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Author Topic:   Noise of 2-stroke vs. 4-stroke
jimh posted 01-29-2003 07:05 PM ET (US)   Profile for jimh   Send Email to jimh  
For the past few days I have been cruising up and down the intracoastal waterway (ICW) in the vicinity of Ft. Lauderdale aboard a wonderful classic Whaler powered by twin 200-HP 2-stroke Mercury V-6 outboards. The following observation was noted several times:

While running on plane at about 25 MPH (the speed limit in most of the ICW in this area) we encountered similar size boats (25-feet) going in the opposite direction, also running on plane, and powered by new 4-stroke engines. Standing at the helm console of our boat, we could easily hear the noise of the 4-stroke engines over the noise of the 2-stroke engines just a few feet behind us.

If 4-stroke engines are supposed to be so remarkably quieter than 2-stroke, could someone please explain why we could hear them clearly at a distance of 100-feet or more, over the noise of our own engines just a few feet away?

Sound transmission decreases according to the square law rule. If one noise source is ten times farther away, its sound pressure level (SPL) decreases by a a factor of 100. If the two sources started at equal level, then for our ears the 4-stroke source should appear to be only 1/100th as loud as the "noisy" 2-stroke engines on our transom. We had no difficulty in hearing the 4-stroke engines as they went past us.

My suspicion is that at typical cruising speeds the 4-stroke engines are not especially quiter than well-designed 2-strokes of similar horsepower.

Any acoustic experts care to comment?

weekendwarrior posted 01-29-2003 07:29 PM ET (US)     Profile for weekendwarrior  Send Email to weekendwarrior     
I have a feeling it has a lot to do with the sound being "different". For example; when riding motocross on my 4-stroke (LOUD) bike, I can clearly hear quieter 2-strokes when the come up behind me. The reason is the sound their motors make is completely different than the sound my bike makes and and it stands out, even though it's quieter and behind me.

I mention this because I also do a lot of cruising in the ICW and I remember being surprised by how quiet boats with the new 225HP 4-strokes were when they pass me. Interesting that you noticed the opposite.

11 footer posted 01-29-2003 09:57 PM ET (US)     Profile for 11 footer  Send Email to 11 footer     
Hi Jim,
Sounds like you had a good trip. I could be whrong but could it "maby" have something to do with a eco across the water? Just a thought.


11 footer posted 01-29-2003 09:59 PM ET (US)     Profile for 11 footer  Send Email to 11 footer     
Hi Jim,
Sounds like you had a good trip. I could be whrong but could it "maby" have something to do with a eco across the water? Just a thought.


whalerron posted 01-29-2003 10:42 PM ET (US)     Profile for whalerron  Send Email to whalerron     
jimh, I have noticed the same thing. This past summer I noticed alot more 4S outboards on the water. At cruising speeds, those 4 strokes are every bit as loud as my new Johnson 90 2S. It could have something to do with the frequency of the exhaust noise. Low frequency noise carries much better than high frequency noise which is more easily attenuated. But to my ear, the 4 strokes and the 2 strokes sound about the same when it comes to outboards running a boat on plane.
kglinz posted 01-29-2003 11:46 PM ET (US)     Profile for kglinz  Send Email to kglinz     
I've got dBa numbers on 225 4 Strokes but none on 2 Strokes.
88 at helm 3000/4000 RPM
99 at helm WOT
89 at helm 3000/4000 RPM
97 at helm WOT
Mercs have different cowling and are supposed to be quieter.
I don't think these numbers mean much if compared to a 2 Stroke, unless it's on the same boat. I would think a 4 stroke would be quieter as it fires half as often as a 2 stroke at the same RPM,but maybe not. 22 years of playing with jet airplanes,and I can't tell anyway.
Kemp Lindsey
Florida15 posted 01-30-2003 12:59 PM ET (US)     Profile for Florida15  Send Email to Florida15     
Weekendwarrior, I think you got your bikes mixed up. Unless you have some special bike that I am unaware of,a 4 stroke dirt bike is a lot quieter than a two stroke. Maybe not as quick, but a lot quieter.
Bigshot posted 01-30-2003 01:40 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Bull dinky....I had an ATC 250SX with a DG pipe on it and that was freakin LOUD. Not the same loud as a YZ250 but louder and you heard that thing from miles away....drove me nuts. Same goes for boats. A chevy I/o with 4" exhaust is louder than any 2.5 EFI Merc. More of a thunder than a swarm of PCP taking bees.
kglinz posted 01-30-2003 02:05 PM ET (US)     Profile for kglinz  Send Email to kglinz     
2 Stroke bike engines are a whole different deal. The muffler is tuned to control the back pressure to the exhaust port for forcing the unburnt fuel - air mix to remain in/to return to the combustion chamber until the piston covers the exhaust port. Noise is not the big issue.
Take a 8 hp Briggs and Straton lawn mower and run it next to a 8 hp Honda and the Honda is much quieter. Its just the muffler. With all the different engine mounts and jack plates it's hard to judge sound.
Florida15 posted 01-30-2003 02:40 PM ET (US)     Profile for Florida15  Send Email to Florida15     
You can make a bike, or a boat engine for that matter, as loud or as quiet as you care to with the proper muffler. I'm comparing apples to apples.
All things being equal, a 4 stroke dirt bike is quieter than a 2 stroke.
weekendwarrior posted 01-30-2003 02:46 PM ET (US)     Profile for weekendwarrior  Send Email to weekendwarrior     

