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Post by prodigit on Jul 25, 2013 10:58:48 GMT -5
Today I drilled out the stock muffler on my ATM50, and was going 50MPH on it, about a 2MPH higher top end than stock.
Acceleration was lower, but top speed increased.
Also idle RPM went up, and idle exhaust gasses don't smell like gasoline anymore;
I drilled 3 (edit:)3/48" holes in the stock muffler right next to the exhaust tip (one on the top, one on the right, and one on the left).
The idle sound sounds a bit heavier, and while riding the exhaust gasses can be heard a bit better, but it's still pretty much silent.
The only theory behind drilling out the stock exhaust tip, is that the tip is often so small, and the exhaust thuds cause the exhaust gasses to go very fast at the tip, causing unwanted back pressure.
The 50cc is pretty ok, but my EVO150 has a smaller exhaust tip than my 127cc, and it's only a tiny bit larger diameter than on my 50cc!
By drilling holes next to the exhaust tip, the gasses will exit the muffler chamber easier. The noise increases but only slightly, mostly only noticeable in idle.
By reducing back pressure, less gasses will try to go back into the exhaust pipe towards the valves, supposedly creating a more efficient engine, but slightly more loud scoot.
I personally think for the ATM50 that the modified stock muffler is good enough for exhaust gasses to exit at a reasonable speed, instead of investing in a performance exhaust where the tip is so wide that the muffling effect is almost gone, and you'd be investing $100 to 'perhaps' go 1MPH faster???
While drilling 3 holes in an exhaust costs almost nothing.
I'm still running tests, but so far I'd have to say that on the ATM-50 the mods are pretty useless to do.
I have the impression that acceleration is lower, but top speed is higher.
I will probably do the same on my EVO150 soon, after I see some more MPG numbers.
Just test riding the ATM for now.
When changing the exhaust to a performance, or drilling it out, there's no need to re-jet, as you're not changing the AF ratio.
Only when changing the air intake (like removing restrictions) do you need to rejet.
If the bike is running fine, and you're just putting a larger airfilter but there's no real huge difference between both airfilters, you could always adjust with the AF screw, and rejetting might be entirely unnecessary.
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Post by Deleted on Jul 25, 2013 11:19:13 GMT -5
I also drilled out my stock before getting a performance exhaust. The performance exhaust seems to perform better but yours is the cheaper option. I run leaner now BTW after the pipe install.
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Post by sailracer on Jul 25, 2013 11:28:48 GMT -5
THREE 3/4" HOLES???!! isn't that almost like removing the whole back cover of the exhaust? Or do you mean 1/4 or 3/8 holes?if you add them all up along with the stock outlet, you have a 3" exhaust  |
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Post by Deleted on Jul 25, 2013 13:53:13 GMT -5
LOL DX
Didnt catch that
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Post by prodigit on Jul 25, 2013 16:45:33 GMT -5
Sorry, yes,  I meant to say 3x 3/8" I first started with 1/8", but 3/8" sounds better. I put 4 holes, but I think it's a bit too much. Besides, the bottom hole is prone to rain entering the muffler, so it's better to just drill 3, one on top, and 2 to the sides, and forget about drilling a hole under the exhaust tip. The extra holes make it sound a bit more gritty, also at idle it very much resembles the sound of a stock 150cc scoot. |
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Post by sailracer on Jul 25, 2013 16:59:05 GMT -5
yeah, I thought it had to be a typo ;D
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Post by ltdhpp on Jul 25, 2013 20:43:18 GMT -5
When changing the exhaust to a performance, or drilling it out, there's no need to re-jet, as you're not changing the AF ratio. Only when changing the air intake (like removing restrictions) do you need to rejet. If the bike is running fine, and you're just putting a larger airfilter but there's no real huge difference between both airfilters, you could always adjust with the AF screw, and rejetting might be entirely unnecessary. You sure about all that? The stock exhaust header pipe is one of the MAIN restrictions on a scooter and is about the same size as the muffler outlet. You can drill all you ant but you will never get rid of that. Some stock airboxes may as well be free flow, in which case an up-jet is absolutely necessary with aftermarket exhaust. Also, most people try both things before posting a comparison thread and making a conclusion about them... If drilling holes in the stock muffler was a ticket to free power, I'd think someone would have already figured that out by now... just sayin'... |
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Post by prodigit on Jul 25, 2013 21:54:36 GMT -5
the more freeflow the exhaust is, the more fresh air/fuel ratio goes into the bike.
Notice that no one really restricts exhaust?
Unless if it's done with purposes of degrading performance.
Mainly air intake is restricted, to reach upto EPA norms. By restricting air intake, they can create a 50cc engine, with fuel economy and EPA ratings of a lower cc engine; but it's also performing as a lower cc engine, restricting speed somewhat, which one could also get by restricting exhaust, which is only done for one reason, to lower sound emissions).
The only reason I can find when using a free flow exhaust, is when the engine is already running lean, the ratio may be even more off when it's free flow. But normally, when the ratio's are correct, when using a freeflow exhaust, compared to a more restrictive stock exhaust, the A/F ratio's in the carburetor should still be the same.
The freeflow exhaust allows less combusted gasses to remain in the cylinder, and more fresh fuel mixture to enter the cylinder.
So, yes, in some sense I think you're right, when the remaining gasses in the cylinder (mainly CO) mix with the fresh mixture, more carbon will mix with oxygen, and make the mixture run richer, as there's less available O2 in the cylinder, and the remaining CO exhaust gasses in the cylinder will compensate for a too lean running bike.
