My First Strato Porting Job

[Chris-PA]

Where in Pa are you?

We're south of Allentown

With too short of a blowdown you will have exhaust pressure in your transfers. Have you done the maps? Here is an older thread that may help you find the best numbers.
http://www.arboristsite.com/community/threads/port-timing-calculator-guide.85786/

Thanks for the link, I had not seen that. Note that the factory blowdown is at 15, which is what I was aiming to preserve but I ended up at 14. I can always raise the exhaust 4 more degree. Also the muffler flows pretty well so that should help to mitigate the issue by allowing cylinder pressure to fall off faster.

I think the other reason for a larger angle between exhaust opening and transfer opening is to reduce the amount of time both ports are open and thereby reduce scavenging losses, but I could be wrong about that. The strato transfers will have only air in them until quite late in the cycle so that is not an issue.

 

[Chris-PA]

Joe (one.man.band) referred me to an interesting document that was someone's master's thesis - do a web search on "MikaelMasterThesis.pdf". It has some neat 3D plots in the back that show the fuel charge does not begin to reach the cylinder until very late in the cycle (like 200* ATDC). I think the purpose of the short blowdown is not on the opening end, but on the other side to hold the transfers open longer to allow the fuel charge time to get there.

 

[HuskStihl]

Joe (one.man.band) referred me to an interesting document that was someone's master's thesis - do a web search on "MikaelMasterThesis.pdf". It has some neat 3D plots in the back that show the fuel charge does not begin to reach the cylinder until very late in the cycle (like 200* ATDC). I think the purpose of the short blowdown is not on the opening end, but on the other side to hold the transfers open longer to allow the fuel charge time to get there.

I didn't understand a word of that, but I like the pretty pictures and I am "subscribing" to see how it works.

 

[Adirondackstihl]

I didn't understand a word of that, but I like the pretty pictures and I am "subscribing" to see how it works.

I'm with Jon..........well........kinda

 

[zogger]

Nice build thread so far man!

Get a brooder light and aim it at your work bench area in the barn when you are working in there, will keep everything and your hands warm for cheap bucks.

 

[Chris-PA]

I didn't understand a word of that, but I like the pretty pictures and I am "subscribing" to see how it works.

I'm with Jon..........well........kinda

With a strato, when the piston rises it pulls pure air through the air valve and down the transfer runners, and an extra rich fuel/air mixture though the carb and into the case. When the piston moves back down it squeezes mostly air back up the transfers an into the cylinder. The rich fuel/air mixture only makes it into the cylinder very late, not long before the transfers close, where it mixes with the air already there to give a net correct mixture (the mixture is not supposed to be "stratified" at the time of combustion).

Because all port durations are symmetrical about TDC or BDC on a piston ported 2-stroke, then opening the transfers early means holding them open later. So an earlier opening of the transfers is also a later closing, perhaps giving more time to get the mixture up to the cylinder.

With a traditional engine people try to reduce blowdown so that exhaust pressure does not "blow down" into the open transfer runners and reverse the flow direction. But with a strato it was already reversed and there isn't any fuel mix in there anyway at that time, so maybe it doesn't matter so much.

 

[HuskStihl]

Thanks for the splanation!

 

[Chris-PA]

Well, the saw is back together and it started and ran. Now I'll have to find time to get it into wood. However, this may be a more interesting experiment than I thought, as I was wrong on the timing numbers of the Zenoah engine. Either the degree wheel slipped or I just had my head up my ***. I got suspicious as the castings didn't look that different, so once I had the Zenoah engine out again I repeated the measurements:
E = 132
I = 137
I(S) = 150 NOT 165 (Strato air inlet)
T = 103

Original Jenn Feng engine:
E = 132
I = 144
I(S) = 155
T = 101

Modified JF engine:
E = 142
I = 144
I(S) = 163
T = 114

So I have lot more duration on the air inlet, and it will be interesting to see what that does. Because the JF engine had more standard intake duration to begin with, I'm now at 19deg more duration on the air intake than on the fuel mix intake. The Zenoah engine has a difference of 13deg. My fear is that I won't be able to get enough fuel into the cylinder with the longer air intake, but we shall see - I'll either be able to tune it in or not. If I have trouble I'll increase the fuel mix intake duration.

 

[Chris-PA]

Some gratuitous build pictures of an obscure saw - Zenoah vs. Jenn Feng clone:

Engine back in:

Air valve and carb:

 

[Chris-PA]

I reduced and smoothed this obstruction at the carb inlet quite a bit:

Everything is different on the JF saws compared to the Zenoahs, even the clutch, flywheel and bar stud thread sizes. The air filter is different too and does not seal well to the air valve, so I added some foam:

Done!

 

[fuzz1500]

Very cool thread !!!

 

[Terry Syd]

I don't know how that air valve works, but I think I see a mechanism on it that makes it open later than the throttle valve. In which case, you should be fine (only 163 degrees and that only occurs well after idle rpm). The original Husky 445/450 had a strato timing of 154 degrees with a 144 intake (the 455/460 also has an extra 10 degrees on the strato).

If you can feed it enough fuel from the high speed circuit she should run fine.

If it does run good, then you may want to go to the next level. At present you are short changing the crankcase with the intake. The engine cannot begin to make crankcase compression until BOTH the strato and intakes are closed.

If you take the intake out to 163 they will both close at the same time and you will have maximised the intake cycle.

If you're reluctant to cut the intake floor lower to get that intake duration, then you can trim the piston skirt as a test. If you don't like it, put a stock piston back in it.

 

[HuskStihl]

I don't know how that air valve works, but I think I see a mechanism on it that makes it open later than the throttle valve. In which case, you should be fine (only 163 degrees and that only occurs well after idle rpm). The original Husky 445/450 had a strato timing of 154 degrees with a 144 intake (the 455/460 also has an extra 10 degrees on the strato).