I think you're thinking of the traditional off road bikes (XR, KLX, etc..). I'm referring to closed-corse race bikes. You should check out the late model 4-stroke race bikes, they'll blow your mind. I would love to post more info but I don't want to drive us too far off topic. If you want to see some pictures check my site here (lets see if I can do this right):

Florida15 posted 01-30-2003 03:32 PM ET (US)     Profile for Florida15  Send Email to Florida15     
Weekendwarrior, you are correct. I am not familiar with the cutting edge racing bikes.
I was simply talking about what I was familiar with such as the TTRs and YZs.
Thanks for the website.
Taylor posted 01-30-2003 04:38 PM ET (US)     Profile for Taylor  Send Email to Taylor     
It seems to me that noise from outboards come from just three sources - exhaust, mechanical, intake.

Exhaust path strikes me as the same. Is it not true that all outboards lack a muffler, and instead use underwater exhaust to muffle the sound? Is there something intrisincly noisier about the exhaust comming from a two stroke vs a four stoke?

Mechanical sounds from the engine are mostly handled by cowl soundproofing, and newer engines seem to have been soundproofing that older ones, at least in my limited experience. But are two stokes less well balanced than four strokes and thus vibrate the boat more? That's the only possible difference I can see.

Intake noise seems to be the area of difference. Two strokes have an intake that goes through some kind of flapper device (what's it called?) into the lower end of the engine. Four strokes go through intake valves in the cylinder head. These differences seem significant. Of course intake noise is also impacted by the intake path, and devices in it like air filters and flame arrestors.

So, to get back to the topic, why would four stroke be intrinsicly quieter than two stroke, from a design standpoing, all other standard soundproofing techniques being equal?

kglinz posted 01-30-2003 05:00 PM ET (US)     Profile for kglinz  Send Email to kglinz     
The only thing I can see is at say 4000 RPM the 2 Stroke fires 4000 times per cylinder per minute. The 4 stroke 2000 times. On a 6 cyl thats 24000 times a minute on a 2 stroke or 12000 times on a 4 stroke. Does this mean a deference in dBa or just a different frequency. Is the actual noise level tied to the amount of fuel burned.
BillD posted 01-30-2003 05:03 PM ET (US)     Profile for BillD  Send Email to BillD     
No technical knowledge here, just my thoughts.

I road on a Maritime Skiff last year with a Honda 225 on the rear. At speed it was every bit as loud as a 2 stroke. At idle it was quieter.

I think 4 strokes have a deeper sound (do I mean pitch) than 2 strokes. This may change the way the engine sounds at higher revs.

