Back Pressure!! Is it good for our briggs single or twin or is it bad?

wanted to create this to see what other opinions people would have about this subject...on my 12hp flathead would it be better off free flow exhaust or something that will hold back a little? let the comments begin and im just curious is all guys...

What I have learned through research is basicly, no, backpressure won't help anything. The exhaust pipe needs to be the correct size for maximum power. Too small will restrict the flow, but too big will slow down the flow also. There are pressure waves in the exhaust gases since the exhaust exits in pulses, not a continuous flow. With the right size pipe and exhaust length, it can be tuned so that there is a slight vaccum at the exhaust valve when it opens. This would only be perfect (most hp gain possible) at a specific rpm. Pipe diameter, length, shape (2 stoke expansion chambers), and even temperature effects things. I checked into the way they figure it out just because I was intrested and thought it was neat. There are formula's to figure it all out but it's pretty complicated. I guess even a formula just gives engine builders a place to start. To make things perfect requires some dyno running and changing the pipe until you get it right.

Of course for our purposes with a off road lawn mower, it doesn't really make all that much difference. If you want it to sound thoaty, use pipe larger than the exhaust port. The larger the pipe the deeper the tone will be. If I can find some websites that have good info on them, I'll post a link on here.

ok ty that makes alot of sense!! i was just curious to that theory u know and thats what i was wanting to know to creak enough vacuum to pull all the way throught the motor to where if its pulling enough it will creat enough flow that when u give it gas it starts to suck more and creates a better throttle response and will make it a little bit better on fuel since its i know pulling the air/fuel through lol but ty yea if u find the links post it i like reading about stuff like this!!!

No the faster you can get the exhaust fumes like "get the h¤&/ outta here" the better. google it, it's a science. Not depending on wether you have a briggs or a tecumseh engine.

On 4-strokes you don't need backpressure, read the term "backpressure". - Tho I do think you still need some sort of pipe, to lead the flames away.

On 2-strokes you DO.

"Back Pressure" has GOT to be one of the single most misused and misunderstood mechanical terms EVER!

Back pressure is probably best properly defined as a restriction to flow. Properly, it would be termed "Static Pressure" in non-engine pipe and duct systems. A resistance to flow caused by a restriction is a bad thing. 4-stroke, 2-stroke, doesn't matter. Bad.

Where the confusion peaks is really in terminology. A piping system designed to draw subsequent exhaust pulses out (scavenging) or increase flow rate (velocity) or even bounce pressure waves around to optimise 2-stroke performance (tuning) is often thought of as having back pressure and therefore beneficial.  It just isn't so.

How does the confusion get created? Well, basically like this: Dude takes the exhaust off his engine and tries to run it. Engine runs poorly. Dude thinks: "Well, I just removed all back pressure and my engine runs like crap, therefore I need back pressure." What really happened? By removing ALL piping and trying to run without any further alteration, the scavenging effect has been lost, the velocity has been lost, AND there's a better than good chance your carb jetting isn't right anymore either, due to the changes in air flow.

A well designed exhaust system is free of restriction (static pressure extremely minimal) yet supports velocity and scavenging. On a 2-stroke, the "tuned pipe" rules the roost, especially on a piston-ported engine. Again- not high in static pressure, but uses different sized piping, divergent and convergent cones, to time the exhaust pulses. Without getting into a bunch of 2-stroke theory, the short version of it is that the RIGHT expansion chamber on a 2-stroke will actually alter the charge coming into the cylinder, and largely improve the scavenging on the way out- improving both power AND efficiency.

The short version- back pressure is bad. For the looong version (grab a coffee), have a look here- http://www.diygokarts.com/vb/showthread.php?t=22264

Wow guys, Impressive responses from all.
I can only add that the best way to fully understand exhaust tuning is to pick up a book such as A. Graham Bell's "Four Stroke Performance tuning". Read it. Twice.   
Then the rest of the learning comes from time in the garage, building different pipes. Test, modify and test again.   

