Tin can opener anyone?

[buttercup]

My first attempt at opening a muffler on my trusty 445, it was surprisingly easy with some good pliers...

The original exhaust outlet diameter is about 12 millimeters (0.47") in diameter.
I was planning to just open up the original hole and use my famous exhaust outlet formula to calculate the ideal diameter, only problem is that according to the formula it should be about 20 millimeters (0.79") in diameter at 50cc - that is huge!
Not sure my neighbors are gonna be happy about that... what should I do - set willy free or be sensible?

 

[buttercup]

The original formula is for a tuned 2 stroke muffler, 10+ inches of exhaust tubing and a proper muffler behind it. That's going to create more flow resistance than a chainsaw muffler, I guess it should ideally be somewhat smaller to create the same effect...

 

[computeruser]

A second smaller opening with a deflector will give you the flow you want without making it unduly noisy.

 

[buttercup]

Ok, I'm not someone who can do anything just a tad, I need something solid to base it on what ever I do.

I did some measuring and investigation and I realized that my formula is meant for a lot hotter engines then our lazy chainsaws, it just couldn't be right - perhaps it would be more realistic for a magnum ported race saw.

Anyway I traced the exhaust port on the inside of the 50cc 445/450 cylinder and then traced it in my cad station to measure the area, I already know the exhaust outlet is about 12mm (0,47") in diameter.

The area of the exhaust port is approx 203,83 mm2, using the formula would have made the exhaust outlet way to big.
The area of the 12mm (0,47") diameter exhaust outlet is 113,1 mm2, it comes out at approx 55,5% of the exhaust port area.
Using the well known suggestion of about 80% would make the exhaust outlet approx 14,5mm (0,57") in diameter.

That's actually not much more opening than it already have so just grinding off the original outlet hole about 1,25mm (0,049") around the edge and make it from 12mm dia (0,47") to 14,5mm dia (0,57") should be sufficient to comply with the 80% rule of thumb.



I also have found one obvious area-restrictive point in the joint between the crankcase and cylinder transfers that I will open up accordingly, and I will polish and smooth the transfer channels in the cylinder a little.
Perhaps together it will all make it just a little more effective engine without it being excessive noisy without any real benefit - if not more environment friendly.