Modding a 7900

[Lakeside53]

Yep... No replacement for displacement...

 

[drmiller100]

My stock 7900 has two 3/4" outlet holes. I removed the internal baffle completely. I have .9 square inches on the outlet. it alone made a big difference in the saws performance.

Now, I am really nit picking...I was busting your balls about the double size comment...I understand what hole the gases flow out of...Your small hole isn't 1 square inch. It is just over 1/2 square inch..

pi r squared

Radius = 7/16 (.4375)
7/16 squared = .1914

.1914 x pi =

.600 square inches

It is double what the stock outlet was at .248 square inches.

sorry, you lost me. Your bone stock 7900 had two exhaust outlets??? I could only find one on mine. Where is the other one??????
On mine, a 1/2 inch drill bit fell through it. a 3/4 wouldn't.
so, I drilled it out to 7/8.

where did you get the 7/16 figure?????

 

[aquan8tor]

Im a neural here, but I think he's talking about the exhaust port size/muffler inlet vs. the size of the muffler outlet. The exhaust port on the piston is bigger, yes? 7/16 is half of 7/8---hence radius instead of diameter

 

[Freakingstang]

sorry, you lost me. Your bone stock 7900 had two exhaust outlets??? I could only find one on mine. Where is the other one??????
On mine, a 1/2 inch drill bit fell through it. a 3/4 wouldn't.
so, I drilled it out to 7/8.

where did you get the 7/16 figure?????

No, it did not have two outlets. I was saying it is bone stock other than the muffler being opened up. i drilled the stock outlet to 3/4" and added a 2nd 3/4" in hole. I added a second scoop to it. I also removed the baffle or cage inside the muffler.

here is a pic

Now, the 7/16 is half of 7/8. To find the area of a circle, you need the radius, not the diameter. The radius is 1/2 of the diameter.

pi(r squared)= square inches in area. pi = 3.141592654

so 3.14 (.4375 x .4375) =.600

 

[04ultra]

Freak we need more pictures.... Just cant seem to figure this out...



..

 

[drmiller100]

Now, the 7/16 is half of 7/8. To find the area of a circle, you need the radius, not the diameter. The radius is 1/2 of the diameter.

pi(r squared)= square inches in area. pi = 3.141592654

so 3.14 (.4375 x .4375) =.600

whatever. ok, you are right my absolute numbers are incorrect. However, I still have indeed over doubled the area of my exhaust outlet over stock.
9/16 hole is .240.
7/8 hole is .600. that is more then twice as big.

an interesting side note. a 3/4 hole is about twice as big as a 9/16 hole. So you have quadrupled your exhaust outlet. At what point is there too much?

 

[drmiller100]

here is a pic

Again, I'm all new to these saws, but looking at your exhaust pipes, that suuuuuure looks too rich to me.
On my saw, there is a little screw on the carb to adjust the highspeed mixture. You turn the little screw with the H on it IN to lean the engine out.

 

[bwalker]

that suuuuuure looks too rich to me.

I noticed that as well, but is it welding spatter? The 7900 I had was very clean burning and the outlet was always cocoa brown, not black.

 

[timberwolf]

Can depend on the oil mix, I know castor and some others when mixed 32:1 make a heck of a mess even though the saw is not runing rich.

 

[bwalker]

Can depend on the oil mix, I know castor and some others when mixed 32:1 make a heck of a mess even though the saw is not runing rich.

Any oil that wont burn clean at 32:1 isnt worth running.

 

[drmiller100]

why would you run 32:1???? why not just buy a decent oil and run it 50:1????

we're running dykes rings, right????

 

[timberwolf]

32:1, everything has it's application.

 

[SWE#Kipp]

50:1 pre mix , Here in Sweden Husky,Jred and Stihl sells it
Alkylat fuel with synthetic oil, it burns very clean and smells nice !!

 

[timberwolf]

Are we wading into another great oil debateopcorn: ?

 

[blis]

50:1 pre mix , Here in Sweden Husky,Jred and Stihl sells it
Alkylat fuel with synthetic oil, it burns very clean and smells nice !!

But its terrible waste of good alcohol

 

[Cut4fun]

Now here was my 7900 muffler running on poulan oil at 40-1, haven't seen a difference in color yet since switching to klotz in this saw. The factory outlet looks like it got some of the fresh paint inside it after the new coat.

 

[Four Paws]

haven't seen a difference in color yet since switching to klotz in this saw.

I bet it smells better, though! :biggrinbounce2:

 

[Freakingstang]

Again, I'm all new to these saws, but looking at your exhaust pipes, that suuuuuure looks too rich to me.
On my saw, there is a little screw on the carb to adjust the highspeed mixture. You turn the little screw with the H on it IN to lean the engine out.

The added scoop has weld spatter on it. (right side) lots of it, was done with a flux core welder...

The left is the stock scoop, with a brown tint. It might look black, but the saw is not running rich. It was when i got it though (used), the spark screen was almost plugged shut....


Yes, screwing the carb in leans them out. Have you figured how to get past the limiters on the jets yet?

Be careful of the rev limiter, you set it on the limiter, you'll burn it up in big wood.

 

[Freakingstang]

At what point is there too much?

Find 80% of the exhaust outlet at the cylinder. After that, according to some is too much. I have gone up to 125% without significant loss. Sure, at some point it just gets louder.


I could care less. It isn't a tuned pipe. It runs a whole lot better than it did stock.

Nothing is too much with that silencer/baffle in there. opcorn:

 

[04ultra]

Freak it looks better now....






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