I absolutely love hood scoops! Always have. They are the essence of muscle car cool. I’ll take a hood with non-functional scoops any day. But real, functional scoops? Now you’re talking!
I remember by youngest brother being with me in my ‘69 GTO Judge, and him saying “Now open the hood scoops!”. Of course, when I did, neither one of us thought we felt any difference. We shouldn’t have been surprised.
I should mention that the boundary layer effect is often talked about when the topic of muscle car hood scoops comes up. While it is true that air moving parallel and close to a surface (your car hood) will not move as freely as if the surface were not there, this really isn’t significant, for reasons that will become apparent shortly.
The “scoop” or “ram” effect of hood scoops is almost inconsequential, even with big scoops at high speed. Some of the Mopar scoops were huge (A12), while my GTO’s probably totaled less than ten square inches. How fast would you have to go to experience a detectable “ram” effect?
The ’68 -’70 Pontiac GTO had these attractive hood scoops. They weren’t usually functional, but were with the Ram Air engines. An under-dash knob opened and closed flaps in the scoops, when needed. It was an attractive and elegant setup.
Buick Gran Sport had these scoops on their ’70 -’73 models. Some may have questioned the low-to-the-hood design, but as you’ll know by the end if this article, that didn’t matter. They were undeniably way cool!
The Air Grabber hood was an ’11’ on the cool meter! A motor mounted on the underside of the hood raised and lowered the scoop.
The Ford hood scoop design was simple yet cool. This was of the ‘shaker’ style that was fastened to the engine itself, and it would rock with the engine.
How Effective?
Probably the best way to illustrate this concept is with a drag car that has a large scoop and is capable of speeds far greater than most street cars. We’ll say our dragster has 61 in2 of scoop area, which is far larger than most muscle cars had. We have to assume an engine air consumption, which we’ll put at 3,400 cfm.
The Critical Scoop Speed (CSS) is the speed at which the volume of air scooped equals the volume of air consumed by the engine. Obviously, this value will vary by engine speed. Said another way, the faster the engine is turning, the faster the car (scoop) must move to equal the volume of air the engine is consuming. The point with this is that at speeds below the CSS, there is no significant ram effect. The vehicle must exceed this speed to start experiencing a ram effect.
The following is a formula to calculate CSS, given engine CFM.
MPH = (CFM x 1.64)/in2, where in2 = scoop area
I hate formulas that have constants, like the ‘1.64’, above. This is a shorthand, but it doesn’t help me to understand what goes into the calculations. So . . . 1.64 = (in2/ft2) x [(min/hr.)/(ft/mile)]
in2/ft2 = 144, min/hr. = 60, and ft/mile = 5,280; So, 144 x (60/5,280) = 1.64 (rounded)
Putting the values for our race car into the formula:
MPH = (3,400 x 1.64)/61 = 90mph
So, at 3,400 CFM, 90mph is the CSS, where the air consumed equals the air supplied by the scoop.
Well, that’s great. But, our drag car can go a lot faster than 90mph. What if we’re toward the end of our run, doing, say, 200mph; what’s the ram effect then? To calculate this, we’re going to picture the volume of air that the scoop will take in, per minute, when traveling at 200mph. This will result in a figurative horizontal “column” of air that has the cross-sectional area the same as the scoop’s and whose length is the distance traveled (here, at 200mph) in one minute.
The cross-sectional area is already defined as 61in2. We divide by 144 (12 x 12) to convert to ft2. Our value is 0.42ft2. At 200mph, our column of air has the length of (200 x 5,280)/60 = 17,600ft. (This is just feet per minute.) We then multiply by the cross-sectional area (in ft2), and 0.42 x 17,600 equals 7,400cfm.
We then divide by the air consumed by our engine during one minute, and get 7,400/3,400 = 2.18. This value is in atmospheres, each atmosphere being equal to 14.7lb/in2 (at sea level). We’re going to ignore any slight adjustment for altitude. So, in addition to the always present 14.7lb/in2 of our atmosphere, we are providing via our scoop and speed, an additional 2.18 atmospheres, or about 32psi. Unfortunately, there are factors that will serve to reduce this boost in real-world situations.
The point is, scoops can be beneficial to race cars, at least at some portions of their race. For much smaller scoops, and much lower speeds, the hood scoops on a muscle car aren’t going to provide any significant ram effect. (See why the boundary layer effect isn’t a concern?) This isn’t the end of the story, though. (Have I mentioned how cool hood scoops are?)
It’s Cold, Baby!
The second thing that everyone already knows about hood scoops is they provide air that’s not been heated by the engine and radiator. That has to be a value, but how much of a value? The table that follows will give you an idea of the relative air density changes at higher temperatures. Just remember that any temperature difference between ambient and under-hood will be minimized by the time the air gets to the combustion chamber, as it will pick up heat along the way. It may still be a significant difference, though.
Of course, denser air equates to “more air”, and this won’t be of much value unless your fuel system can supply the additional fuel needed to achieve the desired A/F ratio. So, can yours? If you’re running a carburetor, the answer is “no”, not very well. If you’re engine is fuel injected, with a MAP system, the same answer applies. However, with fuel injection and a MAF system, the answer is yes, it will be able to sense the difference in air density, something the other systems can’t do. If necessary, slight changes to carb jetting can compensate for the in-taking of cooler air. One writer had an early ‘80s gen III Trans Am and did a before/after measure over a quarter mile. He first ran stock, then he opened up the non-functional hood scoop. He picked up 0.2 seconds just with this simple modification.
