Do we undercarb our sixes?

[SuperMag]

Recently a conversation has popped up that questions whether or not we undercarb our sixes. Since this is where the discussion started, let's look at the 300, and ask the question "Is the 390cfm Holley big enough for the job?"

Let me preface this answer by saying that carburetors don't "make" power. The efficiency of the pumping action of the engine (VE) is what makes power; the carb simply mixes the air and fuel in the amount required by the engine. That having been said, the obvious question then comes up, "What size do I need?" Well, we’ve all seen the calculators that tell you what size carb you need based on the displacement, VE, and RPM capability of your engine. These calculators are crap, because unless you’ve had your engine flow tested, you have no idea what your VE is.

If you are smart, you've built your engine with a goal based on your budget and the inherent strengths and/or weaknesses of your engine. In the case of a street driven 300, a realistic goal given a typical budget and the design constraints of the engine would be about 250 HP. So how big a carb do we need?

The theory and math part (simplified): To make power you burn fuel mixed with air. Fuel contains potential energy, measured in BTUs. This figure is mathematically translatable to HP, or power. In other words, we can calculate how much fuel we need based on the HP figure we set as our goal, and since we mix air and fuel in a prescribed ratio, we also know exactly how much air we need, and air flow capacity is how we judge the capability of a carburetor.

Depending upon the BTU content of the gasoline (it varies a bit depending upon the quantity of additives), the desired air/fuel ratio, and the thermal efficiency of the engine, you will need between 1.3 and 1.67 CFM CFM of air per HP. Picking the middle of the road figure of 1.5 CFM, we get the following calculation for the Holley 390 CFM carb:

390cfm/1.5cfm = 260hp.

In other words, the Holley 390 is capable of supplying enough air to develop 260HP at its rating of 390 CFM at 1.5" HG, and that's more than our 250 HP target.

As a further note, keep in mind that the CFM rating of a carb is not its absolute limit. Start pulling more than 1.5" HG at WOT, and the 390CFM carb will flow considerably more. And one more thing to keep in mind- Nascar has been running 390 CFM carbs on 355CI engines for years making enough power to run at almost 200mph...

Y'all can start debating the merits of under- vs. over-carbing now...

 

[Anlushac11]

8)

I can tell you this much, The Ford 2.3L Pinto motor is 140cid, and has what is considered a crappy flowing head. The stock carb for along time was the Holley 5200. The standard upgrade used to be the 350cfm 2bbl.

When Offy still made the 4bbl intake the 390cfm 4bbl was extremely popular and was considered to have excellent driveability and performance when the secondaries kicked in was considered strong.

If we figure 2.5cfm per cubic inch of motor with the 350cfm then an equivalent setup on a 200 would flow 500cfm. The Falcon Performance handbook says that a stock 200 pulls about 227cfm. All the flow ratings apparently go in the trash as soon as the carb is bolted to a six cylinder since the way the carbs are tested is different from a V8 to a I6. IIRC something about V8's tested at 3" and I6's tested at 1.5" I dont know if its

Yet when Mustangaroo switched from a Holley 5200 to a 350cfm 2bbl on his Oz HEAD he picked up almost 10hp. So somethings got to be happening.

I do understand that was with a Oz head. The biggest problem is for most of us is overcoming the log heads poor airflow. On a log head the best setup is a Offy 3 x 1. The 3 x 1bbl setup flows about 450cfm or better depending on the 1bbl used. And since your pulling air in from 3 different openings in the log the effects of trying to flow air through the log are minimized.

On a Oz head or better then I would go with a 4bbl. IIRC Mustangaroo's son and some others will be running 600cfm Holley 4bbls.

Mt Oz head will be running a 450cfm Holley/Motorcraft 4bbl.

My solution? Everyone should run a Oz, Argie, or Crossflow head.

IIRC one of the Ford guys was saying he was running in excess of 600hp with his 355cid 390 carbed motor.

 

[Kstang]

I have an oz head with a holley 4 barrel (8007) that flows 390cfm. I have yet to run the 1/4 or dyno it yet but it sure feels strong now. It was a little hard to get an idle low enough but we got it worked out.

 

[Anonymous]

1.5 inHG is too much to be pulling at WOT.

