Air flow through hood scoop

[Todd Hedges]

I was thinking, with the cowel induction hood on the old chevy's and the huge air scoop on the 426 Hemi's, how effcive is the size and position of the scoop on the hood of a Cobra?

I feel that the "weak-link" is the ability to get adaquate air flow into the carb.

I'm using a turkey pan with my 750CFM to help caputre air comming in through the scoop, but I can't help but to think that since the scoop is so low to the hood, that an actual vaccum may form at that works against the effeciency of the carb.

Kinda like a paint sprayer.

Anyway, does anyone have any data/experience as to the effeciency of the stock Cobra scoops?

The K&N X-treme air filter top helps, but the 1" filter element is a real choker.

[mr0077]

Todd, I wondered the same thing...if you do the arithmetic, assuming the scoop opening is about 16" wide by 1.5" high, and all the air into the carb goes through the opening, the air velocity through the opening would be about 50 mph if the carb was flowing 750 CFM...the velocity would slow down once inside the T-pan, but it seems like there could be some turbulence inside the T-pan that might upset carb flow without a filter assembly of some sort...any other thoughts?

[ItBites]

You can tape some 1" tufts of yarn around / in the scoop opening and drive the car (have someone film from a car in the next lane) to determine if air goes in or out of the scoop while driving. This is cheap wind tunnel testing, but it works.

I suspect air is going in. The vacuum you refer to (Todd) would be created if the opening were parallel to the flow (Bernoulli principle), but the scoop opening is perpendicular to the flow. However, due to the low aspect ratio of the opening, I would think what is referred to as "edge effects" may reduce the "equivalent hydraulic diameter" to a point where the scoop is not particularly efficient at getting large volumes of air to your engine.

As far as your filter goes, you are correct that a 1"tall element, even with the flow thru the top is not enough for your engine (even if you have a 302). Go to the K&N website and they have a formula there for determining adequate filter area based on engine size and RPM. Carburetor size has nothing to do with it: Since inlet speeds in the venturis are not sonic, the flow in your carburetor is not choked and as an engine demands more flow, it will lower the manifold pressure and resultingly more air will flow into the same carburetor. This will not produce the best HP possible, however, since the manifold pressure is lower, the density of the mixture entering the cylinder is down and resultingly HP. This is why racer engines seem to use carburetors that are way larger than the typical formulas I've seen thrown around in these forums. The larger carburetor will support a higher manifold pressure for any given flow than a smaller carburetor, resulting in increased density of the mixture in the cylinder - and more HP, but only at the expense of signal and trottle response. But I digress... sorry to go on and on.


Dont count on the scoop force feeding your engine and don't count on making the best possible HP with a tiny filter.

Let me know if you do the yarn test...

[Todd Hedges]

ItBites,
Interesting information.
Here's what I come up with from K&N

with a 428 (430ci to cover over-bore) at 6,000rpms I get the following area for the filter:

A=(430*6000)/20839 = 126 SqIN

With the 9" X-treme top if figure the following area;
A=(3.141593 * 16) = 50.23 SqIN

That leeaves me with trying to get 76 SqIN out of the filter:
H=(76/(9*3.14) ) + .75 = 3.44 inches high.

No way his will fit under the hood.
Any suggestions?

9" dia is the largest hat will fit in the turkey pan.
Is there a "drop base" that will fit a 9" round filter
and not interfer with throttle linkage?

Is there anyone else running a taller 9" dia filter
with a 428 in an ERA?

[clayfoushee]

Very interesting thread. I have the same questions you do Todd running a 427 w/ a turkey pan.

[ItBites]

I run a fuel injected BBC bored to 440ci that is shifted at 7500 RPM (or above). I do not run a turkey pan. I have modified a drop-base 14" filter base with a 2.5" element. This is all I can get in the space I have. I'd prefer more.

According to K&N I suspect I'd be short on filtration area too. Understand their size recommendations are likely based on a particular max velocity thru the element that, based on the mechanisms involved in any good filtration system (I forget the three mechanisms - entrapment? impingement? diffusion?yatayata?). Anyhow, beyond a certain air velocity, the filter is not as effective at removing particles of various size categories (bins).

So what? Well, if you run a smaller than recommended filter, the filtration will not be as good. OK, we may have to live with that. I suspect that you could get away with a filter of about half to 2/3 of what they recommend and still not get the velocity way too high thru the element. More frequent filter service might offset the decreased filtration efficieny.

