My First Strato Porting Job

[one.man.band]

from what i read from blair and the ref. paper: the biggest difference in stratified scavenging vs. traditional scav. is the difference in the amount of time before the trans ports begin to flow fuel charge into the chamber. (chris mentions this in an earlier post), the delay is much longer due to the extra air added by the "air" port. the trans port has to move the air at the end of the tunnel out of the way first, so the fuel charge can get through to get lit.

confusing to me is why the goal has become to add more "air". maybe i misread something? totally confused.

regards
-joe

edit: need spelling lessons

 

[one.man.band]

good thread, good luck with the build!

 

[Chris-PA]

from what i read from blair and the ref. paper: the biggest difference in stratified scavenging vs. traditional scav. is the difference in the amount of time before the trans ports begin to flow fuel charge into the chamber. (chris mentions this in an earlier post), the delay is much longer due to the extra air added by the "air" port. the trans port has to move the air at the end of the tunnel out of the way first, so the fuel charge can get through to get lit.

confusing to me is why the goal has become to add more "air". maybe i misread something? totally confused.

regards
-joe

edit: need spelling lessons

Originally the goal was to try to bring the Jenn Feng engine up to what I thought the Zenoah numbers were, but it turned out I had measured that wrong. So now it's just an experiment in porting a strato!

In one sense I look at the strato port as just another path to allow the piston to pull air into the engine - as long as an appropriate amount of fuel makes it to the chamber before the transfers close and mixes with the air in the chamber before ignition, then it is good. The combined area and duration of the two intake paths is pretty large.

I could just increase the traditional intake duration, but that would start to defeat the strato function by bringing relatively more air through the carburetor path, allowing the mix into the chamber sooner and increasing scavenging losses.

Also, as Terry pointed out this way the increased duration is blocked at idle.

So at this point I have increased exhaust duration (and a muffler mod). That means I want the fuel to get to the chamber later so it doesn't have as much time to go out the open exhaust port. So there is increased transfer duration to hold them open for a later fuel arrival, and then more duration on the strato air port to essentially delay the fuel getting there.

It almost sounds like it makes sense, but of course it's still dark and rainy and I haven't tried it.

 

[one.man.band]

thx for clarifying that, see now where you changed the initial dur. number.

with the charging events happening later, do you still plan on advancing the spark? hope you have enough blowdown for the increase in charge volume. if it cant exit fast enough even with the extra exh dur, maybe it will backwash into trans, slowing scavenging even more?

looking forward to the results of the changes you have made.

regards
-joe

 

[Chris-PA]

I advanced the timing maybe 6deg, but I have been wondering if that is a good idea. I have a bag of keys I got from McCMaster-Carr so I can put it back or try different amounts.

 

[Chris-PA]

So I finally got to run this saw today. I didn't have anything to cut cookies off of (and I can't stand to waste wood so I'm not sure I could!), but I had some big frozen oak rounds to noodle. That is actually what I was using the saw for last anyway. I brought along the Craftsman-branded yellow GZ4000 for comparison. The Craftsman is stock except for blending the lower transfers and a similar muffler mod.

It took me a while to get the mixture dialled in, mostly because I was being conservative and had it too rich. These saws really don't like rich. But that told me that the long strato intake duration was not limiting the mixture from getting to the cylinder. Once I got the mixture correct it goes quite well. Compared to the stock timing GZ4000 it definitely revs higher, and can hold it. It does not feel like it bogs easily. Both saws can pull an 18" bar buried while noodling these oak rounds, and neither is slow, but the ported engine has a noticeable advantage.

The muffler stayed clean and I did not notice any more smoke or fumes, so I have not turned it into a piggy, and I feel I did get more power out of it. No doubt still more power could be had by defeating the strato function, but that was not my goal. It is pretty impressive for 40cc

I am looking forward to doing some conventional bucking cuts, and I have some things to drop coming up. That will give me a better feel for it. Plus this engine had very low hours, so I would not be surprised if the rings are still seating. I did pull the muffler afterwards and everything looked good.

 

[one.man.band]

sounds like time well spent.

 

[Chris-PA]

I forgot to mention that the idle was nice and steady too, and it had good throttle response. I think from one perspective you can treat the strato air intake as just another intake path into the cylinder, but one with a valve to shut it off at idle. If you increase the duration of it, it will have the typical effect of lengthened intake duration. For a performance point of view it doesn't matter that no fuel flows through that path, as you can get plenty of fuel through the conventional intake to make up for that.

I have no doubt that if I increased the regular intake duration I'd have even more intake flow, because the area, and therefore the flow volume, is limited through the strato inlet path. But then fuel would get into the cylinder earlier, with more loss out the exhaust port.

 

[Chris-PA]

Just wanted to add the final port timing diagram:

 

[Chris-PA]

I got to put some time on this saw yesterday and today. Today was limbing and clearing branches from an oak and maple that came down in a recent storm, and some time bucking up to 14" diameter rounds. The saw really wants to rev, it's almost scary. I can enrich the mixture to slow it down, but I can't go too far or it's too fat. I'm pondering a theory as to why that is but want to mull it over more. I have always found it very hard to hear the 4-stroking with this muffler, so I'm trying to be careful. If I think it is a problem then I will pull the jug and lengthen the intake duration just a bit, but I'd prefer not to change anything.

Throttle response is also very good (it does have an accelerator pump). When bucking it will hold the revs quite nicely and does not feel like it is bogging down. It still sips fuel, running a long time on the tiny tank and not running out of fuel a lot before the oil tank.

I had to quit when the sprocket tip on a cheap "Power Care" bar exploded and threw the chain (I got free with an ebay saw). I wonder if the rpms got to it? It seemed to be well oiled.

