Small engine no back-pressure

[wlhequipment]

Howdy folks.
So, I had a conversation with a fellow wrench bender the other day, and it made me think a little. His assertion was that a small engine that is well tuned and in good working order (compression, timing etc), without any kind of header or exhaust manifold, does not need a muffler. That the back pressure created by the exhaust system is indeed what necessitates the muffler, and without it, the engine wouldn’t make a lot of noise. Upon hearing this, my initial response would be “BS” But, it made me think a little. At low rpm, and the perfect mixture, the flame front would be moving much faster than the piston, so exhaust gasses should be done burning before the exhaust valve opens. So when the valve does open, the bang is done, and the only noise you hear is mechanical noises of the engine. I was willing to give him that, but he wanted to sell me the same thing happens at high rpm, at which point I did say BS.

Obviously engines make more noise the harder you push em, but what about just idling a perfectly efficient and tuned engine? Wouldn’t it make almost no discernible ignition stroke noise? No “bang”?

 

[Welder Dave]

1. Depends on the engine and application. Some engines produce better power with some back pressure. On some 2 stroke engines the exhaust pipe/expansion chamber is extremely important in creating the necessary HP at the right RPM's. and a muffler/silencer can contribute as well. Have know idea how the "1." got on there and how to get rid of it.

 

[Delmer]

There is a certain modified version of the otto cycle engine that only compresses part of the cylinder displacement, so that the exhaust would be more fully expanded, and hence LESS pressurized when the exhaust opens. I don't think I've ever seen one, and especially not in a small engine, so your friend is full of it, completely. Any small engine I've seen has pressure left in the cylinder, hence noisy exhaust.

 

[Welder Dave]

There is a controlled explosion after all.

 

[HarleyHappy]

Well, if I want to make higher HP on my HD a 2 into 1 creates the best HP.
Open exhaust robs a lot of HP and torque.

 

[John C.]

Ever see the exhaust pipes on a small airplane engine or a top fuel dragster engine? I was told the only reasons that the pipes have a bend in them is to slow down the fresh air hitting the valve heads when the engine is shut down. I've heard the back pressure arguments for years. I've also seen plenty of cars and trucks that had the exhaust piping all increased in diameter in order to vent the gasses easier and faster.

On motor cycles, particularly two cycle engines, the exhaust pipe and "muffler" is made in such a way as to change the horsepower characteristics of the engine. We called them expansion chambers as they didn't muffle anything. I built a few trying to enhance torque in some and/or changing the horsepower curves in others.

 

[stinky64]

Yeah but 31 inch drag pipes with no baffle sound badass c'mon.

 

[DMiller]

Can attest to 2/1 on a HD 103 changes the torque curve, watched in on a HD Shop Dyno. Tested the bike prior, and then after swap, almost 10% increase in baseline torque with a slight 2HP drop. Either get HP or Torque, depends on needs.

 

[wlhequipment]

Back in the late '80s, maybe early '90s, Honda came out with a new technology for their dirtbikes. I'm a dirtbike guy, so I pay attention to crap like this. Although apparently not too well, cuz I can't remember exactly when it was. Anyway, they called it Radical Combustion. The concept had been around for awhile, but nobody put any real R&D into it on a scale that Honda did. They made a 2 stroke engine that had high enough compression and precise enough fuel injection that it ran at almost the brink of detonation. As everyone knows, a regular ignition stroke is more of a "push" and detonation is more of a "bang", usually resulting in some fairly immediate and severe engine damage. Holes in pistons and whatnot. Anyway, the flame front burned in such a way that it reduced emissions by a fairly large amount, and also generated a similarly high fuel economy, putting that engine close to what the engines in street bikes were doing. They even built a few bikes to race in various events such as the Dakar Rally. They didn't win that race that year, but they finished and did well, if I recall. The reason it never really flourished in the marketplace is that the technology was very intolerant of wear. Once rings and lands and bores and such started to wear just a little, the radical combustion didn't happen. Also that was about the time when 4 stroke engines were being developed for dirtbikes, and CARB came out with their regs in what, '93? 94? So Honda mothballed that whole program.

I'm thinking something like this would work in this sort of fantasy discussion. We all know it's BS, but think about it, what if conditions were right enough for the burning to be done by the time the exhaust valve opened (and the sound dissipated etc)? Then you would only have just the mechanical noise, and not the explosion. I was an A&P in a previous life and alot of those smaller older single prop deals just had little stacks for exhaust systems, and no muffler at all. Piper Cub comes to mind, Beechcraft V-(something or other) just had stacks, if I recall. They were noisy, but the prop made more noise than the engine did. At least just as much, so why bother with a muffler? Anyway, those things have nothing to do with my friend's assertion that the engine didn't make noise under certain circumstances.

I thought it would be interesting to bat this around a bit. Not much profit in it, but interesting to talk about.

