Modifying Exhaust Port

[Bugaboo]

I've heard of the rule that your exhaust port can be widened to 68% of the width of the piston. Here's a site that deals with that topic http://www.scooterinvasion.net/forum/vi ... er+porting

Wondering if anyone has advise as to a proper exhaust port width?

[Bugaboo]

here's a nice site on porting terminology
http://www.scooterinvasion.net/forum/vi ... er+porting

[Bear45-70]

You can do that but only if you ring ends don't limit how wide you can make it. Because if you go to far you have a junk cylinder.

[Trafficjamz]

before


after



I went right to the limit

the exhaust port is 35.56 mm wide

72% of bore size 50mm diameter * .72 = max.

[Bugaboo]

You can do that but only if you ring ends don't limit how wide you can make it. Because if you go to far you have a junk cylinder.

Do you mean if the opening of the ring is located were the port will be widened to?

What's the limit on how far you can go up? The exhaust port would have to be closed during combustion and I'm sure there's other concerns I don't know about

[Rooney140]

Do you mean if the opening of the ring is located were the port will be widened to?

What's the limit on how far you can go up? The exhaust port would have to be closed during combustion and I'm sure there's other concerns I don't know about

I guess that the end gap location would be a concern, but the main concern is the rings snagging on the enlarged port. Bigger ports = smaller cylinder surface area = less contact between the rings and cylinder (tension) and an increased chance of snagging.

[Bugaboo]

What about raising the port? what is a safe parameter?

[Trafficjamz]

What about raising the port? what is a safe parameter?

I went with, one ring width up if that helps

I don't know what "max" is .

Making the port wider is the equivalent of using more lift with a 4 stroke cam. While raising the port is the same as increasing the duration of the cam. A wider port retains the low end torque of the stock set up while still allowing the engine to breath better at the top end for more power. I am going the wider route, since the Spree's fixed ratio drive doesn't help the acceleration, I don't want to hurt the low end power at all.

Watch out when widening your ports, more port width = less ring life.
As your piston passes the widest opening of the exhaust port, the ring can enlarge slightly (less of a complete cylinder holding it tight).

[bakaracer]

What about raising the port? what is a safe parameter?

depends on where the exhaust pipe makes power@ a certain rpm. yoiu have to match the intake transfers too.if your not sure what your doing, dont port it. you can screw it up really quick. it only takes 1mm to much to loose power where you need it.

[Bugaboo]

depends on where the exhaust pipe makes power@ a certain rpm. yoiu have to match the intake transfers too.if your not sure what your doing, dont port it. you can screw it up really quick. it only takes 1mm to much to loose power where you need it.

Yeah If I'm gonna probably not do any porting this summer and see how my scooter preforms, but I'm very interested. It's definitely something I'm scared to do right now. Have two spare cylinders, so I'll hack those up before I go for the real deal.

[Bugaboo]

I know that you want the exhaust port smooth as possible. What is the proceedure for smoothing? Some kind of polishing compound or wax?

[Trafficjamz]

I know that you want the exhaust port smooth as possible. What is the proceedure for smoothing? Some kind of polishing compound or wax?

I used 3m flapwheel 120 grit

[Bugaboo]

Ended up just removing the bottom of the outside of the exhaust port and did a little polishing. And rounded the inside of the ports to let the rings move by easier.
Here's what I removed from the exhaust port

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