Porting the exhaust side

[gobucks]

if you port the exhaust side of the cylinder, will this affect anything such as backpressure, fuel mixture, or anything that will cause you to have to adjust anything else?

Also can you just open up the part of the port that is closer to the head pipe and farther away from the cylinder or do you have to open up the part on the cylinder wall too?

[noiseguy]

The idea that the port should be the same cross-sectional area at the cylinder and at the outlet. You shouldn't have to mess with jetting as you're not touching the intake side (yet). It will affect back pressure... that's kind of the point. Which means that running at idle may suffer for better performance... or it may not start at all. Baby steps.

Stage zero (stock) is to clean out all the carbon from the port. This gets things to stock. Clean out your carboned-up exhaust too.

First stage is to "clean up", which is smoothing the rough casting and removing any bumps.

Second stage is widening the port at the cylinder. This doesn't affect port timing, but wider isn't necessarily always better. Again, cross-section should match, and there's a limit to how wide you can make the port.

Third stage is raising the exhaust port. This messes with timing and there's a high likelyhood you will trash a few cylinders experimenting. As long as you're OK with that, go ahead.

Read up on porting in adv tech docs for more info on what to do.

No one has yet posted up porting figures for these engines, which would be known good changes to porting. I'd be interested in seeing any, but am not holding my breath.

[tru72]

going to wide can cause problems, it can start to "hook" the rings and it wont transfer into the upper RPM range..
be careful and have fun!

[noiseguy]

I think that the ring "hooking" could be lessened by making the exhaust port more oval and less square, with larger radiuses in the corners.

Make no mistake that a wider port will shorten ring life... race exhaust ports widths are something like 70% of the diameter of the piston, and need to be re-ringed every weekend.

[Kenny_McCormic]

I think that the ring "hooking" could be lessened by making the exhaust port more oval and less square, with larger radiuses in the corners.

Make no mistake that a wider port will shorten ring life... race exhaust ports widths are something like 70% of the diameter of the piston, and need to be re-ringed every weekend.

But please do recall how a race engine is treated.

[noiseguy]

You don't want a "race tune" porting on a scooter. Without a clutch and a manual transmission, it would be unridable. Racing 125-250cc 2-strokes are tuned to make big power at 11-14K in a very narrow band by sacrificing power at all other RPMs.

[tru72]

very true about the shape noiseguy!

[gobucks]

would opening the outer half of the port where the exhaust bolts up and the end of the pipe to match, do any good if the half closer to the cylinder isn't touched?

[noiseguy]

would opening the outer half of the port where the exhaust bolts up and the end of the pipe to match, do any good if the half closer to the cylinder isn't touched?

Well, the inside and outside need to match... you tell me. My guess is that you'll need to do both.

[noiseguy]

But please do recall how a race engine is treated.

Kenny: You should read the Jenning's books and articles in Adv Tech Docs if you're interested in porting. The best work on this at the owner level is 30 years old; back when 2 strokes were more common.

Here's a good Jennings article on top-end rebuilds; he discusses light porting within:

http://edj.net/2stroke/jennings/topend_rebuild.pdf

I have another, more academic book from SAE on 2-strokes which has a good chapter on design fundamentals. I'll try to find it and get it scanned.

[gobucks]

So, both of the port openings (the cylinder and the bolt up side ) have to match opening areas. So, it won't do any good to make one side of the port (the outside, bolt up side) larger,but not the other, because i'm not sure if i want to be messin' around with grinding down any part of the cylinder wall right now.

[noiseguy]

i'm not sure if i want to be messin' around with grinding down any part of the cylinder wall right now.

Then leave well enough alone.

[evilone]

Have a proffesional do it. I had intake ported and polished, head shaved, and ported exhaust port in cylinder. He threw in the cylinder head. All for $90. Worth it to me.