The SCARR Husqvarna 592XP build thread.

[srcarr52]

So what donor crank are you looking at for a stroker build?

I can’t think of a donor for this as it’s the largest stroke Husky with an internal clutch.

 

[Woodslasher]

I can’t think of a donor for this as it’s the largest stroke Husky with an internal clutch.

Think outside the box, maybe an 066 crank would work?

 

[srcarr52]

Think outside the box, maybe an 066 crank would work?

Let’s get the stock stroke ones done first and worry about crazy mods later.

 

[srcarr52]

I wasn’t happy with my welds on the first muffler, the inside of the welds where cooked and pretty crappy looking. Not a big deal because you can reach them all to clean them up but I figured there had to be a better way. So I created a back purge plate and hit it with 10cfm. Much better welds even when filling a gap from slitting the stock deflector.

 

[moparnut88]

I wasn’t happy with my welds on the first muffler, the inside of the welds where cooked and pretty crappy looking. Not a big deal because you can reach them all to clean them up but I figured there had to be a better way. So I created a back purge plate and hit it with 10cfm. Much better welds even when filling a gap from slitting the stock deflector.

That’s how we do high quality stainless welds on high pressure items. It’s a must. That and inconel both get back purged if being done to code.


Sent from my iPhone using Tapatalk

 

[srcarr52]

This is the lightest, thinnest pin I’ve seen. They probably get away with it because the piston is pretty strong and doesn’t add a lot of bending moment to the pin, and it has a pretty wide needle bearing to spread the load.

 

[srcarr52]

There is some room for improvement on the piston weights. Those extra lightening pockets in the 592 piston almost make up for the 2mm larger bore.

 

[Funky sawman]

So did the 585 with the adjustable carb have a nice steady idle, with crisp throttle response?

 

[Dolkitafreak]

The real question here is do the saws sit still or need a kickstand? The 6 or 7 series will probably have them built in!

 

[Nutball]

Did you measure squish on the exhaust side?

 

[srcarr52]

Did you measure squish on the exhaust side?

Intake and exhaust. It just shows up on the piston wash on the exhaust side only.

 

[srcarr52]

So did the 585 with the adjustable carb have a nice steady idle, with crisp throttle response?

Yup. Pretty crisp out of the box, maybe a touch lean.

 

[srcarr52]

Many know that I like to do things differently. I measure the port timing using a modified dial caliper to get the port height to the squishband and then use a little math to figure out port duration/opening. This way there is no error from the eye of when the port is open and it also allows me to easily calculate the strato port duration.

Knowing the stroke of 39mm and the rod length of 75mm here is all the info you'd need to calculate the port timing, distance till free port of the exhaust and how far the 2nd ring is from getting into the intake port.



The strato port timing on all of the autotune saws have been controlled by the bottom of the transfer port opening into this window of the piston, STP in my notes or the piston strato port height.



Throw it through the math and you get:

Duration/Opening
585 592
Exhaust 160/100 156/102
Trans 125/117.5 117/121.5
Intake 158.5/79 151/75.5
Strato 140.5/70.25 151/75.5

Both saws have over 0.140" before the 2nd ring gets into the intake, so there is no reason why they couldn't have pined both rings over the intake. They chose to pin the top ring centered over the intake and the 2nd ring is off to the flywheel side of the exhaust port. It's pretty far out there so it shouldn't limit the exhaust port width but I'll double check it.

I'll see if I have plates I can use to measure the chamber volume, 53mm and 55mm bores are a little odd. I'm also going to have to make mandrels to cut the base surfaces.

 

[Red97]

When they come back I'll try my oppama tach.


Very interesting stuff.

Thank you for sharing.

 

[srcarr52]

I was pretty skeptical about the compression measurements from these saws but after measuring the combustion chambers and doing a little math they check out.

To measure the chamber I cut a plexiglass disk to seal against the squishband and I fill the chamber with a fluid using a syringe. Many use ATF but windshield washer fluid evaporates really slowly and it's much nicer to work with and clean up. I usually use the blue stuff but all I had was the rain-x brand.



1ml = 1cc (cubic centimeter)

Chamber sizes are:
585 9.8cc
592 10cc

Now using the Ideal Gas Law PV = nRT

nRT is constant so we end up with:

P1 V1 = P2 V2
where:
P1 = base pressure
V1 = swept volume + squish volume + chamber volume
p2 = final pressure
v2 = squish volume + chamber volume

You can simplify and solve for P2:

P2 = P1*CR
where:
CR (corrected compression ratio) = V1/V2

To do this calculation you need a consistent set of units for distance and volume, metric units are much easier to deal with, we'll use mm and cc which will need an extra divide by 1000 to make mm^3 to cc. Since the volumes become a ratio V1/V2 we can use mixed units for the pressure (psi).

So you need to know the bore size, squish clearance, distance from the exhaust port to squish band and chamber volume.

Note P1 is base pressure at your altitude, I'm at ~800' so the base pressure is 14.26psi.

