The Stovebolt Site Forums The Shop Area Engine Shop Why go to dual carbs?

What are the advantages of going to a dual carb set up on a 235? I know it looks cool, but is there another good reason?

No.
Jerry

from what i have read the 235 is under carbed and would benefit from more fuel delivery. ron

They are fun to tinker with if you like to tinker.

Well, I didn't get the screams of outrage I expected from my one-word answer, so here goes- - - - -Extra carburetion just multiplies the potential problems encountered by a single source of fuel and air. I've worked around single, dual, triple carbs, and even one carb per cylinder on some applications, and the more possibilities there are for problems, the more you'll encounter. A single, larger-volume carb or a progressive-linkage 2-barrel might be a wiser choice, IF the engine can live with more venturi area without developing flat spots, sags, stumbles, and backfires due to leaning out at various throttle openings and manifold vacuum conditions.

A carburetor works by creating a low-pressure area in the venturi, so increasing the available space for air to flow by adding a second path slows down the velocity of air flowing through both carbs. Unless you make both carbs much lower-flow units, you just set the stage for NO fuel delivery, just lots of air, at certain times, due to lack of venturi velocity. Not a good idea, in my opinion.
Jerry

I have been reading a lot of the threads on this topic through searches but have found no definitive answers regarding these questions.

I have a 58 Apache which will get a '63 Impala 235 and a 55 Panel with the original 235. I plan to use the pickup as a daily driver, and the panel will be a weekend cruiser.

I gather the gain from 2 carbs is more fuel to an otherwise under fueled engine.

I have read that the number of cylinders divided by the number of jets should be a multiple of 6. So, I surmise that I should go with 2 single carbs. I'll probably get a Fenton intake and use Rochester BCs with an exhaust heat tube to feed the intake. My blind faith with the Rochesters is based on a prior '54 I drove to college with a Rochester carb and I never had problems with it.

Can I expect to have increased mpg's with the 2 carb set up? Does this set up increase fuel efficiency? If so, approximately what kind of mpg? If not, can you elaborate and explain why not.

Luis

What I would like to have is increased fuel efficiency, so I suppose the single carb would be a better route to take. Is that correct?

Luis - yes, if what you want is economy, stick to one, if you're 'delivering more fuel' how can you use less?? ... I don't think the 235 is "under carb'd" [whatever that means], it develops the hp and torque is was designed to and does its job well with economy

and a 63 Impala 235? didn't they quit 235s in 62?

Bill

I met a fellow through the stovebolt and he had a 216 for sale. I went to look at it and he had this other 235 from the Impala. I chose to get the 235 and have to wait to get all of the funds together for the purchase. I neglected to get the casting number off of it. Looking over the casting numbers list from the inliners.org I don't see a production run for 235s in 1963. I hope its a 261 or 292.

I called the gentleman with the engine and it is a 235 out of a '62 pickup.

I've learned a lot through the site and appreciate the input from folks with knowledge.

Lots of opinions, little real information and no wonder it leaves you in a pondry. So here is yet another "opinion" based on my own experience but it backs up what Hotrod Lincoln said. Dual carbs are a pain in the behind. I'd never do it again but like the idea of a single Carter Weber or a Carter 390 CFM 4 bbl. Who needs all the greif of constantly fiddling with the engine to keep it running right? One properly sized carb is enough.
So..suit yourself.

I don't know but my dual Holley Webers that I bought from Tom Langdon really rock! It does take time to set them up but they make the newly rebuilt 261 really scream. Too bad that my truck is so old that it can't keep up with the powerhouse under the hood.

http://community.webshots.com/user/geezer

I have used dual carbs (Rochester B's) on several engines and find them to be relatively trouble free if properly set up. The "cool factor" is the big reason, and having Fenton headers works good too. The two carbs and headers have made my 248 GMC quite responsive, probably has 35 more horsepower than stock.

As 53moneypit mentioned, lots of opinions. I would like to offer another, based on 40 years plus of working on carbs of one kind or another.

First, and foremost, the question becomes:

What does the individual want, and how badly does he/she want it?

The truck engine was basically designed to produce as much low end torque as could be produced economically (build costs, not maintenance costs).

Can the engine be inproved?

On an inline six, the distance from a single carburetor to each cylinder varies greatly. This variance effects engine efficiency. Thus, if one has the perfect single carb (with the perfect intake manifold); a good set of duals will measurably outperform the single unit. How much is measurably??? As much as 8~10 percent; but generally less. These is more to be gained by installing a well designed manifold and a single four barrel (I would suggest the Carter 9400 here, but others may prefer the Holley 390; this post is not about brands). A really good single carb, with a well designed manifold and some exhaust work could be good for 30~35 percent increase at the high end, with a slight loss of low end torque. A different camshaft would increase this percentage significantly more. A good pair of duals could only add 5~10 percent additional to this gain (at increased cost).

