Calculating carb & filter airflow for YOUR engine

[crazy4offroad]

This morning's discussion in my build thread got me remembering an article I read about figuring out what size carb and air filter is best for what motor. Again, onedirt.com's tech articles make it easy.

For carbs, from here:
http://www.onedirt.com/tech-stories...-carbs-picking-the-right-carb-for-your-motor/

Multiply the engine’s cubic inch displacement by the Max RPM of the engine and divide by 3456.

So a 355 SBC that tops out at 6,000 RPMs only requires 616.32 CFM. A 383 that pushes 7,000 RPM would need 775.75 CFM. And a 454 that tops out at 5,000 RPM would require 656.83 CFM.

Next, you're wondering if your 3" air filter is limiting your engine's performance. They also have an article about that too, complete with dyno runs:
http://www.onedirt.com/tech-stories...g-part-1-budget-options-for-added-horsepower/

This got us wondering though, there has to be a easier way to determine the correct Air filter size right? First we need to determine the maximum volume of air your engine will require. Bob Harris, K&N Corporate Accounts Manager showed us the process to establish that,”To determine the maximum volume of air your engine will use, take the displacement in cubic inches and multiply by the maximum RPM you operate the engine at. If the engine is supercharged or turbocharged, divide the pounds of boost by 14.7, add 1 to this answer, and multiply this value by the CIDxRPM value. Divide by 3,456 for a 4 stroke engine, or 1,728 for a two stroke engine. This will calculate the maximum airflow required by the engine.
Sounds really complex but when you look at the equation it’s not that difficult:

CFM = Cubic Inch Displacement x Max RPM x (B) / 3,456 or 1,728
Boost (B) = lbs boost / 14.7 + 1

So for our engine the equation looks like this.
557 CFM = 350 x 5500 / 3,456

So now that we know the volume of air that our engine requires, we can determine how big of an air filter to run. “To determine what volume of air a given filter is capable of flowing, multiply the diameter of the filter by its height, multiply by pi (3.14), and multiply by 6,” Says Harris. “If the filter is a tapered cone (different diameters at each end), add the diameter from the top and base together and divide by two for an average diameter.
Again it sounds more complicated then it actually is. Since we are not using a tapered cone air cleaner, our 2-inch air cleaner equation would look like this:
527CFM = (dia.)14 x (height) 2 x 3.14 x 6
Harris suggested to exceed the CFM required by the engine by approx. 100-150 over what the engine requires. So we might be starving our engine of air with our 2-inch air filter. Based on the equations that K&N gave us we shouldn’t see a whole lot of difference in the numbers produced by the dyno.

So to simplify all that a little, using the CFM ratings we had above for determining carb sizes, we can say...

"So a 355 SBC that tops out at 6,000 RPMs only requires 616.32 CFM."
(diameter) 14" x (height) 2.5" x (pi) 3.14 x 6 = 659.4 CFM
It would be safe to go with at least a 3" filter to gain that extra 100-150 CFM mentioned in the article.
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"...A 383 that pushes 7,000 RPM would need 775.75 CFM."
(diameter) 14" x (height) 3" x (pi) 3.14 x 6 = 791.28 CFM
Again, to be in the "safe" zone of 100-150 CFM above your engine's requirements, a 14" x 3.5" filter would carry 923.16 CFM.
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"...A 454 that tops out at 5,000 RPM would require 656.83 CFM."
Using the calculations for the 383 we can see a 14" x 3" filter would suffice for the requirements of this engine, and give you the 100-150 CFM overhead.
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Hope this helps someone figure out how much air your engine needs to breathe!!!

 

[HotRodPC]

Good Info. Maybe some will now see they don't benefit from the Holley Dominator 1150 dual feed on their 305 that floats valves at 5,000 rpm. They can run 2 1/2 305's with that much carb.

 

[bucket]

The 454 in my crew cab has a 500cfm carb

 

[HotRodPC]

Is it the Holley 2bbl? You know Holley makes some 4bbls in he 400's. Usually used on Tunnel Rams with dual Quad Holleys. They perform well if set up right. I just never had the urge to run a tunnel ram cuz of the all show no go affect. Everyone expects a tunnel ram, or with motor sticking out of the hood, you should be running 8's and 9's in the quarter. Not only that, very very poor for street driving since a tunnel ram is nothing but high end top end and little to no bottom end at all, so you'd have to run a 3500 stall and 4.56 to 4.88 gears with a small street tire and it's not practical at all. I'll stick with dual plane intakes and make sleepers that can hang with many of those tunnel ram guys and noisy motors.

