Why are some car engines so loud?

[coconut62]

Why are the engines of sports cars so loud? I thought a good engine should be very quiet because all the work done must go to moving the car forward. If they produce so many noises, isn't that just wasting fuel/power?

Is the production of big sounds a must for the working of the engines, or the manufacturers just purposely make them noisy to make the drivers look cool?

 

[Simon Bridge]

The noise of the engine is wasted energy.
There is a limit to how efficient you can make an engine ... so there will always be a minimum percentage of power wasted as heat and noise. It follows that the more powerful the engine, at the same efficiency, the more noise it makes.

High efficiency is expensive - so cheap engines may be noisier than expensive ones.
Basically there are a lot of factors that contribute - the above is going to be an over-simplification.

Also look at the role of mufflers.

However - you are also correct: there are customers who prefer a throaty roar from their cars.

 

[jambaugh]

All engines would be loud except that mufflers are used to quiet them. However the muffler restricts the engine exhaust and thereby reduces performance. A sports car will typically have a "sports muffler" which is less effective at reducing noise for the sake of improved engine power. Also the engine power being higher will typically mean more "sound power" coming from the exhaust.

Your point about "noisy to make the drivers look cool" is apt as well. Given the "cod piece" status of muscle cars and the association of noise with engine power stated, there is "prestige" in cruising down the street in a ride which produces a power implying attention getting rumble.

 

[Jupiter6]

Your point about "noisy to make the drivers look cool" is apt as well. Given the "cod piece" status of muscle cars and the association of noise with engine power stated, there is "prestige" in cruising down the street in a ride which produces a power implying attention getting rumble.

I rarely see muscle cars on the street these days. What I do see are the front wheel drive imports with the flatulent resonators, the sole purpose of them being to make noise.

 

[Nugatory]

Is the production of big sounds a must for the working of the engines, or the manufacturers just purposely make them noisy to make the drivers look cool?

I rarely see muscle cars on the street these days. What I do see are the front wheel drive imports with the flatulent resonators, the sole purpose of them being to make noise.

Coconut, you are right that there is a lot of "I want to be cool" going here. If you want to know what really makes for a powerful and efficient motor, you should talk to the people who design, tune, and drive race cars. They care a lot more about what works than what sounds cool, and they even have a word for an exhaust system that makes noise not power - it's not "flatulent resonator" as Jupiter suggests, but he's on the right track.

It's easy to just bolt an exhaust on and get something that runs and makes noise, but entire books like this one have been written about how to do it right. For example, the gas flow is not even remotely uniform. Every time an exhaust valve opens a puff of high-pressure gas is released into the exhaust system, and this sets up pressure waves all throughout the exhaust system. Something similar happens on the intake side whenever an intake valve opens and air goes rushing into the newly opened cylinder. With careful design, we can arrange it so that at some RPM ranges a high pressure wave arrives at the intake just as that valve opens, and a low pressure wave arrives at the exhaust likewise - the effect is to greatly increase the efficiency and power of the motor at that RPM range. And because much of the noise is generated by these pressure waves, this sort of tuning can make a huge difference in the overall sound of the exhaust.

 

[jambaugh]

I rarely see muscle cars on the street these days. What I do see are the front wheel drive imports with the flatulent resonators, the sole purpose of them being to make noise.

Yes, as I said: cod pieces. The source in part of the "status" of that noise derives from its functional meaning for the muscle cars, just as with the cod piece imitating another form of functional endowment.

 

[A.T.]

Is the production of big sounds a must for the working of the engines, or the manufacturers just purposely make them noisy to make the drivers look cool?

Bikers are even worse:

https://www.youtube.com/watch?v=xGyKBFCd_u4

 

[Lsos]

If you think about what happens in a car engine, which is basically a bunch of explosions, then it's actually surprising that they aren't louder than they are. Car engines are pretty inefficient...probably something like 25% efficiency. With careful design you might be able to bump it up a few percent of efficiency, but that's still a lot of energy wasted as heat and noise.

