Porting 394 cyl

[parrisw]

Hey, just got another cylinder for my 394 project, I am doing a little mild porting. So far I've just smoothed the transfers out, getting rid of the big lip there, and just blending it in. What is the best to do so there isn't a edge for a ring to catch on. I'm also planning on going without a base gasket. Should I raise the exhaust as well to compensate. This is my first attempt at porting a Chainsaw, so any tips would be great. I don't want to go to wild for my first time. Defiantly don't want to ruin a 394 cyl.

 

[kevlar]

I take it your parts showed up.Everything look good?

 

[epicklein22]

I haven't ever ported a saw...but have read quite a bit on it and I am about to try a saw soon...anyways, the ring should not catch as along as you are making the transitions smooth with no sharp edges or awkwardness. I wouldn't be touching the transfers for a mild porting besides just a lite smoothing. If you are gonna go without a base gasket, make sure you check the squish before you run it. .2 is very common. Anything under .17 is getting tight. As for raising the exhaust, it is usually not done because the reason you are leaving it out is for more compression and torque. Raising the exhaust puts you back to stock. However, i have read sometimes the exhaust needs to be raised to run optimally. Usually it is fine because you widened the port for flow.

 

[timberwolf]

.2 is very sloppy, .17 is very sloppy. but .020 will serve you well. Though you gain torque buy leaveing exhaust below stock the RPM loss will mean slower cutting times unless you run longer bars.

 

[Tzed250]

:agree2:

Do a search, here and elswhere, about porting a chainsaw cylinder and you should find more info than you can digest in one night...

 

[parrisw]

I take it your parts showed up.Everything look good?

Yup, thanks man. Everything is A-OK, money is on its way as well.

Thanks again.

 

[parrisw]

.2 is very sloppy, .17 is very sloppy. but .020 will serve you well. Though you gain torque buy leaveing exhaust below stock the RPM loss will mean slower cutting times unless you run longer bars.

Ya, I was thinking 20thou. the orignal jug which is messed bad, had been ported quite allot, transfers, intake, ex, no base gasket. I've been looking through the modded work saw section.

:agree2:

Do a search, here and elswhere, about porting a chainsaw cylinder and you should find more info than you can digest in one night...

Thanks tis what I'm doing now.

 

[epicklein22]

.2 is very sloppy, .17 is very sloppy. but .020 will serve you well. Though you gain torque buy leaveing exhaust below stock the RPM loss will mean slower cutting times unless you run longer bars.

d@mn it, I couldn't remember if it is .2 or .020. Looks like I guessed wrong. Thanks TW for correcting me.

 

[parrisw]

.2 is very sloppy, .17 is very sloppy. but .020 will serve you well. Though you gain torque buy leaveing exhaust below stock the RPM loss will mean slower cutting times unless you run longer bars.

If Im gaining torque by leaving the ex alone, and removing the base gasket, maybe it'll pull a 8pin well, to bring chain speed back up.

I don't want to go crazy, cause I may be trading this saw off for something else. And don't want someone else to think they may be getting a messed up saw.

 

[parrisw]

I haven't ever ported a saw...but have read quite a bit on it and I am about to try a saw soon...anyways, the ring should not catch as along as you are making the transitions smooth with no sharp edges or awkwardness. I wouldn't be touching the transfers for a mild porting besides just a lite smoothing. If you are gonna go without a base gasket, make sure you check the squish before you run it. .2 is very common. Anything under .17 is getting tight. As for raising the exhaust, it is usually not done because the reason you are leaving it out is for more compression and torque. Raising the exhaust puts you back to stock. However, i have read sometimes the exhaust needs to be raised to run optimally. Usually it is fine because you widened the port for flow.

Thanks, I'll widen a bit then, and leave the height alone. anybody know how wide you can go on a 394?

 

[Crofter]

I haven't ever ported a saw...but have read quite a bit on it and I am about to try a saw soon...anyways, the ring should not catch as along as you are making the transitions smooth with no sharp edges or awkwardness. I wouldn't be touching the transfers for a mild porting besides just a lite smoothing. If you are gonna go without a base gasket, make sure you check the squish before you run it. .2 is very common. Anything under .17 is getting tight. As for raising the exhaust, it is usually not done because the reason you are leaving it out is for more compression and torque. Raising the exhaust puts you back to stock. However, i have read sometimes the exhaust needs to be raised to run optimally. Usually it is fine because you widened the port for flow.

I think you just left out the zero after your decimal point but I know what you were getting at; twenty thou. Returning the exhaust to stock height after having taken out the gasket will keep the same exhaust duration but because you have brought the head lower you will be compressing into a smaller volume (made the combustion chamber smaller) so you will still have a net increase in compression over stock but you are correct in saying that it will be less than it would with gasket removed without anything done to the exhaust port, so you are pretty close!

Because you will lower the intake port with the gasket removal, you increase intake duration which will be of no benefit if you deliberately create a shorter exhaust duration.( by lowering the jug and not at least restoring original port height).
The increased compression alone will yield a bit higher torque but it may well come in at a lower rpm than the stock saw and TW is correct in saying the max hp may well be lower than a stock saw because of what you have done to high rpm potential. It will grunt louder but it wont cut faster at least in medium or smaller wood.
Lowering the jug also effects transfer timing so you are leaving quite a bit on the table by not correcting it too. Work on the transfers is the most difficult part without the right tools especially on closed transfer cylinders. Simply making them bigger is not always the answer either especially on saws that depend on small base and high velocity transfers to get good scavenging.
There are a lot of different things at work in a two stroke engine and each thing or change to it affects other things, sometimes in opposite ways to what seems common sense. There are ways of screwing things up and I have been hammered on in the past for pointing that out.

