066/394 hybrid build

[Keith Gandy]

Helps "pull" exhaust gasses outta the cylinder.

Scavenging effect? Sorta like a reversed velocity stack?

 

[Deets066]

I was thinkin maybe one could drop the jug more without freeporting but still maintain a larger exhaust port?

It won't let you drop the jug more. The top of the step is still the same exhaust port floor.

I've never tried it, anyone ever done a before and after with just that?

 

[Adirondackstihl]

Scavenging effect? Sorta like a reversed velocity stack?

Sorta.
Some old Homelites had this right from the factory.

 

[Keith Gandy]

It won't let you drop the jug more. The top of the step is still the same exhaust port floor.

I've never tried it, anyone ever done a before and after with just that?

Yes I understand that but it lets u make the port bigger without altering the existing floor height at the window?

 

[Deets066]

Yes I understand that but it lets u make the port bigger without altering the existing floor height at the window?

Yes it does, but does it make anymore power?

 

[Deets066]

Yes I understand that but it lets u make the port bigger without altering the existing floor height at the window?

Is this the secret noodle monster mod??!!

 

[Keith Gandy]

Yes it does, but does it make anymore power?

Heck I dont know

 

[Keith Gandy]

Is this the secret noodle monster mod??!!

. Hell if I know

 

[Chainsaw Jim]

Theroy behind the "step"? please.

Sometimes you can't enlarge the exhaust enough without free porting below the piston. In this case I had plenty of room if I wanted to make the floor flat at that level but the piston doesn't go any lower and is still exposed. If I take the step away and then end up lowering the cylinder I will risk exposing under the piston.

It is a proven method. You're enlarging the outer exhaust to increase velocity of the escaping charge while keeping tolerances in check.

 

[beaglebriar]

I put a step in my 372 just because Randy was talking about it. It was around the same time I started the build. So I figured why not?

 

[Deets066]

Sometimes you can't enlarge the exhaust enough without free porting below the piston. In this case I had plenty of room if I wanted to make the floor flat at that level but the piston doesn't go any lower and is still exposed. If I take the step away and then end up lowering the cylinder I will risk exposing under the piston.

It is a proven method. You're enlarging the outer exhaust to increase velocity of the escaping charge while keeping tolerances in check.

Being more open wouldn't it reduce velocity?
Couldn't the same thing be achieved in opening the port at ex. Flange the same you have and just taper back to the port?

 

[Chainsaw Jim]

Being more open wouldn't it reduce velocity?
Couldn't the same thing be achieved in opening the port at ex. Flange the same you have and just taper back to the port?

I guess it doesn't really gain velocity, it's more like the escaping charge has more open area to escape so it doesn't lose velocity. If it's tighter there the escaping charge fights more to escape when it slams a shallower surface.

 

[CR500]

I put a step in my 372 just because Randy was talking about it. It was around the same time I started the build. So I figured why not?

I think Terry discovered that I wanna say?

Remember Randy giving props to Terry on that

 

[Deets066]

I guess it doesn't really gain velocity, it's more like the escaping charge has more open area to escape so it doesn't lose velocity. If it's tighter there the escaping charge fights more to escape when it slams a shallower surface.

Yeah that's the way I see it.
I've been tapering mine from the cylinder wall to the ex flange, getting bigger as I go

 

[Adirondackstihl]

Pretty sure you can thank Yamaha for the exhaust step as they were using it in the 70's on MX motors.
And I'm nearly 100% sure Homelite was using the concept before that.

 

[Adirondackstihl]

https://flic.kr/p/NHH7ud

 

[mdavlee]

Ace Morgan was the first to grind it intentionally in chainsaw ports that I know of. This was in the late 80s early 90s if I remember what Glen and Jacob J said. Terry got a saw from the west coast that was done like that.

 

[Chainsaw Jim]

I had a chance to fire it up after the trick or treat shift ended tonight. No more air leak or *b-wordy tuning...or silence in my head thanks to this damn tinnitus. I hope this thing cuts as good as it runs. Fingers crossed and video to come tomorrow.

 

[Chainsaw Jim]

Well.... sorry about the hold up. That WB carb isn't going to hold a tune for chit. The throttle shaft is very loose in the carb body and has a large enough gap to suck air. It'll hold a tune for a rev or two then it just goes all over the place. By the time it gets to the wood it's already way off.
I'm going to have to put the carb from my 880 on it to get a good run out of it.

 

[Chainsaw Jim]

I'd be an a-hole if ididn't upload this to share...here's a clip of it with the carb still not right. It's hammering some pretty hard knots too.

I can't wait to get a carb in better shape on it. I'm trying to make my mind up on whether I want to use the modded carb from my 064 pipe saw or just grab the one off my 880. I suppose I'll start with the easiest one to attach.