back pressure

[claynine]

I have been reading about exhaust back pressure.  There is a lot of info out there to be gleaned.  I just have a hard time trying to figure out why Kymco put such a small diameter tail pipe on the Like 200i.  Back pressure  eats up horsepower. This is a fact.  When the exhaust valve opens, the piston has to push the gasses out.  So if there is pressure to push against, hence "back pressure", the piston has to push harder and we loose hp.  The right sized exhaust can create a low pressure area right at the exhaust valve at the right time because of the "pulse" from the last exhaust valve opening.  The pulse has a low pressure right behind it.  There is a lot of tuning to make this happen and changing RPMs messes it up so the manufacturers have to find a happy medium.  The Kymco Like 200i has a tail pipe that has an ID of ~ 1/2".  Tell me if my math is wrong, but 163 cc times 5000 rpms, divided by 2 (since it's a four stroke) = about 107 gallons of exhaust each minute. My garden hose at full tilt runs about 8-10 gallons per minute...I know air is thinner etc., but the velocity of that exhaust must create a bunch of back pressure.  My Like has about 850 miles on it and it performs reasonably well,  so I'm not about to start hacking up the exhaust...yet..., but I wonder how much hp is being wasted due to the pipe.  If anyone knows how after market pipes work for the 200i, I would love to know so I can put this thought to bed.

[91cavgt]

I have been reading about exhaust back pressure.  There is a lot of info out there to be gleaned.  I just have a hard time trying to figure out why Kymco put such a small diameter tail pipe on the Like 200i.  Back pressure  eats up horsepower. This is a fact.  When the exhaust valve opens, the piston has to push the gasses out.  So if there is pressure to push against, hence "back pressure", the piston has to push harder and we loose hp.  The right sized exhaust can create a low pressure area right at the exhaust valve at the right time because of the "pulse" from the last exhaust valve opening.  The pulse has a low pressure right behind it.  There is a lot of tuning to make this happen and changing RPMs messes it up so the manufacturers have to find a happy medium.  The Kymco Like 200i has a tail pipe that has an ID of ~ 1/2".  Tell me if my math is wrong, but 163 cc times 5000 rpms, divided by 2 (since it's a four stroke) = about 107 gallons of exhaust each minute. My garden hose at full tilt runs about 8-10 gallons per minute...I know air is thinner etc., but the velocity of that exhaust must create a bunch of back pressure.  My Like has about 850 miles on it and it performs reasonably well,  so I'm not about to start hacking up the exhaust...yet..., but I wonder how much hp is being wasted due to the pipe.  If anyone knows how after market pipes work for the 200i, I would love to know so I can put this thought to bed.

I am looking for a new exhaust for my Like right now. 

Backpressure is bad, in every sense of the word.  What you want out of an exhaust system is good flow while maintaining good exhaust velocity.  As long as the air/fuel ratio is correct (and since our Like's are fuel injected and have an O2 sensor) then you can run as open of an exhaust as you want without any engine damage occuring.  The trick is to find an exhaust that is well suited for the RPM the engine is turning to help scavenge the spent exhaust gasses out of the combustion chamber without creating excess backpressure.  From what I have read, longer pipes are typically better suited for scooters as a general statement.

I have found one pipe so far for our Like, but it is not sold in the U.S.A. so it must be ordered from England and shipped here.  Total cost would be around $350. 

http://www.ebay.com/itm/130689567631?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649


Even though the picture just shows the exhaust can, I was told via email that it has all provisions for the stock emmissions (O2 sensor) equipment and is a direct bolt on.  I am still searching for others though. 