Keep in mind that when asking a generally vague question such as "is back pressure bad?" you can only expect a generally vague answer in return. The real answer to this thread is:
"Yes. Depending on what your definition of backpressure is".    

Here's a page with a bit of reading on it. They use some technical terms but you can get the jist of it. I didn't read the entire thing but looks life half way reputable info. That and there's a cool picture of some glowing headers on there. Looks like there might be some other neat stuff on that site too.

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm

Been thinking... Here's a couple very general comments I've come up with:

• 
  I'm running a stock muffler on my 12.whatever HP mildly built B&S.  It works very well from idle up to mid-range, then becomes restrictive and starts to limit power output at higher revs. edit: This little can muffler is too short to provide any resonance. Therfore, we can assume that it just simply cannot flow well enough to make power at high revs.
  
  The exhaust outlet for a 12-15hp motor all seem to be about one inch ID.  This is for the 500cc B&S motors and the 125-200cc 4-stroke motorcycles.  This makes me think that the diameter is matched to the power output, not the engine size.
  
  Engine size does have a lot to do with header length (the portion of pipe from the engine to the first change in diameter), but in a roundabout way.  It's not exact, but generally the larger the engine, the less peak RPM's it will be tuned for, and the longer the header will be.
  
  I'm sure someone has successfully built a header for our motors that works well at the desired RPM and doesn't resonate badly or take considerable power away from other RPMs.
  
  The nice thing about exhaust systems is that you can usually see what the top tuners are doing.  You can't hide youre exhaust as easily as you can hide a top secret cam grind.
  
  Somebody on here mentioned and 18" header.  Assuming that it's built out of 1" ID tubing, seems pretty close to what I've observed running on similar machines.
  
  We've talked about burning exhaust valves.  I'm guilty of saying that it's only bad jetting that will burn an exhaust valve (too lean).  However, completely removing the exhaust system or running with a major leak close to the port can introduce air (oxygen) into the system.  Heavy backfiring upon deceleration will sometimes be observed and the exhaust valve will run much hotter than originally intended.  So yeah, Oxygen can assist an exhaust valve in burning.
  
  
  
  

http://www.pedrosgarage.com/Site_5/Heading_Out!.html <-- Good, easy explanation on WHY it's bad on a 4-stroke. We talking stock engines here, not F1 engines running 108 Octane ratings or Jet-fuel's for that matter.

Also they explaines why you can benefit of running 2 - to 1 pipes, then you can go to 2 pipes again on multicylinder engines. But read the link, I can't get down on paper what I mean on stuff like this   

Some stock engines do like a little "back pressure", but can be easily tuned to run great with straight pipes. I'm a firm believer in X-Pipes.

T-Dub13 wrote:

The exhaust outlet for a 12-15hp motor all seem to be about one inch ID.  This is for the 500cc B&S motors and the 125-200cc 4-stroke motorcycles.  This makes me think that the diameter is matched to the power output, not the engine size.
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I think that it basically boils down to volume over time, ie exhaust flow expressed in CFM. While the bike engine has a smaller displacement than the OPE engine, it is spinning 2-3x faster, and therefore expelling as much (if not more) exhaust gasses in a given time period.

Doc Sprocket wrote:

T-Dub13 wrote:

The exhaust outlet for a 12-15hp motor all seem to be about one inch ID.  This is for the 500cc B&S motors and the 125-200cc 4-stroke motorcycles.  This makes me think that the diameter is matched to the power output, not the engine size.
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I think that it basically boils down to volume over time, ie exhaust flow expressed in CFM. While the bike engine has a smaller displacement than the OPE engine, it is spinning 2-3x faster, and therefore expelling as much (if not more) exhaust gasses in a given time period.

Agreed 100%. Great way of breakin it down Doc.     

I know nothing about back pressure lol. whatever you guys say. i will be asking questions about back pressure when i get a exhaust made for my kohler.

