on my shop truck. And it will never see a race track.

Jon.

Yar, too bad you're not a little closer to middle Tennessee. My engine dyno could use a workout!
Jerry