A Tale of Three Saws

[MCW]

I would say the 390 has around 140-150 lbs and the 660 is probably at 150-170 from the ones I had. The stihls seem to have more compression than the huskys.

Spot on. My 390XPG has 155psi but I'm a bit wary the guage was bleeding pressure. My 3120 was 145psi and this saw hasn't seen much work. For some reason my new guage's fitting didn't want to bottom out properly in my Huskys but was fine in the Dolmars.

 

[2000ssm6]

That there isnt JTs work, but that saw is strong too :monkey:
He did do the 5100 though.


Anywhos, Brad, are ya done yet???

I was getting cornfused on those 7900s. JTs ran great but the EC7900 was a jaw dropper! Anyone who thinks the 7900 doesn't run well when ported right, needs to run that one.:jawdrop: That coming from a Stihl man to boot!

My 044 wasn't but a few tenths behind it with a good chain.

 

[parrisw]

Not to take over this thread or anything, but from my limited experience and from limited testing, it's my opinion pop-up's give you very little if anything in a normal woods ported saw. Tightening things up by removing the base gasket or turning down the base is the way to go for a normal woods saw IMHO. I've also noticed some saws seem to like more compression than others. Take this for what it is, as I'm no engineer.

Andre.

I agree and disagree. If your going as far as to mill the cylinder base, I say go all the way and do a popup piston as well, every little bit helps, if your giving the saw more fuel, then you need more compression to take all the advantage of the extra fuel, the higher the cylinder pressures, then more fuel will burn, giving more power.

FYI, I did some machine work on a new 390 about a month ago, it got a .030 popup with squish ending at .019-.020, and the guy reported to me compression tested at 190psi.

 

[mdavlee]

Wow I bet a 390 would really scream with 190 lbs. Mine only had 150 after break in. I don't know if the new owner has checked it or not anymore.

 

[mtngun]

Tightening things up by removing the base gasket or turning down the base is the way to go for a normal woods saw IMHO.

That may work great on a saw that comes from the factory with sloppy squish (Dolmar and Husky), but the new 660s come with tight squish and a lame compression ratio.

Porting alone tends to shift the powerband to higher RPMs, and may actually hurt the mid range. Compression improves torque at all RPMs. Compression is a very good thing on a work saw.

I'm not talking insanely high compression, just an honest 165 - 175 psi.

 

[gink595]

That may work great on a saw that comes from the factory with sloppy squish (Dolmar and Husky), but the new 660s come with tight squish and a lame compression ratio.

:monkey: I'm not sure what the squish on my stock 7900 was but compression was 180-185 stock. My 066 was sloppy with like .054 squish and 150 lbs. Compression.

 

[mtngun]

My 066 was sloppy with like .054 squish and 150 lbs. Compression.

According to Timberwolf and others, the older 066 jugs had smaller combustion chambers and came from the factory with sloppy squish (I wish I could get my hands on one of those old jugs ). Great compression could usually be achieved merely by setting minimum squish.

New 660's (and the 660 BB kit) come from the factory with 0.022" - 0.028" squish. Not much room for tightening up. Combustion chambers on the new Stihl jugs seem to be a little bigger than they used to be, too, though it varies from one jug to the next.

At least the new BB kit has a smaller combustion chamber and produces good compression as-is.

We'll see what Brad finds on the test 660. Have the OEM compression and squish been posted yet ?

 

[blsnelling]

I have not measured anything on the two new saws yet. I will be, probably starting tomorrow. I just ported a later model 066 Friday. Squish on it was about .017 from the factory. I did not check the compression. I did take a lot of pics of it since I figured it was a good example of a Mahle P&C. I don't know yet, but I suspect this 660 does not have a Mahle P&C and know Matt's would like to see pics of it. I thought it might be interesting for reference.

 

[Andyshine77]

According to Timberwolf and others, the older 066 jugs had smaller combustion chambers and came from the factory with sloppy squish (I wish I could get my hands on one of those old jugs ). Great compression could usually be achieved merely by setting minimum squish.

New 660's (and the 660 BB kit) come from the factory with 0.022" - 0.028" squish. Not much room for tightening up. Combustion chambers on the new Stihl jugs seem to be a little bigger than they used to be, too, though it varies from one jug to the next.

At least the new BB kit has a smaller combustion chamber and produces good compression as-is.

We'll see what Brad finds on the test 660. Have the OEM compression and squish been posted yet ?

If you start with 150psi of compression and go up to 170, IMHO you gain nothing, this is from some testing I've done and from some research that I've done. From what I've seen most port their saw, add the pop-up run the saw and automatically think the pop-up gave them something. If you were to port the saw do timed cuts, than add a pop-up, gain 20psi than do more timed cuts you'd see little or no gain IMHO.

If you want sloppy tolerances look no farther than Stihl IMHO, their new P&C have been the lowest quality compared to Husky and Dolmar by a long mile.

 

[Andyshine77]

I agree and disagree. If your going as far as to mill the cylinder base, I say go all the way and do a popup piston as well, every little bit helps, if your giving the saw more fuel, then you need more compression to take all the advantage of the extra fuel, the higher the cylinder pressures, then more fuel will burn, giving more power.

