Stihl 088 woods porting

[Flavien Simon]

I recently bought an used Stihl 088, that I plan to use for milling and falling the trees I will mill. I will probably mostly mill hardwoods, between 2 and 3ft in diameter for my woodworking projects.

After considering asking someone to port it (but it doesn't seem to be done a lot in Europe), watching a lot of videos on Youtube and reading many threads on this forum, I decided to have a try myself. I have no experience about this or tuning motors in general, I only muffler modded my MS261.

It may look risky, but as an engineer who likes experimenting with things I must say I am as much excited by this projet that by my milling plans !

I start this thread and I intend to post some updates about my work and my findings along the way, as it may have an interest to others beginners as myself, and I would gladly accept some advice too. It will also force me to document what I do.

 

[Flavien Simon]

I will start by writing down what I found about my 088 stock condition.

The engine is 121.6 cm3 with a 60mm bore.

Muffler :
Stock muffler on the european version I have looks like a joke regarding the port opening. It consists on a scrwed on plate on a side, with a tube that must measure around 10mm in diameter. I have no precise measurement as I disposed of it almost immediately, but the openning area looked it was even less than 20% of the exhaust port area.

Otherwise, it looks easy with this cast aluminium muffler to design a custom outlet, and simply screw it in place of the stock outlet.

Stock port timings :
I printed myslef a degree wheel and mesured the following :

• Exhaust : opens at 99,5°
• Transfers : opens at 119° so blowdown is around 20°
• Intake : opens at 65°

From what I read and saw on AS, I was very surprised about the intake timing. I have not touched it yet, but it seems there must be something to improve. The only advantage I could think about is that this generates a lot of crankcase compression. Exhaust transfer timings seemed less surprising compared to other numbers I found for stock saws.

Port areas :
I tried to measure them precisely, and I obtained the angle-area from the above :

• Exhaust : 4.3 cm2, exhaust angle-area : 5,7 deg.cm2/cm3
• Transfer : 4,1 cm2 (this is the area of the two upper transfer ports combined, though it is harder to mesure precisely). Angle-area : 4,6 deg.cm2/cm3
• Intake : 4.3 cm2, intake angle-area : 4,1 deg.cm2/cm3

Piston :
Piston skirt width is around 40,5mm, so this will limit the amount of widening one can do to the exhaust and intake port.

Stock compression :
I measured 140 psi, after buying new OEM piston rings. I don't really know what to think about this, this seems not to bad but as you will see on some pictures below, I am afraid compression could be affected by the small wear marks the cylinder is showing.

 

[Flavien Simon]

Before I go on with some modification I already made (I couldn't really wait to start before even testing the saw properly in stock condition...), I will expose what is my rough plan.

I want to do a reasonnable "woods port" : I want to increase the performance of the saw as much as I can, because it will be useful for milling, more of a joy to use, but I also do not want to compromise the longevity of the saw. Or at least I hope. From what I read, such kind of porting will help the saw run cooler while increasing performance without making it a race saw, which is what I look for.

More precisely :

• I plan to increase the muffler opening to 100 to 125 % of the final exhaust port area, from what I read on a thread from Timberwolf
• I do not really want to risk messing with the exhaust timing a lot, so I am considering windening as much as I can without getting to close to the piston skirt edge (this sould be a bit below 65 % of the bore diameter which seems a safe number), and maybe raising it a little, but not more than 0,5mm to 1mm.
• It seems risky to mess with the upper transfers so I will just try my best at polishing them, and I will taper and enlarge as I can the lower tranfers to try to improve airflow
• I consider I will need to work on the intake. I can enlarge it and raise the top (leaving a sort of W shape to protect the piston rings), but it seems I will have to lower it also. From what I read, it is not realy advisable or needed to increase timing above 80°. I read some posts saying that almost every saw benefits from an intake timing around 78° or more, but I may try something more conservatif at the beginning, doing some tests around 70 of 75 degrees.
• I don't have the tools to mill the lower cylinder, squish band, etc. I may just do some testing without the base gasket if that leaves me more than 0,02 to 0.025" of squish clearance, which seems to be a reasonable limit. It seems that if I want the saw to stay cool while milling, it is more advisable to do some porting and work on the airflow, than trying to raise the compression to much.

