Part Three: The Transfer Ports

[Terry Syd]

I spent quite a bit of time clearing that to get that feed equal at the corner.

My lower transfer tunnels can look down right 'industrial', however, the entrance to the cylinder takes me a heap of time to get right. You could call it more than 10 times then the time I spent on the lower transfers.

Think of it as a garden hose - you can increase the size of the garden hose big time, but it will always be the nozzle of the garden hose that creates the flow you see.

Big thing to remember when working transfers - the 'induction stroke' takes place between transfer closing and TDC. If you raise the transfers you DECREASE the induction stroke.

 

[Redfin]

I'd think you could ramp a dual port or step it half way to get a similar effect

You can and I have. You still cannot achieve the same results due to lack of seperation of the feed.

 

[Stump Shot]

You can and I have. You still cannot achieve the same results due to lack of seperation of the feed.

Hmmmm, not exactly, but some of my latest mimic this effect quite well. Took a lot of trials to get where I am now with it and was the most effort I've put into the saws since day one and it's finally paying off in brute power.

 

[mdavlee]

Dolmar 7910...

Talk to me about staggering the transfer ports vs not. I've had suggestions both directions. Im a rookie but I wrapped my head around the porting theory a little bit.

I figured the stagger would continue to direct the charge up into the squish/combustion chamber better than having them flat.

But I'm also considering the timing and the charge going out the other side...

Flat on both with 2 degree stagger for all out HP with the secondary aiming towards the plug.

 

[Redfin]

Hmmmm, not exactly, but some of my latest mimic this effect quite well. Took a lot of trials to get where I am now with it and was the most effort I've put into the saws since day one and it's finally paying off in brute power.

Eggzacry?

 

[mettee]

Flat on both with 2 degree stagger for all out HP with the secondary aiming towards the plug.

Would there be any other variations of this that would provide any other good characteristics?

So keep them flat like this:

P ______ +2 S ----------

And I am looking for 28-30 degrees blow down

@mdavlee thanks for chiming in I was hoping you would see it.

 

[Nutball]

I used to think have the intake side higher, but a couple months ago I thought about how having the exhaust side open first could clear out a bit more exhaust from the center of the piston and maybe get the flow going in the right direction a bit better, while not having too much exhaust get mixed back in to the fresh mix. I had the intake side higher on my 490, and it's really strong, maybe it could have been better the other way.

 

[mdavlee]

Would there be any other variations of this that would provide any other good characteristics?

So keep them flat like this:

P ______ +2 S ----------

And I am looking for 28-30 degrees blow down

@mdavlee thanks for chiming in I was hoping you would see it.

You can taper both of them from the exhaust side to the intake side for a wider power band.

 

[MustangMike]

Don't the ones that come tapered from the factory usually open on the intake side first? And I think Bridge Ports generally make that even more pronounced?

 

[Lightning Performance]

You can taper both of them from the exhaust side to the intake side for a wider power band.

Stihl did just that on most of the modern engines from 029 on and after the 041 era. Look closely it's 2-3@ the transfers window opening, Some models have more. Good tip Dave.

 

[stinkbait]

Has anyone every played with opening one sides transfers a little ahead of the other? I know ya'll have been discussing front or rear first, but what about side to side?

 

[mdavlee]

Stihl did just that on most of the modern engines from 029 on and after the 041 era. Look closely it's 2-3@ the transfers window opening, Some models have more. Good tip Dave.

The 046 had the intake side first opening. I always flattened them out at least.

 

[Nutball]

I'd think that would help cause some interesting mixing, but might not efficiently push out as much exhaust and instead mix it in with fresh air.

 

[mettee]

I think he means right and left side ports, which in my mind would cause issues. I tend to think you want the two charges to impact each other in the cylinder and head up into the squish area.

 

[kfd518]

I think he means right and left side ports, which in my mind would cause issues. I tend to think you want the two charges to impact each other in the cylinder and head up into the squish area.

Quite a few huskys have them staggered from FW side to PTO side.
272 xp and 288 xp off hand come to mind?

 

[huskihl]

Most 2 port Stihls are a couple degrees different from side to side

 

[Stump Shot]

I personally believe the side to side difference is unintentional, two degrees isn't enough to make a huge difference, especially at high rpm. If you watch how a cylinder is cast it's most likely just "off" that much from the process and evidently deemed acceptable by the manufacturer. Similar to the odd exhaust port that isn't quite centered properly and cylinder "numbers" different from one cylinder to the next.
This is just my opinion and not based on any actual fact. Also why porting is all about detailing what the manufacturer could not.
I also do not see(in my pea brain) any benefit to staggering from side to side in any great amount, as some contamination(fuel air mix and exhaust) from one side to the other may/will occur possibly.

 

[stinkbait]

My thought on the offset transfers from side to side is that they are still aimed at the intake wall, but one side will just open sooner than the other. Just maybe it would help create a swirl effect to help keep fresh mix from escaping through the exhaust port. The exhaust gases are going to want to exit the exhaust port anyway since its the path of least resistance. I doubt that there is every a time where there is a lot of exhaust still left in the chamber on a chainsaw without an expansion chamber exhaust. More than likely all the exhaust is expelled along with a good portion of the fresh mix. I'm just trying to think of ways to keep all the fresh mix in with the open box style mufflers. Maybe I just need to try it. Maybe go as far to having the pto side transfers just pointed across the piston top and open later than the flywheel side that is directed up and towards the exhaust.

Like this:

 

[Lightning Performance]

I was referring two the roof slant at the port to cylinder wall window. Side to side height is a whole other issue along with uneven port size and flow.

 

[stinkbait]

I know it's a sore subject for a lot of people, but I think it's worth mentioning for the sake of transfer porting theory. Jms seems to favor heavy transfer mods along with higher crankcase compression. I've seen several times where he ground through to the outside of transfer tunnels and had to use jb weld to seal them back up. And his saws seem to run. I know some will disagree with that statement. So, does anyone think its worthwhile to hog out transfers thus increasing crankcase volume but then making back up for it by increasing crankcase compression?

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