Braapp-Shack/SCARR Engineering 394 build, DONE :)

[mdavlee]

Easiest thing would be to send the jug out while you're waiting and get it machined some.

 

[jimmorrison]

Yeah, that would be the easiest, and what I would do. This guy wants to build his own. Kinda cool and worth the effort.

 

[mdavlee]

I think you'll be at 170-175 with exhaust at 100-101. Last one I did was bumping 200 and exhaust was 97-98. I don't cut as much off as Jason or Shaun.

 

[srcarr52]

Do you think I will retain enough compression with the exhaust at 99 just doing the BG delete? If I recall, I set the exhaust on my 372 at 101, but that's a different engine and of course we can't assume what works for one cylinder will work on another. But my rookie mind thinks the exhaust at 99 degrees might be just a tad aggressive for a plain-Jane BG delete?

Let me re-phrase the first part of that question... Do you think I will retain at least stock compression?

You'll be about even or a little higher with the stock compression with base gasket. Stock squish is around 0.050", remove base gasket you get around 0.025". You'll have to raise the exhaust port around 0.070-0.075" to get 99 (162 duration).

 

[jimmorrison]

Hey Shaun, I am sending you a PM about that mail in machine work.

 

[srcarr52]

It's pretty simple math to figure out what you compression will be PV=nrt, nrt is constant, so P1*V1=P2*V2. You need to know the combustion chamber size which I've estimated at 8.5cc for a 394.

So, stock with gasket... ~145 psi.
Remove base gasket... ~0.025" squish ~175psi ex should be around 105
Raise exhaust to 99 ~160psi
Ex at 101 ~166psi

 

[srcarr52]

Hey Shaun, I am sending you a PM about that mail in machine work.

Haven't I done machine work for you before?

 

[jimmorrison]

Kinda, I bought the whole saw.

 

[jimmorrison]

Cumbustion chamber is easy to figure on a overhead valve V8, how do you do it on a saw?

 

[srcarr52]

Cumbustion chamber is easy to figure on a overhead valve V8, how do you do it on a saw?

Same way. Syringe and a Plexiglas plate. I can also estimate it using the equations in reverse and a known compression and measurements from a good sealing saw.

 

[jimmorrison]

Do you measure the entire volume of the cylinder and then subtract what the piston takes up?

 

[paragonbuilder]

It's pretty simple math to figure out what you compression will be PV=nrt, nrt is constant, so P1*V1=P2*V2. You need to know the combustion chamber size which I've estimated at 8.5cc for a 394.

So, stock with gasket... ~145 psi.
Remove base gasket... ~0.025" squish ~175psi ex should be around 105
Raise exhaust to 99 ~160psi
Ex at 101 ~166psi

Awesome!
Thanks for this Shaun!

 

[jimmorrison]

Do you measure the entire volume of the cylinder and then subtract what the piston takes up?

Duh, it just hit, you only have to measure above the port, and then figure piston travel.

 

[jmssaws]

These saws seem to keep running better the more compression they have where a stihl isn't that way.

It doesn't have to be 200+ to run good

 

[jmssaws]

Do you think I will retain enough compression with the exhaust at 99 just doing the BG delete? If I recall, I set the exhaust on my 372 at 101, but that's a different engine and of course we can't assume what works for one cylinder will work on another. But my rookie mind thinks the exhaust at 99 degrees might be just a tad aggressive for a plain-Jane BG delete?

Let me re-phrase the first part of that question... Do you think I will retain at least stock compression?

I built them from 98.5 to 104 and they run good wherever but there certainly stronger at 98.5 than 103.
I wouldn't worry about compression
If it's 166 or 176lb you wouldn't notice much difference but at 98.5 it free's it up.

The bigger the saw the higher the exhaust can,your 372 is at 101 so a saw That's got 20cc on it will be fine at 99 and 18 degrees of blow down,that's where I like them.
Fast but it has a ton of grunt.

 

[redlinefever]

Are you just going to run a stock carb or doing anything there?

 

[RIDE-RED 350r]

Are you just going to run a stock carb or doing anything there?

As far as the carb, I wasn't planning to modify or replace it. I haven't seen anyone on here modding or upgrading 394 carbs, although just because I haven't heard of or seen it doesn't mean guys aren't doing it.

The info going into this thread is terrific! You guys are awesome!

I'm starting to consider having the machine work done on the cylinder. I admit that I do enjoy doing it on my own, but I might just send that cylinder out.

 

[wcorey]

Speaking of 394's and compression...
Eric and I were using my ported 394 doing a bunch of dyno runs to test some various levels of software filtering.
At some point I noticed the decomp had been sticking open. Going back on the graphs there was no discernible difference in power output whether it was open or closed.
I guess just a lesson in static versus dynamic compression, probably why "Q" or "T" exhaust ports work....

 

[Wolverine]

W/ a gasket delete I got right about 185 after breaking in a fresh Meteor/caber.
Just curious @wcorey what kind of #'s were you pulling?

 

[jmssaws]

As far as the carb, I wasn't planning to modify or replace it. I haven't seen anyone on here modding or upgrading 394 carbs, although just because I haven't heard of or seen it doesn't mean guys aren't doing it.

The info going into this thread is terrific! You guys are awesome!

I'm starting to consider having the machine work done on the cylinder. I admit that I do enjoy doing it on my own, but I might just send that cylinder out.

Do you have a right angle hand peice?