[axy]

I have been reading about exhaust back pressure.  There is a lot of info out there to be gleaned.  I just have a hard time trying to figure out why Kymco put such a small diameter tail pipe on the Like 200i.  Back pressure  eats up horsepower. This is a fact.  When the exhaust valve opens, the piston has to push the gasses out.  So if there is pressure to push against, hence "back pressure", the piston has to push harder and we loose hp.  The right sized exhaust can create a low pressure area right at the exhaust valve at the right time because of the "pulse" from the last exhaust valve opening.  The pulse has a low pressure right behind it.  There is a lot of tuning to make this happen and changing RPMs messes it up so the manufacturers have to find a happy medium.  The Kymco Like 200i has a tail pipe that has an ID of ~ 1/2".  Tell me if my math is wrong, but 163 cc times 5000 rpms, divided by 2 (since it's a four stroke) = about 107 gallons of exhaust each minute. My garden hose at full tilt runs about 8-10 gallons per minute...I know air is thinner etc., but the velocity of that exhaust must create a bunch of back pressure.  My Like has about 850 miles on it and it performs reasonably well,  so I'm not about to start hacking up the exhaust...yet..., but I wonder how much hp is being wasted due to the pipe.  If anyone knows how after market pipes work for the 200i, I would love to know so I can put this thought to bed.

Logic you use would imply that the best tail pipe would be no tail pipe at all because then there would be absolutely no "back pressure".
However, that is not true. In fact, you cannot even rev the engine to 2500 rpm if you remove the exhaust.
There is much more to gas dynamics than volume in a minute.

[91cavgt]

Here is the same pipe, but on Amazon.  The description is much better here than on Ebay, but the price is higher.

http://www.amazon.com/Kymco-Exhaust-System-Streetfighter-Alloy/dp/B00C9ZV73I/ref=sr_1_114?s=automotive&ie=UTF8&qid=1374091369&sr=1-114&keywords=Kymco+exhaust

[91cavgt]

This is the same manufacture as above, but this one has a built in catalyst. 

http://www.amazon.com/Kymco-Catalyst-Exhaust-System-Streetfighter/dp/B00CA03DO8/ref=sr_1_164?s=automotive&ie=UTF8&qid=1374091536&sr=1-164&keywords=Kymco+exhaust

[claynine]

Axy, That sounds right, but like 91cavgt wrote about the scavenger affect.  After the pulse of the exhaust valve opening there is a low pressure area right behind that high pressure of the pulse.  It's like drafting behind someone in front of you on a bicycle or a car behind a truck.  This low pressure area,  if tuned right can be a negative affect on the pressure at the exhaust valve, further reducing the back pressure.  An open exhaust won't do that, or, too big of a pipe will allow turbulence that makes scavenging impossible.  This scavenging effect is why headers on cars increase the HP. But they have to be tuned to a certain length and diameter for that engine and rpm.  There is a lot about this all over the internet.  Read about dragsters and high performance engine on those types of sites.  It is quite fascinating the engineering that goes into the exhaust systems. On two stroke motors the back pressure is tuned differently and they want the pulse to resonate back to the cylinder port to push unburned gas back into the combustion chamber before the piston closes the port.  I am not any type of engineer but not counting any expansion from the combustion chamber, I figure that the gases have to go through the 1/2" tail pipe of the Like 200i at 150 feet per second at 5000 rpms.  I just think that is crazy.  My friend has a Yamaha Zuma that is a 49 cc.  The tail pipe is curiously the same size as the Like 200i. I will be saving up for a pipe like you guys have found, as soon as I hear if it does in fact work the way I think it will.  Thanks a bunch for your in put.  I welcome everyone's point of view.

[Stig / Major Tom]

.  I welcome everyone's point of view.
[/quote]


Hmmmmmm... OK, here's mine. You want more juice outta you 'a scoot. Buy bigger scoot!


And , one more : Kymco has put together a pretty good package for a modest price. with this LIKE 200i. Persons of a certain age and/or certain gender just like tinkering..... (me, too)

[claynine]

It's like a puzzle that I see everyday and it calls to be solved.  If I had a bigger scoot, with a small tailpipe, I would still be asking questions, and wondering...