Need an explanation of hot air and why I am full of it

"PartyPooper2012" (PartyPooper2012) 07/06/2017 at 09:42 • Filed to: None

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As we all know hot air rises. That being said, cold air is denser and lower than hot air. Cars like colder denser air. Makes them go more better.

Why then do vehicles suck air from above and blow it out below? Air intake is usually near the top of the engine and exhaust from headers on is below the engine.

Wouldn’t it make more sense to suck air from below the car and blow it out from above the car. Point headers up at the hood and blow it out that way?

I understand the whole - it doesn’t look pretty and gets in the way of seeing things on the hood. I got it. But people who mod cars for performance - wouldn’t it make things more efficient?

RamblinRover Luxury-Yacht > PartyPooper2012 07/06/2017 at 09:51

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The difference in temperature between air closer to the ground and at top-of-hood height is basically nonexistent. Worse, get too close to the ground, and you could suck air *across* the hot pavement and make it hotter. Also bad, putting hot air at windshield height and heating the passenger compartment for you or others.

In short.... NOOOOOOOOOOOO

random001 > PartyPooper2012 07/06/2017 at 09:52

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PartyPooper2012 > random001 07/06/2017 at 09:53

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That’s not how any of it works....for now...

Demon-Xanth knows how to operate a street. > PartyPooper2012 07/06/2017 at 09:53

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Most actually do pull it in from an area in front of the radiator where there is a high pressure area. Cars with hood scoops and cowl induction draw air from there because the windshield stacks air up. The reason exhaust goes down is due to the strong desire to have the fumes exit behind the driver for breathing reasons. (note the gas mask that were required for vintage dragsters). There ARE actually reverse flow engines where the exhaust is on the top and the intake on the bottom, though these are uncommon due to packaging and heat buildup reasons. The cats and such that are large take up a fair bit of space and the plumbing becomes more tricky.

Full of the sound of the Gran Fury, signifying nothing. > PartyPooper2012 07/06/2017 at 09:53

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A couple of years ago a portion of the local freeway was flooded. Everybody was gingerly driving around the water, but I thought that they were being pussies and so I drove through the shallow end of what was becoming a lake. Bad idea. The car stumbled and sputtered and died. I got it restarted, albeit in limp-home mode; thankfully I was only a couple of miles from home.

Throwing the air filter in the oven overnight to dry it out and resetting the CEL got everything back to normal. And now I have a greater appreciation of the tortured path air takes to get into the engine and how the airbox does a surprisingly good job of preventing undesirable things, like a few hundred gallons of water, from ending up in the cylinders. I may modify my exhaust system, albeit only to increase the noise, but shall leave the intake tract alone.

random001 > PartyPooper2012 07/06/2017 at 09:54

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......

Well played.

RamblinRover Luxury-Yacht > RamblinRover Luxury-Yacht 07/06/2017 at 09:54

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Also - more foreign matter in air lower down, particularly water, and if there’s more exhaust piping under the hood (which routing exhaust up does, if you’re not dumping it on the windshield), it’s harder to keep from dumping the heat under the hood where it doesn’t belong. Also also, hot matter needs cold matter to react against with bouyancy to rise - if you can find cold air in the exhaust being helpful like that at exhaust flow rates for gases, get back to me because you might be a wizard.

Bman76 (hates WS6 hoods, is on his phone and has 4 burners now) > PartyPooper2012 07/06/2017 at 09:54

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And when you go through a puddle... instant water/meth injection... genius!

/s

Nibbles > PartyPooper2012 07/06/2017 at 09:54

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Ambient / “warm” air is more efficient for everyday drivers. It’s less dense. Less fuel required to burn.

OPPOsaurus WRX > PartyPooper2012 07/06/2017 at 09:54

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if u pull from the bottom, every little puddle might as well be a lake

PartyPooper2012 > Demon-Xanth knows how to operate a street. 07/06/2017 at 09:55

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Then there is also the case for cars with engines in the back. That eliminates the heat and toxic gasses. I wonder if a car sucking air from below and blowing it above with engine in the back would perform better.

Rustholes-Are-Weight-Reduction > PartyPooper2012 07/06/2017 at 09:56

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The difference is temperature is insignificant over that height difference. Also if you suck up air from the ground, you would probably have to change your air filter more often

random001 > PartyPooper2012 07/06/2017 at 09:57

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Here’s a short answer. The intake manifold is usually pretty well insulated, and even if it weren’t, the air is moving through it so fast with such a small volume contacting a hot surface that it doesn’t appreciably heat unless there’s a design flaw that allows heat soak, though this is more prevalent on FI engines. Usually the air is ducted from a cooler intake, and that ABS ducting doesn’t transfer heat very well.

As for the exhaust, the point is to make it as far from the intake as possible, and have the shortest run possible to the CC to light it off, and that air gets out the back, which can help reduce wake drag. You don’t want it in the front, you want it far away.

