Understanding port timing numbers

[Mastermind]

I've been around here for a little while now and have noticed something. A lot of members (myself included) ask questions about port timing and they often go unanswered.

Is this because the modders here want to keep the things they have learned (through trial and error no doubt) to themselves. If that's the case who could blame them. I for one am a little apprehensive to alter port timing on an expensive jug without knowing what these changes are going to accomplish.

I have tried different timing numbers on several low dollar saws, and have had a great time attempting to learn all I can. For me, the learning to do a new skill is the best part.

I reached a point in playing with saws that I have more questions than I had when I started. The basic things I needed to know were easy to find using the search function. It's the details that I wonder about these days. And I'm sure I'm not alone.

For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm? And this is the one I wonder about the most, If I have more exhaust duration than inlet should I increase intake timing to match? If I do this will I lose torque? Do certain numbers work better on a given size saw?

Some of these questions I've got pretty well figured out but know others are wondering about. What I'm hoping for is a good discussion on the pros and cons of altering port timing. Of course I'm trying to learn something here, but I'm also looking for a thread we can all use to trade info in for the good of our members.

 

[AUSSIE1]

Some don't like parting with hard earned no's. Fair enough.

Personally I don't do racing etc or have the need to keep work private and wouldn't even scratch the surface of the pro's. I try to share as much as I can, but with that said there is always the fear of giving incorrect info. There is a disclaimer attached!

All saws like different methods/no's etc.

I have put most of my time in the BB kits for the 365 to 372's being cheap if anything goes wrong. A great saw to play with, with the options availible, pistons etc.

Widening the exhaust increases the blowdown and gives increased revs and shifts the power higher. No loss of compression.
Raising the exhaust increases the blowdown and shifts the power higher at the expense of some compression.
Raising the transfers can bring torque down a little and flatter from a high ex duration as long as blowdown allows it.
Increasing the inlet helps balance the need of the transfers and exhaust as long as the duration doesn't excede the crankcase.
Inlet duration usually at or close to exhaust duration.
If revs increase from more inlet duration, it's due to one or more of the other functions needing it.

Remember the disclaimer! :hmm3grin2orange:

 

[Hedge Hacker]

Posts like this make me feel great because i know no matter how long i cut wood or fix saws, i'm never gonna know everything.

 

[chopperfreak2k1]

TRUELY GREAT THREAD IDEA!!! i'm all ready to learn!

 

[mtngun]

A lot of members (myself included) ask questions about port timing and they often go unanswered.

When Timberwolf is around, he has some good suggestions, though you get the impression he only tells you a fraction of what he knows. But, TW hasn't been around much lately.

Seriously, other than TW, I'm not aware of any authoritative technical reference on non-piped two-stroke engines. Most of the reference material like Bell's book is geared toward pipes. Without a pipe, the general tuning trends may still apply, but you won't be able to predict numbers with any certainty.

One of my personal frustrations is that the vast majority of the modding info on the web is geared toward cookie cutting, not milling. It's debatable how much the cookie cutter mods will help a milling saw.

Some people say you shouldn't do any mods to a milling saw. These naysayers invariably seem to be people who are running 120cc saws. You don't encounter many 90cc millers who are satisfied with stock power.

TW and JJ have given me a few good suggestions specifically for milling, but otherwise, I'm on my own, learning by trial and error.

For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm? And this is the one I wonder about the most, If I have more exhaust duration than inlet should I increase intake timing to match? If I do this will I lose torque? Do certain numbers work better on a given size saw?

The only person who may be able to answer those types of questions with any precision is TW. He has computer models and a dyno and lots and lots of experience.

But, all the computer models out there assume you are using a pipe. Lotsa luck finding a model to work with chainsaw mufflers. I assume TW wrote his own program, or else tweaked a program to work with chainsaw mufflers.

One thing we don't see discussed much on AS is the case volume and how that affects optimal porting. I don't know much about that myself, except that it does play a role, and serious modders like TW go to a lot of trouble to measure case volume.