So substituting in the equations for the volumes and simplifying we get:


where (592):
Bore = bore diameter in mm, 55mm.
E = distance from top of the exhaust port to squishband, 1.045" = 26.543mm.
C = squish clearance, 0.022" = 0.5588mm.
CV = chamber volume, 10cc.

So for the the theoretical cranking compression is:
585 = 131psi
592 = 141psi

 

[Wendell]

I was pretty skeptical about the compression measurements from these saws but after measuring the combustion chambers and doing a little math they check out.

To measure the chamber I cut a plexiglass disk to seal against the squishband and I fill the chamber with a fluid using a syringe. Many use ATF but windshield washer fluid evaporates really slowly and it's much nicer to work with and clean up. I usually use the blue stuff but all I had was the rain-x brand.

View attachment 317462 View attachment 317463

1ml = 1cc (cubic centimeter)

Chamber sizes are:
585 9.8cc
592 10cc

Now using the Ideal Gas Law PV = nRT

nRT is constant so we end up with:

P1 V1 = P2 V2
where:
P1 = base pressure
V1 = swept volume + squish volume + chamber volume
p2 = final pressure
v2 = squish volume + chamber volume

You can simplify and solve for P2:

P2 = P1*CR
where:
CR (corrected compression ratio) = V1/V2

To do this calculation you need a consistent set of units for distance and volume, metric units are much easier to deal with, we'll use mm and cc which will need an extra divide by 1000 to make mm^3 to cc. Since the volumes become a ratio V1/V2 we can use mixed units for the pressure (psi).

So you need to know the bore size, squish clearance, distance from the exhaust port to squish band and chamber volume.

Note P1 is base pressure at your altitude, I'm at ~800' so the base pressure is 14.26psi.

So substituting in the equations for the volumes and simplifying we get:

View attachment 317466
where (592):
Bore = bore diameter in mm, 55mm.
E = distance from top of the exhaust port to squishband, 1.045" = 26.543mm.
C = squish clearance, 0.022" = 0.5588mm.
CV = chamber volume, 10cc.

So for the the theoretical cranking compression is:
585 = 131psi
592 = 141psi

tl;dr

[emoji57]

 

[Maintenance Chief]

I was pretty skeptical about the compression measurements from these saws but after measuring the combustion chambers and doing a little math they check out.

To measure the chamber I cut a plexiglass disk to seal against the squishband and I fill the chamber with a fluid using a syringe. Many use ATF but windshield washer fluid evaporates really slowly and it's much nicer to work with and clean up. I usually use the blue stuff but all I had was the rain-x brand.

View attachment 317462 View attachment 317463

1ml = 1cc (cubic centimeter)

Chamber sizes are:
585 9.8cc
592 10cc

Now using the Ideal Gas Law PV = nRT

nRT is constant so we end up with:

P1 V1 = P2 V2
where:
P1 = base pressure
V1 = swept volume + squish volume + chamber volume
p2 = final pressure
v2 = squish volume + chamber volume

You can simplify and solve for P2:

P2 = P1*CR
where:
CR (corrected compression ratio) = V1/V2

To do this calculation you need a consistent set of units for distance and volume, metric units are much easier to deal with, we'll use mm and cc which will need an extra divide by 1000 to make mm^3 to cc. Since the volumes become a ratio V1/V2 we can use mixed units for the pressure (psi).

So you need to know the bore size, squish clearance, distance from the exhaust port to squish band and chamber volume.

Note P1 is base pressure at your altitude, I'm at ~800' so the base pressure is 14.26psi.

So substituting in the equations for the volumes and simplifying we get:

View attachment 317466
where (592):
Bore = bore diameter in mm, 55mm.
E = distance from top of the exhaust port to squishband, 1.045" = 26.543mm.
C = squish clearance, 0.022" = 0.5588mm.
CV = chamber volume, 10cc.

So for the the theoretical cranking compression is:
585 = 131psi
592 = 141psi

Here's a simplified explanation of a similar product.

 

[davidwyby]

So if these new saws run well, a lot of it must be airflow tech, and they’re gonna run even more better once you guys add some compressions n stuff I reckon.

Edit to add: this confirms I need a SCARR saw…

 

[bryanr2]

Here's a simplified explanation of a similar product.

Lol

 

[srcarr52]

We can go even further into the math rabbit hole.

I measured the OD of the chamber at 1.550" (39.37mm). Calculating the volume of a cylinder that size and the depth of what we remove from the squishband would be the amount of volume removed from the chamber.

v = pi (d/2)^2

So if we removed 0.050" (1.27mm) from the squish we'd reduce the chamber 1.55cc. Which with stock exhaust timing, squish at 0.020" would be a theoretical compression of 168psi.

I plan on running about 161 degrees of exhaust duration (exhaust height of 1.015") with 0.05" out of the squish the compression would be 163psi. Then with no grinding on the intake the duration will be 158.5 degrees.