Real talk about saw porting theory (no arguments)

[drf256]

Another pics that's not mine, but it illustrates a nice roof. Many argue a lot is "left on the table" with this amount of shape. The sides can be squarer and the roof flatter, but this is safe.

When you're in the woods cutting and the saw is making you smile, do you care more that another guys saw is a second faster or more that you can have that smiler for a longer time?

 

[drf256]

Off to my day job. I'm sure others will chime in.

 

[mdavlee]

I use brake cleaner to get all the oil residue out so water or air will blow the chips out easier.

 

[EvilRoySlade]

Good talk so far. I'm glad it's just simple conversation.
Does anybody map the ports like in the books? I'm sure it helps understanding. I can definitely say it would help seeing port time/area.

 

[mdavlee]

Good talk so far. I'm glad it's just simple conversation.
Does anybody map the ports like in the books? I'm sure it helps understanding. I can definitely say it would help seeing port time/area.

I did some recently. It was interesting to see some saws the uppers doubled in area.

 

[Dirtmagnet]

This has been an extremely interesting and easily understood thread. Thank you @drf256

 

[redlinefever]

Yes I hope this continues further !!

 

[Al Smith]

If you're going to port map them use lined graph paper .I have but seldom do map .Mac Dizzey seems to like thin card board because it's stiffer .

 

[Mastermind]

Excellent posts Al.

I really don't have anything to add.

 

[Mastermind]

Looks like I kilt the thread. lol

If anyone ever wants to talk about a saw you are porting.......call me. Be ready to tell me what saw you are working on. Stock squish measurement. Stock timing numbers. Then I'll be asking what tools you have....... and things like can you do any machine work? Do you have a right angle?

If I can......I will answer, if I don't, just try again later. 931-310-XXXX

If anyone sends me a PM asking for my notes on any model I do......I'll gladly share that too.

There are too many variables to do a "one size fits all" porting thread.

 

[Chainsaw Jim]

Don't fall for it!!! That's the same number that's been sending me random dick pics for the last three years.

 

[Mastermind]

Don't fall for it!!! That's the same number that's been sending me random dick pics for the last three years.

Ain't nothing random about those pics....

 

[jmssaws]

@drf256 excellent posts

I'll say that there's a lot of talk about numbers and that's a important part of it but a small part of it.

Port shape,transfer direction and roof angle and opening angle,exhaust port width and how flat the roof is are the things that decide how it will run.

If it's 100 123 80 or 99 121 82 you won't notice the difference.

 

[drf256]

You'll hear "it depends" a lot here, cause no model the same.

Finally home.

 

[drf256]

Not sure what you guys wanna hear next. Stop me at any time, or interject with questions or corrections.

Exhaust roof height. Another highly debatable point to argue.

Compression and amount of swept volume are in part determined by the roof height of the exhaust. It's generally accepted that a lower roof height (I'm gonna talk physically, because a lower roof is a higher numerical degree number) creates more torque because there's more time for the combustion pressure to exert force on the piston crown. The issue is when does the pressure fall to a point where the pressure it exerts isn't worth the lower roof.

A lower roof means the blowdown, or degree difference between exhaust and upper transfer opening, can be slightly less than a higher roof-but not a tremendous amount.

So if you go with 15* of blowdown on a 105 exhaust, your transfers will open at 120. So now you've only got 120* of crank rotation to fill the larger swept volume you've created by the lower roof. You can't raise them to have more time to fill because the pressure from combustion pressure won't be vented enough to prevent backfilling them with exhaust. Again, a balance exists and exceptions occur.

Some guys like high exhausts and some (like me) like lower exhausts. The truth is they both work and there are trade offs to both.

 

[drf256]

I'm gonna use the MS360 as my example.

I raise my exhaust to 102-103 after the band cut.

Next is intake.

 

[jmssaws]

Not sure what you guys wanna hear next. Stop me at any time, or interject with questions or corrections.

Exhaust roof height. Another highly debatable point to argue.

Compression and amount of swept volume are in part determined by the roof height of the exhaust. It's generally accepted that a lower roof height (I'm gonna talk physically, because a lower roof is a higher numerical degree number) creates more torque because there's more time for the combustion pressure to exert force on the piston crown. The issue is when does the pressure fall to a point where the pressure it exerts isn't worth the lower roof.

A lower roof means the blowdown, or degree difference between exhaust and upper transfer opening, can be slightly less than a higher roof-but not a tremendous amount.

So if you go with 15* of blowdown on a 105 exhaust, your transfers will open at 120. So now you've only got 120* of crank rotation to fill the larger swept volume you've created by the lower roof. You can't raise them to have more time to fill because the pressure from combustion pressure won't be vented enough to prevent backfilling them with exhaust. Again, a balance exists and exceptions occur.

Some guys like high exhausts and some (like me) like lower exhausts. The truth is they both work and there are trade offs to both.

I like and typically use a low exhaust and have found that exhaust height has almost nothing to do with overall rpm,raising the exhaust moves the power up and the faster you have the peak power the more low end or grunt you loose and you have a saw that lives and dies on rpm and feels very peaky.

 

[drf256]

Intake timing is determined by when the skirt opens and closes the intake floor. This determines two things, the amount of time the crankcase can create negative pressure to pull charge in, and the amount of time that case compression can occur before the upper transfers vent it.

Saws with large carbs and intake tracts need less intake timing to fill the case. Saws with small cases need less time also. And vice versa.

Lower intake floors cause spitback and fuel waste. I believe a lower floor causes better throttle response (correct me if I'm wrong on this point).

The intake floor height will affect case compression as well. If you have an intake floor of 75*, you can raise your uppers all the way to 115* and still have 40* of case compression. But...

If your exhaust is at 105, you'll only have 10* of blowdown which usually is no good.

The bigger the transfers/the smaller the case/quad transfers all like physically lower transfers than the opposite.

In the case of the MS360, the intake is an upside down teardrop. The lowest part winds up at 81-84 after the jug drop. I square it at 78 so that most of the port closes at 78. Inertia and fluid dynamics play a role in what's happening at rpm here as well.

Here's @Backtroller jug

 

[drf256]

Be careful on the intake roof. See where the ring gaps fall on your model. On the MS360 I leave the roof on the left hand side as yo stare down the jug towards the chamber more rounded. The lower ring gap is there at BDC. I like to keep it tucked and see no reason to take a chance there.

Also narrower and rounder ports wear the piston skirt less.

 

[drf256]

Transfers are the black magic of porting. Maybe someone else can help me out explaining it.

It's yet another "it depends"