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Post by gitsum on Jul 25, 2013 22:08:36 GMT -5
Oh my god, where do you get this stuff from?
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Post by skuttadawg on Jul 25, 2013 22:59:24 GMT -5
Prodigit I hope you upjetted to with the extra flow to avoid being lean . A buddy of mine drilled 3 3/8 holes in the end of the muffler and 2 in the airbox and upjetted . It did sound more like a 4 stroke and performed better . I saw a video where a guy cut off the nozzle tip ( IMO many are too small ) , used a Uni bit to bore out the hole , gutted the baffling , rewelded it together along with a larger diameter short pipe to replace the stock tip . I have seen many stock exhausts crack , rust and rot . I think its crazy to spend 200 for a generic aftermarket exhaust for a 600 to 800 4T scoot , but Im sure many aftermarket exhausts are worth the money compared to stock .
On my 2T when I put on a BBK I also installed a Uni and racing pipe with no baffle . Dang its louder than a Yamaha YZ80 , lost takeoff power since it does not build up as much back pressure , but man the midrange and top end are amazing . Exhaust plays a more important factor in performance on a 2T than on a 4T .
Stock airbox do not choke the intake air flow as you would think . Drill one hole and maybe 1 or 2 more later after testing and upjetting . With a Uni or K&N you can not ride in the rain as water would get in the carb and large enough puddle splash could hydrolock the engine . The Uni you can hear it sucking in the air so it will be a lil louder too .
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Post by rockynv on Jul 26, 2013 22:06:41 GMT -5
It is a balancing act to have enough back pressure to maintain good intake vaccume and pulse for acceptable low end acceleration which is usually at the sacrifice of top end speed. You change the back pressure then the intake pulse changes also which then can have a positive or negative effect on the tuning of the length of the intake and port or tube leading to the air box/filter along with the jetting. On some bikes they do take the time to calculate the length and diameter of the intake to tune it to the pulse of the engine with the factory exhaust when they design them making compromises as they go along. You modify any part and go overboard especialty in a system without a lambda sensor and you have to manually retune the entire system to get the job done right.
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Post by jerseyboy on Jul 26, 2013 22:28:15 GMT -5
[replyingto=rockynv]rockynv[/replyingto]Intresting post,, I do all my tuning by ear,smell,and feel for years now,,but I like to read a scientific explanation like that,,good stuff there!  |
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Post by prodigit on Jul 27, 2013 2:00:34 GMT -5
IMHO, the exhaust functions separate from the intake, but the intake affects the exhaust.
By opening up the exhaust, all you're really doing is finding a more efficient way for the exhaust gasses to escape the cylinder, allowing more fresh fuel mixture in the cylinder at the next stroke.
The more back pressure, the more exhaust fumes will be present in the cylinder.
Changing the air intake affects air/fuel ratio, and normally needs to be re-jetted.
As far as exhaust tuning, the tuning is only done on the exhaust tip/muffler. Anything where the exhaust pipe ends and the muffler takes over, is considered outside of the exhaust tuning.
The exhaust pipe has not been modified, only the muffler or exhaust tip.
The gasses escape the exhaust pipe, and get into the muffler.
If the muffler has a too small exhaust tip, then a higher pressure will develop inside of it, as exhaust gasses enter.
The pressure on both the exhaust tip's end, as well as the exhaust pipe's end is equal. meaning, that if the exhaust tip diameter is as large as the exhaust pipe diameter, just about as much pressure will be put on both sides.
Make the exhaust tip larger, and more exhaust gasses will escape the muffler through the tip, than through the pipe back in the engine.
As you know exhaust gasses are exited in pulses, creating pressure and sub-pressure waves. These sub-pressure waves can be eliminated, and become a constant pressure flow, when the muffler does not have a large enough exhaust tip.
I did some tests on my EVO150, but can not say I notice any improvements.
So far there was a strong head wind, allowing my bike to only go 50MPH, however going on the highway with no wind, it reached 60MPH with ease.
I think it has bad gasoline, but I'll change it with the next fuelup, and see if there's any improvement.
Drilling open the muffler, also makes me able to hear exhaust pops on decel (only when the throttle is completely released).
I guess they where always there, only now they can be heard better....
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Post by ltdhpp on Jul 27, 2013 9:59:57 GMT -5
Or, maybe they were not always there, and now you are running lean???
You should always consider all possibilities and experimenting is the only real way to determine your condition.
Have you tried adjusting your needle or at least the a/f screw yet, if it's adjustable? Do it! ;D it may even make it faster...
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Post by gitsum on Jul 27, 2013 10:09:45 GMT -5
Drilling open the muffler, also makes me able to hear exhaust pops on decel (only when the throttle is completely released). I guess they where always there, only now they can be heard better.... No, No, No!!!!! This is completely wrong. The engine is like an air pump. Altering the exhaust for better air flow means the engine can pump more air. More air means more fuel is needed. Pops on deceleration usually means a lean pilot jet. Did you think this was just a coincidence? Can you make the connection now? On a four stroke engine it is true that intake mods usually effect the fuel mixture ratio more than exhaust mods. But exhaust mods still have an affect and you are experiencing one now. Do you get it? Your knowledge of tuning is absolutely screwed up, even though you try to sound like you know what you're talking about. I really hope people that don't know any better aren't reading your garbage and mistaking it for factual  |
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