The other downside (maybe more interesting to us) is similar to that discussed in my earlier post regarding carburetor size. If you have to increase the velocity of the air at the filter boundary because the cross-sectional area of the boundary is small, this velocity comes at a price. The price is that the pressure drop from atmospheric to the other side of the boundary (filter) must be higher (it takes energy to move the air faster, remember momentum = 1/2 * M * v^2) so energy required is a function of the square of the velocity. Lower pressure just above the carburetor (inside the filter housing) will result in lower manifold pressure and then we're back to the density that is available to fill the cylinders discussion above...

Just food for thought: Fuel Injection can be designed to be capable of producing more power than carburetion with ALL other things being equal, because carburation requires signal (low pressure) at the venturis to meter fuel, resulting in manifold densities always below atmospheric pressure, whereas injection can be designed to allow manifold pressure to approach or be at atmospheric pressure and still meter the correct fuel amount. Think about it.

[RACER X #99]

Todd,
If your just street cruising then what you have is probably good enough.
If you want to get serious and make some of the HP that your engine is capable of than you have to dump the turkey pan and modify a drop base 14" filter with at least a 3" element.

One time famous BB FE racer Don the FEz Durner even went so far as to have a custom built oversize scoop to house his 14 X 4" filter for his ERA Cobra. This could be one of the reasons he was so fast on the track. He knew how to make his engine breathe.

It really is funny when guys go thru all the expense to install two four barrel carbs on their Big Blocks and then choke them off with those tiny little Stellings filters.

FYI take a look at the filters the Nascar guys use on their 358CI engines.

Cranky

[Ant]

Cranky,

Are there any picts of Don the FEz Durners ERA with the oversize scoop?
You are on the money with your post, bigger is better. Did Don use a forward facing scoop or reverse?
I have modified my one as mentioned but being taller, and all the posts here guys have noticed vapour etc exiting the front, so I have a big useless bumblebee, bird catcher, I am thinking the original look can take a walk!

Originally Posted by RACER X #99

Todd,
If your just street cruising then what you have is probably good enough.
If you want to get serious and make some of the HP that your engine is capable of than you have to dump the turkey pan and modify a drop base 14" filter with at least a 3" element.

One time famous BB FE racer Don the FEz Durner even went so far as to have a custom built oversize scoop to house his 14 X 4" filter for his ERA Cobra. This could be one of the reasons he was so fast on the track. He knew how to make his engine breathe.

It really is funny when guys go thru all the expense to install two four barrel carbs on their Big Blocks and then choke them off with those tiny little Stellings filters.

FYI take a look at the filters the Nascar guys use on their 358CI engines.

Cranky

[scr11]

Great thread. I'll have to reconsider my K&N 9 inch Extreme lid as well. I've always wondered if I was getting enough flow. I'll do the math.
Scott, SPF#1230 / 393

[btsai]

Would somebody post a picture showing a drop base filter install on their engine? Itbites -- something like yours would be interesting to see.

[REDSC400]

I removed my turkey pan and went with a K&N filter with the Xstream top. This is a 9" filter and top and it stands about one and a half inches tall. This is on a Holley 780 double pumper.

We arrived at this set up while tuning the carb on a chasis dyno this last fall.

There was virtually no difference in measurable HP or volumetric efficiency with the hood open or closed so I would imagine that this means that the ERA air scoop on my car is effective, is not constricting air flow, and is working well with the fans of the chasis dyno running. Now, obviously, sitting still it just allows air to enter the top of the engine bay.

You will notice that many photos of team cars did have air diffusers inside the scoop which appeared to be made from sheet aluminum with round air holes in them. There was too much air entering the engine bay causing the hoods to lift. Early cars had bonnet clamps added to them from the photo evidence I have seen.

The turkey pan on my car, once removed, did increase HP and torque, perhaps because we used a new gasket as the old one was not in great shape upon inspection. I think the turkey pan pays off when the engine AND ambient air temperature are really hot and you are running AT SPEED.

For normal running, with the turkey pan, what was happening was a bogging or lag in stop and go driving from stop or idle. My idle is low as I like the rumpa rumpa gurgle. Again, that lag could have been caused by what appeared to be an old gasket between the carb and pan.