So can dual carbs add horsepower? Yes!
Can dual carbs improve fuel economy? Certainly!
Can this be done by using any old pair of carburetors that one happens to have? If so, you need to go to Detroit; as there is a need for that type of skill.

One other comment I would like to make: dual carbs, if good carbs are used; should require exactly DOUBLE the maintenance of a good single carb :big grin: That is, you have to change TWO air filters instead of one!

Other than that, a properly selected, and set up set of quality duals should require ZERO maintenance for a couple of hundred thousand miles (unless you get some bad fuel). I have personally logged a ton of miles with dual and triple carbs on lots of different engines, from 37 CID four bangers to 390 CID V-8's. Once adjusted, just leave them alone and let them work.

As to the "kool" factor; to each his/her own opinion. I personally add multiple carbs to my own vehicles for better performance and increased fuel economy, and I obtain both. Having said that; you have to drive a ton of miles to get back the cost of the set-up through better fuel economy.

Jon.

Might as well toss my $.02 in. I love my dual Carter/Weber setup on my fresh 235. Primarily because I found a replacement for the old tried and true Rochesters that SEEMED to require regular tweaking. I'm not going to get into the argument about additional horsepower....I would hope that there would be an increase, but I am going to offer my opinion that the DRIVABILITY and SEAT-OF-THE-PANTS PERFORMANCE of my truck has me grinning from ear to ear.

I really don't miss the smell of gas in the cab, hesitation and flat spots and uncertainty of operation from a 50 year old carb(s) that has a sloppy throttle shaft or needs to be baked in the oven to straighten (or not) the (often) warped airhorn. I can understand how a pair of these old girls gurgling, spitting and sucking air might discourage someone from giving the dual (or triple) setup a try. However, I've experienced none of the negative aspects of the one bbl carbs with the Carter/Weber progressive 2 bbls. I set them up correctly, no vacuum leaks, did the UNI-SYN balance, made minor tweaks to the idle mix and now get to enjoy the driving.

As for mileage...the verdict is still out as my fresh engine is still in the "teething" stage....but I have little doubt that I can reach and exceed the 20 mpg mark. If you have the correct combination of engine, cam, carburation, exhaust scavenging, OD ratio, R&P gearing and driving style, I don't think you have to be satisfied with a mediocre performance or mileage.

That's my story and I'm stick'in to it!!!!

Dave

If you only do one upgrade to your 235, that would be the one, together with headers on the exhaust.
It basically allows the engine to breath better.
Gives around 15% increase in HP and torque.

I got a setup like that for many years on my daily driver.
Never have to adjust anything after initial installation and synchronization.
The difference is very noticeable and I get 20 mpg on the freeways.

For street, 2 smaller carbs from a 216 will respond best.

After the conversion, it feels like the stock intake and exhaust are choking the engine.

Well hers just another op, I have ran Dual singles throats 20 yrs on my old 6 popper,pretty
much trouble free {except got pressurer washer to close bowl gasket,my fault}, otherwise, better performance,looks super cool!!

1938 1/2-Ton P/U
1940 1-Ton Dually Panel
424x4

The dyno testing that Frank McGurk did for the May, 1955 Hot Rod article makes it clear that the 2 easiest and most efficient things you can do to get more power from our 6 bangers is to improve the breathing by going to dual or even triple carbs and dual exhaust. I'm going a bit further than that with my 261, but dual pipes and dual carbs are in for sure!

424x4 ...
Got your email - thought others may also want to know....

My truck with the 235 has the stock 3speed with torque tube and 3.55 ring&pinion rear.
It get's 18-20 mpg on the freeway, unloaded with the tailgate down.

The 1200 lbs. lighter coupe with the 261 and the same drive line will get even better milage.
There is more weight on the rear wheels and much less headwind drag at freeway speeds.


I posted graphics of the McGurk dyno curves a while back.
The curves would be pretty similar for 235s, just lower numbers but same curve shapes.

261 street performance gains


Going 2x 1bbl and headers was pretty common back in the 50s.
A lot of guys today use one 2bbl or 4bbl carb instead of dual carbs.
The idea is the same - open up the intake and exhaust and let it breath !

If you have a truck mostly for hauling, the stock intake and exhaust will be fine with plenty low-end torque.