Porting a 365 special

[Matt 1183]

I am new to this site and saw porting and i have some questions..im working on a 365 special and sofar i have widened the exhaust and intake ports and cleaned up the lower transfers taking care not to remove any more material than necessary..i have removed the base gasket . The saw seems realy snappy at low to mid rpm but seems to be lacking a lot at high rpm. Im wondering what the best port timing would be for this?

 

[mdavlee]

What's the stock numbers?

 

[Matt 1183]

The stock numbers are ...exhaust opens at 103 atdc intake closes at 70 atdc and transfers open at 120 atdc..i havent checked the squish yet

 

[Matt 1183]

Squish is 028 without the base gasket

 

[weimedog]

Not bad numbers to start with.. and why a "base gasket delete" really perks things up! Might want to do a little on the intake. Move it to opening in the mid to high 70's BTDC. So if you are at 70 degrees BTDC on the intake, try advancing that by 5 degree's to start with. That along with a muffler mod, 1194 no base gasket build, will make for a snappy saw. Might want to set the no load RPM up from the recommended 12500 on a 365 special to 13000 as well. Then do a plug check after some run time to make certain it's not to lean.

I wonder sometime on the variations in numbers based on how a person checks their port timing. A typical "Stock" w/ no base gasket 372 runs in this range with how I check those numbers:
Exhaust: 100-101 degrees ATDC
Transfers: 123-124 degrees ATDC
Therefor blow down around 23 degrees
Intake Stock usually around 75 degrees BTDC

And I like to leave the exhaust alone.. because changing the "pressure can" (muffler) will a make larger difference in flow & less risky to the health and well being of said cylinder for the average guy with simple tools. Maybe match things up but I'm assuming this is a work saw.

AND I'm assuming no lathe work...just simple tools.

Your blow down numbers are shorter than I am used to seeing at 120-103=17 degrees. In your case, assuming the measurements are correct; raising the exhaust closer to 100 degrees would help it rev but....I really hesitate to go there or recommend that. I have to wonder if how you are measuring the timing is getting those numbers. I use sliver of light or wire depending on the cylinder style...but with the wire method you have to calculate the thickness of the wire into your measurement. And as I said, the intake seems a bit out of whack from what I'm used to seeing.

For a 372 a sold set of numbers measured with "sliver of light method" for a good running work saw would be something like:
Squish: .020"
Exhaust: Open 100 degrees ATDC
Blow Down around 20-21 degrees but that sometime requires some grinding in places that require a good set of skills and a good angle hand piece for decent grinding system. In your case assuming good numbers, raising the exhaust a bit gets you there.
Intake: 78-80 degrees BTDC
Muffle mod with 3/4 inch pipe or equivalent x-sectional area after figuring in for screens.

In the process of building a 365 with this formula. A video is certainly going to follow. SO far the first test case is more than a match of a stock 372 BTW..

 

[Matt 1183]

thanks for the great reply
Im using a degree wheel and a light to get my numbers.. I just did a nwp big bore kit for a 372 using the same method and got 104 exhaust 75 intake and 124 tranfers. And i forgot to add to my first post, i have a dual pipe mm .
So il will port to the numbers you suggest . And i have no lathe but i can take the cylinder to a local machine shop if need be.

 

[weimedog]

thanks for the great reply
Im using a degree wheel and a light to get my numbers.. I just did a nwp big bore kit for a 372 using the same method and got 104 exhaust 75 intake and 124 tranfers. And i forgot to add to my first post, i have a dual pipe mm .
So il will port to the numbers you suggest . And i have no lathe but i can take the cylinder to a local machine shop if need be.

Just run the squish you have, it will be strong and isn't worth the extra risk/money to get it to the .020 if you are .028..while not perfect, it will be fun just the same and its YOUR build.

 

[weimedog]

thanks for the great reply
Im using a degree wheel and a light to get my numbers.. I just did a nwp big bore kit for a 372 using the same method and got 104 exhaust 75 intake and 124 tranfers. And i forgot to add to my first post, i have a dual pipe mm .
So il will port to the numbers you suggest . And i have no lathe but i can take the cylinder to a local machine shop if need be.

