Hi paco

From what I can see through the hole these is nothing on the other side of the muffler cap. Looks like all the muffling is done through the muffler cap and spark arrester. After expanding the hole to 1 + 1/2 inch in diameter, and then running it without the spark arrester in, it sure seems to confirm this too, as it sounds like a straight pipe with megaphone. Very loud without the spark arrester in, but very reasonable with it installed.

I opened up the hole in the muffler end cap to 1 + 1/2 inch with great results! A guy named Jay in the Yahoo XT225 group mentioned that the hole in my original picture did not look to be an inch or more wide, which it indeed turned out not to be that big. The item I was using to measure the hole was not acurate so I have since started using a different method of measuring the hole and have it pretty accurate now. At any rate I opened the hole up to 1 + 1/2 inch diameter. I was able get some additional power out of the engine by doing this. With the spark arrester out it is real loud, kind of fun, but way loud. The bike really pulls great with the exhaust opened up like this, but is too loud. With the spark arrester back in the sound is a bit louder than with the smaller hole, but totally reasonable. When idling or minimal throttle you don't notice a lot of difference from stock. There is a bit more noise than stock at full throttle acceleration, but it is still very acceptable. I am very happy with the results of this mod. More power and a minimal increase in exhaust noise. I will probably take the spark arrester out every so often just to have some obnoxious fun, but for 99% of the time the spark arrester will stay in.

BTW: The reason I did not go ahead and open up the exhaust to the barrel size of 2 inches is that I thought it might make the barrel less stable. With that 1/4 rim still left around the muffler barrel end cap it keeps things nice and tight in there.

Paco's question got me to thinking that it would be interesting if someone replaced the muffler assembly inside the pipe with a silencer and packing instead. Since there looks to be nothing other than the muffler and spark arrester in the pipe, you could remove the muffler totally and insert the right size silencer with packing in the muffler area. At the end of the pipe (not the spark arrester or muffler) around the rim of the end of the pipe there is a weld that is holding the whole muffler assembly in place from what I can see. Theoretically you could grind that weld off and remove the whole muffler from the pipe. Then if you could find the right size of round silencer, you could insert it in place of the muffler and you would have a straight through pipe with a built-in silencer. You could secure it with stainless steel sheet metal screws through the outside of the pipe and you would have it. I'm not going to do it, as I am more than happy with the results of my experiment, but it would be a pretty slick mod.

Hey Z1,

Man, you've been really pushing the envelope on a stock exhaust! I've experimented quite a bit with a variety of reversible stock exhaust mods and carefully tracked the mileage with each one.

The profile of the stock exhaust flairs out like a megaphone and then there is that spherical shape to reflect the exhaust pulses back to the engine. I believe the length and shape of the exhaust is tuned to produce a very flat torque band especially in the lower to mid rpm range, which is where most riding is done.

I suspect what gains you are getting in the higher rpm range by better exhaust flow, equal a loss in low rpm torque due to the loss of the tuned reflected exhaust pulse. Low end torque also requires a certain amount of backpressure.
Which is why the stock hole in the sphere is 3/4 inch... the same as the exit hole in the stock spark arrestor.



This is a home made exhaust mod insert I built from copper plumbing fittings, which yields an average mileage gain of just under 4 mpg due to increased low end torque (a function of mileage). It is free flowing, and VERY mellow sounding.

You might consider making something similar as it would quiet down the bike considerably, and yet retain a smooth straight flow through design which does not compromise low end torque. Now maybe low end torque is not an issue for you, and flat out top end performance is more important, if it is...

...open it up, and put in big jets!

I know you are enjoying experimenting, because I also enjoy tinkering with this amazingly versatile little bike.

