Air flow through hood scoop

[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.

[ItBites]

By the way. Back on my first reply on this thread, I'd suggested the old tuft of yarn test around the scoop inlet area.

Is anyone going to try this? I'd really like to know what happens to the flow in this area.

I think speeds of 50, 100, 150, and 200 mph would give us enough data

[Lew Ledyard]

With regard to cooler air, someone (I think it was Thomas Kirkham) told me that the turkey pans reduce the temperature of the intake air by 40 degrees f.

[aumoore]

I have a 600 Vac secondary Holley on my 302 and am running a 9" Ford Motorsport round open element air cleaner. I started with the stock paper element that came with the air cleaner. Car ran great but I decided to use a K&N filter. The paper element was about 1.5" tall and I calculated that a 2-1/2" tall element would work. K&N had a 2-3/8 element that worked perfectly.

Did the K&N work? I say it probably added maybe 5-6HP which is barely enough to make any differance.

I did add a turkey pan later and it had more effect than the K&N.I would guess at least 10HP due to a cooler running carburator which heats the fuel less and gives denser fuel. Hence more HP. I think the pan helps direct the cooler air from the scoop directly to the carb and keeps some of the hot engine compartment air away from the intake of the carb.

It is a good idea to put a heat shield below the carb to protect it from the engine heat.

[PnRCobra]

You could always switch the manifold to an Edlebrock air gap. This will help keep the air between the manifold runners and the hot oil from the block. Another idea would be to use the X-treme K&N air filter top. It is almost like running with out an airfilter. I did both on mine which I gained more HP and torque in the 12% bracket.

Rich
Sweet as Kandyapple..................Faster than it's prey

[Michael C Henry]

I have the dual carbs. They use 8 1/2" air cleaners.They could use an extreme top or equivilent.They stop at 9".
The original scoop was an after thought when areodiamics was realy getting started.A lot of the builders have thought hard and come up with alterations. On the Shelby comp series they use a larger scoop that has some resembelance the original .I've seen a recreation fo what I was told was the twin suppercharger scoops .It was wider taller and came forward more.The cover car on Summit Racing is a car with a larger higher flow through design ,probably works as cowl induction at speed.The hood scoop was on the original 37 or so SC's.The street roadsters didn't come with scoops.but many were converted.
I'm not happy with my EM scoop. The tunnel wedge 2x4 makes even 1 3/4" tall Stelling and Helling's hit.The front edge under hood and rear at back.If I were in a position( Like going to paint)Change that scoop in order to get the air cleaners up into the flow.I'm still waiting for a couple of extreme tops for my two 8 1/2" cleaners

[Richard Hudgins]

Originally posted by ItBites


By the way. Back on my first reply on this thread, I'd suggested the old tuft of yarn test around the scoop inlet area.

Is anyone going to try this? I'd really like to know what happens to the flow in this area.

I think speeds of 50, 100, 150, and 200 mph would give us enough data

Mr. Bites,

We have done tests on the scoop used on the JBL. It is a big ugly thing. Look here. JBL hood scoop

We did do a flow path test using oil drops to show patterns. (Please note: Oil paths are much easier to observe in real world testing than tuft testing.) Unfortunately. I do not have any photos of the results. Sorry, as photos would show the reality of what is going on with the flow.

There were a couple of test parameters that need explaining:

1. Open bottom (No “Turkey pan”)
2. Closed bottom (With turkey pan.)

The results of the test were as follows.

Case 1. Flow: The oil streams were consistent and did not divulge from the expected flow path. The pressure at the opening was (at 100 mph) 9 inches of water. ( .3251 psi) This was no doubt the internal pressure of the engine compartment. Temperature was 87f. Ambient was 71f.

Case 2. The oil streams were not consistent. Significant divergence was found at the annulus and evidence of stalling and boundary separation was apparent. The pressure at the opening was (at 100 mph) 11 inches of water. (.3974 psi) This was obviously due to flow separation and boundary layer effects due to closed annulus. Temperature was 73f. Ambient was 72f.

