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

[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.

[flipper35]

I had a noticable seat of the pants improvement when I sealed the carb to the hood eliminating the hot air from under the hood. I fabbed an aluminum pan and attached it to the filter assembly, it isn't a turkey pan, and sealed it with Moroso's foam. This allows you to use whatever size element you want including the X-stream top. I know it doesn't have anything to do with ram air because it is noticable at all speeds.

[ItBites]

Lew is correct. Dual fours will raise manifold pressure (provided the total venturi area is more than whatever single four setup you would compare it to) and raise HP (slightly). This will be accompanied by an increase (slight) in total airflow to the engine, but that increase will be small in comparison to the doubling of filter inlet area. So dual filters with whatever carb setup is better than a single filter of the same type.

However... If you are using the stellings (are those the little bitty ones I see so often?) air filter with a 1 inch element or something that small, even doubling it by going to two filters (with dual carbs) will likely not produce optimum horsepower.

Now... I am talking optimum HP here. A post above indicates little change was seen with hood open or closed. I believe that. Removing the turkey pan provided increase, but the scientific method was disrupted because a gasket was changed at the same time and may have affected the results... My point here is this: The increase in HP is likely to be small unless the inlet path is very restrictive. However, racers live and die by slight advantages over the competition. As pointed out earlier NASCARs use huge filters. How much HP does this add? Well if it adds 2 or 3 it was well worth the money to use it in a restrictor plate class... Many people spend major bucks on headwork that gains 7-10%. If a $50 filter can gain 1%, don't the economics work?

The flip side: These cars are space limited and we may have to live with less than optimum intake systems including filters. Heck, some folks even run webers - look great, but too small in series with each cylinder for best HP. Even a 750 on a 428 is way too small for optimum HP at 6000 RPM. Yes, I know about the VE calcs blah, blah, blah, but those are not for optimum HP, they are for best VE or good driveability, which are not the same as best HP. Each of us has to decide where to draw the line and make the compromises we choose. My discussion is only intended to provide an understanding of what the factors in the compromise are and what each otion may or may not add.

I do agree with an above comment (and I see this all the time) where folks spend what I know has to be $15,000 - $20,000 for an engine setup with all the trick bells and whistles and fuel pumps and on and on and then puts a little air filter on it and takes out about $2000 out of the performance. Why put it in there in the first place. I think its mostly for braggin rights and just because the owner likes it. WHich is why we build these things in the first place.

[jdean]

I can't remember the details, but I recall seeing on some thread here in the past a dyno comparison of various sized filters.

THe results showed less than 6-7 HP difference on a 400+ HP engine between a large filter and a tiny one.

I remember because the results defied conventional wisdom on sacrifice made using the small filters us Cobra owners are typically relegated to use.

Anyone recall this?

John

[ItBites]

Actually 6-7 Hp would not surprise me. This would be 1.5%, which is not bad for a $50 mod, consider head porting for $1500 (or more) and a result of 10% improvement, the ROI (return on investment is good).

As I have stated above twice, the compromise made with a little filter may be beneficial for other reasons (space limitations etc), but the HP loss is real. Racers in any class where limits on motor build are in place would kill for 6 - 7 HP

Thanks for some data. I was guessing at 1%, but 1.5% with dyno data is great.


On the cold air subject. Cold air is great PERIOD... I think I recall some folks did a good scientific study looking at temps entering the cylinder (not 1:1 comparison with temps entering the carb) and they found 1% for every 10 degF of drop in temp of the intake charge at the valve. Good reason to seal to the hood, but of course everyone knows that old trick?

Ram air is not a big winner. I recall other data that gained 1% at 100mph due to ram air. I think they had a good inlet too. Our less than optimum inlets would likely show less gain, but some is there to be had at higher speeds. remember dynamic pressure rise is =1/2 * rho * v^2 - (increases with the square of the velocity), so 50mph would result in 1/4 the ram effect gained at 100mph and so on. At 200 mph you might gain 4%!!! Try it and let me know...

[Mr.Fixit]

Another thing to take into consideration is that a too-small filter will make a carbed motor run richer.