 

[bucket]

Is it the Holley 2bbl?

No, it's just an Eddy carb with the choke removed. Throttle response is spectacular, and cold start drivabilty is great... beyond that, the motor is suffocating.

 

[HotRodPC]

No, it's just an Eddy carb with the choke removed. Throttle response is spectacular, and cold start drivabilty is great... beyond that, the motor is suffocating.

It's only suffocating at higher rpm though. That will keep you disciplined and decent mpg though. Actually a well tuned Qjet, and keeping your foot out of it, you can get exceptional mpg. You tune a Qjet right, and drive it as if there is an egg on the gas pedal that you don't want to break, you'll get much better mpg than most other carb set ups.

 

[bucket]

It's only suffocating at higher rpm though. That will keep you disciplined and decent mpg though. Actually a well tuned Qjet, and keeping your foot out of it, you can get exceptional mpg. You tune a Qjet right, and drive it as if there is an egg on the gas pedal that you don't want to break, you'll get much better mpg than most other carb set ups.

Coupled with the weak fuel pump that I still need to change, it runs out of fuel over 75 mph. You can also feel the power dropping right off on a full throttle blast. It can get 10 mpg though.

 

[HotRodPC]

Coupled with the weak fuel pump that I still need to change, it runs out of fuel over 75 mph. You can also feel the power dropping right off on a full throttle blast. It can get 10 mpg though.

I get 13mpg hwy with my 454 and a Qjet. It's a Qjet from a 350, but still a 780cfm carb, and it don't run out of fuel. I've got the speedo pegged at 85 in 2nd gear and it still pulling. If I had to guess, I'd say it hits 3rd about 90mph.
But, do keep in mind, mine has 3.21 gears and only gets 8mpg around town cuz the thing is a sled taking off from a light with a 2.42 ugly first gear in a Th400.
It's nice doing 75mph and you can't even hear the motor running with quiet true dual exhaust with no Cats. Just biggo mufflers. It's not a towing rig, but I have towed a Full Grown Burbon on my 18ft trailer. It struggled a bit but did OK. Towed it at 65-70mph most of the way home.

 

[bucket]

I get 13mpg hwy with my 454 and a Qjet. It's a Qjet from a 350, but still a 780cfm carb, and it don't run out of fuel. I've got the speedo pegged at 85 in 2nd gear and it still pulling. If I had to guess, I'd say it hits 3rd about 90mph.
But, do keep in mind, mine has 3.21 gears and only gets 8mpg around town cuz the thing is a sled taking off from a light with a 2.42 ugly first gear in a Th400.
It's nice doing 75mph and you can't even hear the motor running with quiet true dual exhaust with no Cats. Just biggo mufflers. It's not a towing rig, but I have towed a Full Grown Burbon on my 18ft trailer. It struggled a bit but did OK. Towed it at 65-70mph most of the way home.

Yeah, I know. Remember that I have the Burb version of your truck, but with 3.73 rear. The crew cab dually is a entirely different animal with more power (till it sucks the fuel bowl dry), more weight, more height, more tire, and more gear. I'm very happy to get 10 mpg out of it. I have a 670 street avenger I need to rebuild and stick on it. I've also thought about a twin TBI setup... that would be cool.

 

[HotRodPC]

And it's a Fo Buh Fo, you're doing good to get 10 out of that. Dual TBI? That would be interesting. Special aftermarket computer I assume? Or just double up the connector and have the second one run as a ghost like the first one? Guess that isn't possible is it? Would be cool.

 

[bucket]

And it's a Fo Buh Fo, you're doing good to get 10 out of that. Dual TBI? That would be interesting. Special aftermarket computer I assume? Or just double up the connector and have the second one run as a ghost like the first one? Guess that isn't possible is it? Would be cool.

Last year at a local car show I saw a '77 Monte Carlo with a dual TBI 383 and thought it was slicker than fish snot on a door knob. I waited for the owner to come around so I could talk to him, he got an entire kit from Howell Engineering I think it was. It's basically just refurbed GM components with a new harness and custom prom, IIRC. It's not so fresh in my mind anymore, but I think you could just buy a couple key items from them and piece the rest of the system together on your own.

 

[thaifighter]

what about open air filters compaired to factory snorkels? I asume the calculations are for open air filters only as the amount of air is restricted by the diameter of the snorkel and the speed at which it is being driven given the ram air effect if so what would be the advantages and disadvantages to running open air over the snorkels?