Carb Size

[Todd Hedges]

How do you determine the most efficient carb size for an engine?
for example:

Currently have a 428 FE with Edelbrok alumn. heads at 10.6:1 compression. No porting or crazy valve sizes.
This motor has a Stock - Holley HP4150 - 750cfm carb.

The engine seems to run rich as evident by the black soot in the side pipes. Is the carb restricting the proper amount of air flow in the mixture or are the jets to rich?

What is the best way to determine proper mixture/jets on an engine that is installed in the vehicle?

The engine builder adjusted the carb while on the dyno, now that the engine is being choked by the restrictive - small air filter that fits under the hood scoop I think it is too rich.

Thanks for any help

[wilf leek]

The way to see what is happening is to measure and look at intake manifold vacuum. If, at high revs and wide open throttle, you still have more than, say, 1-2" of vacuum, then your carb and/or filter is strangling the airflow.

So buy/borrow a reasonable quality vacuum guage, and tee it into a port on a manifold runner (not on the carb) and see what's what.
Once you have sorted that, then you should go get your carb set up properly. 750cfm carb on a "stock" 428 should be OK.

[Ron61]

Todd,

I have an auto math book that gives all kinds of formulas for figuring out the cfm of the correct carb. However it isn't accurate for all applications. If you remove the air filter and run the car for a short time does that seem to relieve the rich condition. I have 850 double pumpers on both my small and big blocks and they run very clean but I did have to go to a K&N air filter. On the big block I am running 86 jets in both the front and rear so it gets huge amounts of gas as I can watch the gas gauge go down when I start to open it up. But the exhausts are a nice gray color and never any gas smell or flooding. I did have a little trouble with the small block until I took the filter off and drioe it for a few miles and what a difference that made.So I then started looking and found a 15 inch long air cleaner that gives me plenty of clearance and with the K&N filter I have never had another problem. I also gained around 5 horsepower which I badly needed.

Ron

[Tongue Pirate]

There is a carb size calculator in a small block ford windsor build book I have at home. As far as carbs go really just about any Holley or Demon carb double pumper should work as long as it is not too small. They are set from the factory to run properly on demand which means the fuel is metered out according to the amount of air being pulled through the venturi & the jets are sized for the maximum amount of air that can pass through the carb. This is why it is important that the carb is set with factory jets. The only adjustments that should be needed are the idle mixture & nozzle size. However if you do go too big & can be a little sluggish on response & may not open vacuum secondaries, which is why I said double pumper. Ideally you want to go as small as you can without going too small. Carter/Edelbrock carbs take quite a bit more tinkering to get set properly. If you would like to see this calculator send me a PM & I can mail it to you tonight.

[Jeff Frigo]

Simple, get a different air cleaner, you are probably loosing 50 hp. with it.

[rdorman]

The problem with most of the charts is you really need to know the volumetric efficiency of the engine to use them effectively. The only way you are going to get that information is on a very good dyno. Recently I ran accross this equation:

Volumetric Efficiency = 5,600 x HP x 100% / (RPM x CID)

Don't know how valid it is....... HP is at the fly wheel.

Second the opinion on the little air clean. Those Stellings OEM style cleaners may look nice but they are too small. I have a 15x3 k&n set up that fits nicely under the hood. I am even going to one of their air filter lids and a stub stack. You can hook a vacuum gauge to the air cleaner if you want and just see how restrictive it really it.

The carburator operates on pressure differential. So every thing from a restrictive air cleaner, undersize, oversize carb, tempature, barometric pressure, humidity and quiet possibly the phase of the moon will effect not only the CFM that you see but power valve, vacuum secondary operation etc. It is my understanding the HP series is jetted a bit rich because of its intended application. This on top of the DP generally being a bit rich. This all adds to what was said about the vacuum etc.

So what is the right size? That is one BIG depends. I can run everything from a 600 CFM vacuum secondary to a 850 double pumper and each one has its benefits(on a cleveland). Max HP, go towards the large size. Best drivability towards the low side.

There is a ton of info on sizing your jets. An aftermarket O2 sensor is about the best. Aside from that a new set of plugs and a very good magnifying glass like the one that doctors look in your ear with. Or just take the thing to a good tuner. Tuning a Holley takes a lot of patience but you will be rewarded in tuner heaven!

What would I do if I where you? Change my air cleaner and tune what you have. 2 cents.

[Tongue Pirate]

I agree don't waste your money on a new carb! What you have is probably good!

[Bob In Ct]

Hey Todd:
I have the book "Super Tuning and Modifying Holley Carburetors" if you want to borrow it. It lists all the carbs and jet sizes and so forth. Most carbs are set a little rich by the manufacturer since a too lean set-up can burn valves and pistons.

Keep in mind if you're not running with PCV the engine will run a little richer than it would with one.

Bob

[jwoodard]

you're air cleaner is probably restrictive. The best way to tune your carb is to get the car on a chassis dyno with an O2 sensor. Your carb is probably perfect size wise, and once on a dyno you can get it dialed in perfectly. I just did this with my car and gained over 20 RWHP and a like amount of RWTQ. Also made a huge difference in throttle response.

Jim

[Todd Hedges]

Thanks for all of the help.
This place is great.
Bob in CT, I hope to meet up with you this year for some track time at the Rock this year.
Happy Holidays.