 

[parrisw]

I think you just left out the zero after your decimal point but I know what you were getting at; twenty thou. Returning the exhaust to stock height after having taken out the gasket will keep the same exhaust duration but because you have brought the head lower you will be compressing into a smaller volume (made the combustion chamber smaller) so you will still have a net increase in compression over stock but you are correct in saying that it will be less than it would with gasket removed without anything done to the exhaust port, so you are pretty close!

Because you will lower the intake port with the gasket removal, you increase intake duration which will be of no benefit if you deliberately create a shorter exhaust duration.( by lowering the jug and not at least restoring original port height).
The increased compression alone will yield a bit higher torque but it may well come in at a lower rpm than the stock saw and TW is correct in saying the max hp may well be lower than a stock saw because of what you have done to high rpm potential. It will grunt louder but it wont cut faster at least in medium or smaller wood.
Lowering the jug also effects transfer timing so you are leaving quite a bit on the table by not correcting it too. Work on the transfers is the most difficult part without the right tools especially on closed transfer cylinders. Simply making them bigger is not always the answer either especially on saws that depend on small base and high velocity transfers to get good scavenging.
There are a lot of different things at work in a two stroke engine and each thing or change to it affects other things, sometimes in opposite ways to what seems common sense. There are ways of screwing things up and I have been hammered on in the past for pointing that out.

Thanks allot. Good post.

So by reading on what your saying, I should put it together with a base gasket in there? If I leave it out, what will I need to do to make it work? Raise the ex back to stock height? Sorry now I'm just more confused then ever?

 

[epicklein22]

That was a good post crofter. Looks like I know the basics, but need help on details.

Parrisw, when I read about people porting their saws, it always seems they leave out the base gasket, but a lot of them don't touch the transfers. It sounds like that if you raise the exhaust back to stock after removing the gasket along with widening it, plus some work on the intake, you should have a strong saw. Some of the potential will be lost though by not touching the transfers, but too much can be lost by messing them up. I mean this is your first porting job and it sounds like transfer work take knowledge and especially experience.

Hope it ends up being a strong saw!!

 

[timberwolf]

Raising it back to stock height is a safe bet. Rarely but not saying never would I go lower than stock on exhaust port.

If you widen out the port you can go to ~ 33 mm giving you a safe 1.5mm or so on each side to keep the port sealed by the piston skirt. This also is still a safe 60% of the bore diamiter.

Like this that saw will pick up some torque, but also carry the torque at a slightly higher RPM, I would expect a gain of about 1000 RPM in the wood when combined with a opened up muffler.

 

[troutfisher]

be careful when raising the exhaust port, take it too high and it'll fall on it's face in the cut. A degree wheel, or some other way of measuring the port height is a must.

 

[Crofter]

be careful when raising the exhaust port, take it too high and it'll fall on it's face in the cut. A degree wheel, or some other way of measuring the port height is a must.

I agree, it is easy to go too far or forget about what effect a largish chamfer afterward will have on timing. Also was thinking that the original exhaust timing numbers will dictate how important it is to raise the exhaust. Some saws have quite long durations stock. Original blowdown time varies a lot too and even from one identical saw to the next. Having things in degrees both numbers you get from someone else and numbers you are grinding in makes a better guarantee that you will arrive at the same kind of performance the other guy reported. Also using the same port shapes and widths has an influence on performance that has to go along with initial port opening. Just a matter of detail here too epicklein!

it is easy to go back and take more off: it is real hard to put it back on so err on the safe side!

 

[troutfisher]

Several people warned me about getting crazy with a porting tool, but it never really sank in until I fooked up a couple saws. They sounded real good until I tried to cut wood.:censored:

 

[timberwolf]

Could not agree more, likely the biggest porting mistake going is raising the exhaust too much.

That is the nice thing with a basic mod, just raise the exhaust the thickness of the base gasket and/or what ever amount you lowered the jug and you know you will still be good. Good place to start.

To raise it without measuring and any idea of what numbers you are trying to achieve is a recipe for a screamer that falls on it's face soon as it hits the wood. Generalities here but huskies can ussually take a couple degrees more exhaust, but I would be careful on newer stihls, dolmars or solos, some have pretty high numbers to start and quickly loose torque and compression.

 

[parrisw]

Could not agree more, likely the biggest porting mistake going is raising the exhaust too much.

That is the nice thing with a basic mod, just raise the exhaust the thickness of the base gasket and/or what ever amount you lowered the jug and you know you will still be good. Good place to start.

To raise it without measuring and any idea of what numbers you are trying to achieve is a recipe for a screamer that falls on it's face soon as it hits the wood. Generalities here but huskies can ussually take a couple degrees more exhaust, but I would be careful on newer stihls, dolmars or solos, some have pretty high numbers to start and quickly loose torque and compression.

Thanks again. So if I raise the exhaust the thickness of the gasket. How much do I compensate for gasket compression? And for measuring the exhaust height can I just measure from the bottom of the jug to the top of the ex port?

 

[timberwolf]

Don't sweat it, if you raise the exhaust 0.5mm or about 20 thou and take the gasket out you will be good. Start first getting the width of the port you want then blend in the roof to give a nice oval with slightly flattened area top center.

Any good high temp automotive sealer will work in place of the base gasket, Permatex MotoSeal 1, Yamabond, Hondabond ect or just red or copper silicone sealer.