Let's Talk Transfers

[drf256]

The more I'm learning, the less I know with these engines.

I used to think that each port had its rule and that was it.

It's definitely more of a symphony than the individual instruments.

Like Sophia Lauren-each individual feature on her is not so attractive, put it together and Wow.

 

[Backyard Lumberjack]

395 after machine work is around 110 degrees on exhaust opening. I run them at 100. When you raise exhaust you gain blowdown. Raise the transfers back you lose it.

got it, thanks...

 

[drf256]

Let's use a saw as an example. It may be clearer and easier to understand.

On an 026. It's a dual port. Timing form the factory is around 96-98 exhaust, 120 transfer, 70 intake. Small tight case, small carb Venturi and intake boot.

 

[drf256]

So I take .060 out of the band. I get to an exhaust of 105*.

The transfers move somewhere into the mid 130's. The intake 74.

So now I have good case compression, but really low transfer duration.

So I raise my transfers up.

Now the questions begin

 

[drf256]

I'm gonna wanna raise the transfers to the point where I get maximum flow into the cylinder, but not so much that I expel mix into the muffler.

I've created a larger swept volume by lowering the exhaust roof.

I'd like some more intake duration with the tiny carb. If I lower the intake floor, I lose a few degrees of case comp.

So now I'm stuffing the case betterer, maybe. So I've got some extra volume there with charge inertia, again, maybe.

So now I'm gonna want to go with a higher transfer because I have more to push but with less pressure, or do I want a lower transfer duration because the lower pressure may enable some exhaust to flow down the tunnels?

 

[drf256]

So with 150* of exhaust duration, if I carve the uppers in at 120* below TDC, I get 120* of transfer duration.

That 80% of my intake and exhaust duration that I'm trying to push through two tiny slots.

The cross sectional area is likely bigger than the carb Venturi though.

 

[Stihl working Hard]

Really interesting thread guys thanks for all the valuable input

 

[Ironworker]

After cutting the base and chamber for compression if you don't raise them back up you don't get the gains. I've never tried to widen them to get the time area but the exhaust is usually getting raised 2-10 degrees so that added to blowdown would make most a turd.

On the last saw I did, a 346, Brad cut the base and made a pop up, all I did was hog out the lowers,raise, widen and polish the exhaust, widen the intake very little, and just widened up the uppers a touch, the saw screams and holds torque with the bar buried. I did this out of being impatient, my Chinese right angle dental tool had not arrived yet so I could not raise the uppers so I figured I'd just take the less is more approach and if I wasn't happy I could go back in.

 

[drf256]

On the last saw I did, a 346, Brad cut the base and made a pop up, all I did was hog out the lowers,raise, widen and polish the exhaust, widen the intake very little, and just widened up the uppers a touch, the saw screams and holds torque with the bar buried. I did this out of being impatient, my Chinese right angle dental tool had not arrived yet so I could not raise the uppers so I figured I'd just take the less is more approach and if I wasn't happy I could go back in.

So you pretty much fully ported it.

 

[drf256]

Popups still have their place.

You maintained the excellent OEM entry angles by using the popup and not having to raise them.

Why did you raise the exhaust and how much?

IIRC, a stock 346 is around 105*.

 

[Ironworker]

So you pretty much fully ported it.

Yes

Popups still have their place.

You maintained the excellent OEM entry angles by using the popup and not having to raise them.

Why did you raise the exhaust and how much?

IIRC, a stock 346 is around 105*.

The base was also cut so it did drop a little, I raised and widened the exhaust to get the extra burnt gas out. I personally believe that timing numbers are overrated. I believe in simple, gas, air, compression, explosion, power, repeat, the more gas and air coupled with compression the more power.

 

[Mastermind]

Good thread gents.

I'm really enjoying the conversation.

 

[wigglesworth]

I've widened and raised the transfers in them since the mains are restricted compared to the 48 and 50mm bore. On those I just raise them and don't widen.

I've pondered a bit on the 3-series saw transfers. Omitting the xpw jug (it's a chop saw jug anyways), you have a 47, 48 and 50mm jug, and coincidently, the larger the bore, the larger the transfers.

I really don't think husky did that by accident. I mean, the 48mm jug has the same casting, even says 50mm on the base. Why would they shrink the transfers on em? De-tune em? Increase transfer velocity? Why is there a direct relationship from bore size to transfer size with these jugs?

I "think" I know why, but Imma hold out for a bit.

 

[huskihl]

My head hurts

 

[awol]

I've pondered a bit on the 3-series saw transfers. Omitting the xpw jug (it's a chop saw jug anyways), you have a 47, 48 and 50mm jug, and coincidently, the larger the bore, the larger the transfers.

I really don't think husky did that by accident. I mean, the 48mm jug has the same casting, even says 50mm on the base. Why would they shrink the transfers on em? De-tune em? Increase transfer velocity? Why is there a direct relationship from bore size to transfer size with these jugs?

I "think" I know why, but Imma hold out for a bit.

Are the tunnels restricted on the smaller jugs, or only the port opening into the cylinder?

 

[Mastermind]

Everything we know should be under constant self examination.......

 

[Mastermind]

Are the tunnels restricted on the smaller jugs, or only the port opening into the cylinder?

They are smaller from base to outlet.

 

[Magic_Man]

Lots of good info here.

 

[jmssaws]

Holy crap! Do you guys sleep?

I went to bed early and wake up to read 3 pages,I need to eat and process.

Great thread!
Al we need to go to a bar and talk saws for a week. Interesting!

 

[jmssaws]

Everything we know should be under constant self examination.......

Everything we do changes something else and then that changes something else.