PartyPooper2012 > OPPOsaurus WRX 07/06/2017 at 09:57

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Surely there is a way around it. Heat the water with very high electrical current. Cause H2O bond to break and you get pure oxygen. Boom. It works better.

Might be slightly combustible and might require crazy amounts of electrical current... but those are minor details

PartyPooper2012 > Nibbles 07/06/2017 at 10:00

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mmhmmm... this is why they sell cold air intakes that suck ambient air through a pipe and call it cold air intake.

PartyPooper2012 > Bman76 (hates WS6 hoods, is on his phone and has 4 burners now) 07/06/2017 at 10:01

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Water could be used to cool off air going in. Just need to catch it and eject it before it ruins the engine

Demon-Xanth knows how to operate a street. > PartyPooper2012 07/06/2017 at 10:03

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As a Corvair owner whom has done work on the engine, I’ve actually thought about that. The issues that arise are that it requires drawing dirty, turbulent air from low down compared to much cleaner air. As well as not having to deal with issues such as road spray and snow. With today’s engines and safety requirements placing the engine rearward increases many design compromises as during an accident the heavy lump wants to go through the passenger compartment as well as limiting storage space since a rear area is much easier to open up than the front which already has steering gear taking up space. See the Clio V6 trunk as an example.

PartyPooper2012 > Full of the sound of the Gran Fury, signifying nothing. 07/06/2017 at 10:03

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Been there. Done that. Needed a tractor to pull the car out of a river.

I get the water danger. especially in flooding roadway. I am talking about average day driving. I think maybe cars could benefit from having air from below and exhaust out above the car.

Nibbles > PartyPooper2012 07/06/2017 at 10:04

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yarp

Smallbear wants a modern Syclone, local Maple Leafs spammer > PartyPooper2012 07/06/2017 at 10:06

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#1, the difference isn’t going to be huge in any case. Not worth pursuing for 99.99% of people.

#2, if anything it will be hotter closer to the road because it’s getting baked by the sun all day... as anyone who’s walked across asphalt barefoot in the middle of summer can tell you. It’s heating the air immediately above it the most. Go a little higher and the heat dissipates.

#3, many cars do suck from below, but the filter box is still up the top for packaging and serviceability reasons. Those that don’t, often have forward-facing inlets above the rad taking advantage of a little ram-air effect. That’ll make more impact than the very slight, if any, density change will.

#4, on the exhaust side, “hot air rises” isn’t what you need to be thinking of, it’s “less dense stuff rises above dense stuff”. All that’s in the exhaust system is hot exhaust. It has nothing to rise above or fall beneath. It’s also under lots of pressure due to it’s own expansion and the action of the pistons, so it’s coming out regardless. At first glance, I could see the direction of the tips (the angle of the rest of the system isn’t going to do anything) making a difference, as the thinner air from the exhaust would be fighting the denser air in a “turn-down” exhaust, but reality is that’s not pure air coming out of there. You’ve got lots of heavier particles as well, so while an up-turned tip might get rid of the air you’d be making it harder for the heavier stuff. Best thing to do imo is straight out the back, but it won’t make much difference either way. Look at cars on cold days... the exhaust more or less sits at the level it came out. It’ll rise a little bit, even with the surrounding air at it’s densest. On warm days it ain’t gonna rise at all unless the air currents make it.

On another note, can we agree on the idiocy of the pretty-looking “cold air intakes” that do nothing but suck underhood air?

Demon-Xanth knows how to operate a street. > Smallbear wants a modern Syclone, local Maple Leafs spammer 07/06/2017 at 10:08

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You mean hot air intakes?

If only EssExTee could be so grossly incandescent > PartyPooper2012 07/06/2017 at 10:09

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The <2ft altitude difference between mounting the intake low or high is not going to have an appreciable temperature difference.

Full of the sound of the Gran Fury, signifying nothing. > PartyPooper2012 07/06/2017 at 10:11

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Well, my Mazda’s air intake is actually quite low in the engine compartment, pulling in from less than 12 inches from the ground.

I would guess that the straightness of the exhaust system is better for performance in that it has fewer obstructions to restrict flow. Other than off-road vehicles that may get submerged, I’m not sure what the benefits of a high-mounted exhaust system would be...

PartyPooper2012 > Smallbear wants a modern Syclone, local Maple Leafs spammer 07/06/2017 at 10:11

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this idiocy is what made me think of this in the first place.

If you gonna suck air just like a regular air intake does, call it air intake.

If you gonna suck air and cool it somehow, then call it cold air intake.

PartyPooper2012 > Full of the sound of the Gran Fury, signifying nothing. 07/06/2017 at 10:14

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What I know is that the exhaust is much hotter than ambient temperature for the most parts of the world. Maybe we can leave the intake where it is, but maybe blowing the exhaust out around the hood might make it more efficient. Tho some suggest heavy particles in exhaust might have more of a difficult time getting out, I have doubts that the force of exhaust won’t be enough to blow it out.

