Exhaust porting

[axo]

[newbie mode=ON] Ok i ported the case for the boost port, now i want to get the exhaust port on the cylinder very shiny and smooth..
I have read some guides on exhaust porting but seem to be too general and still have some doubts . Here is a good video on it: http://www.youtube.com/watch?v=ZCVU18g_3xQ (it has 3 parts)

So i have this new taiwan cylinder, and the port edges are very sharp, out of shape and very rough on the inside.
I understand i should not enlarge them so i do not change the timing. But i read the exhaust port could be enlarged like one millimeter to the top
of the cylinder, maintaining the round shape so the piston rings will not get stuck in there.
The video shows really nicely how to do the job.. but my question is if there is any kind of calculation i should make with my cylinder or is it the same for most of them like "just go and grind about one millimeter and it should be good" ?
Mine is a 82cc Taiwan bore.

And how does it work...? my guess was that if you enlarge the exhaust port upwards, you give the rings less room to squeeze air to the top of the cylinder, so you take compression out of it right? like punching a hole in the middle of a manual bicycle pump... it would compress less air ...

[/newbie mode]

[axo]

Come on guys... let it go... give me the obscure dirty little secrets you have on hacking the chit out of the cylinders with a round file in hand.

hehe.. no prob.
I have been reading a lot of older posts on the subject.
Seems more like you guys do it with your stock cylinders, or know what to do if the cylinders are Malossi or Polini, i guess because they kind of maintain the same shape and measurements from the factory.
But because this Chinese piece of iron i have here, with so many irregularities, i guess i would have to measure everything myself just to check the timings and then decide what could be done with it... not right now... may be later if i learn more on the subject.
By now i will just smooth it out and try to correct the very sharp edges but thats all.

[Bear45-70]

You can widen the port without changing the port timing. Making it taller DOES change the timing. There was a thread on it with pics at one time. Have you looked in Wikispreedia?

Here it is, right where it should be. it is for a Spree but the principle applies to ll 2 strokes.

http://hondaspree.net/wiki/index.php5?t ... st_-_Spree

[SPREEtown]

okay, about to reveal my ineptitude: raising the exhaust port will allow exhaust gases to enter th cylinder sooner, meaning a greater volume of gases, meaning more compression/power? while widening it will increase/improve the flow, in a different way?

if you couldn't tell my basic understanding of the 2 stroke engine is lacking a bit..

[Wheelman-111]

Greetings:

Raising the exhaust port allows the burnt mixture to begin leaving the chamber sooner, effectively terminating the expansion stroke earlier in the cycle. That sounds counter productive, until you consider that it also means that the transfer of fresh mixture can also begin a bit earlier. Through the complex and simultaneously-occurring migrations of gases outlined here, the net result of earlier exhaust port opening is a higher-revving engine. Horsepower = Torque x RPM.

[SPREEtown]

oh...

haha thanks for the info. definitely cleared some things ups.

[axo]

Ok thank you for the links guys
Today i worked on it with the dremel (not exactly a piece of cake job), removed some very rough spots on the exhaust port and chamfered some very sharp edges to help the break in process. I polished the exhaust port up to 800 grit sandpaper so now it is pretty smooth and shiny.
Even after looking at that guide on the wikispredia, i believe that is a good guide for stock cylinders that are known to have limitations from the manufacturer, but not just for "any" other one because it is a 2 stroke engine. Like you wouldn't just raise the port 2 rings up on let say a Polini cylinder right?... they already might be shipped with tight timings because these are high performance parts. You could just go ahead a wide up the port but you might hurt the life on the rings, or may be performance.
I have no clue about the settings on my Taiwan setup and no one here could give an opinion or said to have experience with it, so i wouldn't risk changing the timings because of my own ignorance. I do not know what the limits are on this.

[Wheelman-111]

Greetings:

Port timing numbers are recorded in my Flash 4.2 Build. This was with a 44.1 ministroker crank however. Sprees open very late, and their power - such as it is - is better down low. It has to be in order to run the single-speed tranny. Stock port timing is more aggressive on the Elite models, and I suspect more so on the Aftermarket kits. That translates to an engine that packs more torque up higher on the RPM scale, hence more Horsepower. It'd be interesting to compare the timing between Polini's Contesta and Corsa kits.

[elitedio]

You are light weight. Your power band may be narrower and at higher rpm but it can be compensated for with lighter rollers and lighter clutch springs if needed.

I dont like using the dremel tool for fine work. What you don't want are waves in the port. Waves are much worse than roughness. I use small riffler files, they give a much smoother finish. They work pretty well on cast iron.

[axo]

You are light weight. Your power band may be narrower and at higher rpm but it can be compensated for with lighter rollers and lighter clutch springs if needed.

I dont like using the dremel tool for fine work. What you don't want are waves in the port. Waves are much worse than roughness. I use small riffler files, they give a much smoother finish. They work pretty well on cast iron.

The Dremel was used to remove the big stuff.. but it is an akward tool to use because of its size. I ended up smoothing out everything with files, sandpaper and patience.

[Bear45-70]

For small fine dremel work, I like this one from Harbor Freight. It is about a 1/4 of the size of a regular Dremel tool.

http://www.harborfreight.com/80-piece-r ... 97626.html

[axo]

Wow.. thats just 10 bucks but i wonder if 16k RPM would do against the 35k of my Dremel tool. The dremel couldnt cut the boost port on less than three quarters of its maximum setting.

[Bear45-70]

Wow.. thats just 10 bucks but i wonder if 16k RPM would do against the 35k of my Dremel tool. The dremel couldnt cut the boost port on less than three quarters of its maximum setting.

You just gotta go slow and use the right bit or wheel. It's about finesse not speed or brute force.