No muffler = better vac!!?

[rhino1]

OK so I posted about the muffler breaking off my 24 HP Honda the other day. As I was in a bind and nobody in this area carries the part, I rigged a couple of Ford tractor exhausts onto my machine.

Holy moly, I bet the vac went up 15% easy.... Now the thing is loud as hell, but I don't work at night anyways. Sounds like a tractor pull in the driveway, but the wand is screamin'.

I can't believe that crappy muffler was bogging down that Honda so much, but I guess it was.

 

[Nate The Great]

OK so I posted about the muffler breaking off my 24 HP Honda the other day. As I was in a bind and nobody in this area carries the part, I rigged a couple of Ford tractor exhausts onto my machine.

Holy moly, I bet the vac went up 15% easy.... Now the thing is loud as hell, but I don't work at night anyways. Sounds like a tractor pull in the driveway, but the wand is screamin'.

I can't believe that crappy muffler was bogging down that Honda so much, but I guess it was.

From what I was told, the engine needs XX amount of back pressure in order to run correctly. Alot of damage can happen if not.

 

[Chris Sheldon]

Nate is right on the money!

Be careful about running that exhaust out of the engine too fast; you can burn up the valves.

Too much back pressure is a bad thing, but an engine does need to have some back pressure.

 

[Larry Cobb]

Chris;

Back pressure should be measured on all engines and blowers.

All of our TM's have to meet back pressure specifications for warranty certification.

Too much on an engine will definitely lower the performance.

When I raced automotive engines, we went to great lengths to eliminate back pressure.

I fail to see any reason why low back pressure, would damage valves of any engine.

Blower silencers can also rust & clog easily, restricting blower CFM.

Larry

 

[Chris Sheldon]

Larry, here is an excerpt from a great article on this subject:

"Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. "

And I agree Larry that back pressure should be measured and that too much will hamper performance, its just not a good thing to totally eliminate back pressure. Its the same thing as people thinking their engines should run as cool as possible. Yes, too much engine temp is a problem, but an engine that runs too cool presents its own issues.

-Chris

 

[Chris Sheldon]

its just not a good thing to totally eliminate back pressure.
-Chris

why?

"...........the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. "

 

[Greenie]

There IS something to be said for keeping a factory muffler on an engine, you are assured the back pressure is correct and will not have an issue with warranty etc...

 

[Chris Sheldon]

There IS something to be said for keeping a factory muffler on an engine, you are assured the back pressure is correct and will not have an issue with warranty etc...

Well said.

 

[Chris Sheldon]

All that means is that the AFR should be adjusted.......

I am assuming you mean air-fuel mixture?

Its not that simple. Sure, on some carbs you can adjust the air/fuel in the idle circuit, but there is no way to adjust the high speed circuit. The only thing you can do is re-jet the carb, but how do you know what size jet to run? What is the relationship of back pressure to jet size or air/fuel mixture?

 

[rhino1]

OK, so I will keep an eye on it and be sure it doesn't run any hotter. I think maybe the original muffler has to be causing too much backpressure for this motor to run so well now, not saying it didn't run good before, but with the tractor mufflers it just seems to run better. I'm sure the tractor mufflers are causing at least some backpressure, otherwise they couldn't put them on a tractor engine?

Something else I noticed: used to be when I shut the engine off after it ran for a while it would make a small backfire, it doesn't do that now.

To hell with the warranty anyways, I'll let you know when I burn it up.

 

[dealtimeman]

i think he ment air-fuel:ratio but just maybe

 

[floorguy]

go to a muffler shop and get a small car one...

same great vac, yet less noise

 

[SRI Cleaning]

More air flow would just make it siphon more gas from the carb.

 

[Lee Stockwell]

Much adoo about nothing.

My grandfather said the "experts" warned about the early V8 engines that the pistons would wear unevenly because they were laying at an angle.

The low backpressure issue is a MYTH as well. Sorry.