After all, 18 wheelers have exhausts pointing up anyway. They have no issues.

Future next gen S2000 owner > PartyPooper2012 07/06/2017 at 10:16

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Yes hot air rises and cold air is denser. Unless you have a house or an enclosed structure where the mass flow rates in and out are very low relative to the volume you need big swings in elevation to notice the air temperature difference. The ground isn’t significantly colder than a tree house but you will notice a difference between the base of a ski lift and the top of the mountain.

Any difference you might see be lowering the intake and raising the exhaust aren’t greater than packaging constraints. Think about this, if you place to thermometers close to each other but spaced about 2 feet vertically apart, they won’t register different temperatures.

As for the exhaust, the flow characteristics (ie velocity, pressure, path......) are more important than going up because the exhaust is warm.

Passengers draw in air where it is convenient and where it won’t suck in water. Along with packaging constraints, that dictates the intake and exhaust layout.

Smallbear wants a modern Syclone, local Maple Leafs spammer > Demon-Xanth knows how to operate a street. 07/06/2017 at 10:18

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Exactly... hence the “”

PartyPooper2012 > Future next gen S2000 owner 07/06/2017 at 10:21

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Yeah. I guess the temp diff isn’t significant so maybe we can leave the air intake where it is. however blowing the exhaust out of the engine could be improved.

Some suggested toxic gasses and heat build up - but both can be remedied with engines in rear.

I am just wondering if same engine running with headers pointing conventional way vs headers pointed up... which engine would run better?

Demon-Xanth knows how to operate a street. > Smallbear wants a modern Syclone, local Maple Leafs spammer 07/06/2017 at 10:22

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I will say that in most of the circles I’ve been to, when people refer to it as a cold air intake they spent time making duct work to get air from outside of the bay. But the one pictured, that air is straight from the radiator and exhaust manifold. You’d be hard pressed to find a worse filter location.

Smallbear wants a modern Syclone, local Maple Leafs spammer > PartyPooper2012 07/06/2017 at 10:27

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I prefer the name “performance intake”... but yeah. I mean, an intake could be located to get cooler airflow, but in doing so you’re really just moving it somewhere that it’ll suck less underhood air.

Things that DO make a difference:

- Smoother tubing with less restrictions.

- Bigger inlet. Falls into the same category as above.

- Relocating the inlet for more outside air.

- Relocating the inlet for ram-air effect.

- Insulating the intake system to keep the air from picking up so much heat from the engine.

Even still in many applications a good chunk of these will be little more than grasping at straws... and it’s pretty easy to make things worse. You don’t know, without testing, how much effort the engineers put in on a given component. Unless something is a blatantly obvious pain point chances are you’d f something up by changing it.

Full of the sound of the Gran Fury, signifying nothing. > PartyPooper2012 07/06/2017 at 10:37

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I would think that you’d want the engine exhaust as far away from the cabin air intake as possible, and as such, exiting from the hood sounds like a bad idea if you want to keep the occupants alive. There are probably laws that prevent this as well.

Obviously a vehicle like this isn’t bothering with many of the laws that the manufacturers have to deal with.

I think that with large trucks it’s about efficient use of space, good flow and minimizing OEM parts inventories. With the differential(s) and fuel tanks (and APUs and other things) close to the frame rails you would have less space around the chassis to run the exhaust close to the ground in a simple fashion. Putting the exhaust pipes up high at the back of the cab is just a easy way to used otherwise wasted space. Sure, you could design an exhaust system that routes around everything in the chassis, but with the myriad options on a modern large truck it just makes more sense to have a more basic setup with a minimal number of parts in inventory. And then there’s the ease of servicing an externally mounted exhaust at the back of the cab.

Anyone can design a system that is complex, but it takes a great deal of skill to design something simple, something I learned in school. Or, as Leonardo da Vinci said, “Simplicity is the ultimate sophistication”.

Future next gen S2000 owner > PartyPooper2012 07/06/2017 at 10:38

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I am just wondering if same engine running with headers pointing conventional way vs headers pointed up... which engine would run better?

That depends on the flow characteristics of the exhaust routing, which is heavily determined by packaging requirements.

jimz > PartyPooper2012 07/06/2017 at 10:40

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Wouldn’t it make more sense to suck air from below the car and blow it out from above the car.

your talking about a difference in elevation of maybe 18 inches. that doesn’t amount to a hill of beans as far as air density and temperature go. Also, a low-mounted air intake puts the engine at a far higher risk of hydrolocking in wet weather.

as for “hot air intakes,” that’s just people who don’t know any better. If you look at the vast majority of OEM intakes, the airbox might be inside the engine compartment, but they’ll have a duct for the airbox to pull air in from either the fender well or just behind the grille.