Part Six: The Exhaust System

[Mastermind]

We've covered the exhaust port.

http://opeforum.com/threads/part-one-the-exhaust-port.7279/

The intake port.

http://opeforum.com/threads/part-two-the-intake-port.7290/

And the transfer ports.

http://opeforum.com/threads/part-three-the-transfer-ports.7306/

And compression.

http://opeforum.com/threads/part-four-compression.7318/

And ignition timing.

http://opeforum.com/threads/part-five-ignition-timing.7331/

Now let's talk about the exhaust system. Mufflers, cones, tuned pipes, etc.

How much is too much outlet?

Is there a "golden rule" to follow?

Why do we need that damn fire screen?

How does a tuned pipe work?

Is there any way to keep the engine quiet and still make solid gains in performance?

Ask lots of questions fellers.

 

[Johnmn]

Is 80 percent the exhaust port the rule to follow? How about single vs dual port does that change the rule?

 

[Fruecrue]

Subbed

 

[dustinwilt68]

I myself like a gutted muffler that is not "to" loud, dual port is generally my go to style with a 288 deflector, I also like a fire screen. I dnt run race saws, I just cut firewood, I want performance but not at the cost of my ears and setting the woods on fire. Jmo

 

[Mastermind]

Is 80 percent the exhaust port the rule to follow? How about single vs dual port does that change the rule?

Blowdown time changes everything. With a shorter blowdown more opening is needed. At least that's what I've seen.....

There really aren't any hard and fast rules.

When I'm looking to get all I can......and the noise is no worry. I like to do outlets on top and both sides. I think that a flat front helps because of wave reflection, so that's why I like side and top outlets.

 

[Johnmn]

I usually gut the inside baffle if there is one and open the outlet as big as I can under the factory deflector then install the spark screen again. So far I have enjoyed the single port sound and performance.

 

[Backtroller]

I always wondered about the frequency or resonance (not sure how you would term it) and how that bothers individuals, with normal hearing, differently.

For example, when I de-catted my 6421 and opened the exhaust slightly, the sound (drone, frequency, resonance) bothered me. It wasn't that much louder, the "new" sound of it bothered me. When I installed the 7910 cylinder and matched it to the exhaust outlet size it didn't bother me anymore. Now that Mike ported it, its louder, but it's not as bothersome as it was when it was a 6421. I'm not sure how that can be?

My MOFO 026 should be awful with as much area is open on that can, but it's really not bad. Granted it's only 50cc but still a larger percentage of that muffler is open.

 

[Adirondackstihl]

Subbin'

 

[Mastermind]

I always wondered about the frequency or resonance (not sure how you would term it) and how that bothers individuals, with normal hearing, differently.

For example, when I de-catted my 6421 and opened the exhaust slightly, the sound (drone, frequency, resonance) bothered me. It wasn't that much louder, the "new" sound of it bothered me. When I installed the 7910 cylinder and matched it to the exhaust outlet size it didn't bother me anymore. Now that Mike ported it, its louder, but it's not as bothersome as it was when it was a 6421. I'm not sure how that can be?

My MOFO 026 should be awful with as much area is open on that can, but it's really not bad. Granted it's only 50cc but still a larger percentage of that muffler is open.

At one time I had 9 ported 372XPs.

Over a few months of using them a lot I grew to hate the higher pitch sound they made.

I still have one.......that I rarely use. lol

 

[Motorhead]

How can 80 percent of exhaust port opening be calculated

 

[Mastermind]

How can 80 percent of exhaust port opening be calculated

The last grade I passed in school was the 7th. So.......I have no idea. lol

I did get a GED and go onto trade school later. But math still is not my strong suite.

 

[p61 western]

Can you get away with using the same rule for the exhaust port for the muffler opening, or would it be to much?

 

[Mastermind]

Once you get past 100% idle quality can suffer. But I've seen saws running fine with double the port area for outlet.

 

[Mastermind]

I do a lot of Husqvarna mufflers like this.

 

[Mastermind]

 

[Manofstihl066]

That's some clean muffler work.

 

[Mastermind]

That's some clean muffler work.

Some of the guys here make my stuff look like crap.

I do try to make them look good though. These Husqvarnas will be sold new in the box with a full warranty. So, in this case looks matter.

 

[Manofstihl066]

Some of the guys here make my stuff look like crap.

I do try to make them look good though. These Husqvarnas will be sold new in the box with a full warranty. So, in this case looks matter.

Oh yea I have seen some really nice muffler mods on this site. But I do like the simple effective look.

 

[Simondo]

How can 80 percent of exhaust port opening be calculated

I get as close as i can in Sq mm aria of the port by the usual .. top to bot ..times.. side to side measurement and then convert into circular aria with a conversion program. Im mainly using round tube so the last bit is needed to know the tube size that equates to the % of the port. If your going for a oblong muffler output then % of Sq mm aria is all you need.

 

[Al Smith]

I gut them,no baffles,deflectors screens .Maybe another hole total of around 80-85 percent are of the ex port .
You get the hole{s} too big you just might over run the carb .A bigger carb is not always the answer either .
BTW those old kart engines that ran megaphones had huge often multiple carbs ,they didn't start real easy and in a race they splattered the racers behind them with castor oil and fuel .By the end of a 20 lap feature event those guys looked like oil field workers .Damned things would idle at around 4000 rpm .Much less they loaded up and flooded .The noise was deafening even in the stands .