 

[Welder Dave]

Flow can have a lot to do with it too. That's why aftermarket mufflers on 4 stroke dirt bikes generally have a larger outlet and less restriction. Unfortunately it makes them louder too. It's interesting on 2 stroke enduro bikes they are much quiter than their MX counterparts. They have less power but the noise difference is huge.

 

[Welder Dave]

I used to hear people say a single exhaust on a semi has more torque than dual exhaust but I don't think it makes any difference on a diesel as long as the exhaust is sized properly.

 

[chidog]

Howdy folks.
His assertion was that a small engine that is well tuned and in good working order (compression, timing etc), without any kind of header or exhaust manifold, does not need a muffler.
What? A small engine with no exhaust system, just like most engines we are familiar with, will just have a port opening, to outlet the exhaust gases!

That the back pressure created by the exhaust system is indeed what necessitates the muffler, and without it, the engine wouldn’t make a lot of noise.
What? The exhaust system is just an exit for the spent high pressure gases, its the valve popping open and the fast release of those expanding gases that makes the noise, not back pressure. There is pressure created by compression and the combustion process, there is atmospheric pressure that the spent by products of combustion are forced into.
And those forced exhaust gases are much higher than atmospheric pressure or they would not exit, its an acoustic wave that makes the noise. So did he think the atmospheric pressure would over come that?
I didn't really understand the logic.


Obviously engines make more noise the harder you push em, but what about just idling a perfectly efficient and tuned engine? Wouldn’t it make almost no discernible ignition stroke noise? No “bang”?

The guy has no handle on simple physics. Test it just remove the muffler on a small engine and enjoy. My comments in bold.
He talks if it was well tuned. Just tell him to listen to a top fuel dragster they are very well tuned and have fairly short exhaust systems with no mufflers.

A single exhaust on a semi may have more power / torque, because of the pressure wave tuning effect of the long exhaust system.
(need to expand the above to see everything I wrote)

 

[Welder Dave]

It would be interesting to see a dyno comparing a single exhaust to dual exhaust. It would seem all the big 500+HP trucks wouldn't be running dual exhaust but almost all of them do.

 

[wlhequipment]

From what I understand, which may be limited, a little back pressure creates a more torquey seat-of-the-pants-wise experience, while less back pressure creates a higher horsepower experience. Whether or not that’s true, I really don’t know. Again, going back to motorcycle engines, I can certainly tell the difference, but those are the only engines I’ve ever tried any performance mods with.

I wouldn’t say those top fuel engines are well tuned. I’d definitely say they’ve figured out how to get fuel through em though! Don’t those engines consume fuel faster than it comes out of the gas station pumps? I read that somewhere. That is impressive. Also run at 5000 HP or some crazy high number like that? Talk about an expensive hobby. I try to build my engines so they last longer than 30 seconds. It’s better for customer retention

 

[HarleyHappy]

Can attest to 2/1 on a HD 103 changes the torque curve, watched in on a HD Shop Dyno. Tested the bike prior, and then after swap, almost 10% increase in baseline torque with a slight 2HP drop. Either get HP or Torque, depends on needs.

Sure but what really sucks is seeing the dyno numbers after you just spent a grand on true duals cause you really like the look. Lol
With my upgrades I’m pushing 98 hp and 93 torque on my 103”.
Lol
Wanna buy some true duals?

 

[DMiller]

Top fuel are dumping so much fuel thru those engines nothing is a real factor except getting the burn AWAY from the engine as soon as possible.

 

[mitch504]

Talking about torque, a 350 truck engine has smaller valves than a car engine, less hp, but more torque.

 

[chidog]

The old Torque and Horse power deal. All dynamometers measure ONLY torque. That figure is converted to Horse power by factoring in RPM's. So it is all about the RPM's that determine what torque makes what horse power.
No exhaust back pressure is desirable for max power out put, the goal is to fill the cylinder with fresh fuel and air, not force exhaust by products back in to dilute the fresh charge. And is the reason for tuned exhaust systems and valve overlap, to help "SUCK" the bad stuff out.

 

[Welder Dave]

Top fuel is currently around 11,000 H.P.! I remember about 15 or so years ago it was about 6500 H.P. They shortened the 1/4 mile to 1000 feet to make it safer. They're still going 300+ MPH.

 

[wlhequipment]

Isn’t it funny how almost all engine conversations eventually turn into top fuel / formula 1/ moto gp stuff? Those engines that run in the rarified air of top competition don’t have much in common with the engines I run into. What’s good for that goose is not necessarily good for this gander (or even applicable). Really impressive technology though. 11K HP. $hit that’s hard to get my brain around! My dodge is supposed to have almost 400HP, and I’m thinking if it actually has one darn pony over 300, I’d be real darn surprised.

But now we got electric vehicles right? So eventually nobody will even know what valve overlap is. Cylinder scavenging? What’s that? Hah.