Part One: The Exhaust Port

[XP_Slinger]

I think you are giving it a smaller area to vent when it first opens. Yes it's flat but not near enough area I think

What I was thinking is make the flat your target opening number. Not raising the flat beyond your desired port height.

 

[XP_Slinger]

On a 7900 I've got to tear down several that my loggers have ran the piss out of. On this particular saw. This is what I like. And remember I build these differently per the customer. but the ones that seem to run the best and wear great. Are like the left pic and I'll explain. The right pic is more arched with straighter side walls making for a huge exhaust. But has a nice curve so not to hang a ring. But both let's say 70% width. The left one is super flat on top but then has a big corner that drops a a steeper arc to mid port. This style seems to flow just as well but not pit as much skirt wear on. The bigger the port seems to really wear on the bottom of the skirts faster.

Thoughts.?
Apologize for the pics as the suck. But hopefully I got what I was trying to say across. lol. View attachment 68359

That makes a lot of sense. Gets the flat advantage without risking a snag. Thanks for sharing

 

[Miller Mod Saws]

Maybe this will help. This is a 1/4" burr. See how it matches the factory corner. I go straight out to 70 percent. The 2 black marks will be super flat and then arc down to where I went straight out. Should have marked those in red. Lol.

 

[Motorhead]

Ok fellas, bear with me on the drawing as I'm not the artsy type. But, anyone try a shape like this? Would give a flat area for rapid release of pressure and potentially better scavenging vs an oval but the ring would be safe. Not as optimum as flat all the way across but maybe a way to get part of the flat advantage. Justathinkin on this flat thing and the idea popped in my head
View attachment 68356

Home lite has that on a 410

 

[Miller Mod Saws]

This may not be the ticket. Both seem to perform equally or so seems in my test. Flatter with steeper arc vs little more arc with bigger straighter sides. But the flatter more arc seems to wear the piston less.

Am I crazy lol.

 

[CJ Brown]

Has anyone tried forming more of a bell shape where the port and the cylinder wall meet? So instead of the exhaust flowing past a sharp edge into the port, it flows around a bit of a curve?

 

[Deets066]

Has anyone tried forming more of a bell shape where the port and the cylinder wall meet? So instead of the exhaust flowing past a sharp edge into the port, it flows around a bit of a curve?

You would loose compression and ring seal

 

[GoBigBlue1984]

I know of a couple 66 work saws that blow that theory.

I've had stock 660 jugs at 95/96

 

[CJ Brown]

You would loose compression and ring seal

Well what I mean is, stay within your timing and width numbers with the edge of the bell. Your port area at exit of the cylinder would actually decrease, then increase as you traveled further towards the muffler flange.

I am just trying to relate what we did porting v8's 30 years ago. Exhaust ports in those engines would neck down just above the valve then widen as you traveled further towards the port exit. The thinking at the time is gasses do not like sharp turns, and even though the the port was a bit smaller above the combustion chamber, the flow was actually greater than if you simply ran the port straight out from the chamber. Think of the 3 or 5 angle valve job, with the angles blended for smoother flow.

 

[Deets066]

I was thinkin similar to this, for a work saw

 

[EvilRoySlade]

I have a few angles on this as I learn.
1) if you could design your own cylinder for any saw using the stock crank and piston, would any of you have target numbers, angles, volumes of ports. It's many steps backwards in the design but essentially it's what we are doing.
2) An engineer I believe would decide on certain goals and try to achieve them. Production is much different than play as we all know. So...peak torque at 9k or 12k, torque peak spanning 700rpm or 3k rpm.

I know those are general and all encompassing but it comes back to the exhaust;
As many have stated it varies by engine size, to connect with others comments I believe it's very much crank angle/cylinder volume/corrected compression ratios. It's been stated already but there has to be enough of an explosion to drive the piston down but only until the crank has passed an angle that the rod isn't needed as much for all of the leverage, that should be when the port opens. Now... if you can tell me the common denominator, I will tell everyone you're a solid fella.

 

[EvilRoySlade]

Well what I mean is, stay within your timing and width numbers with the edge of the bell. Your port area at exit of the cylinder would actually decrease, then increase as you traveled further towards the muffler flange.

I am just trying to relate what we did porting v8's 30 years ago. Exhaust ports in those engines would neck down just above the valve then widen as you traveled further towards the port exit. The thinking at the time is gasses do not like sharp turns, and even though the the port was a bit smaller above the combustion chamber, the flow was actually greater than if you simply ran the port straight out from the chamber. Think of the 3 or 5 angle valve job, with the angles blended for smoother flow.

I agree with this, gas doesn't follow sharp angles without disruption.

 

[mdavlee]

Work saw 066 = 100 on the exhaust

Race saw 066 = 92-94 on exhaust

94-96 is my work saw numbers.[emoji12]

 

[mdavlee]

Not trying to criticize at all. Just trying to learn.

Wouldn't that roof be considered too flat?

Think where most of the spring in the ring is. I've never hung a ring in exhaust. A few in transfers

 

[mdavlee]

I measure card stock to 70% and hold it rounded on the cylinder wall to get my width. Imo, 70% straight across is getting wide for a worksaw. 70% rounded is close to 65% straight across

I've took some 50mm bores to 38mm across the bore.

 

[tree monkey]

that's 76%

 

[tree monkey]

cast ring or steal ring?

 

[jake wells]

can't grind this port because you run into a problem called piston ring locator pins

 

[drf256]

My 346 could stand to be a bit flatter

View attachment 68358

I believe that is perfect myself.

 

[drf256]

This is a roughed in Husky 262 port. This is what I look for personally. I'm a work saw guy, if I try a race saw, I may take it more to the limits.