Let's Talk Transfers

[srcarr52]

My theory is because some of these new designs use basically ambient air for the purge portion of the transfer it might slow the burn rate down until that air it heated by the combustion process .If that holds true it would explain why for instance certain Stihls like the 201 and 441 are slow on the uptake acceleration .It seems to me with either one of those saws they run better after a portion of usage because of that .
It also makes sense that because of the use of pure air they would use less fuel than conventional designs which blows the unburned fuel right out the exhaust------best I can come up with on that subject .

Husky stratos are not slow to accelerate at all. In fact they are down right quick.

 

[CR888]

My stock 550xp has lightening fast throttle response, my internet ported ms261 is a turd.

 

[Terry Syd]

Perhaps the common usage term of TDC to BDC would be the 'displacement' of the engine, but the swept volume is for lack of a better term, the 'sucking volume' (intake cycle). I note that Wikipedia uses a four-stroke to show swept volume and would be from TDC to BDC. The intake valve opens just before TDC and closes ABDC - that is the INTAKE CYCLE for a four-stroke.

So, where does the INTAKE CYCLE occur on a two-stroke? It doesn't happen in the cylinder, it happens in the CRANKCASE. It occurs when the piston is RISING in the cylinder. Whereas a four-stroke will intake from TDC to BDC, a two-stroke can only intake from TRANSFER CLOSING to TDC.

When you raise the transfers on a two-stroke you will shorten the intake stroke.

It is always preferable to widen the transfers to get an increase in time/area rather than raising the transfers. If you can't get the engine to work by widening the transfers, then you raise the ports.

 

[Mastermind]

Perhaps the common usage term of TDC to BDC would be the 'displacement' of the engine, but the swept volume is for lack of a better term, the 'sucking volume' (intake cycle). I note that Wikipedia uses a four-stroke to show swept volume and would be from TDC to BDC. The intake valve opens just before TDC and closes ABDC - that is the INTAKE CYCLE for a four-stroke.

So, where does the INTAKE CYCLE occur on a two-stroke? It doesn't happen in the cylinder, it happens in the CRANKCASE. It occurs when the piston is RISING in the cylinder. Whereas a four-stroke will intake from TDC to BDC, a two-stroke can only intake from TRANSFER CLOSING to TDC.

When you raise the transfers on a two-stroke you will shorten the intake stroke.

It is always preferable to widen the transfers to get an increase in time/area rather than raising the transfers. If you can't get the engine to work by widening the transfers, then you raise the ports.

Sorry Terry........I disagree.

There are no cut and dry answers that fit every model.

 

[mdavlee]

I'll let you know...I'm gutting mine tomorrow. I've read about good results in other places. Really helps tame the idle

The one I didn't guy the guy said out pulled a 385 and 390 with 33" bars. If you want the numbers I'll send them to you. I did cut the base and chamber a little.

 

[Terry Syd]

It is always preferable to widen the transfers to get an increase in time/area rather than raising the transfers. If you can't get the engine to work by widening the transfers, then you raise the ports.

Come on Randy, you don't disagree with that quote do you?

The concept is the same for all ports, increase the area before changing the timing. If you had a narrow exhaust port, say that was 55% of bore, I'd expect that you'd widen the port before you would start raising it.

Granted, some engines just aren't amenable to wider transfer ports, but a 10% increase in width (and thus a 10% increase in area) is not a big leap for many four port jugs.

 

[Mastermind]

I prefer not to widen any transfer ports......

 

[mdavlee]

With no machine work I've widened transfers on saws that I didn't plan to lower. I also widen a few that are more towards play saws where flow is the key and torque isn't as important.

 

[drf256]

Many here have hogged out uppers, only to find a deterioration in performance.

I think every model is different, period.

I'm afraid of catching a ring, mostly.

 

[drf256]

I'm still reading Blair's book, which is much more comprehensive than Bell's or Jennings.

He calls flow though finger ports "indifferent". I'm still trying to understand that one. It's during a time where he is describing directional flow from transfers to increase transfer area.

I believe that based on the cross sectional area of a finger port, they aid is some flow and some increase in transfer area. I think they flow quite differently and will need more time to do so. I still see them as being set with more duration than the mains.

If they flowed like regular ports, than I'd agree that they should be placed lower to lift the charge. I see them filling the small dead space where the loop scavenge misses near the edge of the piston on the intake wall at BDC.

It's probably why most see gains with fingers place 2* higher and along the intake wall.

If one were to use a true boost port with full surrounding walls, I'd say they should be set physically lower than the mains.

Charge from fingers has to eminate like its "peeled" off the cylinder wall.

 

[XP_Slinger]

What r u gutting? I gutted a 241 from the plastic divider on the back of the carb to the port window. It ran good but on a long idle it would stumble when u dumped the trigger. I added a stock intake manifold with baffle intact back in and it stopped. The #s were 105/118/74. I sold the saw the the other day and the guy was shocked how well it ran and ask if the high rpm would damage it? Lol!!

I'm gutting all strato dividers...Filter mount, carb and intake. I'm also considering reshaping the cylinder intake to favor the main intake.

 

[XP_Slinger]

The one I didn't guy the guy said out pulled a 385 and 390 with 33" bars. If you want the numbers I'll send them to you. I did cut the base and chamber a little.

I would very much appreciate your numbers. I'm putting in a lot if time researching these XT's and any information I can get my hands on is helpful.

 

[Keith Gandy]

I'm gutting all strato dividers...Filter mount, carb and intake. I'm also considering reshaping the cylinder intake to favor the main intake.

I left the divider on back of carb intact but gutted the manifold and intake

 

[XP_Slinger]

I left the divider on back of carb intact but gutted the manifold and intakeView attachment 12084View attachment 12085

Nice work. The intake on my 372 strato looks COMPLETELY different than the one in your pic. My strato ports are above the main intake passage. I'll post some before and after pics of mine later this evening

 

[XP_Slinger]

Starting to wonder if I should start a 372XT thread for this stuff.

 

[Keith Gandy]

Starting to wonder if I should start a 372XT thread for this stuff.

Please do. Im waiting on my cylinder being cut from Randy and Im building my 2007 372xp

 

[XP_Slinger]

Please do. Im waiting on my cylinder being cut from Randy and Im building my 2007 372xp

Done

 

[Angelr]

I'm new to this, I should not say, but I think I need it to learn more. I understand that if I increase the transfer time, logically increasing them, I shorten the time of admission, with what I get a higher number of rpm but a loss of torque, to have a lower entry of mixture to the case

 

[Lightning Performance]

So is it considered best to hit the chamber with fresh charge then flush from the intake side 2nd, or open the intake side then charge the rest of the chamber?

Yes

I would very much appreciate your numbers. I'm putting in a lot if time researching these XT's and any information I can get my hands on is helpful.

And...

 

[Mastermind]

Yes

You just replied to a 5 year old post Kenny.