Putting a muffler in the exhaust (and intake) streams obviously reduces this noise, but it's actually at the cost of even more efficiency, as it's a restriction to the air flow.

 

[Ranger Mike]

sound emmissions

Why are the engines of sports cars so loud?
Loud is relative. Compared to what? If you asking why a high performance street car is louder than a small commuter car, then here are some reasons.
All street cars must meet a maximum sound level limit dictated by local city ordinances and laws. Violators will be ticketed for anti noise violations so there is a maximum sound level permissible.
To answer your last question first, manufacturers will increase exhaust noise slightly over that of a production vehicle as a selling point but only to the point that internal ride comfort is not compromised and the overall noise level is well under the maximum permitted noise emissions.
Let us look at the physics of the internal combustion engine. We are converting chemical energy to mechanical energy to propel the automobile. We have a large air pump sucking in air and adding fuel ( gasoline most common) to make a combustible fuel/air mixture that is ignited and converted into linear force that is changes to rotary force thru mechanical advantage of design. One by product of the ignition of the fuel air mix is exhaust gases ( having considerable pressure), heat and sound energy.
In order to meet the local laws regarding noise emission, a muffler is placed in the exhaust system to reduce the amount of noise emitted. When this is done, the exhaust flow is restricted as is the overall performance of the engine. We are restricting flow of the spent fuel air mixture which means we are reducing the efficiency of the engine SOME WHAT but not to the point of serious detriment of efficiency. We are in effect “ wasting fuel/power” as you say. Now let us compare the “ Sports car” to the daily grocery getter.
The typical sports car has a higher weight to horsepower ratio compared to the little commuter box we see at the local Wal-Mart store. We may have a turbo charger added to the small 4 cylinder of we may have a 6 cylinder engine replacing the 4 cylinder. When you have a larger engine you have a larger sound emission problem ..it is that simple.
In some cases a dual exhaust system is used with two mufflers. Anyway that is my take on the matter for what it is worth.

 

[Danger]

Anyway that is my take on the matter for what it is worth.

I'd say that it's worth quite a lot given your history with cars.

Since Coconut wrote "If they produce so many noises...", and everyone has focused exclusively on exhaust, I'm just going to briefly mention some of the others. I refer here to Detroit Iron, not rice-rockets, because that's what I'm familiar with.
Lsos and, to a lesser degree, Nugatory touched upon the subject of intake, but didn't really address it. Most musclecars incorporate some sort of free-flowing intake such as open-element foam filters, velocity stacks, etc., which create a very noticeable "whoosh" and sometimes whistling somewhat similar to a vacuum cleaner. If the car is running a blower, you can multiply that effect significantly and add in the distinguishing whine of the gears and buzz of the drive pulleys (or turbo whine, if that's the way you swing).
Lots of "tuners" prefer mechanical lifters, which can make an annoying racket. Personally, I'm willing to forgo the potential performance increase in order to avoid the hassles of spending half of my life under the hood with a wrench. Anti-pump-up hydraulic units for this boy...
Timing equipment, be it double-roller chains and sprockets or dual-idler gear drives, contributes to the overall under-hood ambience, as do the various accessory drive belts and pulleys for such things as the alternator and water pump. Even airflow through the radiator can be noisy. Piston slap can be annoying, and can be good or bad depending upon the reason. (My Roadrunner sounds like a coffee can full of rocks because the pistons are mounted backwards for increased efficiency. It only adds about 35hp, but that's still worth the auditory assault.) There are more factors, but that covers the most predominant ones.
Some of those things increase the power of the engine, and some rob from it, but the robbing ones are necessary for streetability.

edit: I made a somewhat misleading statement regarding the pistons. The decrease in frictional losses due to them being installed backwards accounts for about 35hp. Because of their design, though, doing so also kicks the compression ratio up from 12 to 12.5. That's worth even more.

