Sizing Airflow for intake
Thread Starter
Racer
Joined: Jul 2007
Posts: 444
Likes: 0
From: Roselle IL
Sizing Airflow for intake
I have purchased an aftermarket L-88 hood to accomodate the new 496build. I plan to create a cold air intake for it. However, the structural brace at the rear of this hood is solid so I will need to either drill a series of holes or slots in it to accomate air flow into the chamber. I plan to build a chamber that will have the air cleaner contained within it. Essentially, I will put a plate below the air cleaner to seal the with the chamber.
My question is how much air flow do I need into this chamber and how do I determine how much air flow can fit through a given hole? I thought that a series of 1/2" or 5/8" holes would maintain the structural ridgidity of the brace over long slots. My initial logic was to mainain an opening area at least 1.25 greater than the area of the four carb barrels. However, I am not sure that is enough and I don't have any real experience here.
Any guuidance that can be provided?
My question is how much air flow do I need into this chamber and how do I determine how much air flow can fit through a given hole? I thought that a series of 1/2" or 5/8" holes would maintain the structural ridgidity of the brace over long slots. My initial logic was to mainain an opening area at least 1.25 greater than the area of the four carb barrels. However, I am not sure that is enough and I don't have any real experience here.
Any guuidance that can be provided?
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
My Cowl induction hood is completely open at the back with a lower cross brace. You want the opening very near the window to get the ram air effect. To not create a restriction I would think that you would need 10X the area of the throttle bores.
This is a fellow forum members air box design which I also built for a prior motor. I hope that he doesn't have the patent on this!
This is a fellow forum members air box design which I also built for a prior motor. I hope that he doesn't have the patent on this!
Le Mans Master
Joined: Apr 2006
Posts: 5,403
Likes: 95
My Cowl induction hood is completely open at the back with a lower cross brace. You want the opening very near the window to get the ram air effect. To not create a restriction I would think that you would need 10X the area of the throttle bores.
This is a fellow forum members air box design which I also built for a prior motor. I hope that he doesn't have the patent on this!
This is a fellow forum members air box design which I also built for a prior motor. I hope that he doesn't have the patent on this!
Thread Starter
Racer
Joined: Jul 2007
Posts: 444
Likes: 0
From: Roselle IL
I was thinking of a similar setup to the one in the picture, but building it out of fiberglass and attaching it to the hood. Then putting the foam at the base of the filter for a seal. I think I like your version better; Certainly a bit easier to build.
the crossmember on my hood goes all the way up to the hood deck, unlike a real L-88 hood (it is a vanacor). The hood is thin, so I don't want to remove a large chunk of the brace and I believe that round holes preserve a good amount of ridgitiy of a brace. I am now struggling with the issue of airflow. What kind of area for the airflow will be required to support the carb/engine without restricton.
I don't care too much about ram effect. I care mostly about getting colder air into the engine to improve HP and mostly to resist detonation.
the crossmember on my hood goes all the way up to the hood deck, unlike a real L-88 hood (it is a vanacor). The hood is thin, so I don't want to remove a large chunk of the brace and I believe that round holes preserve a good amount of ridgitiy of a brace. I am now struggling with the issue of airflow. What kind of area for the airflow will be required to support the carb/engine without restricton.
I don't care too much about ram effect. I care mostly about getting colder air into the engine to improve HP and mostly to resist detonation.
Team Owner
Joined: Apr 1999
Posts: 21,953
Likes: 1,445
From: Reno Nevada
2024 C3 of the Year Finalist- Modified
You have to remember that our motors shake and TQ flex. So you have to have a rubber seal.
My aftermarket hood is cut out on the whole indented area next to the rear top of the hood
My aftermarket hood is cut out on the whole indented area next to the rear top of the hood
Last edited by gkull; Feb 28, 2008 at 09:10 AM.
Advanced
Joined: Jan 2007
Posts: 62
Likes: 0
From: LaGrange Georgia
The answer to your airflow question isn't as straightforward as you might have hoped. There are a couple of things that will come into play when determining this:1) the air velocity coming through the hole and 2) the pressure drop across an orifice. The first equation is pretty simple: Q=VA where Q is flow in CFM, V is velocity in ft/min, and A is the area in square feet. So theoretically any size hole could flow a given amount of air if the velocity is high enough. The problem lies with the amount of pressure drop a very small hole and high air velocity would cause. You would have to find an equation to find the pressure drop through that hole at your given flows, pressure, velocities, etc. (probably bernoulli's). To make a long story short, I would say just make it (or the sum if more than one hole) as large as at least the cross sectional area of the opening on the carb so that you don't cause an increase in backpressure on the engine causing it to work harder to suck in air.
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Thread Starter
Racer
Joined: Jul 2007
Posts: 444
Likes: 0
From: Roselle IL
Matt,
This seems hard to swallow. I have seen the air cleaner intake horns on some those 300+ HP factory engines that couldn't have had a surface area any greater than 10 square inches.
Does anyone know what the surface area on an air clearner horn for the 400+ HP factory engines?
I would like to plan for a 900 CFM airflow, but I don't know the velocity nor how to equat that with area.
This seems hard to swallow. I have seen the air cleaner intake horns on some those 300+ HP factory engines that couldn't have had a surface area any greater than 10 square inches.
Does anyone know what the surface area on an air clearner horn for the 400+ HP factory engines?
I would like to plan for a 900 CFM airflow, but I don't know the velocity nor how to equat that with area.
Last edited by lr172; Feb 28, 2008 at 02:06 PM.
Thread Starter
Racer
Joined: Jul 2007
Posts: 444
Likes: 0
From: Roselle IL
The total surface area of the carb is 10 sq. inches. I think I can easily cut enough slots to create 15 square inches without looking too ugly. It sounds like fewer and larger slots would reduce resriction and velocity. Did I understand that correctly?
Does this seem like a safe plan?
Does this seem like a safe plan?
Advanced
Joined: Jan 2007
Posts: 62
Likes: 0
From: LaGrange Georgia
The total surface area of the carb is 10 sq. inches. I think I can easily cut enough slots to create 15 square inches without looking too ugly. It sounds like fewer and larger slots would reduce resriction and velocity. Did I understand that correctly?
Does this seem like a safe plan?
Does this seem like a safe plan?
Race Director
Joined: Oct 2000
Posts: 15,053
Likes: 79
From: central FL, near the beach.
1st thing before u buy a hood:
FIND OUT THE COWL INTAKE SIZE!
my ecklers hood has
20"x2.25
enough for 900 HP!
me, i cut w/hole saw 6,
1.875" and
3
1.125 for
~20 sq in or 400hp
can u use anemic holes? sure! it's your car, and it MIGHT work satisfactory.
FIND OUT THE COWL INTAKE SIZE!
my ecklers hood has
20"x2.25
enough for 900 HP!
me, i cut w/hole saw 6,
1.875" and
3
1.125 for
~20 sq in or 400hp
can u use anemic holes? sure! it's your car, and it MIGHT work satisfactory.
Last edited by Matt Gruber; Feb 29, 2008 at 12:50 PM.