0.50 lb/hp/hr
390 ft^3/min
13:1 air/fuel
Air = 0.0765 lb/ft^3
Fuel = 46.81 lb/ft^3

0.50/60 = 0.0083 lb/hp/min

390*0.0765 = 29.8350 lbs/min air
29.8350/13 = 2.2950 lb/min of fuel
2.2950/0.0083 = 276.51hp

Soooo. At 0.50 lb/hp/hr and assuming a 13:1 air/fuel ratio, 390CFM will support 277hp. Sounds like a nice number, but you'll have to be pulling those 390cfm at 1.5inHG to do it. If you were putting out 200hp, you'd need 282CFM (once again at 1.5inHG). Using this conversion <img src="https://www.dwyer-inst.co.uk/htdocs/images1/flow/flowmetercurves-formula_pic1.gif">, 390cfm @ 1.5inHG is 225cfm @ 0.5inHG. With a still-somewhat-restrictive 0.5inHG vacuum it would support 160hp. Big difference. To support 200hp with 0.50inHG, you would need 489cfm (@ 1.5inHG).


-=Whittey=-

 

[SuperMag]

0.5" HG at wide open throttle is a nice number for a race engine, but I wouldn't care much for the manners of such an engine on the street. If to meet your horsepower goal on a street motor you are relying on a carburetor so huge as to give you 0.5" HG at WOT, I feel you've made too many compromises.

I can tell you this much, The Ford 2.3L Pinto motor is 140cid, and has what is considered a crappy flowing head. The stock carb for along time was the Holley 5200. The standard upgrade used to be the 350cfm 2bbl.

If we use Whittey's handy conversion formula, that 350cfm carb translates to 247cfm when compared to the 4V...

 

[Anonymous]

1inHG pressure drop for 200hp would be 345CFM. 0.75inHG pressure drop for 200hp would be 398. Of course that is 200hp. Intake, exhaust and a mild cam will easily surpass that.
230cfm @ 6inHG will support 141hp. Hehehe.


-=Whittey=-

 

[SuperMag]

230cfm @ 6inHG will support 141hp. Hehehe.

Yeah, I know.

 

[jwhoss76]

Okay I'm following so far. But what happens if you overcarb? Wouldn't it cause flooding or stalling due to too rich of a mixture? Just curious.

 

[Anlushac11]

8)

Depends on how rich it ran. If it was set to good a/f ratio then it would stumble and bog on low end but mid range and high rpm would be good. Since most of our I6's are not built for for high rpm its pointless to have it make most of its power there.

With it over carbed you would have horrible low end throttle response. And it would probably bog and sputter.

 

[Anonymous]

From what Ive read Im starting to think less and less of all the formulas and much more of practical experience. For example 302 V8 have used carbs from about 220 cfm for a Stromberg to 780 cfm succesfully. 650 to 750 cfm with just a mild cam, and better extraction on a 302 works well so why did the manafacturers settle on the Strombergs 220 cfm? dont ask me.
Even if an engine cant burn all the extra from an oversupply of fuel in some instances its seems that there may still be some beneifit??? I have heard that too much fuel can strip the cylinder wall of oil but I find this hard to believe as the oil at higher rpm is continually re-circulated back to the walls at a faster rate (maybe the cause of visible fuel oil mix fumes out of exhaust under these conditions as extreme example).
As always if you drive hard you need to change the oil more often especially if there is concern about getting too much fuel as it has been known for some time that fuel by products contaminate and breakdown oil reducing its ability to lubricate properly.

 

[Anonymous]

Another reason many of the formulas for carb cfm ratings often do not apply, is that they are "conservative by design". They are based on the idea of "what works in most cases". This does not necessarily have all that much to do with HP but drivabivablity and perhaps longetivity also. It is based on the idea that a smaller carb "will when bolted on almost always work better than a bigger carb"
The trouble with formulas where big variables are involved is that it is almost impossible to get a formula that will work consistantly. When this occurs it is often easy to go for the play it safe method
It is amazing what carb combinations people have come up with when they have dared to take a bit of practical advise and at some risk applied it, occasionally with dramatic results.
[/quote]

 

[Anonymous]

A couple points i'd like to make...

If you overcarb an engine, you are not necessarily going to run lean. Also, you could size your carb off the primaries only and figure when those would rail out and leave the secondaries up to the task of making your WOT hp. Unless, of course, you run a mech secondary carb.