If you can feed it enough fuel from the high speed circuit she should run fine.

If it does run good, then you may want to go to the next level. At present you are short changing the crankcase with the intake. The engine cannot begin to make crankcase compression until BOTH the strato and intakes are closed.

If you take the intake out to 163 they will both close at the same time and you will have maximised the intake cycle.

If you're reluctant to cut the intake floor lower to get that intake duration, then you can trim the piston skirt as a test. If you don't like it, put a stock piston back in it.

I definitely have absolutely no ******* idea what u'r talking about, but that post makes you sound really, really smart. If I read this stuff for another 10 years, I might get a basic understanding of how a 2-stroke works. Thx boys!

 

[Terry Syd]

Mate, I've been messing with engines for more than 50 years, ya gotta pick up something during that amount of time.

 

[Chris-PA]

I don't know how that air valve works, but I think I see a mechanism on it that makes it open later than the throttle valve. In which case, you should be fine (only 163 degrees and that only occurs well after idle rpm). The original Husky 445/450 had a strato timing of 154 degrees with a 144 intake (the 455/460 also has an extra 10 degrees on the strato).

If you can feed it enough fuel from the high speed circuit she should run fine.

If it does run good, then you may want to go to the next level. At present you are short changing the crankcase with the intake. The engine cannot begin to make crankcase compression until BOTH the strato and intakes are closed.

If you take the intake out to 163 they will both close at the same time and you will have maximised the intake cycle.

If you're reluctant to cut the intake floor lower to get that intake duration, then you can trim the piston skirt as a test. If you don't like it, put a stock piston back in it.

Thanks Terry - The air valve linkage is sort of an idle/WOT on/off switch. It's not intended to be a linear, progressive opening, rather to pop open all at once above idle so it doesn't really delay the strato port rather it just defeats it at idle.

My goal for this was to be a mild port job that preserved the strato effectiveness, but I've accidentally gone further off into the weeds than planned due to my erroneous measurement. I'm not really worried about the 163 as a total intake duration as I have other saws that run really well like that. But I had intended to maintain the relationship between the air port and the fuel port timing, and now I'm pulling relatively more air in. As long as I can tune it right and it doesn't lean out at higher rpms then I guess then it should not hurt anything.

In truth I do not yet know why they run a longer duration on the strato air intake than they do on the regular intake, but there must be a reason.

I have no problem dropping the intake to get it closer to the strato, but it is all buttoned up now so I figure why not see how it runs first? I had no issues starting it with the carb settings from the Zenoah engine. The idle is off but it revved up OK, so it will probably run fine and then it's just a matter of how much power it makes.

Oh, and the ignition is advanced about 6deg, mostly out of habit. It has a real key so I can put it back if needed.

 

[Terry Syd]

Yeah, see how it runs now. You can always start adding intake duration by trimming the piston skirt. Take it up to 154 degrees and see how it runs, then take it up to 163 degrees - you can always pop in a new piston to take it back to where it really worked well.

These sequential strato intakes have a tremendous advantage over the conventional two-strokes as the time/area of the intake can be considerably increased without compromising the idling/transition of the engine.

 

[Chris-PA]

Yeah, see how it runs now. You can always start adding intake duration by trimming the piston skirt. Take it up to 154 degrees and see how it runs, then take it up to 163 degrees - you can always pop in a new piston to take it back to where it really worked well.

These sequential strato intakes have a tremendous advantage over the conventional two-strokes as the time/area of the intake can be considerably increased without compromising the idling/transition of the engine.

Oh, I was being dense - the strato ports are blocked at idle so the longer intake duration doesn't effect idle.

 

[Terry Syd]

Yep, the strato port is blocked at idle so a longer duration on the strato port doesn't affect the idle. Whereas around 160 degrees is considered fairly safe for the carb intake, you may be able to take the strato port out to say 170 or more without having any idle problems and get even more air in the cylinder.

Remember, it is the air that is difficult to get into the engine. It is really easy to put more fuel in, just turn up the mixture screws. However, getting the air in to burn that fuel is the problem, thus we see super and turbo chargers on engines to try and cram more air in.

You just have to try different things and see what works. What works is just that - what works. You could end up with 163 on both intakes or maybe 155 on the carb intake and 170 on the strato intake. The great thing about working with a sacrificial piston is that you can always go back to stock with a new piston and start over if you go too far.

 

[Chris-PA]

Yep, the strato port is blocked at idle so a longer duration on the strato port doesn't affect the idle. Whereas around 160 degrees is considered fairly safe for the carb intake, you may be able to take the strato port out to say 170 or more without having any idle problems and get even more air in the cylinder.

Remember, it is the air that is difficult to get into the engine. It is really easy to put more fuel in, just turn up the mixture screws. However, getting the air in to burn that fuel is the problem, thus we see super and turbo chargers on engines to try and cram more air in.

You just have to try different things and see what works. What works is just that - what works. You could end up with 163 on both intakes or maybe 155 on the carb intake and 170 on the strato intake. The great thing about working with a sacrificial piston is that you can always go back to stock with a new piston and start over if you go too far.

Yes, this is what I was thinking of. If you look at the Master's Thesis document I mentioned above, the fuel didn't get to the cylinder until quite late. That's OK as long as it does get there in time to mix properly before ignition. I can imagine a scenario where the fuel mix doesn't reach the cylinder in time and then you would not be able to make the mixture rich enough with the H screw - but this would also mean that you'd have a lot more flow through the strato port than you would through the carb and it would be very unbalanced. I'm probably a long way from that.

I'm starting to like where these port numbers came out in concept - now I need to see how they work in practice. Damn these short days and all the rain we're having!

 

[diggers_dad]

opcorn: