Hot Rods 3x2 setup on a 671 Blower

This is a Rochester 3x2 set up with no idle circuits on outbound carbs, Offy intake on a 327/L79 350 hp long block. On the ch***is dyno it was lean near and at top end. I used to know the cfm on these but I forgot. . . Hope this info is helpful.

 

Pontiac used Tri-power till 1967 when the 750 cfm quadra-jets came out. 1966 Pontiac's used three 2GC's all the same size, which are fairly big, and the Q-jets still ran better.

 

A blower does not care what is on top of it. The engine does. Lippy

 

^^^^and the very reason I was happy, stimulated, pimple faced teen.

 

Let's Do the Math!

383 ci turning 5000 rpm on the street ***uming 85% volumetric efficiency is going to require 0.5*383*5000*0.85/1728 = 471 cfm in naturally aspirated form. Now, how much boost are you planning on stuffing into that motor? 1 atmosphere (atm) is 14.7 psi, so the NA motor uses 1 atm of air, or 470 cfm @ 5000 RPM. The same motor running 14.7 lbs of boost at 5000 rpm is using 2 atm of air, or (2*470) 940 cfm.

I am ***uming that at nearly 15 lbs of boost, that Chebbie 350 will have properly **** the crankshaft out the bottom, so lets say you keep it to a more reasonable 8 lbs of boost, since it is a street motor and all.

8/14.7 * 470 = 727 cfm at WOT. 3 Rochester 2G's tuned for about 250 to 275 cfm each should get you there. The thing you will have to figure out is at what pressure difference that 275 cfm of flow occurs at. As stated before, with a roots blower, you want to minimize the incoming air restriction as much as possible. Most 4bbl carbs are rated at 1.5 in/Hg, and most 2 bbls are rated at 3 in/Hg pressure difference.

Note: These are theoretical numbers. There are many more factors and variables in play here, and your mileage may vary.

 

Why does everyone forget about the big Pontiac tri-powers?

 

Why do people use those goofy formulas to try and predict carb size on an engine ??? GM spends millions of dollars on research and development and have never used such small carbs on a stock run of the mill engine, much less on a high performance one.
I think 3x2's are going to be way to small, although 6x2's would be nice. I was just watching a video of this sbc running with 8 94's, ran really good.

 

^^^You make a couple of good points Swade!

​

 

Every post he makes has two great points. I am bewitched by his posts. Okay, his avatar.

 

Remember 2Vs are flow rated at a different depression that 4Vs. Three 185 cfm 2V s is equivalent to only 397 cfm 4V.

(A 2V is flowed at 3" hg depression; a 4V is flowed at 1.5"hg depression.) Hope this helps.

 

They didn't?
https://www.jalopyjournal.com/forum/threads/the-general-motors-tri-power-story.1084407/

 

I'm surprised that Frenchtown Flyer didn't expand his answer a bit.

The conversion factor is 1.414. If you want to compare cfm ratings between 2 barrel and 4 barrel carburetors you either multiply of divide by that number.
Since most of us are familiar with ratings of 4 barrels, to convert a 2 barrel rating to a 4 barrel, divide by 1.414,

Thus, a 650 CFM Holley 2 barrel is the equivalent of a 460 CFM 4 barrel.

3 - 650 CFM 2 barrels would be like a 1379 CFM 4 barrel or 2 - 689 (700) 4 barrel carbs

3 - 500s = 1060 in 4 barrel world

3 - 350s = 742 CFM 4 barrel

So you can see that in a street application you can easily get enough CFM to feed a Blown 383 stroker with a TriPower set up.

 

I would not opt for progressive linkage.
Why progressive? So you can get better fuel economy? Really? With a blower and three deuces?

That's adding complexity and reducing reliability of the linkages.
I run a few inlines with a straight 1:1 linkage.
I tend to believe that blower will be a giant Mixmaster and disperse fuel.

 

Thank you!
I did not realize there was a difference in CFM ratings between 2 barrel and 4 Barrel carburetors.

I definitely need to keep hanging out with you guys.

Hey guys. Try this while you're stuck at home wondering what else to eat. Super easy, super good. Hot cook-style chocolate pudding poured over vanilla ice cream. You're welcome. LOL

 

If there is a math test at the end of this cl***, count me out! LOL!

 

The test is: You build it and see if you can make it work.
The difference I see between today's builders and the hot rodders of the past is: now days guys ask if this or that will work. They want the blessing of the guys who've been there and done that.
In the olden days guys tried it. If it didn't work then you tried something else. That approach can be frustrating and expensive. But when you make something cool and different than everyone else, there is no better feeling.

 

Wonder what happened to reading spark plugs? And don't give me this stuff about ethanol either. I read NGK's on alky and nitro, Lol.

 

Three JET Rochester 2G Stage 1 Carburetors, at 500cfm, each.

Straight linkage.

 

The formula for calculating how much CFM (cubic feet per minute) an engine requires is: CFM = Cubic Inches x RPM x Volumetric Efficiency ÷ 3456. Any ordinary stock engine will have a volumetric efficiency of about 80% Blowers like fuel

 

Hello,

For a street set up you might want to talk to others that have the same set up as you are thinking of installing. It looks to be a little less than most have seen. Progressive linkage is to run the center carb, with the outside two carbs kicking in at a pre-set point in the acceleration. But, the center carb is used 80-90% of the time, so, it may work.

The other choice is to set the linkage to make all three come on with the pedal. That is a lot of carbs, but may do the trick. Most would laugh as the center carb is probably the easiest to adjust and make it run fine. Obviously, it will be easier to adjust, but may not be enough. That is where the linkage can be set to help out.

All of the above comments are valid and it takes all kinds of experiments and discussions to get something right. Sometimes, talking to a person with a vast background in multiple carbs on any motor and understand what works and what will have problems.

We were fortunate that our local speed shop, Reath Automotive was the top shop in all of So Cal and the Joe Reath was a wealth of knowledge. He suggested the multiple 97 Stromberg carbs as a way to get as much power out of our 283 SBC motor without going to Hilborn injectors. Those Hilborn injectors back then were compe***ion only and not for daily driving.


Our 1940 Willys Coupe was supposed to be street legal hot rod, so the Hilborns as nice as they were, did not or would not work back then. So, the advice was six 97 Strombergs on our SBC motor. It took a while to adjust all of the linkages and set a good throttle response. But in the end, it worked and gave us the power for the SBC 283 motor. It was a good installation and worked on our street driving around the neighborhood to get all of the bugs out, before heading to the drags.

Jnaki


Similar motor to our 292 ci 671 SBC in our 1940 Willys Coupe.

After a couple of months of racing on the weekends and the sometimes run on the streets, it was lacking something to be more compe***ive. It was as fast or faster than most hot rods on the street, but, not in our designated B/gas cl*** compe***ion. So, we went to a new model 671 supercharger and used the 6 Strombergs on top.

That set up got us close to the national record for C/Gas and we were on our way to the nationals after another week or two of runs and finishing speed parts to go faster, yet. It took a lot of adjusting and timing to get that larger 292 ci SBC motor with all blower spec parts working well. But, the instant power was no match for anything else, for our first time build. YRMV
Still in B/Gas, until added welded in steel plates over the rear tires inside of the trunk area and water in the stock gas tank, gave us the weight to get into the C/Gas cl*** compe***ion.