Carb dilemma

T

triumph/buickv6

Member
Joined
Nov 11, 2009
Hello all... I have a 72 Triumph Stag with a rebuilt '82 4.1 L with an upgraded cam with Edelbrock performer manifold. Currently I have a 500 cfm Edelbrock carb with factory set up. I am about to give her over to a shop to put her on a dyno and dial everything in. My question is... Is this too much carb for this engine? Would I be better served with a 390 Holley? Or should I stick with the 500 cfm and just have them dial it in?
I dont plan on doing any 1/4 mile time trials, mostly cruising and surprising a few people at red lights.

Thoughts??

(PS thx for all the help a year ago while I was getting this all together!)
 
IMO if anything it is on the small side. I run a Holley 600 on my N/A 4.1 and it does great. In my experience it is hard to over carb or over cam a N/A 4.1L engine. The Edelbrock intake is a great manifold for even fairly serious engines. I think it will run well. What does the car weigh?
 
Throw the edelbrock carbs away and get a holley. You'll get much more out of it and it will work better. If you really want to wake it up get a spread bore for it. The small primaries and large secondaries of a spreadbore will give it a lot better throttle response as well as mileage. The factory 4 barrel carb could easily flow 750 CFM so you're a little on the small side to me.;)
 
Any carb with 600 cfm will work better on a 4.1 than a 500CFM. I have run 600 CFM Hollys, Edelbrocks, and a Quadrajet from a 350 Chevy. They all ran good when set up and jetted properly. The Holly had the best throttle response. The Q-Jet and Edelbrock needed the least amount of maintenance. I run an Edelbrock now. I like being able to tune it without pulling the float bowls. The metering rods and springs are really easy to change. The jets are a little harder. One way to tell if a carburetor is too small is to hook up a vacuum guage and make a full throttle run. If it shows any vacuum, it's too small and restricting airflow. Do not run a 390 CFM Holly. Way too small. I tried it on my first 3.8 in 1977.
 
There's so much misinformation when it comes to CFM's and carbs. The stock air flow, ("powerband"), on the .231 engine at best, even with an OEM high performance well tuned engine is somewhere in the neighborhood of 370 CFM@5500 RPM, (WOT). Stock carburetor CFM ranges are from 227 CFM, (the Duel-Jet which was the most common carburetor used on the .231), to 750 CFM, (the Quadra-jet used on the turbos and .252 engines).

It is virtually impossible for the OEM V6 to pump any more air than 370 CFM maximum without modifying it well beyond the scope of what it was designed for. The Q-Jet was built with a 350 SBC engine in mind, (which will pump 567 CFM@7500 RPM peak. So..... forget all that CFM rating nonsense on carbs. Look at what the engine can pump based on porting, piston volume, intake and CID.

If you have a carb that's rated at 750 CFM's the .231 engine will only pump 370 CFM@ WOT. Even if you put a carb thats rated at 1000 CFM's it will still only flow 370. THE ENGINE DETERMINES THE FLOW NOT THE CARB!
 
Freddie's Buick said:
There's so much misinformation when it comes to CFM's and carbs. The stock air flow, ("powerband"), on the .231 engine at best, even with an OEM high performance well tuned engine is somewhere in the neighborhood of 370 CFM@5500 RPM, (WOT). Stock carburetor CFM ranges are from 227 CFM, (the Duel-Jet which was the most common carburetor used on the .231), to 750 CFM, (the Quadra-jet used on the turbos and .252 engines).

It is virtually impossible for the OEM V6 to pump any more air than 370 CFM maximum without modifying it well beyond the scope of what it was designed for. The Q-Jet was built with a 350 SBC engine in mind, (which will pump 567 CFM@7500 RPM peak. So..... forget all that CFM rating nonsense on carbs. Look at what the engine can pump based on porting, piston volume, intake and CID.

