Chainsaw Porting Theory

[drf256]

Fascinating stuff for sure.

If you're getting less fuel in and less out, how does a full circle crank produce more power? More RPM from the same saw? It has to be.

It seems like the 044/046 hybrid might unlock some on the full circle crank secrets. There you have a tighter than normal case on a cylinder. It seems to want to pull rpm and likes different timing.

FWIW Jason, I see tons of case compression, but a lower volume. I bet u need lower physical transfers. If you increase the intake, you'll probably get tons of spitback and negate the effects of all that work. It should need less, not more time to fill the case.

Making more power has to be more work over time, so an increase in RPM.

 

[XP_Slinger]

On the crank case compression subject...what are your opinions on when the intake should close in relation to transfers opening? I haven't seen this discussed yet and in my mind, this cycle, if timed improperly, could absolutely ruin an other wise good running engine. Thoughts??

 

[drf256]

Case compression time.

Interesting topic. I'd love to here some opinions as well.

I've heard that a good ROT in 40* or better is good, but I'm sure it's like everything else porting. It depends.

I know from reading Blair, that at least in motorcycles, there was a trend towards smaller tighter cases and less intake duration. The hey found that the transfers needed to be physically lowered or the charge was so violent, it pushed itself out of the exhaust. It also made the engines peakier with a less broad power band.

Finding the balance is where it's at.

At least for me, I see intake durations of under 150 in a small case allowing me to run more blowdown effectively. I'm seeing many of the pro builders here balancing the intake and exhaust durations as well.

 

[drf256]

I'm also seeing transfer durations between 70-80% of the balanced intake and exhaust durations.

 

[XP_Slinger]

Case compression time.

Interesting topic. I'd love to here some opinions as well.

I've heard that a good ROT in 40* or better is good, but I'm sure it's like everything else porting. It depends.

I know from reading Blair, that at least in motorcycles, there was a trend towards smaller tighter cases and less intake duration. The hey found that the transfers needed to be physically lowered or the charge was so violent, it pushed itself out of the exhaust. It also made the engines peakier with a less broad power band.

Finding the balance is where it's at.

At least for me, I see intake durations of under 150 in a small case allowing me to run more blowdown effectively. I'm seeing many of the pro builders here balancing the intake and exhaust durations as well.

Interesting info about too much case compression making the transfer entry too violent. It will most certainly depend on the balance referred to many times in this thread, but learning information such as you posted above helps, A LOT. Cause and effect brain storming is where it's at. Thanks for that post.

 

[drf256]

Hey, I could be wrong also. I really don't care.

Free discussion and thinking is where it's at IMHO

 

[XP_Slinger]

When I hatch a theory in my head I almost always assume it's wrong...then I work to prove it. Either way it turns out, I learn something. Case in point my 372XT that I'm working on. I raised the transfers and widened the exhaust yesterday and didn't get the gains I was hoping for. Now I'm rethinking the absence of my base gasket as a possible contributor to slower velocity out of the raised transfers due to it closing later than with a gasket installed. The saw doesn't run worse, but now I'm rethinking all the way back to square 1.

 

[drf256]

I've been told that I'm wrong, but I see losing a gasket and cutting the base as reducing case volume.

 

[Stihlbro]

Did someone say, no????

 

[drf256]

Good morning Joey.

 

[Stihlbro]

Morn'n!

 

[Stihlbro]

So is case compression the topic for today?

 

[Stihlbro]

I've been told that I'm wrong, but I see losing a gasket and cutting the base as reducing case volume.

I agree

 

[drf256]

I agree

Agree that I'm wrong?

Yes.

Let's do it.

CASE COMPRESSION

 

[drf256]

I'm gonna start drinking early today. I have to screw up some port work.

 

[wigglesworth]

I've been told that I'm wrong, but I see losing a gasket and cutting the base as reducing case volume.

Depends on where u measure. If your only measuring what's in the actual crankcase, then no. If you measure from the upper transfers down, then yes, it does reduce the total trapped volume inside the crankcase, before the transfers open.

Clear as mud?

 

[czar800]

I've been told that I'm wrong, but I see losing a gasket and cutting the base as reducing case volume.

When your delete the gasket and cut the base that physically removes volume. I don't see it any other way...

 

[wigglesworth]

So is case compression the topic for today?

Good Morning boys and girls, i'm your
substitute teacher, Mr. Buckwort....

 

[czar800]

Depends on where u measure. If your only measuring what's in the actual crankcase, then no. If you measure from the upper transfers down, then yes, it does reduce the total trapped volume inside the crankcase, before the transfers open.

Clear as mud?

That helps thanks!!

 

[drf256]

Depends on where u measure. If your only measuring what's in the actual crankcase, then no. If you measure from the upper transfers down, then yes, it does reduce the total trapped volume inside the crankcase, before the transfers open.

Clear as mud?

When your delete the gasket and cut the base that physically removes volume. I don't see it any other way...

That's how I see it. But I've been told I'm wrong by some respected sources.

So, we cut the base. We reduce trapped volume. We increase pressure at the same intake closing point, or we have the same pressure and less volume?