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Poor boy's answer to a couple of porting questions.

  • Thread starter Werks 4 da Man
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Werks 4 da Man

I didn't want to derail Al's thread so I'll respond here.

These are some good threads you have going Al.

Couple questions

I noticed you didn't do anythng to the intake port, other than polish it. I always thought or read that and from little porting I have done that if you alter the exhaust the same should be done to the intake port to keep the same amount of air entering the cylinder to be the same as what is existing out the exhaust. In other words if you widen the exhaust you should widen the intake the same amount. Does it even matter one way or the other because the amount of air volume is determined by what amount the crankcase can take in and hold.

I like your explaination about the muffler and how it works, much easier than some of those books I have been reading on the subject.

Now I will raise another question that might contriversial.

Ever since I have found out about modding muffler the norm seems to be to gut the muffler totally. I question this practice myself . I understand the why's of doing this for better flow and more power. Mufflers in most of today's saws seem to construsted not only to reduce noise but conduct heat away from the port and cylinder and tranfer it to the outer portion of the muffler. Most of the newer Sthils have a heat shield behind the muffle which to me acts like the exhaust manifold on a car engine. This is just my opinion and oberservation is that by gutting the muffler it loses it ability to conduct heat and weakens the muffler's internal structure. When i mod a muffler I try to improve the flow by either opening the baffles more or cuting out just small pieces in the direction of the exhaust flow.

This may seem to be on the conservative side but I also feel that gutting a muffler you affect the timing of the scavaging side of the cycle a little. This is just my own opinion is all. I don't have the answers because when it comes to modding saws I am just a shade tree mechanic but I am always open to a good disscussion and to learn more.
Alot of guys have read books and purchased software to try to arrive at informed values for each.

As far as the intake goes... chemically and physically the air/fuel entering the saw differs from the spent exhaust gas. In as much, there can be no linear comparison of the two.

A simple analogy for me when considering the intake is a straw. Consider a dampish morning when there is around 7% higher humidity in the air than there will be midday. The same amount as the fuel in the intake charge.

Suck through the straw until you fill your lungs. Now do the same without the straw. Now you have enough information to decide how big you want your intake to be. Well maybe not, but in can explain a little about how it works.

I also see your point about transfering heat via the muffler. This is definately Newton's Second Law of Thermodynamics question. Without going on a tangent, I would say that removing only the front of the internal baffle on a stihl is a good compromise, but if your oulets are large enough then the faster escaping gases would remove heat more efficiently than the wicking that the steel contact affords.

Big outlets, no baffle. Small outlets, baffle.

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