Another bone for thought -- take the Dolmar 7900, which most people consider a best-in-class powerhead. Look at the 7900's port timing and you will find ..... absolutely nothing remarkable. Makes you think, doesn't it ?

My only firm conclusion to date is that I've never had a saw with strong compression that didn't cut well, and I've never had a saw with low compression that impressed me. Dolmars and Solos generally have strong compression. The latest 066BB has strong compression. I like that.

 

[AUSSIE1]

There is alot of misconseption about cookie cutters, even from some who play with porting. Cookie cutting is basically a way of measuring mods that you have made, whether it's a race saw or a firewood saw.

Some may go to the lengths that TW does, others alter durations etc without even using a degree wheel or port map. What ever works for you as there isn't any law. Some have that many parts at their disposal, if a cut goes wrong, piff it in the bin and grab another.

TW has considerable knowledge, but that doesn't make others redundant.

 

[troutfisher]

For instance, will a longer intake duration raise rpm but cause a lose of torque? If I have amble blowdown time could I give up some of this by raising the transfer roof, in turn increasing torque and rpm?

I think of transfers as "bleeding off base pressure" . As the piston moves down, the base compresses, and once the transfers start to open that pressure is released into the cylinder. It takes some time for that air to move. When RPM's increase that time window gets smaller, so I need to raise the transfer a bit to keep the actual time the port is open the same. Just my 2 cents, may be wrong

 

[parrisw]

I'm in for this discussion.

Where's John with some rat tail file port job discussion?

 

[Mastermind]

We have a great discussion going already. Thanks for everyone's thoughts and comments so far. This is one aspect of the tuning process that we just don't see many threads on. Thanks.

 

[mtngun]

TW has considerable knowledge, but that doesn't make others redundant.

Agreed.

I've learned a lot from you, Brad, Tree Slinger, JJ, and the other experienced modders.

But as near as I can tell, most modders, including myself, are doing what TW calls "guess porting." We make an educated guess, cross our fingers, and hope it works.

Eventually we find a recipe that works well, and then we tend to stick pretty close to that favorite recipe forever and ever.

But to be able to answer "what if" questions with any certainty, TW is the only one in the chainsaw world who seems to be able to do that. He has the computer model, the theoretical understanding, and the dyno. The rest of us are lacking at least two of those things.

Troutfisher, thanks for the explanation of transfer timing.

I would really like to get into the computer modeling someday. If for no other reason, than to be able to play with "what if" scenarios and see how they change things. That should be educational.

That was what I found with computerized reloading software. It wasn't so much that I used the software to find a final load, because the software wasn't that reliable, but being able to run many different "what if" scenarios and seeing how it changed the outcome gave me a pretty good feel for what worked and what didn't. I was able to learn more in one year playing with the software than I had learned in the previous 20 years of "guess reloading."

 

[Mastermind]

One thing we don't see discussed much on AS is the case volume and how that affects optimal porting. I don't know much about that myself, except that it does play a role, and serious modders like TW go to a lot of trouble to measure case volume.

I'm in complete agreement with this statement. I would truly enjoy learning more on this subject as well.

In the Husqvanra 357xp vs 359 etech comparison I did awhile back, the 357xp had crank stuffers while the 359 did not. After running both saws I found the xp made more rpm stock than the 359, but the 359 (ported) had a lot more torque. I can't say with any certainty how the xp would do ported. (It was not my saw, and was only muff-modded and the squish was lowered to around .020). I would like to be able to say I know what the stuffers did for the performance of this saw but could only parrot what I have heard others say, so a discussion on this subject would be welcome by me for sure.

 

[mtngun]

I think of transfers as "bleeding off base pressure" . As the piston moves down, the base compresses, and once the transfers start to open that pressure is released into the cylinder. It takes some time for that air to move. When RPM's increase that time window gets smaller, so I need to raise the transfer a bit to keep the actual time the port is open the same.

