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372 bottom feed debate

David Young

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I want to share my thoughts on something that. We hear about a lot. I don't have an answer but I will share my thoughts and observations.

I think husky design the bottom to be of variable volume at the right time. We all know when the piston is at tdc it opens the sides to the upper transfers. This means at tdc. The middle of the wrist pin and slab side increase case volume at a theoretical peak intake flow. (Intake is most open).

When the piston travels down. The intake closes then the sides of the piston are closed off just before the transfers open. The Base is now of a smaller volume because the piston travel down and the sides are sealed.


If we raise the lower walls this is defeated
Like Randy says the engineers are not stupid.


Hope the pics help.
 

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metallic

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I thought that design was just intended to reduce friction losses between piston and cylinder. I wonder if the fresh charge entering the crankcase even gets to the transfer tunnels prior to the piston descending from TDC?
 

idiotwithasaw

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Are you saying don't remove or modify that part of the skirt. If you were to modify the lowers, what then would need to be done to the uppers to offset it?
 

Four Paws

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I want to share my thoughts on something that. We hear about a lot. I don't have an answer but I will share my thoughts and observations.

I think husky design the bottom to be of variable volume at the right time. We all know when the piston is at tdc it opens the sides to the upper transfers. This means at tdc. The middle of the wrist pin and slab side increase case volume at a theoretical peak intake flow. (Intake is most open).

When the piston travels down. The intake closes then the sides of the piston are closed off just before the transfers open. The Base is now of a smaller volume because the piston travel down and the sides are sealed.


If we raise the lower walls this is defeated.

OK, educate me.

Right before the intake opens, the crankcase is at max vacuum...the lowest pressure it will ever be...when the port cracks open, fuel and air rush in.

At TDC, the inertia of the intake charge has been established, and fresh fuel and air are flowing in...hopefully equalizing to atmospheric pressure at the least, and preferably above atmosphere if the intake flow inertia can stuff in some more charge. The piston pin and slab side pockets open to the upper transfers and case volume increases - I buy it.

But as the piston moves down, it traps this fresh charge. It never moves through the transfers as it is blocked off...it is trapped. For the sake of discussion, that space is now full and now the piston again moves to TDC. When the upper transfers open, the space in the piston pin and 'trap door' created by the piston slab side pockets can't take any additional charge volume. It is full of charge at exactly the same pressure as the rest of the case. Flow stands still in this zone - nothing moving in to the trap door, nothing moving out.

No doubt this piston and cylinder are of good design and performance. I just do not think this trapdoor adds anything in OEM trim. I would consider it adds to the equation if:

It opens to the case shortly before BDC and the charge trapped is above case pressure at this point. It would then give a little more punch to the transfers before the compression begins, and would be ready to accept a fresh charge near TDC.

My view and interpretation.
 

tree monkey

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those transfer tunnels are by far larger then those pockets in the piston.
 

David Young

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OK, educate me.

Right before the intake opens, the crankcase is at max vacuum...the lowest pressure it will ever be...when the port cracks open, fuel and air rush in.

At TDC, the inertia of the intake charge has been established, and fresh fuel and air are flowing in...hopefully equalizing to atmospheric pressure at the least, and preferably above atmosphere if the intake flow inertia can stuff in some more charge. The piston pin and slab side pockets open to the upper transfers and case volume increases - I buy it.

But as the piston moves down, it traps this fresh charge. It never moves through the transfers as it is blocked off...it is trapped. For the sake of discussion, that space is now full and now the piston again moves to TDC. When the upper transfers open, the space in the piston pin and 'trap door' created by the piston slab side pockets can't take any additional charge volume. It is full of charge at exactly the same pressure as the rest of the case. Flow stands still in this zone - nothing moving in to the trap door, nothing moving out.

No doubt this piston and cylinder are of good design and performance. I just do not think this trapdoor adds anything in OEM trim. I would consider it adds to the equation if:

It opens to the case shortly before BDC and the charge trapped is above case pressure at this point. It would then give a little more punch to the transfers before the compression begins, and would be ready to accept a fresh charge near TDC.

My view and interpretation.


Josh,

Thank you great point, the pockets will be trapped just like the bottom of the piston.

but I am struggling, it was done for a reason. the 385 and 357 don't have this extension. the 385 has a piston style like a poulan pro. the 357 has a 372 piston but to a smaller degree. The 346 has the extension too but the piston is more like the 372.
there are changes and I doubt it is because husky engineers are whimsical.

I know Randy says his testing shows its better to leave it alone.
anyone else done testing?

Great discussion.
 

Four Paws

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Josh,

Thank you great point, the pockets will be trapped just like the bottom of the piston.

but I am struggling, it was done for a reason. the 385 and 357 don't have this extension. the 385 has a piston style like a poulan pro. the 357 has a 372 piston but to a smaller degree. The 346 has the extension too but the piston is more like the 372.
there are changes and I doubt it is because husky engineers are whimsical.

I know Randy says his testing shows its better to leave it alone.
anyone else done testing?

Great discussion.


I agree, great discussion.

Scott, what kind of gains did you see from modifying that 385 piston with windows and flat sides?

David, I don't look at the charge in the bottom of the piston exactly the same way as I look at the charge in the 'trap door'. The charge in the bottom of the piston is not truly trapped. It can flow up through the transfers if the pressure in the combustion chamber is less than the case. I will say, the likelihood of that happening is slim as it would require transfer efficiency approaching 100%.
 
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