Those aren't bad numbers at all. 104.... more torque than RPM's but good blow down numbers. And you could push that intake closer to 80 degrees BTDC. Larger exhaust numbers means a little more time where the piston is under pressure. 104... is pushing the point of diminishing returns in my mind, would rather see 100-102 and certainly wouldn't like 105 but yet again hesitant to suggest changing that. My smaller displacement saws I like those bigger numbers for the exhaust because I look for torque. I get that with larger exhaust numbers and pushing the compression and/or squish numbers. Again assuming simple tools. If a lathe is part of the equation, trimming the squish band and cylinder base will move the build to the next level of performance. But is it worth the money???

AND I have to say this...because I never trust anyone's timing measurements...I always would rather see them err on the side of conservative numbers, even leaving things alone if they aren't too far out in these online discussions. If they are standing in my shop & we are measuring where I can see the method and results...that's a different animal. (I don't do work for hire such as many of the builders here advertising.. BTW, just for my saws and a few locals. )

 

[mdavlee]

I like a higher exhaust but I'm gaining a good bit of compression with machine work.

 

[weimedog]

I like a higher exhaust but I'm gaining a good bit of compression with machine work.

AND your machine work moves the cylinder therefore exhaust down relative to TDC. Question might be if you had a number like 104 for exhaust before machining.. dropped your cylinder some...where would you want your exhaust to be when you completed your build?? Closer to 100?

My bet is for a "cut" cylinder numbers like this with how I measure, are pretty good for a 372 work saw:
Exhaust 100 degrees ATDC
Blow Down 20 degrees
Intake 80 degrees BTDC

Partly because the machining gets a smaller combustion chamber, better squish band characteristics and therefor torque to spare.

But this is with how I measure port timing.. can't stress that enough.

 

[mdavlee]

Some but I go higher on exhaust than stock on almost all builds.

 

[weimedog]

I've seen exhaust number in the mid to high 90's ATDC on cut cylinder 372's. Still with blow down in the 20 degree range & intake the higher side of 80 degrees approaching 81. They run strong! I still err on the conservative for on line conversation and for my personal saws... my 372's exhaust all sit around 100-101 BTDC ( basically stock as I leave them alone) with my method of measurement, but they aren't meant to be impressive to anyone but me out in the woods. They run stronger than stock too with my other changes & therefor if asked its what I say because its what I do. Understanding there are some really fast saws out there with different more aggressive numbers.

But as I mentioned before when in "online conversations"..I always question measuring technics. So when I read 104...I hesitate to suggest hacking that back to 100 "posters method of measurement" as that's a significant change. And I've never seen a cylinder stock with larger than 102 for the exhaust with how I measure those numbers. So in "online" conversation I tend to try and use relative vs. precise numbers as you can't be certain there is a "one to one" relationship with how the person on the OTHER side of the conversation gets their numbers...

What if what I measure as 102 is seen as 104 with the other person? I say cut it to 100....thinking that's a 2 degree change and through variations in measuring he ends up cutting it 4 degrees ..... just don't want to go there is all.

 

[Matt 1183]

I've seen exhaust number in the mid to high 90's ATDC on cut cylinder 372's. Still with blow down in the 20 degree range & intake the higher side of 80 degrees approaching 81. They run strong! I still err on the conservative for on line conversation and for my personal saws... my 372's exhaust all sit around 100-101 BTDC ( basically stock as I leave them alone) with my method of measurement, but they aren't meant to be impressive to anyone but me out in the woods. They run stronger than stock too with my other changes & therefor if asked its what I say because its what I do. Understanding there are some really fast saws out there with different more aggressive numbers.

But as I mentioned before when in "online conversations"..I always question measuring technics. So when I read 104...I hesitate to suggest hacking that back to 100 "posters method of measurement" as that's a significant change. And I've never seen a cylinder stock with larger than 102 for the exhaust with how I measure those numbers. So in "online" conversation I tend to try and use relative vs. precise numbers as you can't be certain there is a "one to one" relationship with how the person on the OTHER side of the conversation gets their numbers...

What if what I measure as 102 is seen as 104 with the other person? I say cut it to 100....thinking that's a 2 degree change and through variations in measuring he ends up cutting it 4 degrees ..... just don't want to go there is all.

Any recommendations on advancing ignition timing?

 

[weimedog]

nope... not that I'm going to post. Have to have an undetectable edge when Spike60 comes around with his saws.. Have over sized keys and a file

 

[mdavlee]

Any recommendations on advancing ignition timing?

1/3 of the key is about normal.

 

[Matt 1183]

1/3 of the key is about normal.

1/3 of the key is about normal.

ok..thanks