OK, since you did a great job of explaining transfers, I'm going to pick your brain a little more.

Let's use the current 066BB as a guinea pig.

And let's say that I this 066BB is used primarily for milling, which means heavy loads at 7000 - 10,000 rpm without bogging. Bog-resistance is very important on a milling saw.

The jug comes from the factory like this, give or take a few degrees:

169 degrees exhaust duration
178 degrees intake duration
32.5 degrees blowdown

Someone suggested raising the transfers to 25 degrees blowdown, and that does sound like a more conventional number than 32 degrees blowdown.

But how is 25 degrees blowdown going to affect the ability to mill at 7000 - 10,000 rpm ? Will it shift the powerband to higher RPMs, and make the saw more prone to bog under heavy load ?

I imagine the answer will be "it just depends."

 

[troutfisher]

169* of exhaust means it must open at 95.5* atdc
95.5 + 32.5* of blowdown = 128, the transfers open at 128* atdc
180-128= 104* of total transfer time, that sounds low to me.

if you raise the transfers to 25* of blowdown that means the transfers will open at 120.5* atdc. That means you will have 119* of total transfer time. That sounds a lot better to me. I agree with raising the transfers.

That said, I have never ported a 066BB, or milled with a chainsaw, so I'm not sure about setting up a motor specifically for it.

 

[troutfisher]

without bogging. Bog-resistance is very important on a milling saw.

As for Bog-resistance, I would think that would be a function of the chain and gear setup. I have bogged 40 horse motorcyle engines in the cut, and run 346xp's that I could set the dawgs and pull on. Maybe one tooth lower on the sprocket, skip chain, square ground chain, ect. might make it less prone to bogging.

 

[Metals406]

This is just my own opinion, but I view porting as optimization. . . The path of least resistance if you will.

I like to eyeball a jug, slug, and case and think about ways to get the most amount of charge, through the intake and out the exhaust in the most efficient way possible.

Scientific it is not, but it has worked well for me.

I have yet to degree a saw, or figure out it's numbers. Perhaps I'm leaving some things on the table -- but one can also spend a lot of time behind the grinder (and machining) for gains as small as a few tenths of a second. Unless I plan on racing, it's just not worth the time for me.

When I port, I also want just as much saw longevity as one could expect out of stock.

It's kind of a bean counter concept. . . Do you build an airplane that flies 20 knots faster, but you have to rebuild it 3 times as often?

I also think the 4-stroke concept is lost to a lot of people. That burble you hear in the air, is fuel and air that will be used during work. Ported or not, I always want my saws to cough a little in the air, and clean up in the cut. That to me is optimal tuning, not a certain RPM "target" number. I let atmospheric conditions, altitude, etc dictate where the saw wants to be at that particular time. Maybe one day it's 500 rpm less than the time before, maybe it's 250 rpm more?

Like I said, just mah two pennies.

 

[mtngun]

As for Bog-resistance, I would think that would be a function of the chain and gear setup. I have bogged 40 horse motorcyle engines in the cut, and run 346xp's that I could set the dawgs and pull on. Maybe one tooth lower on the sprocket, skip chain, square ground chain, ect. might make it less prone to bogging.

Yes, but ...... if it bogs with a 7 tooth, lotsa luck finding a 6 tooth ! ! !

The milling equivalent to skip chain is the Granberg grind, where half the teeth score and the other half cut. It doesn't necessarily cut faster, but it does rev faster and makes life easier on the powerhead, similar to skip chain. The downside is the cost if you buy it, or the time consuming grind to make your own.

My limited experience so far with "bog-resistance":

-- a stock 066 was gutless for milling, and seemed to have a narrow powerband. I didn't have a tach at the time, but it had to be fed carefully to keep the revs up.

-- the old, low compression 066BB was prone to bog below 8400, but rarely had enough power to spin more than 9500 in the cut. Talk about a narrow powerband !