 

[jambaugh]

I rarely see muscle cars on the street these days. What I do see are the front wheel drive imports with the flatulent resonators, the sole purpose of them being to make noise.

Just to clarify, resonators are noise reduction devices. They however are tuned to have max effect when the engine is at at the higher RPM when the car is at competative speeds. Also more cylinders per displacement tends to even out the fundamental oscillating pressures which are the source of ehaust noise.

The trend toward smaller lighter and more efficient cars has really push the engineers hard to keep within acceptable noise levels. If you then demand sports performance (street legal) then you find quiet is the first thing sacrificed. It will be as close to the regulatory limits as the engineers can make it.

 

[Danger]

It will be as close to the regulatory limits as the engineers can make it.

And those regulations vary not only with jurisdiction, but also with the attitudes of enforcement personnel.
After my tooling around town for several months with straight pipes on the 'Runner, an RCMP officer (and I knew them all from serving them as a bartender) waved me over and said "Will you be putting mufflers on that thing any time soon?"
"Yeah, okay, if I have to."
Next day I purchased a pair of Baby Thrush header mufflers (they have flanges that bolt directly to the header output) and dropped the then unnecessary pipes so that the exits were directly beneath the front seat.
There was actually an increase in decibels, because the 4 metres of piping (per side) eliminated more sound than the straight-through Thrushes, but suddenly it was legal.

 

[Ranger Mike]

Good one, Danger...ah the days of glass packs...

 

[Danger]

Good one, Danger...ah the days of glass packs...

Believe it or not, I never ran those. Too many instances of them blowing out. Also, they were very expensive back then. Thrush mufflers, and Turbo Thrushes and Baby Thrushes, are pure steel. (Hmmm... I never thought of this, but maybe they aren't available outside of Canada. Thrush is a Montreal company. If you see a woodpecker smokin' on a huge cigar, that's the Thrush logo.) They last pretty much forever. In the case of Baby Thrushes, they sound about the same as Cherry Bomb glass packs. It might just be a sign that I watch too much TV, but I'm starting to become intrigued by MagnaFlows. I suspect that the prices won't be equitable, though. My first pair of Baby Thrushes cost me $20, back in '75. The next pair were the header-mounted Baby's that I bought after Bart pestered me, in '86 or so. I was shocked that they had gone up to $20 per muffler rather than $20 per pair. (All of those purchases were through different branches of Canadian Tire.)
All Thrushes had a neat feature that you in particular would appreciate. The endwalls were extremely flexible, almost as if they were made out of lead or plastic rather than steel. I don't know how they did that, and probably wouldn't understand it if it were explained to me (metallurgy is not my strong point). Anyhow, you just put the thing down on the floor, step on the muffler body to hold it down, and then bend the inlet and output tubes to whatever angle you want. The endwalls just flex to where they need to be and then stay there. There are also tube size adapters of every denomination included in the package. The material is crushable, to be sealed with U-Bolt clamps, or easily welded.
All in all, I think that they're one of the best Canuck inventions since the Arrow.

 

[coconut62]

Oh my, so many terms I've never seen before O_O

 

[Danger]

Oh my, so many terms I've never seen before O_O

If any of them confuse you, which would be quite understandable, we'll be glad to explain them. It's easy to fall into old jargon and forget that people unfamiliar with the subject won't know what we mean.
I'll start with a couple. A "blower" is a centrifugal supercharger, originally used for aerating mineshafts, that crams extra air/fuel mixture into the engine and is driven by a belt/pulley system from the crankshaft. That drive system, which looks way cool, is what distinguishes it from a turbocharger which is powered by exhaust gas. It isn't as efficient as a turbo, but has a brute-force approach to the matter that we really old gear-heads appreciate. You'll see them on Top Fuel dragsters and funnycars. That's what's sticking out of the hood of Vin Diesel's car in the movie "The Fast and the Furious". You can't sneak up on somebody with one, because they're louder than a raped banshee.
"Lifters" are the little cylindrical bits that ride up and down on the camshaft lobes to move the pushrods and thence activate the valves via the rocker arms. Hydraulic ones are self-adjusting using oil pressure. The older style are not, and require frequent adjustment by the driver/mechanic.

