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390xp advice

Jake Dykstra

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Morning gents.
Starting to play with my 390xp and need some porting tips.
Starting with a hwy 57mm pop up just to play with ( and I need practice porting).
First off I'm getting that heavy piston down to stock weight. 8 grams to go. Half way there!
Looks like I'll need to clearance the case for the larger piston.
Transfers look very small so lots of work ahead to get the air up top but mostly done with that already.
Exhaust is open up a bunch.
Muff mod needs to be done yet.
Took some doing ( because I haven't clearances the case yet) but the numbers are
E 102
T 132 ➕
I 78
I'm thinking leave it at 102 for now since it's not a long stroke saw and will wear a 32 or 36 inch bar.
I'm think 120 on transfers...18 BD to help fill that big bore?
Leave intake alone for now as I may cut the squish in the future.
Timing advance for sure

Anyway please let me know if this saw likes more BD or less and a high or low exhaust.
Thanks
 

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I just got a 57mm kit myself. From other Hyway kits, beware the brittle plating. Diamond only.

What’s you squish? Most Hyway land around 0.040”.

102 exhaust is probably a little low but if you can avoid grinding the exhaust the jug has a better chance of survival. That’s the most common plating failure area. Unfortunately you’ll probably be a 104 if you drop the squish where it should be.

57mm is a pretty big bore. I would port more to 395 numbers. 20 degrees blowdown.

I haven’t done it yet, but I think I’ll raise the lowers when I try mine. Most of the transfer constriction is there. I’ve had problems doing that on 372 Big Bore. The cylinder warps and the flange rubs the piston. But they run much better until they die!
 

Jake Dykstra

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Was shocked to see how small the lowers were. Decided to cut them down because the restriction is at the base of the cylinder.
Widened the exhaust.
Fingers in the future if It seems like it needs more transfer.
 

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Jake Dykstra

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I just got a 57mm kit myself. From other Hyway kits, beware the brittle plating. Diamond only.

What’s you squish? Most Hyway land around 0.040”.

102 exhaust is probably a little low but if you can avoid grinding the exhaust the jug has a better chance of survival. That’s the most common plating failure area. Unfortunately you’ll probably be a 104 if you drop the squish where it should be.

57mm is a pretty big bore. I would port more to 395 numbers. 20 degrees blowdown.

I haven’t done it yet, but I think I’ll raise the lowers when I try mine. Most of the transfer constriction is there. I’ve had problems doing that on 372 Big Bore. The cylinder warps and the flange rubs the piston. But they run much better until they die!
Yeah I was thinking 18-20* BD. I did a 044-046 BB and it definitely liked less blow down. I like wide open exhaust so reversion is less likely.
I'll probably end up at 98-99* exhaust roof after lathe work.
Haven't had the flaking trouble yet with the hyways I've done but I'm a very slow porter. Like a 40 hour week slow. Lol
 

Ketchup

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That’s exactly what I was thinking on the lowers. Looks like you could work the back angle of the primary to match OEM.

Compared to a 54mm 460 this has 3mm more bore, 12cc more displacement AND constricted tunnels. 395 likes 80/100/120 with a wide open muffler and it’s 56mm.

Good news on the squish.
 

Jake Dykstra

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That's why I was thinking this...but much nicer and more even. Walls only .140 thick so can't go deep.
Maybe I'll relocate ring pin to the middle so can go wider??
Sucks the bottom ring is the one that's closest to center
 

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Ketchup

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Eh. Maybe. Bottom fed quads rarely want boost ports.
 

Jake Dykstra

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Case clearance work done.
.010 gasket installed.
Port timing from hyway: with .010 gasket
E 101
T 130
I 77
Squish 22
 

Ketchup

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Those numbers are a lot like a Dolmar 7900. Maybe Hyway knows something we don’t. I’d be tempted to bolt that thing down and run it. That moght show you where you want to go next.
 

Jake Dykstra

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I so wanted to but my bigger is better mentality has gotten the better of me!
I kinda wanted to do another one anyway.
 

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Ketchup

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Well, being excited is half the fun. Maybe you should do the finger ports.

Where’d you land those uppers. 121?
 

Jake Dykstra

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Yep. I've always liked 100/120/80 or there abouts on my 372 saws.
Heck my old red lever 044 is 99/120/71 so it must be the magic combo...right?
 

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I have had success adding angle to the back of the transfers on AM jugs. It’s a pain, but it really improves power. It takes some long ball burrs and patience. Like the red line.

IMG_1820.jpeg
 

Jake Dykstra

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Yeah I got up in there and aimed it back a little. Took so much out of the roof of the transfer tunnel trying to keep roof angle similar I kept waiting to see light.
 

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Jake Dykstra

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I have had success adding angle to the back of the transfers on AM jugs. It’s a pain, but it really improves power. It takes some long ball burrs and patience. Like the red line.

View attachment 412163
Just cracked it off. If sounding wicked strong is an indication it shouldn't disappoint.
Holy crap it got some compression for sure. Haven't even cut the cylinder yet. Already busted the recoil starting it. Lol. Gonna put the decomp in on this one
 

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