As I already started to grind the exhaust and to modify the muffler, I plan to post a few pictures of the results and some testings in a few days, with these two first modifications.

 

[Flavien Simon]

I attached two pictures of the exhaust port. The first first one is stock, it also shows the wear marks on the opposite side of the cylinder.
The second is after modification, it just needs some more polishing to remove the file marks, and I still need to chamfer the inside and match the muffler and gasket to the outside shape.

At the moment I just increased the port area by 14%, to 4.9 cm2, by widening the port and grinding the top just a little. It leaves me with something like a 0.1" margin from the piston skirt side.

I noticed the stock port shape seemed nice and round, so I tried not to alter it much.

 

[the woolfe man]

I attached two pictures of the exhaust port. The first first one is stock, it also shows the wear marks on the opposite side of the cylinder.
The second is after modification, it just needs some more polishing to remove the file marks, and I still need to chamfer the inside and match the muffler and gasket to the outside shape.

At the moment I just increased the port area by 14%, to 4.9 cm2, by widening the port and grinding the top just a little. It leaves me with something like a 0.1" margin from the piston skirt side.

I noticed the stock port shape seemed nice and round, so I tried not to alter it much.

Where did you ex land at after your work,looks neat esp 1st time
And what carb is it running?

 

[Flavien Simon]

Where did you ex land at after your work,looks neat esp 1st time
And what carb is it running?

Thanks !
I will need to check that, but I think it is a HT-12E carburator. It has a factory setting of 1/2 turn for the H screw, and 1/4 for the L screw

 

[the woolfe man]

See how you go if it doesn't tune properly after mods let me know.You may already know this would be good to check what coil you are running some are unlimited and some are limited.

 

[the woolfe man]

sent you a pm Flavien

 

[Flavien Simon]

Here is a video of the test after the exhaust modification. I tuned the saw around 12500 rpm, which may be a bit rich from the sound, but I didn't want to go to high on a first test.
I noticed that the saw was definitely reving more easily and more reactive on the throttle, but I may also have found that I lost a bit of torque, which is not easy to say. Compression (measured the engine being cold) went from 140 psi to 130 psi, after this mod, when I checked after the test cuts.

On this test I use a 36" GB bar with 3/8 Oregon chisel chain and a 7 teeth sprocket, which will be my setup for milling.

 

[Flavien Simon]

Here is another video after I completed the modifications I intented. The saw was not broken in at all that time so maybe I was not very careful running it already in some big piece of wood.
Carburator was tuned rich around 13200. The comparison with the other video is not fair, as I changed the chain from anti kickback Oregon chisel chain, to some Stihl chisel chain. As I tested the two chains with the same state of tuning that in the previous video, I must say that most of the improvement comes from the chain, not the further porting of the intake and transfer ports.

 

[Flavien Simon]

Here are the numbers corresponding to the previous video :

Muffler :
As in the picture attached

Stock port timings :

• Exhaust : opens at 99,5°
• Transfers : opens at 120,5°
• Intake : opens at 77°

The slight work I did on the top of the transfer port was compensated from the removal of the base gasket, so the exhaust timing is still basicaly the same. I also raised the transfers a little. I lowered the intake port a lot.

Port areas :
Still not easy to measure but doing my best so I can assess how much I increased the area :

• Exhaust : 4.9 cm2 (+14%), exhaust angle-area : 6,5 deg.cm2/cm3 (+14%)
• Transfer : 4,7 cm2 (+15%) Angle-area : 4,6 deg.cm2/cm3 (+12%) -- there was a mistake in the previous post, stock angle area was 4,1 deg.cm2/cm3
• Intake : 6,2 cm2 (+44%), intake angle-area : 8,0 deg.cm2/cm3 (+95%)

Compression :
The removal of the base gasket left me with a squish band around 0.025". The compression I measured, after porting and honing the cylinder a little, was at 150 psi.

Now I will try to stop tweaking with the saw, let her break-in and see how I am pleased with the results. This is not easy, as all my previous reference with saws is based on my MS261, so I cannot really compare to anything else especially a 088 stock.

Any thoughts maybe based on my current numbers, and the videos ?

 

[Flavien Simon]

Some pictures of the final port work.