Exhaust question

[thatkidrob]

I'm working on building my own muffler and Ive heard that you must have back pressure on the engine other wise you can ruin it, as well as that the amount of back pressure you need may vary with different engines. So do you need back pressure, and if so how do you determine the amount you need as well as create the exact amount you need?

 

[Russ2251]

Ive heard that you must have back pressure on the engine other wise you can ruin it

Rumor has it that zero back pressure ruins valves. I have yet to witness this so called "fact" and therefore I call it bogus. If someone can prove otherwise (with sources), I'll post a retraction here.
I've run different brands/sizes of 4 cycle single cylinder engines for many summers with exhaust just pouring out of the block and NEVER had any problems what-so-ever.
Most folks here run straight pipes about a foot or so long.
Is engine in question 2 or 4 cycle?

 

[crazycart]

Most 4 cycle motorcycle engines depend on backpressure for power, i once unhooked the pipes on my trike at the bottom where they connect before the silencers and i lost alot of power (although its sounded louder and meaner) it wasnt worth the power loss.Some engines react differently.

 

[pickledchang]

from what ive heard about running with no exhaust, the damage to the valves isnt caused by lack of backpressure as much as it is by being exposed to the cold outside air. i guess another rule of thumb is if you are going to run a straight pipe, make like a 2 foot section, spray paint it, run the motor for a while, and then cut the pipe right where the paint stops burning off. i believe this is around 16" for a ~5hp motor. i just built my straight pipe, but havent had the chance to try it out because of how loud it is, lol.

 

[ed1380]

Most 4 cycle motorcycle engines depend on backpressure for power, i once unhooked the pipes on my trike at the bottom where they connect before the silencers and i lost alot of power (although its sounded louder and meaner) it wasnt worth the power loss.Some engines react differently.

ive heard something similar. they could only reach 13k rpm with no pipes. 18k w/

i also agree with russ

 

[Russ2251]

from what ive heard...

I love it...

 

[thatkidrob]

Is engine in question 2 or 4 cycle?

4 cycle

 

[thatkidrob]

And i live in Florida, so i doubt the valves would be ruined because of the outside cold.

 

[Jerryburger]

I'll bet that a loss in performance comes from not properly tuning the exhaust and intake tracks to work together. (No pipe vs. having a pipe or muffler, etc.) If you remove restriction at the exhaust end, it'll influence how much intake charge there is- some how, usually through mixture adjustment or jetting, you can compensate for the changes you've made. I would say to Thatkidrob to run whatever exhaust he can fabricate that isn't too loud or too restrictive, and then jet the intake charge to suit. I agree with Russ about the cold air f-ing with exhaust valves.... maybe it could've happened back in the days of really crappy metallurgy...?

 

[Kenny_McCormic]

The performance loss is because it runs leaner with a free flowing exhaust, I was just working on a tecumpseh trimmer missing the muffler with an emissions carb, it was too lean to run without the muffler but it sounded **** cool when it hit.

 

[pickledchang]

I love it...

i love it. good stuff. guess i didnt get the email that an online forum wasnt for contributing thoughts/ideas/opinons/information and that only the top gurus of the field sent from the heavens above were allowed to contribute. could you forward it to me if you have it?

And i live in Florida, so i doubt the valves would be ruined because of the outside cold.

the air in florida is still cold compared the the heat the exhaust port endures if this is a factor. but who knows, since no one in this thread has provided any 'facts'.

 

[pickledchang]

and since its for a 4 cycle, then go ahead and build whatever you want for an exhaust. just dont make it too restrictive, obviously. if noise isnt an issue, just go with the straight pipe.

 

[thatkidrob]

I just took the muffler off and ran it through the pipe, and holy crap is it loud, but once i got up to higher rpm's it starting spiting out fire (the pipe going to the muffler is very short), so i shut it off and put the muffler back on. I'm still doing research on how exhaust systems work (i don't wanna screw the engine up).

 

[pickledchang]

you can still do a straight pipe, and then do a big, oversized muffler to help quiet it down a little and supress some of the flames, heh. how long of a pipe did you try?

 

[jr dragster T]

you can still do a straight pipe, and then do a big, oversized muffler to help quiet it down a little and supress some of the flames, heh. how long of a pipe did you try?

You shouldnt have flames...

 

[Kenny_McCormic]

Popping and spitting suggest the carb needs adjustment.

 

[Russ2251]

(i don't wanna screw the engine up)

You won't.

 

[pickledchang]

You shouldnt have flames...

obviously it could indicate a problem with mixture, timing, or exhaust valve and whatnot, but with a "very short" pipe, why not?

 

[thatkidrob]

yes it was a VERY short pipe, only about an inch

 

[pickledchang]

i would also consider that very short, heh. as long as your motor is running well, those flames will go away and wont be quite as loud if you switch to say a 12-16" pipe.