-=Whittey=-

 

[Anonymous]

Sorry Whittey. Cant make much out of a lot of the techno stuff in the first entries but to summarise does a four barrel flow what it says it is able to flow or are there all sorts of factors to include. Im assuming a 650 does flow nearly twice as much as a 500 which is a 347cfm and 450 and 390cfmm 4 bbl are able to flow that much on the right application. Excuse me for my ignorance in having to ask you to explain.
Regards Tim

 

[Anonymous]

You are correct (assuming the 500 is a 2bbl and not a 4bbl).


-=Whittey=-

 

[CobraSix]

Maybe it's also my ignorance here...

With a NA engine, it seems to me that the cylinders will only draw in so much air/fuel mix for a given stroke. Just because a carb can flow 1000 CFM doesn't mean the engine will draw that much. It's not like the carb is forcing 1000 CFM into the engine.

Just wondering...I can only imagine that it has to do with the fact you're using the idle circuit more that may not give enough fuel.

Slade

 

[Lazy JW]

One thing that needs to be understood is the fact that a carburetor needs to have a reasonable amount of air velocity in order to do an adequate job of both mixing the fuel with the air and keeping the fuel in suspension. If the velocity is too low, then it won't suck the fuel into the air stream properly (yes, I know it doesn't suck the fuel in, the higher atmospheric pressure pushes it in) and you have a stumbling condition until the engine moves more air or you squirt more fuel in with the accelerator pump. With a small bore one barrel carb, such as my YF Carter on the 300, it is very easy to maintain good velocity, even when suddenly opening the throttle at low speed. This carb gives good driveability and throttle response, but obviously runs out of breath at higher speeds. That's what four barrels were invented for, to give good low speed driving, and then open the secondarys when the engine is turning fast enough to maintain good velocity through the venturies. Whittey does an excellent job of describing this in detailed technical terms. Keep up the good work
Lazy JW

 

[SuperMag]

One thing that needs to be understood is the fact that a carburetor needs to have a reasonable amount of air velocity in order to do an adequate job of both mixing the fuel with the air and keeping the fuel in suspension. If the velocity is too low, then it won't suck the fuel into the air stream properly (yes, I know it doesn't suck the fuel in, the higher atmospheric pressure pushes it in) and you have a stumbling condition until the engine moves more air or you squirt more fuel in with the accelerator pump...

Bingo. Known as the "kicking-in" or "seat-of-the-pants" phenomenon. That's when some guy does something goofy like stick a Holley 850 on a 302 and tells you how awesome it is 'cause you can really feel it "kick in" at 3000 RPM. Nothing is actually kicking in; that's just the carb starting to flow enough air to become effective.

And this is also the reason why one should pick a vacuum secondary 4V carb for the street over a mechanical secondary 4V. A vacuum secondary carb will open the other two throats when it is ready and not before. It bases its 'decision' on throttle position and engine vacuum. A mechanical secondary carb on the other hand is directly linked to the throttle, so the secondaries open at a predetermined throttle position only. So if you snap the throttle too far open at low RPM on a mechanical secondary 4V you will kill the air flow velocity through the carb until you build enough revs for it to "kick in."

So why even fool with mechanical secondaries? They're great for finely controlling engine revs at 7000 RPM on turn two at Talledega...

 

[CobraSix]

Makes more sense now...thanks.

Slade

 

[Anonymous]

While carbs may not be able to supply any more cfm to an engine than it can take (pull actually) they can supply more fuel under acceleration when the vacume reaches a set point and the powervalve cuts in to enrich the fuel mixture.
Also its seems logical to me that a small one or two barrel carb could place a restriction on the amount of air that can be pulled through all the runners verging at that one congested point (the carb). This can have an advantage as well, but usually at lower rpm.
At WOT at higher rpm when the secondaries open up on a oversized four barrel the result is less restriction at this point and presumably more air flow and cfms can get through the carb to the engine. It stands to reason that it should be able to do this better through a four barrel than it can from a smaller two barrel.
Of course carbs come with there cfm rating but I would definitley gamble at going bigger with a four barrel than I would need. So long as the secondaries did in fact open sufficiently I would expect there to be some gain. The bigger carb could also meet the demand for extra fuel that the freeing up of restriction would cause to engines increased power output (a small multiplying effect???)
Its all just pure conjecture on my part but there seems some logic in it. At least to me. Anyway I hope to put my theory to the test some time in the near future.

 

[jwhoss76]

That being said I now see the point in multicarb setups. You acheive higher velocity due to more direct flow paths, you don't have to fill the manifold first and wait for the individual cylinders to draw from the main body. I never would have thought of it in quite those terms. Thanks guys.