Dual Carbs - Looking For Honest Answers

[nonstop]

Hi all!

 

I have searched up and down and always come up with conflicting answers.  My truck is a daily driver, and my commute has recently gotten longer.  It includes freeway (short bursts) and hills.

 

I am looking to get better mileage (I know, old truck and being realistic about it will likely not get too much better).  I recently replaced my exhaust with a kit that goes from 2 inch to 1 3/4 inches out of the muffler and was surprised how much power was lost.  I will be fixing that this week.  
 

So on an impulse buy, I bought an Offy dual carb intake, justifying it by telling myself the engine will be more efficient.  Of course I will put dual exhaust if I go forward with it.  I do not mind putting in the work to make it work, BUT I was hoping to hear some real world experience.  

Did it help with power?  Did it help with efficiency?  Did it help with both? Am I dumb and likely just going to have a cool looking piece hanging off the engine if I go ahead with this?

 

I would probably be better off redoing the rear and going from 4.11 to 3.73, but those are practically impossible to find.  I DO have a set of 3.55s but the hills would likely negate any of that.

 

Any help would be appreciated!  
 

Thanks,

 

Nick

[bkahler]

Nick,

 

I can't answer a lot of your questions because my 51 B3B dual carb set is not yet fully on the road although it does run and drive.  However, I can tell you that I've also done a lot of reading and research as you have and most of what I've found is dual carbs will help with mileage and performance, assuming you've upgraded the exhaust.  

 

Based on what I've read and what I've experienced so far just driving my truck up and down my driveway and around my property I don't think you can't go wrong with the dual carb conversion.  I would highly recommend converting to 3.73 which is what I have.   My 218 is bored .060" over making it a 226 with a 230 cam.  The head might have been shaved slightly but only enough to ensure it was flat.  
 

My setup is dual carbs on a Rusty Hope modified intake with split 2" exhaust into to the muffler with a single 2" outlet from the muffler, i.e. 2 in 1 out.  It was surprisingly easy to get the idle down to around 500 rpm.  

 

I do think your biggest improvement would come from a change to 3.73.  The best way to improve mileage is to reduce rpms.  

 

Brad

 

 

 

[Los_Control]

11 minutes ago, nonstop said:

I would probably be better off redoing the rear and going from 4.11 to 3.73, but those are practically impossible to find.  I DO have a set of 3.55s but the hills would likely negate any of that.

Weeeel lets see here ..... the 3:55 ..... I'm assuming you have the gears to use on your stock rear end. You do not want a modern rear end.

 

I would think it would perform better on the flats, do what you need to improve fuel mileage. Hills might be a problem.

But, how big of a problem would it be? ..... I'm guessing, you just might have to try it once & see for yourself.

I'm assuming you have the hogs head with the 3:55 gears ..... so pull axles swap hogs head and put it back together .... not a major ordeal.

I swapped one on a 1961 Dodge truck, non tapered axles in about 1-2 hours.

 

So now you have a better gear ratio. I have heard mixed messages about the 3:55. There is a guy who ran both 3:55/3:73 .... He claimed he loved the 3:55 while his engine ran well.

As it got older & less compression .... it no longer had the ooompf needed to push the 3:55 up the hills. .... He had a T5 transmission & would need to downshift from 5 to 4 on the hills. ...... With 3:73 he could just stay in 5th on the hills on the freeway.

 

Comparing apples to oranges if you have a stock 3spd. ..... Point is. If your engine runs strong, you may not have a problem at all.

If it does struggle on the hills, now you have a good reason to add chrome & dual carbs, headers & straight pipes.

 

Another option would be to install shorter tires. ..... 3:55 you are in the ball game .... not ideal but doable.

IMHO

[47 dodge 1.5 ton]

 My set up may not be as good as yours so don’t know without trying. 
  When I installed dual carbs, and dual 2 1/4” exhaust on a heavier truck it was noticeably stronger. How much?? Maybe 10-15 hp don’t really know but it is much more snappy, easier to make higher rpm’s, don’t press as hard on the flats, up hill around my neck of the woods still let’s me use all the gears I have. Cant speak on the economic factor but believe there is less load on the engine in general. The exhaust I would guess is the best gains if measured alone. The dual carbs have a certain sound that makes it worth the effort and a tinker factor that I enjoy so it completes the package. Also makes great added conversation.

  If I wanted to stay original 100%, everything ran great, maybe not. In my case, intake had a crack over the heat plate that needed repair so I was already in there.