And finally, I do not remember the name of the principle, but we all learned it in High School physics, where the sound of the siren on a police car changes when it is approaching you or moving away from you. Could this cause the engine sound to appear louder on any boat coming in the opposite direction?

Just a few thoughts.

Perry posted 01-30-2003 06:06 PM ET (US)     Profile for Perry  Send Email to Perry     
When my neighbor flushes his Evinrude 90 2 stroke in front of his house, I have to close my windows because of the noise . When I flush my Honda 90 4 stroke, it sounds like a Honda Civic is reving its engine. Huge difference.
Doug Weaver posted 01-30-2003 06:09 PM ET (US)     Profile for Doug Weaver  Send Email to Doug Weaver     
That's the doppler effect Bill. Sound waves are compressed as the source approaches you and then stretched as the source moves away from you. As for the 4-stroke being louder than the 2-stroke, my only experience is sitting on the transom of a large catamaran with twin Honda 130-HP's travelling at 30 knots or so and carrying on a conversation without needing to raise my voice. My Outrage has an 225-HP Yamaha that I can hardly hear myself think over sometimes. I have read in a boating magazine article that 4-strokes are comparably loud to 2-strokes at WOT. Maybe that guy in the ICW was just gettin' on it Jim and you guys were not.
Doug Weaver posted 01-30-2003 06:15 PM ET (US)     Profile for Doug Weaver  Send Email to Doug Weaver     
Another thought Jim. I have noticed that the degree to which a motor is trimmed will influence the sound level emitted by a boat's motor. Also, tight turns seem to allow some more of that exhaust noise to escape as the wake is altered and pushed to one side of the boat or another, leaving a vacuum if you will where the noise of the exhaust and gearcase are less in contact with the water and possibly emit sound to the air which would normally be muffled by the water. Or, maybe he just bought a nice set of cowlings and threw them on a pair of old mariners to mess with you;)
jimh posted 01-31-2003 11:37 AM ET (US)     Profile for jimh  Send Email to jimh     
These observations are unscientific, but they do seem to be counter to the conventional wisdom that 4-strokes are always MUCH more quiet than 2-strokes.

It could have been that the 2-stroke Mercury engines were loafing along at 2800 RPM to put the 25-foot Classic Whaler hull on plane, while the 4-strokes were being run much faster to keep the other boat running at a similar speed.

What I am hearing here (pardon the pun) is that at cruising speeds there is not much difference in Sound Pressure Level (SPL) between 2-stroke and 4-stroke engines.

Arch Autenreith posted 01-31-2003 11:47 AM ET (US)     Profile for Arch Autenreith  Send Email to Arch Autenreith     
If someone's driving the farm tractor in the field I can hear them talking above the dim roar of the engine. If I'm the one sitting on it I can't hear anyone talking to me.

Could this be a similar comparison/phenomenon?

T Party posted 01-31-2003 11:56 AM ET (US)     Profile for T Party    
I think that's right, Jim.

When I'm not on my Montauk (now deceased 2 stroke 1991 Johnson 88SPL), I'm frequently on my neighbor's 18' Scout with a 4 stroke Yamaha 115. At idle, his engine is barely audible. There's a major highway that runs across a bridge about 200' from his dock, and I cannot hear the motor over the passing traffic. On plane, his motor seems to me to be every bit as loud as mine was. As proof, he has a stereo and we occasionally listen to loud party music. You cannot hear his stereo over his motor when he's on plane, though it's plenty loud at idle. The timbre and tone is different, but the decibels are probably about the same.