Thanks,
Lee

 

[dgardner]

Yes, eliminating backpressure can alter the AFR, but only on certain engines, here’s why (this comes from tuning VW-powered sandrails back in the day):

IF the intake runners are the right length, and IF there is enough valve overlap (time when both intake and exhaust valves are open at the same time, seen more in higher-rpm engines), and IF there is sufficient exhaust backpressure, then what is known as a ‘reversion pulse’ is created.

When the intake valve opens, a pressure wave literally pushes the air backwards up the intake plenum. As the air flows backwards through the carb, some of the fuel already drawn into the airstream goes too. In the VW’s I used to work on, you could see a little cloud of fuel in the carb throat because of this. When the pulse ends, the air flows back into the carb, drawing some more fuel on the way back in, enrichening the mixture even more. The carb jetting was set so this ‘double whammy’ resulted in the correct AFR.

In these type of engines, changing the backpressure WILL alter the mixture. Reducing backpressure can reduce or eliminate the reversion pulse. Without the pulse and resulting extra fuel, the mixture will lean out if jetting is not changed.

Do the small engines we see on truckmounts have this reversion pulse? I honestly don’t know. Probably not.

However, if you’re at any significant altitude above sea-level, and assuming your engine does not have a high-altitude kit installed, it is running rich anyway – as altitude increases and air density decreases, the carb metering gets confused and runs too rich. It gets pretty complex when you start accounting for all the variables.

All this only applies to carbureted engines. Modern injected engines monitor all this stuff and adjust the AFR on the fly. Changing the exhaust on one will have little to no effect on the AFR.

 

[dgardner]

Larry, here is an excerpt from a great article on this subject:

"Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. "

And I agree Larry that back pressure should be measured and that too much will hamper performance, its just not a good thing to totally eliminate back pressure. Its the same thing as people thinking their engines should run as cool as possible. Yes, too much engine temp is a problem, but an engine that runs too cool presents its own issues.

-Chris

Chris, I read that same article; it has been copied ad nauseum across the web. It is interesting, but contains a few inaccuracies (aside from misspelling stoichiometric). Automobile engines do not run at precisely stoich because at that point exhaust temps soar, and NOx emissions rise. They typically run slightly richer than stoich at part-throttle cruise. The 14.7:1 ratio is also a kinda theoretical number. It depends on the exact fuel formulation, which varies regionally as well as seasonally.

Also, best power is not obtained at stoich – actually closer to 13:1.

That brings up one huge difference between an engine used in an auto and one used in a slide-in. A vehicle uses a bunch of horsepower during acceleration, then much less during cruise. At cruise you can lean out the mixture and advance the ignition timing for better mileage. A slide-in engine, however, never ‘cruises’. Once you bring it up to operating RPM it always takes the same high horsepower because the blower is loading it down. Off-highway motors are tuned differently because of this, and rarely (if ever) incorporate vacuum advance.

And Anthony is correct – if reduced backpressure caused more air to be drawn in to the combustion chamber, the carb would simply do its job and add the extra fuel. When you open the throttle, the same thing happens – more air, and thus more fuel.

So, you have to be careful about applying information geared towards vehicles to industrial engines, not everything is the same.

 

[Chris Sheldon]

An engine on a TM is VERY similar to a vehilces engine; sure they are managed quite differently, but the basic operating principles are the same. So having worked in both the industrial and automotive sides, I can tell you that engine theory is much the same in both. In fact Dan, on my first day of diesel school, 18 years ago, we tore apart a small Briggs engine to learn the basic parts and fundamentals of an engine! Imagine that!

The bottom line is the empirical evidence: I have seen valves get burned up from shoddy exhaust systems, not often, but I have seen it.

An engine is designed to cool the valves at a particular exhaust velocity, so what happens when you increase that velocity too much? How can you be sure that the exhaust valve has had adequate heat transfer to the cylinder head in this scenario? You don't know.

I think this is more plasuible than the "leaning" issue and makes more sense to me and is what I was taught.