  I just have a stock 236 for a test before replacing with a 265 if desired later down the road, factory (2) carb intake, factory truck B&B’s, and Langdon’s headers. I did have to upgrade linkage as the original was worn severely. 
 Everyone has their own opinion on what’s best for these engines so I just listen and don’t stress my thoughts. IMO— heading in the right direction for improvement.

 Gears— If I do a pickup someday will stick with 4.10’s and add an OD unit most likely. 
 Best of luck!

Edited April 11, 2023 by 47 dodge 1.5 ton

[greg g]

I have a 56 230 in my car.  Factory rat4d at 125 flywheel hp.  Dual carter's on a Fenton intake, bored .030, head milled .040, .010 decked from the block. The fellow doing the machining calculated the compression ratio at 8.7 to 1, stock cam, lightened flywheel,balancec,  with single exhaust, 2 1/2 all the way with dynaflow turbo flow muffler.  With about 5000 miles on the rebuild, it pulled a chassis dyno of 127 hp at the rear wheels at 3300 rpm.  It runs 4.11 gears with 225r75 15 tires.  Banging around town it gets 17 mpg. Hiway miles without overdrive gets 19, with od, does 21.  So more power, yes, better efficiency, yes.

 

My test is to find a level road, drive in 1 to 1 3rd gear. Then at 30 mph, floor it to 50 mph, timing it.  My times hover right at 7 seconds with every thing being up to snuff. This with spare tire, tool box, half tank or more of gas, and my over large butt aboard.  Don't know what the trucks weigh but my coupe is 2970 if I remember.

 

I did remember reading the offy intake was not the best of the bunch for flow characters, but better than stock.

 

 

[bkahler]

9 minutes ago, greg g said:

I did remember reading the offy intake was not the best of the bunch for flow characters, but better than stock.

 

 

I read somewhere that with the offy intake, if the carburetors were raised higher, they will perform better.  Something like a 1" to 2" spacer/runner between the carbs and the manifold.  If you look at the custom designed intakes the carburetors always sit higher above the cross tube than the offy does.  

 

 

[nonstop]

Thank you guys!!! I think I will try out the 3.55s (scared because there are a lot of hills, although I am currently able to pull them in 3rd), and will still hold onto the manifold and exhaust to add on to the engine!

 

But please, keep the info coming!  It is making me feel better about my plans!

[Los_Control]

If I wanted to do the swap .... I would search for 3:73.

If I already owned the 3:55 .... I would try to work with them.

 

Thats just me.

[bkahler]

I'd suggest the 3:73 as well.  My 51 came with 3:73 which I believe was the first year that ratio was available.  My only problem is the ring gear was trashed so I found a 3:73 diff out of a mid 90s Jeep Cherokee and installed it.  

 

Lots to think about

 

[Veemoney]

My 47 has a 230 with stock manifolds and single B&B. I shaved the head to bump compression as well.  It came with a 4.10 gear which I changed out for 3.54 gearing from a 48 car. Truck is a 4 speed and I use 2nd,3rd & 4th now with the new gear set. I honestly think if they had a 3.23 option for the original rear it would be ideal for me but then I don't have the long steep grades to deal with. 

[PT81PlymouthPickup]

My opinion is get a 3.73 differential!  I'm running a 3.55 behind my .040 over bored 201 block in my 39 PT81 with a T5 trans. It will go up big hills but requires much shifting and I sometimes get a line of modern vehicles lining up behind me. It will cruise wonderfully at 65mph or better on a relatively flat roads. When I had my original trans with a 3.90 differential It would climb a mountain but at 55mph sounded like she would blow up from high revs. I have mixed feelings on dual anythings. Really sweet to look at, but the little extra performance we get comes along with more things to give me aggravation. Twice the carbs and twice the exhaust means twice the things to fail. I must admit I'm jealous of those cool looking dual installs. I started to go the dual route by accumulating some parts, but decided if I do anything more to the old girl, I'll install a 3.73 differential. I still have original rear end with a 3.90 gear in perfect condition if anybody wants it? (Free)

[Veemoney]

I would add in my opinion that anyone considering installing highway gears should shave the head to bump up the compression as well otherwise your not getting the most out of the gear change. When I run a compression check on each cylinder shortly after shaving ~.080" from the head I was getting 130-135 PSI on my motor. Very noticeable improvement when driving at speed and mashing the pedal.