 

[OCR]

You're right, a "blower" is a centrifugal supercharger.

http://en.wikipedia.org/wiki/Centrifugal-type_supercharger

But... I'd bet my Hemi you're talking about...

http://en.wikipedia.org/wiki/Roots_type_supercharger

When they first started using them, they were called a "671"... lol

They came off a Detroit (Jimmy) by the same name.

http://en.wikipedia.org/wiki/Detroit_Diesel_Series_71



OCR

 

[Danger]

But... I'd bet my Hemi

You have a Hemi...?!
Adopt me! Put me in your will! I admit that I used to ignore you, but now I love you...

Indeed, I was a bit slack in my explanation, but I'm trying to ease a neophyte into the scene. To me, a blower is a 671 or maybe 871 (or a 471 if you have a Jeep, but that's like decaf) Roots supercharger. It must use a 3" Gilmer belt. (I saw one once that had a triple V-belt drive; it was just embarrassing.) The numerology, by the way, is that a 671 is capable of pressurizing 6 cylinders to 71 psi. You can extrapolate the rest.

edit: I just checked your profile. I can't believe that there is yet another member on here who's older than me. This is getting silly.

 

[Ranger Mike]

old??

is ok Danger...not many people alive today can describe Mr. Otto and his wonderful invention in my case, i had to keep after Edison on that light bulb thing.

 

[Danger]

i had to keep after Edison on that light bulb thing.

Well, I'm certainly glad that you talked him out of using hair for filaments. The stench when one of those things broke was horrendous...

 

[jim hardy]

671 blowers are positive displacement not centrifugal. Roots type as mentioned earlier..

http://members.shaw.ca/Luke76/Photos/Blower671-2.JPG
picture courtesy of this guy
http://www.evolutionmachining.com/i...6-71-blower-converting-for-small-block-chevy/

as to loud exhaust - the roar appeals to young men.
I had a 1956 Royal Enfield 700 twin with Dunstall megaphones. The kindly policeman said "Son , I had just stopped a guy for being a little bit too noisy. You embarrassed me from two blocks away..

 

[Danger]

Jim, the terms "centrifugal" and "positive displacement" are not mutually exclusive. A Roots is both. That would not be true if the air was squished between the rotors for acceleration, but it is in fact swept around the inside of the case.

 

[jim hardy]

Jim, the terms "centrifugal" and "positive displacement" are not mutually exclusive. A Roots is both. That would not be true if the air was squished between the rotors for acceleration, but it is in fact swept around the inside of the case.

Fair enough.

I thought it was sucked in by the separating lobes on top and squished out between approaching the lobes on the bottom.

http://www.sdrm.org/roster/diesel/emd/history/roots-1.jpg
http://www.sdrm.org/roster/diesel/emd/history/roots.html

Roots* blowers’ technology is built on a heritage of engineering and manufacturing expertise dating back to the Roots brothers’ discovery of the principle of the rotary positive displacement blower in 1854. Today, GE Oil & Gas’ Roots blowers are manufactured in a variety of bare shaft models with distinct features and in standard factory packages to meet an array of application requirements.

http://www.ge-energy.com/products_and_services/products/blowers/

 

[Danger]

I thought it was sucked in by the separating lobes on top and squished out between approaching the lobes on the bottom.

