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Discussion Starter · #1 ·
I finally got that relic up and running (1984 Z50R). It has a rebuild head, new piston and rings, and everything electric is brand new good stuff. The valves are adjusted correctly, it has good compression, and the electrical seems to be as good as its gonna get as far as setting the gap on the points. Its a dog?? This motor makes my XR50's look like fire breathing torque monsters. Are Z50R's just slow in comparison, or is something not adjusted right? The top speeds are similar between the XR's and the Z...but the Z is pokey as all hell. I might be parting this bike out....again. Can't make up my mind. A couple people have suggested dropping teeth on the front sprocket...but i dont want the bike to top out at like 10 :roll: Will a slight bbk wake it up?
 

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Discussion Starter · #3 ·
I forgot to put the exhaust gasket in....turns out 50's need back-pressure to make torque! Who'd a thunk it?? Anyways, it runs a lot better now, and its getting "pimped". Tonight it recieveda couple coats of black gloss on the tank...got masked off...and the flakey peweter flames got laid down. I recovered the seat with some black vinyl, and the rims are prepped for paint. I'll post some pics of it up when its all done.
 

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Okay, so why do you need backpressure on a 50 to make torque? Please enlighten me if I'm wrong, but on a 4-stroke engine, doesn't exhaust backpressure rob the engine of power??? That IS why (during a port job) we enlarge the exhaust port, right? to allow the engine to breathe better (exhale better).
 

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Velocity, I don't the technical answer you maybe searching for, but I do Know that you do need some backpressure for an engine to work properly. During I race that I was in, a guy ran up my back tire and snapped the rear can section from the pipe, (where the header tube and the can meet). Well, my 88 ran about as fast as a stock 50 and super loud. So, I guess you need a small amount of back pressure. Z88r can probably give you the technical version to your question.
 

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Discussion Starter · #7 ·
LOL.....Velocity. Take your pipe off and ride around....and see if you can find any torque. Porting the head lets it breate better, but there is a point of no return where an engine breathes too freely and loses power. Like an import car going from a stock exhaust to a big ol' fart can....unless its model specific, those exhausts will normally make a 4 banger lose some of its torque. What little it has to begin with :roll: .



Anyways...here is a pic of the little bugar.

 

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Back Pressure Effect

You can't control your overlap without changing the cams, but the effects caused by changing the back pressure are the same. When you reduce back pressure, it is equivalent to increasing valve overlap, and when you increase back pressure, it is the same as decreasing the amount of valve overlap. That's why some people will say, "you need a muffler for torque", or, "you'll have more high-end, but less torque, if you run straight exhaust". They are right, but a muffler's purpose is to reduce sound output, not horsepower! By reducing back pressure in an exhaust system, you increase high-end horsepower at the cost of low-end torque.



You can compensate for this by increasing the velocity of the intake charge. Increasing the intake velocity has the added side effect of increasing back pressure, because there is more air to be evacuated during the exhaust stroke. Note that if you increase intake velocity past the limits of the exhaust system, the gains you achieve are diminished to the point of being non-existent. That power will be there when you do upgrade the exhaust system, which is why something as simple as upgrading the exhaust system can result in huge horsepower gains.



You can decrease back pressure by increasing the size of your exhaust ). Increasing the size of the exhaust pipe and decreasing its length also helps.

Valve Overlap

During the combustion process, when the exhaust valve is open, all of the compressed (and depeleted) air-fuel mixture spills out from the cylinder as fast as it can, through your exhaust pipe. The problem with this is, just before your exhaust valve is about to close again, your intake valve opens up, allowing the fresh air-fuel mixture to rush into the cylinders. This is called overlap, and one of the things you take into consideration when choosing a cam, because it can be used to your advantage.

hope that helps explain it
 

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Just a thought.

I do believe that backpressure is needed to make the engine run as designed. When you start porting and altering or modifying exhaust flow you also have to modify the intake side of the engine. Stock intake with modified exhaust won't run to it's fullest potential until both sides are tuned to opperate efficiently together. Think of it in terms of a water valve. If you have tiny pipes feeding the valve, and large pipes exiting the valve it's gonna A) lose water pressure as opposed to matching pipe sizes, and B) only flow as fast as the small pipe will allow. I could be way off base here but just something that i thought i should throw out. Does that make sense or am i way off base?
 

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accurate until he forgot to mention exhaust scavenging which is what determines a lot of how the exhaust system develops power. When you have a pipe on there the pipe helps increase the velocity of the exhaust gasses which help pull the exhaust gas out of the cylinder thus allowing more intake charge to enter.
 

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the sound wave has two sides

like a pulse and creates the desired affect you are relating to

but that way of thinking is more two stroke theory than 4 stroke theory

where you are trying to get the spent fuel out in one stroke. a two stroke actually has scavaging ports in the cylinder itself.
 
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