A Tale of Three Saws

[rms61moparman]

Matt, I don't think that's uncommon. There's at least one machine shop that would be a lot more productive if it weren't for these threads (actually this site general) :monkey:

Productivity is over-rated!

Especially considering the QUALITY at your fingertips when you employ the "Worlds Greatest Machinist"!!!!!

 

[Arrowhead]

How wide did you go on the transfers? Are you posting pics of the finished port job?

 

[blsnelling]

How wide did you go on the transfers? Are you posting pics of the finished port job?

Didn't take any. They're probably 50% wider than stock, just the rear ones.

 

[TommySaw]

We'll find out soon enough. Honestly, I have only ported a couple 385/390s.

Well I just got one if you'd like more practice:greenchainsaw:

 

[MCW]

Well I just got one if you'd like more practice:greenchainsaw:

I'd be upset if it's faster than mine though

Good stuff, you won't be disappointed mate.

 

[Andyshine77]

20 psi makes a huge difference when you're cutting under commercial conditions and making a living with a saw. 20 psi might mean the difference of getting another 25-30 trees on the ground in a straight falling job. That translates into $$ when you're cutting stuff that's going 30-40 trees per load and you're getting $125 per load.

Well if you say it works it must work. I haven't seen much gain in the few saw I've seen run, but that's only a small handful of saws.

 

[MCW]

Well if you say it works it must work. I haven't seen much gain in the few saw I've seen run, but that's only a small handful of saws.

Hi Andy.
I know it's only one saw and the results are more than likely variable across the board but increasing the compression on my original 7900 just by removing the base gasket made a distinct and noticable improvement to power and throttle response. I did not touch one other thing on the saw, not even the tune.

 

[Andyshine77]

No, I'm strictly a newbie at the saw modding game, and have never raced, and probably never will. You've been playing with saws far longer than me.

Trust me you're way above me here.

I can tell a difference on my milling saw. I can even tell a difference in how the saw runs on a cold day, when the air is denser, vs. a hot day. Compression gives it more grunt, it doesn't bog as easily, and it's easier to operate.

Anyone who has ever cut wood at high elevation, where the saw loses compression and power, has noticed a difference.

I'm not sure how much compression you gain with dens air, but you definitely gain power because denser air = more required fuel, and that means more power.

Whether a cookie cutter would see a difference in times, I don't know, perhaps depending on the diameter of the cookie. I give Brad credit for using big rounds in his tests, so the saws have to work hard.

There are compromises with any mod. A pop-up, or merely tightening the squish, increases intake duration, which may actually hurt the low end. Without a dyno, or at least a good computer model, we're just fumbling around in the dark.

However, TW does have a dyno, and he does have computer models. That's why I posted his power curve for the 460. Unlike me, TW is in a position to say whether a particular mod helps. If a pop-up didn't help, I don't think TW would do it. And what he has said about the 066/660 is that, in the old days, all you had to do was drop the squish, but with the new jugs, a pop-up is the way to go.

158 psi doesn't sound so bad, but that would only blow 135 psi where I cut. If it were my 660, I would absolutely do a pop-up to get the compression back up.

:agree2:

 

[FATGUY]

How does port timing affect compression? I assumed that the compression "pulse" you see on a compression gauge happens after the ring(s) are above the ports and the piston and ring(s) have sealed off the ports from the combustion chamber/squish band. Anybody want to bring me up to stupid on this?

 

[TommySaw]

I'd be upset if it's faster than mine though

Good stuff, you won't be disappointed mate.

maybe I could get a pop-up in mine and help this thread

 

[Andyshine77]

Hi Andy.
I know it's only one saw and the results are more than likely variable across the board but increasing the compression on my original 7900 just by removing the base gasket made a distinct and noticable improvement to power and throttle response. I did not touch one other thing on the saw, not even the tune.

Hi Matt.

I'm not questioning the fact that more compression = more power, I questioning does 10, 15, 20 pounds really make much difference, that's all really. Whan you lower the cylinder you also slightly change port timing, but only a slight amount, but it does help as you now know.

 

[edisto]

How does port timing affect compression? I assumed that the compression "pulse" you see on a compression gauge happens after the ring(s) are above the ports and the piston and ring(s) have sealed off the ports from the combustion chamber/squish band. Anybody want to bring me up to stupid on this?

You're right. But if you raise the roof of the exhaust, you change the volume of air that is trapped, and if you lose the gasket, you compress the same volume of air into a smaller space.

 

[AUSSIE1]

When the piston is moving up, compression doesn't start till the top ring reaches the top of the exhaust port. If you raise the roof of the exhaust port to gain more time, compression starts later giving you less (less area to compress).

 

[FATGUY]

You're right. But if you raise the roof of the exhaust, you change the volume of air that is trapped.

interesting..... is this to say that compression ratio is no longer a function of the volume of cylinder at BDC and TDC? but rather a function of volume above the exhaust port and volume at TDC?

 

[blsnelling]

you're right. But if you raise the roof of the exhaust, you change the volume of air that is trapped, and if you lose the gasket, you compress the same volume of air into a smaller space.

+1

 

[edisto]

interesting..... is this to say that compression ratio is no longer a function of the volume of cylinder at BDC and TDC? but rather a function of volume above the exhaust port and volume at TDC?

All those silly holes in 2-strokes break up the relationship between displacement and compression somewhat. There are 2 different types of "compression ratio" for 2-strokes, but the only one that has a lot of meaning is the trapped volume.

 

[blsnelling]

interesting..... is this to say that compression ratio is no longer a function of the volume of cylinder at BDC and TDC? but rather a function of volume above the exhaust port and volume at TDC?

You're thinking 4-strokes here, where the valves are closed for the complete compression stroke of the piston. The cylinder is wide open on a 2-stroke until the exhaust port closes.

 

[Andyshine77]

I was talking with Mike about this the other day. My question is how do you come up with the compression ratio in 2 smoker? Is it just volume above exhaust port and psi?

 

[FATGUY]

OK, so here's another couple I'm going to lob out there. Why does it take X number of "pulls" to get the compression on a saw? Surely the volume of the hose can't be responsible for it, can it? Secondly, has anyone ever figured out if either compression or compression ratio can mathematicaly be derived from one another?

 

[edisto]

Dang! The answers are flying quickly! It's an AS intervention for someone falling off the 4-stroke wagon!

Nik, fill a couple of coolers full of beer, put 'em in your truck, and I'll do the same. If we leave at the same time, I'll be at the end of your block in 17 hours, and so will you, and we can sort this out in a proper manner.