[deadwood1584]

14 x 3 with drop base and extreme lid and 6 x 12 deflector--works for me at 11000 feet--breathes good and deflector keeps fuel from being sucked out of top of filter at speed
[IMG][/IMG]

[BMK]

Hi all



Yes agree an interesting Thread.




Good to see all the information. I used the 14" top. Similiar to 'deadwood'. Then due to clearance used a 2" base. Have had no lag under heavy accelaration. Putting out 424HP at rear wheels.

Have learnt not to get angry and stamp my foot. The headspins send me dizzy..

Again thanks for the information.

Cheers

Bernie

[Lew Ledyard]

If you wanted to maintain a stock appearance, wouldn't a dual carb set-up help the situation since instead of one tiny air filter, you have two?

[clayfoushee]

Lew, but don't you also need more air with duals?

[Lew Ledyard]

I don't know if you would need significantly more air with dual carbs. I would think that the amount of air needed by an engine is a function of displacement and RPMs, not of the size or number of carbs. Larger or more carbs might let the engine breath at its full potential. Therefore, if an engine is drawing a constant amount, and it has to do so through a restrictive aircleaner (which flows at Xcfm) , it seems that the engine would have an easier time drawing such amount with two carbs and aircleaners which could flow 2Xcfm. I have no experience or knowledge in this field whatsoever, so I might be way off base.

[davids2toys]

This is a most interesting thread for me. I presently have the 9" air cleaner 2 3/4 k+n filter element, turkey pan set up. I love the way it looks. I have found out that this filter is only capable of flowing 470 cfm, clearly not enought for a 428 with a big Holley DP. So I am looking for a way to get a lot more air and keeps the traditional look under the hood and on top of the hood
More research the other day, I found out that the long oval cobra air cleaner flows no better than the little one I have on there now(reduced height for hood clearance). I am shocked no one has designed a drop down base for the big oval air cleaner, I called k+n and the have a 3" filter element for this, it measures 20.81Lx9.5Wx 3H, it will flow 950 cfm, this to me would be perfect! Now, to make it perfect, would be to design a turkey pan to mount to the intake manifold flange(like the present Turkey pan), only shaped to accomadate the long oval filter.
Now, as far as providing air for this much increased cfm potential, I know under the hood air availability is a real question mark, especially at speed. Dyno guys have told me they have much increased numbers just taking the hood off and doing nothing else. I was thinking of a RAM AIR setup coming from the two 7x3 holes in the front, ducting it outside the wheel well and then entering it into the engine compartment thru the wheelwell housing. Location would be about 1/3 of the way back on the valve cover.
Mount a flanged adapter on each side of the new tukey pan that would accomodate a 3 inch brake ducting hose.
I think this could be a real clean, very effective and very good looking set up.
Pegusus has ducting and adapters, I am sure there are more sites that have more choices, this is as far as I have gotten so far.
Has anybody attempted anything like this yet, I don't want to recreate the wheel if has been done already.
To compliment this I would opt for new sidepipes with the spiral core option that I talked to Maurice at Unique about, to free it up so I can take advantage of all that available intake air I would now have. Then of course it would be off to a dyno to dial it all in.
Related Thread On Unique forum
http://www.uniquecobra.com/forums/showthread.php?t=5525
Any input would be appreciated!
Thank You....Dave

[Lew Ledyard]

Take a look in my gallery. I started with a Summit oval aircleaner with a filter top and dropped the base. I still run a turkey pan. The exhaust no longer has that unburnt gasoline smell, so that tells me the aircleaner flows better than the S&H that it replaced.

[davids2toys]

Originally Posted by Lew Ledyard

Take a look in my gallery. I started with a Summit oval aircleaner with a filter top and dropped the base. I still run a turkey pan. The exhaust no longer has that unburnt gasoline smell, so that tells me the aircleaner flows better than the S&H that it replaced.

Nice car Lew, yes I saw that aircleaner in Summitt, I think it was an Edelbrock actually. Does it have a drop down base, I did NOT think it did, but you say it does?
I have heard of spurttering issues with the open top air cleaners with our cobras and the scoops we have on our cars affecting the airflow.

[Lew Ledyard]

It does not come with a dropped base. The entire base sits above the carb. However, the flat base on it can be modified so that it drops. I think mine is dropped about 3/4". I have no sputtering issues.