Small 4 stroke engines don't require much back pressure to run extremely efficiently simply because the displacement is so small it doesn't have much of an effect. It's good to have a pipe that is more then a few inches long at least due to something called reversion.

Reversion has to due with the duration of a cam, its basically the overlap between the exhaust and the intake valves opening and closing causing a fast withdraw of air occurring in the lower RPMs when there isn't much flow happening.

In essence having to short of a pipe can cause the cold air to hit the hot exhaust valve making it brittle by a heat treating effect which can create cracks, chips, and more fracturing type of damage to the valve over a period of time.

backpressure is needed to a certain degree. mainly so you dont toast your exhaust valve

willis923 wrote:

backpressure is needed to a certain degree. mainly so you dont toast your exhaust valve

No, please re-read the thread.  What we're saying is that back pressure is bad.  Reversion is bad and having too short of a pipe is also bad.  All for different reasons really.

Generally speaking:  Here are the points we're trying to make.

Back pressure will hinder performance.  (Stock or restrictive muffler will hinder the engine's breathing capabilities).
Too short of an EXH pipe can induce reversion. (Too short of a pipe will allow the sound waves to arrive too soon).
Reversion can hinder performance and introduce cold air (containing oxygen) to the EXH valve. (burnt valve and poor performance).

So, when someone says that these motors like back pressure, what they mean is that they will run better with a stock muffler than with no muffler at all or a badly designed shorty pipe.  It's about reversion and pressure waves not back presssure.

This means that to design a "performance" EXH system, one needs to have a pipe that is free-flowing and the right diameter and length.

The lower RPM the EXH is tuned to, the longer the pipe will be.

These low RPM motors (so I've been hearing specifically about the 28's) like a pipe that is about 1" in diameter .  This is because they don't breathe well compared to modern 500cc 4 strokes which can use up to 1.4" diameter header pipes (think volume).  The pipe length at least 18" long is all about timing of the EXH pulse waves (think timing).

From my experience with building many other kinds of performance motors, this seems about right for our flatty's, maybe a little short still.   

So can I run 1.25" pipe thats around 15" long on a twin cyl? Briggs v twin to be specific?

You could try it. I don't know the twins, so it depends on the cylinder cc and the exhaust valve size mostly. Then the RPM you're tuning to. Assuming it's a 1000cc twin (two 12hp cylinders) and the top RPM is around 5000, then I'd try a 1 or 1.125" diameter pipe by 18-20" long. Two into 1 collector to a 1.5" single pipe to a glass pack.
If it falls flat on top then you can experiment by cutting the header pipes shorter, or enjoy the low end torque. It's best to have an idea of the kind of performance you're tuning for. (top end speed, low end torque etc...)

It's all pretty much guesswork, guided by experience and using the general rules as guidelines. Exhaust tuning by the seat-of-the-pants-dyno is easy, cheap and fun.

An interesting redneck method (not necessarily correct) my wife's brother in law said works for exhaust tuning... is to make the pipe longer than you'd want, paint the pipe, run the engine at desired speed until paint starts to burn (not at the engine) and chop the pipe off at the burn. Take that with a grain of salt as I heard it at least second hand.

that seems about right, for a redneck anyway.. lol

TheRainbowBoxer wrote:

An interesting redneck method (not necessarily correct) my wife's brother in law said works for exhaust tuning... is to make the pipe longer than you'd want, paint the pipe, run the engine at desired speed until paint starts to burn (not at the engine) and chop the pipe off at the burn.  Take that with a grain of salt as I heard it at least second hand.

I remember that method. Never tried it, so I just did some research... Seems there is some validity to it, but it's primarily used on multi-cylinder engines to find the "Crossover point" where the header pipes should be connected. Either a crossover tube or a collector, it isn't entirely clear. Also one guy commented that the burn point was dependent on the lendth of pipe he began the experiment with. I'm thinking that it was most likely he was finding the 1/2 wave and full wave points.  

If Red were to do this, I'd recomment he place a 2-into-1 collector at the burn point and see what happens.