Running without back-pressure?

cmb271

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Jan 5, 2015
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Okay, I haven't posted much in a while because after loosing the key that holds a sprocket onto the 7G transmission of my 49cc 4 stroke and not having the means to replace the transmission I lost interest in investing time into my motorized bicycle. Now that my car is currently sitting in my yard after being hit by a pick-up truck from the side I've decided to look back into the motorized bicycle way of life.

Instead of going with the 4 stroke again I've decided to just buy a 2 stroke kit and a new bicycle since the old one has seen better days. I haven't order either yet as I'm waiting for my paycheck on Wednesday so until then I'm just in planning mode on what I'm going to do. I'm going to try and do it right this time, being given a clean slab allows me to look at what made my previous motorized bicycle builds fail and how to avoid them.

I still have the parts to my old 2 stroke build and among the parts is my old straight pipe exhaust. The muffler portion of it broke at the weld leaving me with a straight pipe. I enjoyed the noise it made and was wondering on the new build if it'll cause any damage to the engine if I ran it as a straight pipe exhaust compared to using the default muffler that will come with the kit.
 


butre

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Dec 2, 2013
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if you're not going to use an expansion chamber then back pressure is unimportant. the only thing no muffler will hurt is your ears, and maybe your wallet if a cop is having a bad day and wants to ticket you for it.
 

jaguar

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Nov 25, 2010
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there is no power advantage to using a straight pipe without muffler so just use the new one.
 

cmb271

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Jan 5, 2015
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there is no power advantage to using a straight pipe without muffler so just use the new one.
I'm honestly not worried about performance enhancement I'm more worried about reliability, I don't know what the pros and cons of running a 2 stroke without a muffler.
 

Frankfort MB's

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Apr 3, 2016
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Just take the cap off the stock exhaust and there you go! A race bike!! Garrenteed 20HP gain!!

Nah just noise;)
 

HeadSmess

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May 17, 2010
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noise noise noise noise and more noise@extremely loud levels dot com

its just about bearable if you dont have the outlet pointing in your face or straight at the ground.

drill a hell of a lot of holes in the last four inches, wrap it in fibreglass insulation, and wire a drink can with ends removed around fibreglass... mmm, dodgy swiss muffler :)
 

KCvale

Motorized Bicycle Vendor
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Apr 18, 2010
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3,518
Well, 2-strokes have no valves. They do intake and exhaust at the same time.
No exhaust pipe, or a strait open one, simply robs you of power and makes a really big mess.

All the unburnt fuel/oil mix goes out the tailpipe.


 

45u

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Feb 18, 2016
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Well, 2-strokes have no valves. They do intake and exhaust at the same time.
No exhaust pipe, or a strait open one, simply robs you of power and makes a really big mess.

All the unburnt fuel/oil mix goes out the tailpipe.



This the truth and nothing but the truth. Pipes have a LOT TO DO with how well a 2 cycle runs.
 
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HeadSmess

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May 17, 2010
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3,033
agreed agreed, if you want PERFORMANCE from a two stroke, you get a well designed tuned pipe/expansion chamber. its simply a supercharger for 2 strokes.

if you only want to reduce noise, you stick on the stock standard exhaust as equipped.

if you couldnt care less and just want to get home, or around for a few days until parts arrive... remember that virtually no 2 stroke outboard engine runs a muffler...AT ALL! they actually run with negative back pressure, as the propwash is "sucking" on the standard exhaust outlet, ie, the middle of the prop.

no engine will be damaged by running with no exhaust system (look at the old aircraft...they had little 6 inch pipes only to clear the fuselage) AS LONG AS you tune the carb to suit. they just maynt produce all the power they can....
 
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