From these tests, it is obvious that the only real benefit is temperature. Ram air effect is negated by the flow properties and stalling of the flow at the annulus.

There are also a number of problems with boundary layer effects and the curvature of the front section. It would be quite easy to fix these problems, but then it would even less like a Cobra then the JBL does now.

Therefore, things were left as is.

Note: we did test the small scoop configuration as well. This unit provided no flow enhancement. It actually reduced pressure at the annulus. This was due to boundary layer effects. In other words, at 100 mph, the flow is over the unit due to thickness of the boundary layer, therefore giving a net reduction in pressure.

That’s all I know.

[Ant]

Richard,

With Case 2, maybe using a large style tray semi sealed around hood scoop, with some exhaust capabilities/relief ducts off rear of tray, but I guess with the turkey pan there is a pressure build up affecting the air path? I read on Cobra Mustangs they fit a similar forward facing intake sealed system, with a relief valve exhaust that helps flow through!?!?

This seems to be a lot of work and nowadays they seem to have this problem solved by keep the air intake totally separate with eg 4" tubing and relocation of air filters in front of the radiator and you have your cold air system!

[Edley Rondinone]

I just hope my annulus never has "flow" problems!!! lol !!!

[Excaliber]

he he,,,,,,that was funny Edley.

Very informative Richard, I always appreciate your expert analysis of a problem!

Ernie

[aumoore]

Richard
Come on now Edley was just making a joke and it was actually kind of funny.

Now that I have said that could you translate that for us non-engineers?

[Wayne Maybury]

One thing I have often wondered about this subject of air filter size is the following. When a formula is used to calculate the required filter size, is it referring to the size of the opening, that is, the height times the circumference of the filter container OR is it referring to the actual surface area of the filter element which is much greater due to it's flutted shape?

Has anyone ever run their car on the 1/4 mile with and without the air filter? If so, did it make a difference?

Wayne

[G-Pete]

I read this thread from the first to the last post, what I see is a bunch of guys try to explain and calculate fluid dynamics.

What I don't see, are any real numbers - like dyno sheets. With or without turkey pan / larger or smaller filter.

To bust some of the bubbles here, the efficiency of an air filter is altered by 25% in the first 6 month (not in Nevada or West Texas - more like 35%).

Many numbers swirling around about CFM and RPM. All manufacture numbers are based on a stoichiometric air–fuel mixture. Well, most carburetor guys just can dream of it. So, take 25% of the number and let this the base of the calculations.

A good motor performance is not created on the internet nor just on paper. Many "internet engineers" don't realize that. They even believe Wikipedia is correct in all fields.
Trial and error is the key, so if someone would provide some numbers here to make this thread more substantial.

Thanks

[davids2toys]

I agree with you to a point, but as a start in the right direction, the airflow is what it is. So if my K+N filter wont flow more than 450 cfm brand new(per K+N tech dept), and engine needs 750cfm to perform to its intended potential, at this point it is not rocket science!
I love the look of the turkey pan and I have the biggest filter (height and width) I could possibly run with the turkey pan. I will be going with the spiral flow exhaust this year which will flow much more than my present setup, this will further exaggerate the starving for air in high performance situations. I will most probably go to a cool looking ram-box setup and a dyno service to get it all perfect!
Dave

[maxrpm]

I picked up 1-2 tenths in et and approx 2-3mph by sealing the scoop to the carb. But figure in that I also upped the jetting 2 sizes at the same time in anticipation of more air. I put the air cleaner on for a pass & dropped 2mph, my air cleaner is the small round 9"x2". I'm in the process of fabricating a filter housing for installing a 9" xtreme flow K&N filter in there, I'm a firm believer in the K&N after experimenting with it in my modified Dodge Cummins dualie. I'm not thrilled about running no filter at the track.

[flipper35]

Richard, I heard the Challenger T/A has its scoop up off the hood because of the boundry layer. It's modeled after the scoop on the belly of a P-51 for those that don't remember what they look like.

Even if the scoop were up in to the moving air, how much would it help due to ram air?