If you have a carb that's rated at 750 CFM's the .231 engine will only pump 370 CFM@ WOT. Even if you put a carb thats rated at 1000 CFM's it will still only flow 370. THE ENGINE DETERMINES THE FLOW NOT THE CARB!
Click to expand...

so what exactly are you saying? that 600+ cfm is too much?
 
dr_frankenstein said:
so what exactly are you saying? that 600+ cfm is too much?
Click to expand...

No...the .231 will still handle it. It's just a wasteful effort. You will not get anymore speed torque or any advantage other than what the engine calls for. You can put a Duel-jet in there and it will do just as well as a Q-Jet or a Holley 600. You can't overcarb it you can only under carb it if you're running at a constant high rpm. But you're essentially doing nothing by going past the engines demand.

What it boils down to is that one can start with the engine's CFM value @ WOT. Then you can use any carburetor equal or higher. From that point on the choice for a carb is squarely based on it's tune-ability and simplicity not CFM rating.
 
Just wondering how you came up with the 370 CFM max for the 231 engine Freddie? The reason I ask is because the stock turbo's rated at 350 CFM and the 86-87 has a higher rating than that.
 
It's actually closer to 368 but I rounded it up to 370. The formula is:
Code: 
(CID X WOTRPM X 1)/3456=CFM
OR:

Code: 
(231 X 5500 X 1)/3456=367.622
The numerical value of 1=100% Volumetric Efficiency. In other words a highly tuned performance engine which is the goal for most of you. 75% is a low performance engine. 350 CFM on the stock turbo makes perfect sense to me since the WOT RPM I used is for a High Performance cam. The stock RPM is probably a lot lower and so is the VE making the stock turbo fairly matched.
 
I think in reality, were all very correct.

the engine probably does only use 450-500cfm at max.... this doesnt supprise me. But I think how the engine comes to each breath is what set these motors apart.

the buick has to be purposely over carbed in order for it to recieve proper booster signal. those big wide 120* firing pulses make for long shallow breaths. thus the pressure drop the carb is normally rated too, now isnt what the carb sees- its actually lower then normal. so now that honking HP950 Holley you got looming over the engine actually breathes like a 600 at that pressure drop........... that holley 600........ now 350.......

interesting.....
 
I'm not sure If I get what you mean Doc. But my understanding is that there is no such thing as "over-carbed". Think of it this way: Your foot size is 9", you'll have great difficulty fitting into an 8" shoe. But you can fit into a size 10" (albeit rather loosely). That's the same principle with carbs.

I know that math is boring to some, but former Rochester Products Division guru, Doug Roe (a carb engineer), offered up that formula in the Q-Jet book. Based on that 450-500 CFM on a .231 is nearly impossible. 370 is max realistically. (That is if you dont intend to install a flux capacitor on it).:)
 
What I will point out i that mathmatical formula are only used to "calculate" to size of the carb and don't always predict how an engine will work in application. Smokey Yunik prooved this many times and on one of his tests with the new (in the 80's of course) 4.3 chevy V6 he ended up using 2 holley 500 CFM 2 barrels to get the best power level out of them. This was on a modified V6 marine intake intended for 1 square bore carb and he made a new top for the intake so he could use 2 cross mounted carbs.;)
 
You're not getting my drift. It has nothing to do with calculating the carb. One matches the carb to the power of the engine. Once you know what the engine puts out, then you match a carb. The carbs are rated from 227 to 1000 or more CFM's. Anything over the engine's power band is doing absolutely nothing.
 
I understamd completely, but what I'm getting at is the calculations aren't always right in a given situation. You and I both know that you can't feasibly go over 100% VE unless you add some sort of power adder like a turbo or supercharger. You can also under carb an engine and produce a great deal of power. Nascar does this to restrict the speed on the cars. Mathmatically a bumble bee can't fly either, but it does.;)
 
were all saying the same thing.... just in different forms.

what we need to all agree on, is that the engine will function better, with a the larger carb then with the smaller one - not that it can use anymore of the carb thats there, but because of mixture efficency

do we all agree?
 
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