Unscientific and anecdotal, I know, but that's my experience.

kglinz posted 01-31-2003 12:21 PM ET (US)     Profile for kglinz  Send Email to kglinz     
Of course you could always stoop so low as to buy one of those Sea Ray lookin Whalers, with the Euro Transon and the head. They're quieter at the helm.( Can't prove it, just an opinion )
where2 posted 01-31-2003 01:10 PM ET (US)     Profile for where2  Send Email to where2     
I believe if you reference any 2-stroke to 4-stroke comparisons, you'll find that above an idle, 4S do make nearly as much noise level as the comparable 2S. However, if the same 4S boat had passed you in an idle area, you wouldn't have heard him.
djahncke posted 02-02-2003 08:57 PM ET (US)     Profile for djahncke  Send Email to djahncke     
The human ear's sensitivity to sound drops off pretty quickly above several thousand Hertz. It could be that the 4 cycle engine produces a lower pitch sound at high speeds and as such is perceived by the human observer to be louder.
gss036 posted 02-03-2003 02:40 PM ET (US)     Profile for gss036  Send Email to gss036     
Could someone with some practical experience give an opinion. I would interested in hearing from them. I have heard several 4s inside the harbor at idle, but not on open water.
I think it would have something to with how the exhaust is funneled out through the prop and how far under water it is at the time.
I know my 2s changes sounds with the trim and I am assuming that is because it puts the noise farther behind the boat when it you are timmed up shallow.My 2 cents?
Bigshot posted 02-03-2003 03:32 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I have owned over 30 2 strokes from 4-225hp from Johnson, Evinrude, Mercury, Mariner, Yamaha, & Suzuki. The quietest 2 stroke was my 90hp Yamaha(other than say a 7.5hp OMC) but that thing was LOUD compared to my Zuki 4 stroke. At cruise it is not whisper quiet but with my face above the windshield all I hear is wind noise, can anyone here say the same for their 2 stroke?

As far as how it sounds when I pass someone....who cares, I passed them did'nt I? ;)

I remember when I had my 13' and a 35hp Johnson on it, I thought she was pretty quiet compared to my dad's 140 Suzuki. When I had this last 13' with the same vintage 25hp Johnson....I was shocked at how loud that thing was. Darn things sound like they are gonna throw a rod at any moment. Certain engines are louder than others and certain boats make the engine seem louder than it is on other boats due to many factors. All the people who have been on my Montauk ALWAYS comment on how quiet it is. Maybe Zukis are the most quiet but I would like to hear a 2 stroke that is quieter than it.....even at WOT.

newt posted 02-03-2003 04:13 PM ET (US)     Profile for newt  Send Email to newt     
Bigshot, I can say that at WOT the wind is louder than my Merc 90 2s.

Of course with all those 2s ponies, I probably go a lot faster than the 4 strokers. :)

Bigshot posted 02-03-2003 04:26 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I meant at cruise....even downwind.
hooter posted 02-03-2003 05:06 PM ET (US)     Profile for hooter    
This one's had me thinkin'. If you put two women back t'back and get both o'them talkin' at the same time, they's both goin' t'understand perfectly what each o'them is sayin', be able t'repeat it verbatim. But if you put a guy and his wife back t'back and just set her t'talkin', he cain't hear a thing she's sayin'. This is b'cause he cain't SEE her talkin', and he's thinkin' about his boat or about his car tire that needs changin' or about some bug he was lookin' at that mornin' when he picked up the newspaper. It's all in the line o'sight.

Followin' this along, jimh, when yer at the helm of yer boat, you cain't hear yer own engines as well as the boat goin' by, that yer LOOKIN' at, b'cause you cain't SEE yer own engines and you CAN see the other guy's engines.

It's childishly simple:-!