-- the new, high compression 066BB would lug as low as 6000, or spin as high as 10,500. Despite its questionable port timing, the powerband was awesomely broad and forgiving. As a bonus, it consumed less fuel than any other 066 top end I've run.

-- but then after I widened the 066BB's intake, it would bog below 7000, while not gaining anything on top, and drinking a lot more fuel.

-- my stock Efcos had a broad, user-friendly power band with conservative port timing and decent compression. A mild woods port shifted the power band to higher RPMs -- cuts a little faster as long as you keep the revs up, but not as forgiving if you lean on it.

So far my favorite recipe is all the compression I can scrounge up and sane port widths and port timing. Seems to give a broad, user-friendly powerband. I'm always looking for something better, though.

 

[ronT2]

OK, since you did a great job of explaining transfers, I'm going to pick your brain a little more.

Let's use the current 066BB as a guinea pig.

And let's say that I this 066BB is used primarily for milling, which means heavy loads at 7000 - 10,000 rpm without bogging. Bog-resistance is very important on a milling saw.

The jug comes from the factory like this, give or take a few degrees:

169 degrees exhaust duration
178 degrees intake duration
32.5 degrees blowdown
Someone suggested raising the transfers to 25 degrees blowdown, and that does sound like a more conventional number than 32 degrees blowdown.

But how is 25 degrees blowdown going to affect the ability to mill at 7000 - 10,000 rpm ? Will it shift the powerband to higher RPMs, and make the saw more prone to bog under heavy load ?

I imagine the answer will be "it just depends."

Those are some interesting timing #s, I’ve never seen a longer intake duration than an exhaust duration. Could the longer intake duration and later transfer opening have something to do with case volume or is it more of a function of where and when Stihl wants the power to come in, probably both. I realize it’s an aftermarket BB kit but I’d imagine they would model it after the OEM jug.

Maybe I’m looking for someone to come along and say something like….because the case volume is greater relative to the 066s swept volume the intake needs to be open a little longer to fill the case and the transfers have to open a little later so intake charge has time to compress?????

I don’t know, I’m just thinkin which usually gets me in trouble. I’m also in the same boat as Troutfisher, I’ve never worked on an 066 but really enjoy this kind of thread.

 

[troutfisher]

Yes, but ...... if it bogs with a 7 tooth, lotsa luck finding a 6 tooth ! ! !

I'll bet Danzco would have one, or make one.

http://www.danzcoinc.com/html/pin_drive_sprockets.html

 

[chopperfreak2k1]

sorry for the thread jack, but i want to give a sincere thanks to you guys for sharing your knowledge here with the rest of us. thanks!

 

[mtngun]

I realize it’s an aftermarket BB kit but I’d imagine they would model it after the OEM jug.

Yes and no.

From what I've read between the lines of various comments Grande Dog has made over the years, the NWP design process originally consisted of something along the lines of sending the Taiwanese manufacturer an OEM P&C, with instructions to "make it like OEM, except 2mm larger," or words to that effect.

Thus in the beginning, the BB timing and compression ratio was nearly identical to the later 660 jugs (159 intake, 173 exhaust, and 21.5 blowdown on the last OEM jug I measured).

Over the years, the BB design has been tweaked. For example, there were problems with the piston skirt hitting the case, so they shortened the skirt -- which increases the intake timing. That seems to be how the 066BB ended up with radical intake timing -- by accident. I don't think there was a lot of serious thought put into it.

I'm not sure how the current 066BB ended up with more blowdown than the OEM ? ? ?

BTW, Stihl seems to follow a "cookbook" approach to saw design. For example, most of the pro Stihls have very aggressive exhaust duration, apparently to compensate for the restrictive muffler. Most of the recent big Stihls have an anemic compression ratio, apparently to compensate for poor gas or poor operators.

Efco also seems to follow a cookbook recipe for all their saws, only their recipe is different than Stihl's. Modest port timing, but decent compression. I like the Efco recipe better.