Take Care,


Greg

[ January 24, 2006: Message edited by: Greg ]

Hi Greg

I do not notice any loss of low end torque doing this mod. If fact the opposite is true; there is better pulling power than before all through the range. I too was apprehensive about modifying the exhaust thinking that there would be a trade off somewhere along the power range, but in fact the mod has increased the power throughout the entire range. I too figured Yamaha had done their homework to figure out exactly the right amount of back pressure needed when designing the exhaust the way they did, but that turns out not to be the case. I think Yamaha was more focused on creating a fool proof spark arrester that would pass all the government regulations rather than being focused on creating an exhaust that metered out the correct flat torque curve with a minimal amount of noise. The bottom line from my experiments is that the stock exhaust is way too restrictive for this engine. It is so restrictive that it basically nullifies any tuning effect of back pressure on the power curve. The restriction and back pressure is so great with the stock pipe that it does not come close to the threshold of where you can tune the exhaust with too little or too much back pressure. Plain and simple it is plugged up so bad in its stock form that it would be like running a marathon and trying to exhale out of only one nostril of your nose, with your other nostril plugged up and your mouth closed. Even with the 1 + ½ hole I ended up drilling in the exhaust, which is almost like running an open pipe without the spark arrester in it; the power curve is only enhanced with no ill effects to the low end power. The way I can really tell about the low end power increase is when I go up long incline hill and I am able to pull one higher gear than I usually would, even with the new 16 tooth countershaft sproket I installed. I can also tell by riding on flat ground and choosing two gears higher than what I would normally do, allowing the engine to lug down low and then opening up the throttle. With the stock pipe I would feel very little increase in power if I lug it down that low and then open the throttle up. I would normally need to gear down to get an increase in power in that situation. With the exhaust opened up I can open up the throttle and get more pulling power without having to go down a gear. I do suspect there is even more power gains to be had by increasing the intake flow through the airbox and rejetting the carb to run richer. Yamaha probably figured the intake airbox volume specifications taking into account the limited exhaust flow, and the ability for the engine to only require that limited intake of air, so there are probably gains to be made there for sure. I will report back as my progress toward XT225 Nirvana continues.

Hey Z1,

Ok... you are the one who knows, as I haven't actually tried what you've done.

I'm just going by my own experience in that when I did the exhaust mods, I couldn't trust my own subjective perception that more noise felt like more power. Which is why I did impartial mileage tests on everything I tried and compared them to the bone stock mileage, because they gave impersonal objective answers as to what worked the best.

However... I might just try what you did...

The only thing holding me back is a persistent aversion to doing irreversible mods on expensive parts.

Thanks for sharing your experience and describing it so completely, I'm really enjoying reading your words.

Take Care,


Greg

Hi Greg

Keep in mind I am not really focused or concerned about keeping the MPG up on my bike. So if that is your aim I would not go down this road. My focus is in allowing the motor to develop all the power it can throughout the range while keeping the exhaust pretty quiet. I would expect that by the time I am done with rejetting the carb and opening up the airbox to increase the flow there, I will indeed lose some MPG in the process. More power usually does equate to more fuel used. Allow more fuel and air to enter the combustion chamber, and you get more power.

Hi Folks,
I ended up boring a 7/8th inch hole in the back of the pipe. I chose that size as it matched the size of the opening in my custom endcap. With the endcap out the sound is very racy but crazy loud for anything other than pissing off the neighbors.It has that "burble" on downshift that sounds amazing. However as I value whats left of my hearing and try not to be the neighborhood pariah ,I put the 7/8th inch cap back on. I have a 130 main jet. a 1/16th inch spacer on the needle. a 40 pilot at 3 turns out.Stock airbox and snorkel with the screen removed. I found you do get a noticeable increase in upper rpm power and no noticeable loss on the bottom. Perhaps even a small increase but thats pretty subjective.I plan on temporarily removing the snorkel and see if it makes a difference. Just my nickels worth Z1XT225 you will notice an improvement with a larger main jet. I ride mostly at sea level to about 2000 feet and going from a 125 to a 130 was a big boost.
All I can say about the exhaust mods is that is one constipated stock pie that can easily be improved with a new endcap and about 20 min wrk with a drill and a cone shaped grinding bit!!!

It must be close to lunch time .I said pie instead of pipe. So much for "project literacy"