Technical No hood-open scoop air cleaner & af ratios

Been looking at my 31 tudor with a 440 tunnel ram and deciding on what type of air cleaner to run.

I've been leaning towards the surfer style scoop but then I started wondering if as speed increases and your packing the scoop with air does it have a tendency to lean out the a.f. ratio ?

Now I understand that as more air passes thru the carb it is sucking gas thru the ventura's but is there enough pressure at say 65 mph to actually lean out the mixture as I would think your raising the pressure of air, something like happens with a blower.

Also thought of the air cleaners with the side inlets but wondered if the air rushing down the sides of the motor/vehicle would create a vacuum/low pressure in the carb and again mess with the a.f. ratio ?

Now IF both cases are true I could see were town driving at lower speeds thus lower air pressure into the scope or less "vacuum" as the air passes the side inlet carb cleaner would not be such a problem.

Now why would this really matter ? Well suppose your driving a stick, you tool around town with the tach reading 1600 rpm as you run down main street in 2nd gear, then 20 minutes later your driving 60 mph in o.d. at the same 1600 rpm, will you have different a.f. ratios and is there enough difference in them to be a problem ?

So what's your thoughts on this gentlemen? Anyone ever install a meter to see if there is a difference? If so did you find anything of note ?

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I don't think about it, I just drive...no problems!

 

Driving through town, a scoop in any location is not working it.
The diagrams of wind tunnel tests, that I have viewed, show face forward pressure only at the lead hood edge, or high up off the body.
Rear facing scoops mid hood and back to the windshield benefit from the build up of air, at speed. The obvious cause being the windshield.

Aaww just reread the title... hmmm

 

No the carb is literally a calibrated metering device, it will give the appropriate amount of fuel regardless what's going on at the air cleaner unless it is not tuned correctly.

 

Not entirely correct, pressure does affect the calibration. That's why carbs need to be rejetted for different altitudes or even barometric changes if you are racing and need it to be spot on.

 

But carbs don't really need to be rejetted for different elevations....at least not in my experience. Idle speed, yeah.

 

A couple of our HA/GR racers have got frog mouth scoops on their V8 flattys and found that they ran shithouse the quicker they tried to go. They turned the scoop to the rear and they ran better.

 

Yes ever driven a station wagon with the back window open?

 

If the scoops face forward, air entering the scoops creates a positive pressure before the air enters the venture. Bernoulli's principle explains it. In the venturi, the air flow is restricted, and the pressure is reduced resulting in an air/fuel ratio that the system isn't designed for. With the scoops facing rear, there is a constant air flow. Air boxes in NASCAR type cars are designed for constant air flow.

 

Thank you gents, there is some of the situations posted above that have me curious, especially @alanp561 high pressure on the top of the carb.

I looked at the idea of it somewhat being like a blower with the high speed air being crammed into the scoop at hiway speed and trying to force it thru the carb, the difference I couldn't figure out is that with the scoop the pressure is built above the carb and a blower the pressure is built below the carb and whether it would matter either way.

And like @squirrel stated would it be enough to cause a problem.

Another point is that of @spanners as I've heard thats why Nascar and others face rearward on their air cleaners.

All good points and I do appreciate the thoughts and info shared !!

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Me and my buddies have been running sealed ram air scoops on big block Chevies and Chryslers for years on the street and strip and have never had an engine go lean on us. That only becomes a problem above 150 and still not enough of one to matter.

 

The pressure increase possible if you manage to convert all the airspeed into pressure at 200mph will produce a pressure increase of around 3%. This is for 200mph of undisturbed air flow, meaning very high up above the hood of a car, where the flow is undisturbed by the car itself, not a scoop just around hood level where the flow may be crap, turbulent and slow.
This changes with the square of the speed, i.e. at half the speed you get a quarter of that pressure, ~0,75%. Half again, 50mph, will produce practically nothing, along with all other legal road speeds. And as I said, that's with a great scoop in a good location, 99,99% of what you find on the street is nowhere near that good.

Actually, it is far easier to produce a vacuum when working with moving air, don't be surprised at all if a poorly designed scoop actually tries to suck air OUT of the carbs at high speed, or if turbulent flow disrupts the function of the carb.

Anyway. Don't most car carbs have the float chamber ventilation connected to the mouth of the carb? That would mean that any pressure changes at the inlet produces the same change where the fuel is, so higher air pressure is compensated by higher pressure on the fuel, forcing more fuel through the carb jets - and vice versa if the air pressure is reduced. Not perfect, but most of the time good enough to compensate for things like clogged air filters.

 

Been running carbs with scoops sticking through my hoods for many years, and never an issue or difference I can tell between around town and cruising down the freeways at 70 mph. I think this is just a waste of time to even worry over.

 

this may be something we are over thinking. Just run it if it leans out fix it.

 

I heard through my cousin's neighbor's brother-in-law's nephew, that an exposed scoop can pack so much air into the engine that it will actually aerolock and seize the engine, bring it to a complete and abrupt halt. I believe it 'cuz this guy has never been stumped on ANY scientific question: he even knows where Sasquatch lives, so I believe him!
Just run the damned thing and don't over think it! Put some window screen on it to keep the bugs out!

 

The boundary layer in airflow created by the radiator at 65MPH will likely be well above the scoop. 2nd, your scoop will likely "stall" the airflow when it fills up which essentially creates a stable inlet pressure anyways. It's only gonna eat what you give it. Ram Air systems feeding a dedicated box are pretty much the only way FORCED AIR AT SPEED actually works to change A/F ratio.