FYI, I did some machine work on a new 390 about a month ago, it got a .030 popup with squish ending at .019-.020, and the guy reported to me compression tested at 190psi.

I agree with you 100% But can you tell me how much power the saw gained from the pop-up? did you do testing before you added the pop-up than after you added the pop-up? I'd honestly like to know, as I only know one saw builder that did this, and he no longer uses pop-up's in most woods saws, as his testing showed little to no gains in most saws.

 

[parrisw]

Wow I bet a 390 would really scream with 190 lbs. Mine only had 150 after break in. I don't know if the new owner has checked it or not anymore.

I agree with you 100% But can you tell me how much power the saw gained from the pop-up? did you do testing before you added the pop-up than after you added the pop-up? I'd honestly like to know, as I only know one saw builder that did this, and he no longer uses pop-up's in most woods saws, as his testing showed little to no gains in most saws.

Yes, your right I would have to assume, a .020 popup wont net much. And ya 20psi wont net much of anything. Allot of saws seem to be around the 140-150 mark stock, if you can get that to say 180-190, that is going to help allot. I built a car once and went from 8.8:1 to 11.2:1 compression ratio, OMG what a difference, that wasn't the only thing I did, you can hear it in the ex note when you up the compression allot. Check this vid out, listen to the idle, I guarantee a stock 395 doesn't sound like this at all.

Pay no mind to the mediocre chain on it. This saw has allot of balls down low.

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[rms61moparman]

I agree with you 100% But can you tell me how much power the saw gained from the pop-up? did you do testing before you added the pop-up than after you added the pop-up? I'd honestly like to know, as I only know one saw builder that did this, and he no longer uses pop-up's in most woods saws, as his testing showed little to no gains in most saws.

I would guess it would all depend on what you started with.

A saw with good 150+ compression to start with wouldn't see nearly the gain that a saw with 120 to begin with, would. (IMHO)
The 365 I did started with somewhere in the 150-160 range and went to 190+ with just a cylinder shave of .023.
I don't think there would be ANY gain in cut times with a pop-up on that saw and it would, in all probability, actually slow it down unless you brought the rest of the engine up to a higher level to match the compression. (i.e. more port work, bigger carb, transfer work, ign. timing etc.)

I can however see it adding a lot more strain on the starting system of the saw AND the operator!!!LOL


Mike

 

[Andyshine77]

Yes, your right I would have to assume, a .020 popup wont net much. And ya 20psi wont net much of anything.

Precisely what I was saying.

 

[Andyshine77]

I would guess it would all depend on what you started with.

A saw with good 150+ compression to start with wouldn't see nearly the gain that a saw with 120 to begin with, would. (IMHO)
The 365 I did started with somewhere in the 150-160 range and went to 190+ with just a cylinder shave of .023.
I don't think there would be ANY gain in cut times with a pop-up on that saw and it would, in all probability, actually slow it down unless you brought the rest of the engine up to a higher level to match the compression. (i.e. more port work, bigger carb, transfer work, ign. timing etc.)

I can however see it adding a lot more strain on the starting system of the saw AND the operator!!!LOL


Mike

:agree2: Excellent post Mike!

 

[AUSSIE1]

Well it's a balance of various things.
Some saws with lousy comp readings that respond well to some reasonable timing no's need that comp increase. 385/90's for instance. A combination of mods is what it's about.
A 10=20psi may not necessarily show up in the cut, but help sustain a cut, even if ever minute.

 

[the westspartan]

The best argument I have heard for running a stock (non-pop-up) piston, in a true work saw, is ease of piston replacement. If your saw is set up for a pop-up and for whatever reason the piston needs to be replaced, you must get the new one turned down so that it matches the old one. It is a heck of a lot easier to just toss in a new one instead of taking it down to the machine shop first.

 

[parrisw]

The best argument I have heard for running a stock (non-pop-up) piston, in a true work saw, is ease of piston replacement. If your saw is set up for a pop-up and for whatever reason the piston needs to be replaced, you must get the new one turned down so that it matches the old one. It is a heck of a lot easier to just toss in a new one instead of taking it down to the machine shop first.

Too true. But I have a lathe, and that was my argument, that why not do it?

 

[mtngun]

If you start with 150psi of compression and go up to 170, IMHO you gain nothing

Here's a detailed build of a 660 woods port that starts at 150 psi and ends up at 170. Timed cuts were done at several steps along the way.
Timberwolf's 660 build

If I remember correctly, even TW once said that in some cases, dropping the jug to raise compression may actually slow the saw down, unless port timing is adjusted to compensate.

Since we are talking about the 660, which has excessive exhaust duration, its exhaust port timing may actually benefit from dropping the jug.

It's ultimately up to the customer whether he wants a pop-up. If he wants a 145 psi 660, that's his call.

No argument about the quality of recent Stihl jugs. If only Stihl would make better jugs, with sane timing and a healthy compression ratio ...... As it is, they're getting spanked by the competition.

 

[the westspartan]

Too true. But I have a lathe, and that was my argument, that why not do it?

If you have a lathe and the saw in question is yours, it is a different story!

 

[parrisw]

If you have a lathe and the saw in question is yours, it is a different story!

Yes, that was my argument. I think next build I'll do no popup just to see.