Tri Power CFM

Does anyone know how many CFM a stock tri power set up was. 750 CFM total?

More. It's up over 1200. The end carbs are 1 3/8" venturi and the center one is in the 1 1/4" range. 435-381-435. If your center carb is later model, it is a bit smaller.

That can't be right. over 400 CFM for a 2 barrel? You sure?I was under the impression that they were all 250 CFM. But the middle one is smaller so it would make sense that it was less CFM. If a stock tri power motor was using 1200 CFM what's the big deal about my NON stock motor using 1100?

Yeah, the 2 end carbs were huge! You would never be able to smoothly idle on one of those. 1 3/8" venturi end carbs were used in all years of tri-power. The center one changed according to year and engine size.

The 1100cfm dual power is fine with your setup. In fact, you can go bigger if you wanted to as long as it's jetted properly. It's the guys that try to put dual 850s on stock 350s that goof things up. More bigger is not more better! That's Mopar mentality.

I hope my dual 550s will do alright for me! I called the local carb shop and asked him what he thought. He asked me if I got any advice from someone who actualls ran what I was trying to run and I said "no" so he said "just got with what you have and go from there." So I'm going to rebuild the carbs stock and see how that goes.

There are no "actual" CFM ratings that I know of for the Tri Power setups. I've heard anywhere from 750 to 1200 combined for the package. I'm guessing it's somewhere in the 900 range.

Here's a good resource for tri-power info.
http://www.pontiactripower.com/

That site is pretty good but it doesn't have much for '59s.

Not much changed between '59 and '61

Looks like most all of his stuff is for mid 60s tri power stuff. And if you read his FAQs he tells you there's no definite CFM rating on the carbs as I did state earlier.

Just thinkin about this, how is there no certain CFM? Doesn't CFM depend on the diameter of the openings in the bottom?

It could be calculated but there's just no numbers out there. Also, CFM would also be based on te engines ability to recieve air.

I don't quite understand CFM numbers. Is the number specified using a terminal velocity of air entering the venturi?

From what I understand a 500 cfm carb is a 500 cfm carb whether on a race car or a lawn mower. The CFM numbers should not be dependent on the engine.

I'm not sure how they calculate it but I know it has to do with the diamater whatever the damn holes at the bottom are called.

Like my car has 2 550 CFMS, that sounds like way too much CFM but it seems to run great and not bog down when you rev it up off idle. I'm surprised

But being an amount of volume over a specified amount of time, CFM has to be based on air flowing through it. That's why I ask if it's based on a terminal velocity of air.

Because technically, if an engine is not running the CFM would be 0 since there's no air passing through the venturi.

Right but I'm sure a say a Chevy 550 AFB will flow the same amount as a Buick AFB with 550 cfm. There has to be a standard measuring procedure. You know what I mean?

I know they will have different flow numbers on different motors, but I'm talkin what they're rated at stock.

Of course a stock motor with dual quads won't flow as much as a race motor with dual quads, even if the carbs are rated at the same amount.

There has to be a formula to calculate CFM that is independent of the motor the carb is goin on.

I just did a quick search and all I can come up with is calculating the proper size carburetor for your engine. I can't find anything on how to calculate the factory CFM of the carburetor itself.

As i understand the cfm rating means cubic foot/minut at a certain manifold vacum and a specific temperature.
I remember reading these numbers somewhere, but i think i remember the figure 76 inches om mercury (not sure), and a temperature specified in farenheit. A flow test bench would properbly have a suction pump to create this vacum, and either a temperature champer or you compensate for the actual measured temperature.
The temperature has a lot to say, since the specific density of air (or any gas) changes significantly with temperature, according to the law of ideal gasses :

V * P = n * R * T

Where :

V is the volume, P is the pressure, n is the number of gas molecules, R is a constant and T is the temperature.

If the pressure is constant too, it follows that the volume of a given number of molecules ( a given weight of gas), has to change with the temperature. Since the specific density is the weight of a given volume, the density changes with temperature. (shit, i hope this makes sense )...

So what i am trying to say, is that the cfm rating means : This particular carb. will flow X cubic foot of air per minute, if you subject it to a vacum of 76 inches of mercury (i think), at a temperature of (?) farenheit.

To be exact you would also have to specify a fixed absolute pressure for this to make sense, and my best guess would be that this is standard atmospheric pressure.