Sort of yes and sort of no, as shown in your diagram. A Roots isn't actually a compressor; it's just a funny looking fan. The air comes out at atmospheric (with a bit of heat expansion) if there's no resistance. The compression aspect arises when that air finds itself trapped in an intake manifold with all of its little molecular friends being crammed in on top of it. The rotors aren't about to give up just because the air is resisting (hence "positive displacement"), so the manifold and blower together form a compressor.
There is a variation of that, though. One of the newer models (perhaps an Eaton) uses tapered rotors that get narrower from intake to outlet, so there actually is internal compression with that design.
I think that perhaps this is just a terminology glitch.

 

[jim hardy]

Thanks Danger...
My study of pumps has been cursory, just enough for me as an EE to peacefully co-exist with genuine mechanical engineers. I was aware some pumps use a mix of centrifugal and positive displacement principles. It's always a shock when in school we're taught things are black or white and find out in the real world they're all shades of gray.When I was about fourteen I saw a 6-71 blower apart on a friend's father's workbench. I've been trying to figure out those rotors for fifty five years now !

Is that Eaton you mentioned what they used on Ford Supercoupe?

 

[Danger]

Is that Eaton you mentioned what they used on Ford Supercoupe?

I couldn't tell you. The only time that I ever saw it was on an episode of "Musclecar" or "Trucks" on the Speed channel several years ago. I'm not even sure that it was an Eaton. Might have been a new Cragar. I'll try to find out.
Also, keep in mind that I have no formal education whatsoever (all of my knowledge is from turning wrenches and stomping pedals to the wood), so I might be using the term "centrifugal" from my perspective as a professional writer rather than in the proper engineering context. I think that I'm right about it, but that might be a delusion.
I'll say for sure that I consider Ranger to be the best authority currently haunting PF, so I hope that he weighs into clarify the matter. There was another major gearhead around a few years ago, but he seems to have vanished.
As for the rotors, there is a major factor that doesn't show up in either the photo or the diagram. The major diameter of each lobe has a slit milled along it into which a strip of Teflon is inserted. Those strips act like piston rings to seal the rotors to the shell.

 

[SteamKing]

A Roots blower is one type of supercharger:

http://en.wikipedia.org/wiki/Roots-type_supercharger

Roots blowers can operate at a max. pressure ratio of about 2:1, meaning that an atmospheric intake can be compressed to an output of about 28 psi.

Roots blowers were used to provide supercharging and scavenging for two-cycle Detroit Diesel engines. The blower sizes '4-71', '6-71', etc. refer to the particular Detroit Diesel engine on which they could be fitted. (For DD engines, the first number is the number of cylinders, the second refers to the displacement per cyl. in cu. inches)

 

[Danger]

Roots blowers can operate at a max. pressure ratio of about 2:1, meaning that an atmospheric intake can be compressed to an output of about 28 psi.

The pulley ratio in the drive system is variable. In practical applications for street vehicles, there's usually a 20-30% underdrive relative to the crank speed. (I've seen them overdriven, but that's insane.)
Before vehicular use, and the numerical designations, they were used for pumping fresh air into mines and other industrial applications.

I saw, back in the early 70's, an AA/ funny car sneeze its blower about 10 metres into the run. The damned thing flew about 30 metres into the stands and almost killed a couple of spectators. I notice that they now encase them in flak jackets.

 

[mender]

Two main types of superchargers: positive displacement and centrifugal, the difference being the method of raising the pressure. You can get cross-overs or hybrids but typically for automotive use they are distinct groups.

The Roots is a example of a positive displacement air pump. The compressor part of a turbocharger is a centrifugal air pump. A turbocharger uses exhaust (via the turbine stage) to drive the centrifugal compressor stage, while a centrifugal supercharger uses belts or gears to drive the centrifugal compressor stage. A Paxton supercharger is an example of a belt-driven centrifugal supercharger.

 

[mender]

Roots blowers can operate at a max. pressure ratio of about 2:1, meaning that an atmospheric intake can be compressed to an output of about 28 psi.

Top Fuel runs them at about 55 psi boost. Max is around 75psi.