You asked the time, here's a clock...

 

The air cleaners themselves are most likely canceling out any kind of ram air effect, especially if using those tiny 2.25 x 6 inch elements that come with those scoops.
I have a couple cars with forward facing scoops and have never noticed a lean condition while driving at speed.
I have wondered if turbulence inside a scoop without air cleaners could cause a disrupting difference with the air trying to go down the throat at the rear carb vs the front carb though.

 

I run the coupe with scoop forward and guess what, It doesn't care!

 

Trans Ams ran the shaker backassward so I could watch the little flapper door open and close.

My olds has air scoops under the front bumper that do a fine job of picking up leaves and bits of straw that I have to remove from my filter so they must be moving air. On a rainy day, following a semi on the superslab they provide water injection as well.

 

I had one of those 70 Mopars with a functioning "grabber" hood scoop facing forward. It was pretty well centered on the hood that opened with the flip of a switch (around 3.5" -4" high and about 10" wide). The air filter housing was sealed to the under hood duct work with a rubber boot.

At (what the speedo said was) 100 mph, the scoop was pulling in enough air, it was trying to lift the hood. Both rear hood hinge corners of the hood were bent downward and the center of the hood at the cowl was about an inch above the cowl. I didn't notice any gap until the speedo read over 80 mph.

What that tells me was that the hood scoop was at least pulling in air at speeds over 80 (or whatever it may have really been), and there was more air in the scoop then the (383) motor could use (since it was lifting the hood enough to bend it), be it air filter restrictions, or carb restrictions, or actual motor use.
It is a fact the forward facing scoop mounted at the center of the hood actually was pulling in air at speed.

 

All good points above. And appreciated!

On the thought of boundary layers, that's why I was sure to make clear it was hood-less.

With a hood I could see that the effect would be lessened by the boundary layer, where as without the hood there isn't anything around the scoop to create a boundary layer.

Also I had considered the fact with as straight as the grille and shell is if the air would infact be deflected and straight flow into the scoop would be deflected, BUT, always a but, with no boundary layer produced by the hood would the air coming thru the radiator add to the pressure so that the grille and shell would be less of a concern as there is no air "stacking" inside the engine compartment.

I agree, it probably doesn't make a whole lot of difference and the world may never know for sure but it was just something I was curious about and wondered if anyone else had thoughts on it.

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Well... It says the under hood pressure was higher than the over hood pressure. There can be a lot of different factors involved there, high speed air flow over a surface can turn into something similar to an airplane wing, where you intentionally use the air speed to reduce the pressure.
It's a very interesting point, absolutely, but not necessarily proof of a high pressure under the hood - just higher than whatever there was above it.

Grab some tape and a bunch of short pieces of yarn, tape the yarn to whatever part you want to check air flow direction near, or to pieces of metal wire standing off a surface if you want to check the flow further off a solid surface. Take a drive and see how the yarns move with the air flow. Don't be surprised if it goes in the opposite direction of what you expected in quite a few places.

 

Did you miss the point the scoop lifted with a flip of a switch? The car could be driven with the scoop opened or closed. I often ran with the scoop closed if I was planning or driving fast, I didn't like to watch the hood bend. With the scoop closed, the hood remained flat well over 100 mph.
Gooogle "Air Grabber" on a 70 Plymouth.

 

Gene I was told of this myself by an older gentleman yrs ago, said he had the same problem. My boss told him it was an easy fix " just keep it under 90". The gent just laughed.

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Somewhere, there is some research as to the "ram" effect on the Ram Air Pontiacs from the late 1960's and early to mid-1970's.

I don't remember the specifics, and too lazy to look for the papers; but do remember that at street speeds any benefit from the ram air was in the mind of the owner. There WAS a benefit to the engine, however; as the air entering the scoop was cooler than the underhood air. This caused the necessity to jet the ram air carburetors differently than the non ram air versions.

Someone (Chevy? Olds?) called the scoop a cold air package, which was more accurate than ram air.

Jon

 

I guess I could be in trouble if I ever get this sucker rolling at high speed lol

 

I remember a magazine test back in the 70’s or 80’s where they used a container with fluid in it and a small clear hose taped to the hood in various locations. The way it worked, air was forced through the tube which forced the liquid in the container to rise. They moved the intake tube several times before deciding the best spot to put a scoop. This was all homemade stuff they used, there is equipment designed for this purpose but right now the correct name for it escapes me.

 

Shark Week! My wife says that means something in womanese? What's new? Cool scoop regardless!

 

Womanese eh lol

I actually cut the scoop apart and reshaped it to sit exactly where it is and to hold 3 inch tall breathers, I put a bigger blower on the truck and the damn thing is an inch taller than the old blower, so ally work went out the window.
2 steps forward and one back they say lol



And get this, over the weekend at a swap meet I bought a couple aluminum air cleaner spacers to replace the plastic ones I had on the J, damn things were a 1/2 inch taller and again lifted a scoop higher than I wanted it to be.
These scoops are for the birds and not the Peacock variety lol

 

Chrysler had several studies in their Direct Connection performance manual about air flow and boundary layers. A couple of their cars were noted to have scoops directly in the higher flow or where the layer diminished. I still think you'd see an area at least ½ taller than the existing hgt of the radiator "blocked" at speeds approaching 65+.