How a given setup works on a given engine is a question of many more variables than actual cfm rating, since the CFM rating of the system (valves, heads, manifold, carb. and airfilter) will depend on how well they are matched. I think this is the reason that it's hard to put a number on a set of tripower carbs. The bottom line is that you would have to test a complete system to make a fair comparison between a tripower and - say a quad, since the air dynamics inside the manifold is properbly quite different.

The reason that too large cfm rating is problematic is that the airspeed through the carbs at idle is too low to make a even mix of fuel/air, leading to a poor idle. Theoretically the tripower should take care of this, because the middle carb handles all this, and the endcarbs are activated only at or near wot. Wish i had one....

Sounds like you got it right. And I understand how that works. I would just like to know the calculations and the set variables (IE - temperature, density, etc) you need in order to make the calculation complete based off a specified carburetor.

Hi gang, this is my 1st post. Found this site while googling for '59 Ponts, and joined right away! I have had my Series 7000 (as Canadian builts were officially called out by GM) since April of 1969. Time is tight right now, but I hope to be an active member as things straighten out for me; I have a lot of stories about my baby, and have been a self-employed mechanic for 35 yrs, so hopefully I can entertain or help. It's a Parisienne 2 door hardtop, one of about 11 or 12 hundred produced. Canadian Ponts were 100% Chevrolet, chassis wise, with the body panels altered to fit the 119" wheelbase. Much to talk about, later, but I'm jumping in here to help clear up this tri-power talk.

The CFM rating of a carb is determined on a bench and is a property of the carb; how the engine uses it is a property of the engine build. HP Books has an EXCELLENT reference "Rochester Carburetors" by Doug Roe. The main point here is that 2-bbl carbs are rated at 2 in of mercury and 4-bbls are rated at 1 inch. This is a factor of 2, so to find equivalent ratings, you have to mult or divide by the square root of 2 (1.414) because air flow varies as the square of the depression. So when you add up the 2-bbl ratings and get, say "1200" cfm, the "4-bbl equivalent" is 1200/1.414 = 848 cfm. Now that sounds more reasonable, eh? (Canadian, right?)

Gotta go now, Peace & Love from Jacques

haha nice, Welcome to the site and thanks for clearing that up!

I hope you tell us all about your Canadian Pontiac!

Hi gang, here's my 2nd post, to correct my 1st one. The devil is in the details!

4-bbls are rated at 1.5 inches of mercury (not 1) and 2-bbls at 3.0 (not 2.0). But the ratio is still 2:1 & thus the rating conversion is still 1.414. This is the danger of recalling stuff read years ago, and for which I apologize.

By the way, here are some numbers for Rochesters. The obviously "small" 2-bble (1-1/4" flange, 1-7/16 throttle bore, 1-3/32 venturi) is 278 CFM. The "large" one (1-1/2" flange, 1-11/16 throttle) comes in 4 flavors depending on venturi size: 1-3/16" = 352 CFM; 1-1/4 = 381; 1-5/16 = 423 and 1-3/8 = 435. Remember, these are 2-bbl CFMs.

Did 4-bbl '59 American Pontiacs use Rochesters or Carters? I can give the CFM data for Rochester if anyone wants to know, but I have always found the Rochester 4GCs fussy and unrewarding. I totally agree that nothing is "sexier" than a tri-power, but for all around carburetor wonderfulness, nothing comes close to a Quadrajet. The list of technical superiorities is long, and once set up they are rugged and dependable. QJs were used on Pontiacs towards the end of the sixties, and if the manifold can be bolted on to a '59, well, DO IT. I can't tell from here in Canada whether it would be simple or need some work-around, but a trustworthy wrecker or machinist down there should know.

Next time I post, I'll give the simple formula for figuring out what CFM setup you actually want/need.

Later, Jacques

I think Pontiac only used Carters for 1959. I don't really know but seems like when you do see 4 barrels (super rare for some reason) they're Carters.

I know Buick used Carter and Rochester. I don't know if any of them used Quadrajet.

I had only heard bad things about Qjets. I've only heard good things about Carters and mixed feelings about Holleys and Edelbrocks. IDK, I had an Edelbrock on my Buick 401, it ran awesome, took it off, put it on a 74 dodge 360, ran awesome. I think it was a 650? CFM.

How does CFM work with dual carbs? I'm running 2 550 CFM Carters.

According to my original shop manual your right - all 2brl carbs (including tripower) were rochesters, 4 brl were Carter AFB. I bought one on E-bay, but cant find a manifold.

Exactly, the 4 barrel manifolds are damn near impossible to find. I've only seen maybe a handful on ebay.

Ad to that 100$+ shipping, because it's pretty heavy...