hooter posted 02-03-2003 05:27 PM ET (US)     Profile for hooter    
Forgot to make the followin' observation: ain't it remarkable that without a single learned opinion, jimh, yer still got moren'yer money's woith with ova two DOZEN replies t'yer question?
andygere posted 02-03-2003 07:22 PM ET (US)     Profile for andygere  Send Email to andygere     
I don't know about the rest of you, but I like the sound of outboard motors. When I have a fouled plug or a gummy carb, I hear it before I notice any other symptom. I for one, do not want an outboard that whispers to me all the time. I want one that speaks with a clear, confident voice!
TampaTom posted 02-03-2003 09:53 PM ET (US)     Profile for TampaTom  Send Email to TampaTom     
Loudness (by itself) is a subjective measurement (ambient noise, altitude, objects, distance, frequency, wave length, etc.) A 4 stroke frequency is lower than a 2 stroke.
Low frequencies, because of the big distance between the peaks and troughs of the wave... tend to go 'around' objects that are in their way. They can maintain shape and still bend. High frequencies are narrow, because of their higher power, and very tightly packed. They have the tendency to 'bounce' off of objects in their way. Foghorns have a very high amplitude, and a very very low frequency. That way they won't be blocked by ships or rocks in the water. It ALSO means that they tend to 'bend' along the curvature of the Earth.
jimh posted 02-03-2003 10:11 PM ET (US)     Profile for jimh  Send Email to jimh     
Sound Pressure Level (SPL) is not subjective. Perceived loudness as sensed by the human ear is subject to some influences, however, since we hear all sounds with the same ear, all the influences affect all the sounds. It is not like I listen to my engine with a different ear than I listen to the other guy's engine.

The ear's perception of sounds below about 400 Hz is diminished, thus the theory that the 4-stroke's lower pitched sound would be perceived as louder is in opposition to well-known facts about human hearing and the loudness curve of the human ear.

At 3000 RPM, a V-6 two stroke has 18,000 power strokes in a minute. There are 60-seconds in a minute, thus the major "note" of the engine would be
around 300-Hz.

At the same 3000 RPM a v-6 four-stroke has only 9,000 power strokes in a minutes or a major note of about 150-Hz.

We need to compare the path loss at these two frequencies to see if there is a significant distance over a short path like 100-feet.

lhg posted 02-04-2003 05:21 PM ET (US)     Profile for lhg    
I have always thought that outboard engine noise, at planing speed, at least the kind JimH is talking about, is the mechanical noise, coming from the engine powerhead. This seems to be related to engine RPM's, reed valves, type of induction (carbs being more noisy than EFI) and cowling design. Idle sound of conventional 2 strokes seems to be related to powerhead design and idle relief design, plus cowling design.

Then, on a 4-stroke, there a lot more moving parts, which is where the noise comes from at speed, anyway.

A few examples:

Recently I have been reading that Mercury's version of the Yamaha 225 4-stroke is quieter than Yamaha's own version. So this probably means Mercury has a superior cowling and air intake design. I also know that my twin 6 cylinder 115's are a LOT louder than my twin 6 cylinder 200's. The difference here is clearly cowling design and carb induction.

As for RPM's, we all know lower RPMS mean less mechanical noise. That's a major reason why I like to overpower a Whaler. Running in overdrive produces less engine noise than running in second gear.

As for the difference in carb induction vs EFI, I am convinced the EFI is lot quieter.
My 1989 Mercury 200's, with bucket one piece cowling, were MUCH noiser than than my 1997 EFI's, with basically same cowling design. The difference is HUGE, on same boat.

And Nick, this summer I drove a Montauk with a single Yamaha 90 on it. It was louder than the twin Merc EFI 200's on my 25! Seriously!
Admittedly, one is closer to the powerhead on a Montauk than on a bracketed 25.

newt posted 02-05-2003 11:59 AM ET (US)     Profile for newt  Send Email to newt     
No kidding, first time out with the new Merc 90 2s, at WOT, the wind noise completely drowned out the motor.

I remember thinking, "why all the fuss over engine noise, when it's the wind that drowns out conversation?"

Then again, I have 15 miles of smooth river to navigate to get to the ocean, so I spend a lot of time at WOT. (actually I run at around 5000 rpms to spare the engine a little).

Bigshot posted 02-05-2003 01:25 PM ET (US)     Profile for Bigshot  Send Email to Bigshot WOT you roughly shorten the life of an engine 90% to only 10%. At 75% of WOT you supposably get 90% of engine life.
Dr T posted 02-05-2003 02:03 PM ET (US)     Profile for Dr T  Send Email to Dr T     

The reason has to do with the way the sound propagates. In general, sound from a point source will propagate as a sphere only if there is nothing in the way to stop the wave propagation. From a directional source, the wave will tend to propagate as a cone in the primary direction of the horn.

