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Let's Talk Transfers

drf256

Dr. Richard Cranium
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I wanted to start a thread to brainstorm a bit about transfers. All about them.

Location, function, timing, etc....

The more I read on the subject, the more I see the transfers as being the black magic of saw builds.

Smaller cross sectional ports than either the intake or exhaust. They open faster because of higher piston speed as one approaches the center of the stroke.

I don't know if all generalities about them apply to all saws. I'm sure there are exceptions that work well and make everyone scratch their heads.

There's tiny tight transfers that work better than a big open one, there's really physically low ones (like on Dolly's) that work well, there's really high ones (044/046) that work well, etc...

Let's discuss. This may certainly be an area where some PRO's don't want to reveal their thoughts. It's nice to get "good numbers" from someone. But understanding the "why" is more important to me than the actual numbers.
 

drf256

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Being the port in between the two others, transfers seem to be much more interdependent than the other ports.

Case volume, intake closing point, exhaust roof geometry all seem to affect transfer shape and location for optimal performance.

The OEM's have moved towards 4 ports. 4 ports seem great because you are getting the best of both worlds. Good velocity and good area all mixed into one design.

4 ports seem to run at a much physically lower region than 2 ports. Why? They can flow more and fill the cylinder the same as a 2 port in a smaller amount of time? Because of this, there's less risk of backfilling?

Then there are the varying entry angles of the quad ports. It seems that the ex sides enter flat and purge the piston crown off, but the mains focus the flow towards the intake wall.
 

Redfin

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I was pondering how a near zero blowdown jug would act recently. Im sure its not feasible for anything useful but it crossed my mind.

Edti. The primary charge would pressure would have to be pretty dang close.
 
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Brewz

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100% agree on needing to know the how and why on the subject.

I am learning and unfortunately have little to offer information wise.

I do have lots of questions and thoughts.

Something that interests me is if its better to open the transfers all at once to blast the fresh charge in all of a sudden or to angle the transfer to direct the initial charge to the intake side and gradually open the transfer over a few deg of downward movement sweeping the spent charge out with a higher pressure at the opposite side to the exhaust port. I really like the thought of the later.

I guess the optimal setup is going to where we open the exhaust and let the combustion chamber pressure drop to a point where it will be at the same or slightly lower pressure that the fresh charge sitting in the transfers. I imagine that if the transfers are too high, you will open them when there is still a very high pressure in the combustion chamber which will slow the fresh charge entering.
Too low and your wasting time getting fuel/air in there.

thoughts?
 

Brewz

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Re-reading my post, I think I raised a subject I had not thought of before...... Pressure!

We all talk flow and velocity but no one ever talks pressure.

I guess the power gains seen by a muffler mod are actually the ability of the pressure to drop faster in the combustion chamber as the exhaust gas can get out faster and easier.
This then offers less resistance for the incoming charge from the transfers, allowing more of the fresh charge to be packed into the combustion chamber.

I think this is a great subject to further explore.

A motor is an air pump and we can improve flow in and out but its the transfers that determine how much fresh charge makes it to the combustion chamber and how much spent charge stays or leaves.
This has to be a major factor in torque production.

Its easy to make a motor spin, making it pull harder is the difficult bit.
 

Simondo

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I wonder how much difference the "swept volume " of the cylinder has on transfer design. Smaller cc engines moving less air than a big cc engine so i guess must put a different requirement on port/size/shape and number ???
 

Brewz

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I would assume that if everything is proportionally equal and identical on a 50cc and a 100cc motor, the optimal requirements would be the same but flow restriction is a strange beast. I doubt as size increases, flow restriction is linear assuming identical motor, just different size
 

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Ok
Another thought relating to pressure.
More a question actually

Is the saws compression we measure the pressure you generate in the combustion chamber with no bang, or the pressure we generate in the case to push the fresh charge up the transfers?
 

drf256

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Lately Ive leaned more toward staying with what the factory did and just change the # and clean everything up alil. Less is more approach
Agreed that the factory has resources for research we would never have.

We are given a factory saw that's a balance of performance, economy, reliability, endurance and emissions.

We are given that set sum of criteria. In the end, the sum remains the same. We are getting less of one to get to the other.

So we are simply trading.
 

Keith Gandy

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Agreed that the factory has resources for research we would never have.

We are given a factory saw that's a balance of performance, economy, reliability, endurance and emissions.

We are given that set sum of criteria. In the end, the sum remains the same. We are getting less of one to get to the other.

So we are simply trading.
Thats my conservative approach and I usually go alil short on my mark until I run the saw as I dont wanna go too far South with the saw on the 1st try
 

Mastermind

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Being the port in between the two others, transfers seem to be much more interdependent than the other ports.

Case volume, intake closing point, exhaust roof geometry all seem to affect transfer shape and location for optimal performance.

The OEM's have moved towards 4 ports. 4 ports seem great because you are getting the best of both worlds. Good velocity and good area all mixed into one design.

4 ports seem to run at a much physically lower region than 2 ports. Why? They can flow more and fill the cylinder the same as a 2 port in a smaller amount of time? Because of this, there's less risk of backfilling?

Then there are the varying entry angles of the quad ports. It seems that the ex sides enter flat and purge the piston crown off, but the mains focus the flow towards the intake wall.

You have your terminology reversed Al. The mains are closest to the exhaust port, secondaries next to the intake.
 

Brewz

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You have your terminology reversed Al. The mains are closest to the exhaust port, secondaries next to the intake.

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?
 
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