My point from making an issue of this was for people to understand that there can be ramifications to altering your engine when you aren't familiar with engines. Just leave the thing alone if you are counting on this machine to earn a living.

 

[dgardner]

An engine on a TM is VERY similar to a vehilces engine; sure they are managed quite differently, but the basic operating principles are the same. So having worked in both the industrial and automotive sides, I can tell you that engine theory is much the same in both. In fact Dan, on my first day of diesel school, 18 years ago, we tore apart a small Briggs engine to learn the basic parts and fundamentals of an engine! Imagine that!

The bottom line is the empirical evidence: I have seen valves get burned up from shoddy exhaust systems, not often, but I have seen it.

An engine is designed to cool the valves at a particular exhaust velocity, so what happens when you increase that velocity too much? How can you be sure that the exhaust valve has had adequate heat transfer to the cylinder head in this scenario? You don't know.

I think this is more plasuible than the "leaning" issue and makes more sense to me and is what I was taught.

My point from making an issue of this was for people to understand that there can be ramifications to altering your engine when you aren't familiar with engines. Just leave the thing alone if you are counting on this machine to earn a living.

Wow Chris, my post was not intended to irritate. I don’t disagree that all internal combustion engines are similar, your diesel and briggs both have pistons, valves etc. But the way fuel is admitted and metered could not be more different between the two, and the discussion on fuel mixture is all about management.

If you read my post, you will see that we agree – that mods to the exhaust can burn valves under the right conditions. We also agree, though I didn’t state it, that leaving things stock is the best route to take if you’re counting on the engine to make a living. Unless, of course, you have the instrumentation, tools and knowledge to do it right.

The one thing we may disagree on is that exhaust velocity plays the major role in valve face temperature. In fact, the temp is primarily determined by the EGT (which heats the valve up) and the amount of time the valve is in contact with the seat when the valve is closed (which cools it down). Some heat is also conducted up the valve stem, which is why you sometimes see sodium-filled valves – because sodium is a superb conductor of heat. Gasoling burning at stoichiometric can approach 1600 DegF – and will run two or 3 hundred degrees cooler at more reasonable AFR’s.

Another major cause of burned valves as you know, is when a bit of carbon or other foreign object gets stuck between the valve face and seat, preventing the valve from fully closing (and cooling) against the seat. Changes in mixture, velocity, temperature, or other things can break crud loose and cause this problem, mostly in well-used engines. So sure, there can be several reasons for burned valves

 

[dgardner]

It occurs to me that some may not be aware of what a ‘burned’ valve is or the mechanism by which it happens. First, here is a pic I found of a burned valve:

Burned is really a misnomer – as you can see a burned valve is really one where material has been eroded from the face. This happens when the valve does not seal properly. When the plug fires (in a gas engine) and the mixture ignites, tremendous pressures and temperatures are produced. If the valve leaks, the flame screams through the opening like a torch and literally scours the valve material away.

So, valves do not burn unless they first leak.

The number one cause of valve leakage is a worn guide and/or valve stem. The valve can wobble from side to side and does not hit the seat square as it closes. It leaks, then burns. Other things can cause leakage, such as a bad valve job where the machinist cuts the valve seat and makes it too narrow, or fails to recondition/replace the guide, if the valve edge is ground too thin, or if foreign material is caught preventing the valve from closing.

High temperatures cause the valve to run hot, which breaks down the oil film between the guide and stem, which causes extreme stem/guide wear, allowing the valve to wobble, which causes burned valves.

What does this mean to the O/O? Heat is your enemy! Anything that raises valve and guide temperature will shorten the life of your engine and cause burned valves. Clogged cooling fins, blocked ventilation paths, lean mixtures, etc. can all contribute to the problem. Changing the oil/filter, by reducing valve guide wear, can help. If you plan to rebuild that engine rather than replace, do it sooner than later. Waiting until the valve seats deteriorate to the point they start to burn will just be more expensive to fix.

If your valves burn quickly, something is wrong! Now that you know the why, finding your problem may be easier.