Since you are sitting in the boat, not only is the noise pointed backwards through a directional horn (the prop hub), but also, you have the transom of the boat to block the propagation in your direction. You are within the cone of silence for the noise source.

[Some structures, like high volume industrial cooling vents are specifically built to direct noise away from specific areas (like a directional RF antenna). When I was at the University of Texas back in the 70's, the cooling fan for the tokamac in the mall out side the physics, math, and astronomy building was built to direct nois straight up. you could talk in a normal voice 5 feet away from it, but put your head over it and you would go deaf. I recall the noise cone hit the building at about the 9th or 10th floor.]

On the otherhand, the guys with the 4 stroke were shielded from their own noise. However, you were within their noise cone.

I suspect that they were making comments about your loud two strokes.

This same propagaion can be seen by taking a look at the wave propagation through a gap in a breakwater, for instance.


Whalerdan posted 02-05-2003 02:05 PM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
I agree with newt. The wind noise, at power, is louder than my 98 Evinrude 90hp.
gnr posted 02-05-2003 02:35 PM ET (US)     Profile for gnr  Send Email to gnr     
Some of the noise you heard coming from the "other" boat was actually the echos from your own engines (I know, I know, an unbelievably small, almost undectable amount, but some just the same).

I think it's pretty hard to draw any informed conclusions from the data you present. Way to many variables.

I do know that my 3.5 cubic inch 2S chainsaw is a heck of a lot louder then my 350cc 4S ATV at any rpm.

I also can easily pick out the 4S snowmobiles over the 2S snowmobiles as they pass my house about a quarter mile away.

jimh posted 02-06-2003 12:35 AM ET (US)     Profile for jimh  Send Email to jimh     
I like DrT's analysis, with the directive source accounting for the perceived loudness differences.

I also think sound carries downwind much better than upwind. Many times we have sat in the quiet cockpit of our sailboat having afternoon tea and listened to other boaters--usually husband and wife couples--who were trying to anchor in the same secluded cove. Shouting at each other over their local engine noise, their voices carried downwind to us with total clarity. I am sure they had no idea how clearly we could hear them, based on the content of some of their remarks. :-)

gnr posted 02-06-2003 08:42 AM ET (US)     Profile for gnr  Send Email to gnr     

Is that an emicon I see at the end of your last post?


Bigshot posted 02-06-2003 09:35 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Holy moly......Jim used a smiley!

One time(at bandcamp:) we were parked on a sandbar in NJ drinking a few coldies. The only boat around was across the waterway about 2-300ft and they were a young couple having sex. Anyway after checking them out wiith the binoculars we started telling stories about weird sexual stuff that we heard or read, etc. Later these people pulled anchor and parked next to us and told us we are really funny, etc. We did not reciprocate nor show them the binoculars.

Moral is be careful which way the wind is blowing because you might sound like you are sitting next to someone who is a football field away.

Dr T posted 02-06-2003 11:56 AM ET (US)     Profile for Dr T  Send Email to Dr T     
The wave sound carries over water is interesting. Often there is a cooler layer of air close to the surface (like a thermocline in water) and the boundary layer traps the sound between the surface of the water and the boundary layer. This acts as a wave guide for the sound.

Since sound propagates as a wave through the air mass, the flow in the air mass will move the sound wave with it in the direction of flow. The confinement to the boundary layer will conserve the sound energy, and so serve to effectively amplify the sound.

I suspect that Peeping Nick was downwind of the the couple. One can only guess what the couple were saying about their their audience.

Bigshot posted 02-06-2003 01:34 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I was upwind.
newt posted 02-06-2003 03:35 PM ET (US)     Profile for newt  Send Email to newt     
Bigshot, I have followed the discussions on engine life, but still cannot resist running hard!
Dr T posted 02-06-2003 03:41 PM ET (US)     Profile for Dr T  Send Email to Dr T     

Now that I read what I wrote, I should have said upwind. Your comments would be blowing in the wind to them, but their answers, my friend, would not be blowing in the wind to you (with apologies to Bob Dylan).


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