67mustang302

So, I had some spare time and got curious, so I did a little investigating. We all know what overcarburetion is, right? It's when you have that big shiny perty thing with all the barrels that you say makes you go faster, then you step on the throttle and get a puff of black smoke and a car that runs like a pig. Well, many of us are aware of the vast amount of overcarbureting that goes on, but the question is how much is too much? Many of you know from my posts I'm an advocate of "less is more"

Well, I did some calculating based on known information about NASCAR Nextel Cup engines, a highly developed race enginewhichDESPITE the fact they STILL run flat tappet cams and a single restricted 4bbl carburetor actually produce very similar mean cylinder pressure numbers(the widely accepted best basis for comparing 2 different engine's ability to produce torque) as Formula 1 engines do(widely accepted as the absolute apex of the evolutionary chain for n/a automotive engines). They produce approxamtely 800hp from 358 cubic inches at around 9,000rpm. Using a BSFC(brake specific fuel consumption, getting into thermal efficiency now) of .45, which is considered good for a well tuned and developed performance engine(though a Cup engine may be slightly lower which is better), this puts us in the ballpark of around 105-110% volumetric efficiency. Using the old tried and true carburetor cfm calculation(I use one that has a pulldown menu for single vs dual plane intake, and gives a cfm range to take into account varrying engine builds), this comes out to around a minimum of 1,050cfm. However, they only run 830cfm carburetors....yet they still make that power with a carburetor that's seemingly several hundred cfm too small.

Now, I know some of you are thinking, "Yes, but carburetor cfm ratings are based on the depression they're flowed at, so flowing a carburetor at a higher or lower depression will yield a higher or lower cfm rating." Yes, that's true. However, they use Holley based carburetors, so it would stand to reason that they use the industry standardcfm ratings at the same depression for a 4bbl carb(1.5" Hg), and if they wanted to flow them at a weaker depression, it would have to be quite a bit weaker to yield a cfm rating that's some 20% or more lower. I can't see a purpose in doing so though.

So it would seem that we don't need nearly as large a carburetor as we may think. Just one that's well set up to the engine

Discuss

remicks

I just read a similar article in the months MMFF. It was a super stock 94 GTR with an obviously highly modded 473 (bored/stroked 460). They were producing over 800hp with a little 600cfm carb. They used that instead of the stock size they were allowed to use which I believe was a 830cfm. Plus the girl who drove the car was incredibly hot. It shows that its a matter of the internals of the carb and tuning.

gothand

I would, but I'm going EFI so I've got new issues to worry about. If it is any consolation, I am going with 24# injectors (19# stock)and only a 65 mm throttle body (60 mm stock) on a motor rated 120 hp more than stock.

my77stang

great logic, but with their restrictor plates they might as well use a 600 cfm carb LOL.

btw, flat tappet cams have lighter lifters which means its easier to control valve float at high sustained rpms

67mustang302

LOL, yeah, restrictor plates are like lettingan engine breathe through a straw, still though, they're running a way smaller carb than you'd think they need AND a restrictor plate, and STILL managing to make over 2hp/cid and over 100% VE. So why do people with less than half as much engine use carbs that are almost as big as what a Cup engine uses? If "too small" is good enough for a Cup engine to make that kinda power, shouldn't "too small" hold even more true for a street car?

And yeah, flat tappets are quite a bit lighter, but don't have the breathing advantage of a roller, so just from an airflow standpoint, it's really dang impressive that those guys get so much power from so little. Flat tappet cam + small carb + restrictor plate and they still get that power output from 358 cubes.

EFI is a whole different ball of wax too, a lot more forgiving than a mismatched carburetor

fast66

Thats why Im a BIG fan of my 570 Street Avenger, even up to a warm 347.
Incidently, here in Australia, the 570 S.A.is actually refered to as a 465.S.A. Different measuring standard?

my77stang

yeah, you guys are upsidedown over there

i've got my 570 S.A. thats going on my 289 - i'll have the motor in the car very very very soon and i'll let you guys now how it does

Gary H

So, where does a 3HP per cube 500 cube pro stock fall? With 2500+ cfm's worth of carbs? One reason NASCAR uses a smaller carb is fuel economy, they dont have an umlimited amount of fuel for a race. Not to mention pit stops slow them down.

67mustang302

Yeah, prolly different depression for cfm measurng down under.

And yeah, I calculated for the Pro stock guys too, at the mid 2k cfm range and up they fall right around the middle to the larger side of the required cfm range, but not outside the range so as to be overcarbureted. Certainly the NASCAR guys have to worry about mileage when the Pro Stock guys don't, which is one reason for the smaller carb(that and the rules require like 780-830cfm for Nextel Cup). I just found it interesting that the NASCAR guys are running VERY small carburetors for theiraplication and still managing to make the power they do, so it would stand to reason that on a street car where mileage, throttle response and low rpm performance are an even bigger concern than a Cup car, that the carb could be just as small and still work well. The thing that strikes me is that if they can still get that much top end power and maintain that high a VE with such a small carb for their app, then what abut a street car that has quite a bit lower VE, 90-95% at best in most cases. Certainly the slight loss in top end from a carb on the smaller side would be more than offset by the gain in midrange, bottom end and mileage, not to mention throttle response and drivability.

FWIW I like my lil 570 SA also, runs great and makes plenty of power

Starfury

I think 90-95% VE is rather optimistic. Most of our older flat-tappet engines with poor head castings are probably closer to 80 or 85%.

I run a 600CFM carb on my 9.3:1 CR 331 and it works great. Good throttle response above 2k (cam doesn't make power until then, even with the Rhoads Lifters) and it pulls hard all the way to 6200. Even at 100% VE, my 331 would barely pull 600CFM at 6200rpm.