Aftermarket Cold Air Intakes and Air Filters
Modern cars pull air from the front fender area and send it through a rectangular filter, mounted in an under-hood filter box. There are sometimes devices intended to attenuate certain frequencies of noise emitted from the input tract. The removal of these likely won’t provide any advantages, other than to make your intake noisier, which you might like.
It’s my belief that 99% of the time, these factory systems are best left alone and that there is really little benefit to be had by replacing them with a larger, after-market type. There might even be some risk.
The aftermarket cold in intakes won’t guarantee that your mass airflow sensor (MAF) is receiving the laminar airflow that it needs to accurately measure the volume of air flowing through it. Laminar airflow is when the air is flowing in the same direction, or in “layers”. An aftermarket system could possibly compromise this.
The only valid reason for replacing the factory setup is to address a (significant) obstruction to airflow. Is this system a “significant obstruction” to airflow? No, it’s not. Pure and simple. If you have cash to spend on your engine, there is little bang for the buck here!
What’s even worse is when someone replaces their factory cold air intake with one that has a cone filter that’s pulling hot air from the engine compartment. This is actually providing lowered performance, for the dubious benefit of “looking cool”. It may look cool, but it’s not providing the cool air your engine wants. (A little play on words there…) Don’t do it!
This looks “cool” but now the engine is breathing hot under-hood air. Before this dubious mod, it was taking in cool air from the wheel well. (Isn’t the idea of modifications to improve performance? This stuff is so hard to keep straight!)
Note that an aftermarket cold air intake looks like this picture, but there is a box around the filter, that mates to the wheel well opening, much like the OEM unit.
The classic Trans Am hood, first seen in 1970, was intended to pull in cold air from the base of the windshield, though it was a bit forward placed for that. I always viewed this as an adaptation of the Chevy cowl induction. Early on, in 1973, Pontiac was forced to close the air intake. The feds determined that too much noise was emitted from the intake track under some conditions! Noise? Crap, we can’t have that! (Good thing we have those fat ole DC boys looking out for us all!)
Many air filters are kinda crap. They generally have way more element area than they should have, resulting in pleats that are too tightly packed to make use of the element area. I’d much, much rather have a reusable filter that I can clean and oil occasionally. You don’t need (or want) a HEPA filter here!
This might conceivably be worth a couple of Hp, as well, but the benefits lie elsewhere. If you ever have the chance to chassis dyno your car, you might remove the air filter entirely and see what the Hp difference is. If it’s not lost in the noise, it will be insignificant, and then only at high rpms.
I’ve used K&N filters for thirty years. The cleaning and re-oiling is simple and oddly satisfying. There’s something about washing the dirt off and spraying a new, clean film of oil on.
Note the metal mesh that holds the element in place. All of the element is exposed to airflow. Regarding airflow, I vividly remember reading a book back in the ‘80’s that made the statement that a dirty K&N flowed better than a typical new paper element filter. I’ve always believed it.
There are some newer air filters that have a cotton-type element that has pleats that are held open by the supporting structure. I would buy one of these, given the choice, over the pleated paper type. I ‘suspect’ that maybe some of the paper-element filters have so much paper, even though it’s forced into closed, touching pleats, to be able to advertise “X square inches” of filter media.
A Bit of History
You might be aware that paper element type air filters didn’t always exist. Before them, engines had oil bath air cleaners.
These were metal cylinders that contained an amount of engine oil in their lower part. They were designed so the incoming air had to make a hard U-turn right above the surface of the oil bath. Particles suspended in the airflow tended to not make the turn, but rather to continue on into the oil, where they were captured. This was fairly effective but required the occasional, messy, replacement of the old oil with new.
Around the 1940’s, many automakers were offering a buyer the choice of a “standard” element type filter or, at extra cost, an oil bath air cleaner. The oil bath cleaner was preferred in dirty and dusty areas, with its longer periods between service.
The element type wasn’t like the ones we’re used to today. The element itself was different, and it was required to be coated in oil. The owner therefore had to occasionally pull the filter, wash off the old oil and dirt, and then re-oil and reinstall the filter. (Sounds like a K&N filter …)
Oil bath air cleaners are still used, primarily in the industrial arena, with equipment that operates typically in a dirty environment (bulldozers, loaders, backhoes, graders, etc.)
This diagram shows how the airflow forces dirt and dust particles to separate from the air and embed in the surface of the oil. The heavier-than-air particles can’t follow the sharp turns of the flowing air.
I was recently addressing an issue with our vacuum cleaner, which didn’t seem to have the suction it ordinarily did. I located the offending hunk of animal hair that was stuck above the “cyclonic chamber” and all was well. (With two dogs and four cats, it’s a miracle this isn’t a weekly thing!) I’ve marveled at how well this thing can separate the majority of dirt from the air so effectively. It then occurred to me that this was working in much the same manner as an oil bath cleaner, in that the dirt-laden air is forced to make a U-turn, which the air makes effortlessly, but the dirt doesn’t. The dirt then ends up in a part of the chamber that is sealed, except for the input port. Pretty darn efficient!
The Take-Away
If you have an older car with a round air filter, I’d use a K&N type filter and leave it at that. Same for a newer car with a filter box and a rectangular element. Don’t mess with the factory cold air intake unless your engine is making an ungodly amount of horsepower relative to the stock engine. Even then, do not pull hot under-hood air into the engine. That is bad, very bad.
Appreciate hood scoops on classic cars, functional or not, and effective or not. If they are functional, they will allow your hi-po mill to breathe cool, fresh air. This is good, very good.
They were the essence of cool from an era of excess that will never, ever be equaled.