Then I pulled the muffler to take a peak and it looked perfect in there. So I am very happy with this mod and the saw is a blast to use. At 9.5lbs this will continue to be my go-to saw - with an 18" bar it does the vast majority of what I need for firewood cutting.

 

[weimedog]

I really hope you can visit one of our GTG's this year....those saws are really interesting as well as your approach.

 

[Chris-PA]

I really hope you can visit one of our GTG's this year....those saws are really interesting as well as your approach.

Thanks, I appreciate it and it would be fun - the PA one too. I will look at the dates and see, but with the warming weather I'm about to get really busy around here and it is hard to get away. I'm already in full panic mode looking at all that needs to be done, plus the extra stuff due to damage from this winter. I just noticed a header that cracked from snow load on the front porch - now I have to jack it up, cut out the remains of the old one and make a new timber (it is older than modern dimensioned lumber) to piece in. And I used more wood than I expected so I'm trying to get ahead on firewood.

I will see if I can make time though.

 

[Chris-PA]

 

[zogger]

That thing cuts GREAT for 40 ccs!! You are doing an excellent job with these saws.

 

[Chris-PA]

That thing cuts GREAT for 40 ccs!! You are doing an excellent job with these saws.

Thanks! I had fun running it today. I got out the Ryobi 10532 (same as a GZ400), which has stock timing but a muffler mod and a little timing advance just for comparison, and it definitely pulled at lower revs than this saw (still very nice though).

 

[mopar3]

from what i read from blair and the ref. paper: the biggest difference in stratified scavenging vs. traditional scav. is the difference in the amount of time before the trans ports begin to flow fuel charge into the chamber. (chris mentions this in an earlier post), the delay is much longer due to the extra air added by the "air" port. the trans port has to move the air at the end of the tunnel out of the way first, so the fuel charge can get through to get lit.

confusing to me is why the goal has become to add more "air". maybe i misread something? totally confused.

regards
-joe

edit: need spelling lessons

The air pushes the exhaust out and some goes out with it. In a standard non strato engine unburned fuel goes out the exhaust. The idea is less unburned fuel out the exhaust.

 

[295 tramp]

Great build
For a saw that many would pass on to add to their collection you have probably started a new infectious CAD disorder.
I can tell the prices are going to jump on saws that most thought were cheap or throw aways.
I have run across poulan pro 305's that have adjustable oilers and passed on them.
Great Job
Randy

 

[WHSH]

First some background: I've had fun porting my plastic Poulan clamshells. Some have worked out great and I use them regularly, some have been dogs and/or failed, and I learned a few things. But I wanted a strato engined saw, and I figured what saws were what and got a few broken RedMax GZ4000s, which are the best small saws in my opinion. The very first one is actually a clone made by Jenn Feng in Taiwan, and after some minor repairs and a muffler mod it ran great, but this summer it ate a rod bearing and that was it. I stuck a Zenoah engine from a Ryobi RY10532 in it and it runs great. But I picked up a Jenn Feng Troy-Bilt branded carcass with a good engine, and decided to do that engine, put it back in the McCulloch and then fix the Ryobi.

It turns out that the Jenn Feng saws share almost nothing with the Zenoah saws. You can swap the engines, but all the JF made castings and forgings are actually different. As are all the case parts, air valve, filter and the Walbro carb. A couple of other parts will swap, but are not quite the same. When I looked at the Zenoah engine from the Ryobi all I did to it was to remove the base gasket (squish ended up right at 0.020") and blend the lower transfers to match the case. I did not touch the ports as I did not want to screw up the strato function and I know how well they run already. The timing from the Zenoah engine was:

E = 132
I = 137
I(S) = 165 (Strato air inlet)
T = 103

After removing the base gasket on the Jenn Feng engine I measured:

E = 132
I = 144
I(S) = 155
T = 101

I like the strategy of a short exhaust duration to maintain cylinder pressure, and a long intake duration. With strato the short blowdown should not matter as there should be little fuel in whatever goes out the open exhaust port. But I was disappointed that the intake duration was short and decided to port it. Here is the saw it's going in:
View attachment 327326

I am a little confused by your numbers. I am assuming you are measureing duration and not degree when opening. If i do the math from your numbers belows is what I get. I am not shure what I, Is, T are.

ex 114
Trans 111.5
Strato port 97.5
T = 128 what is T?
Non of this makes sense to me, can you help me out and give me an idea of how I should be looking at you numbers?

 

[Chris-PA]

I am a little confused by your numbers. I am assuming you are measureing duration and not degree when opening. If i do the math from your numbers belows is what I get. I am not shure what I, Is, T are.

ex 114
Trans 111.5
Strato port 97.5
T = 128 what is T?
Non of this makes sense to me, can you help me out and give me an idea of how I should be looking at you numbers?

Sorry for being unclear, I should have spelled it out better. The numbers I listed were duration. Take a look at the diagram in comment #50 ( http://www.arboristsite.com/community/threads/my-first-strato-porting-job.250199/page-3#post-4725119 ) - I tried to show it better there.

E = Exhaust
I = Intake
I(S) = the strato air intake
T = Transfer

What I did on this saw was to increase the duration of the strato air intake while leaving the regular intake duration alone.

 

[one.man.band]

The air pushes the exhaust out and some goes out with it. In a standard non strato engine unburned fuel goes out the exhaust. The idea is less unburned fuel out the exhaust.

my comment in post #57, was not directed as to the how a strato air port functions.

it was directed to the novel way that chris decided to port this saw.

to my knowledge, no one has written about using this method before.

ie. chris's post #60 last sentence.

hope it clears that up.

regards
-omb