Spuds Porting and Tuning Thread

[Gerbutt]

There are many takes on this but the general consensus is that you open it up 70% of the area of the inside opening of the exhaust port.

 

TBH, it isn't THAT critical and IMO, much depends on how loud you want it.

 

I have two steel tube diameters - the smaller is used on sub 60cc saws, the larger on 60cc+ simple:thumbup:

 

Cheers dude. What size is the smaller tube?

 

Also whilst I'm here, how much would it be for you to buy a new exhaust for my 346xp (50cc with nothing in the old exhaust), port it and send it to me?

[spudulike]

Cheers dude. What size is the smaller tube?

 

Also whilst I'm here, how much would it be for you to buy a new exhaust for my 346xp (50cc with nothing in the old exhaust), port it and send it to me?

 

The 346XP is a very simple mod, I just take the arrestor screen out and bore out the hole by around double.

 

The smaller diameter pipe is 16mm OD.

[Gerbutt]

How do you get something under the deflector to bore it out bud? I did mine like that and made a right pigs ear of it? I put two slits in mine, folded it back, made the hole 'a bit too big I think' and then hammered it back down again!

[spudulike]

I put the degree wheel on my 066 after spraying up the crankcases. I am taking the port duratiion measurements of the cylinder before modification so here is what I have: -

 

Exhaust duration - 173 degrees

Transfer duration - 123 degrees

Inlet duration - 160 degrees

 

So, what are these figures? Basically they are the degrees the crank goes through when turned top of port to top of port when the crank is turned through BDC.

 

What can they be used for - they give us an indication of when the ports open and close and more importantly, the duration of the ports and the blowdown on the engine.

 

From these figures you can work out that you have 93.5 degrees of compression from the top of the exhaust port and TDC.

 

You can work out that the blowdown figure is 25 degrees.

 

Blowdown is where the power stroke compression is exhausted through the exhaust port, the pressure in the top of the cylinder drops to around 5psi and the transfers can push the exhaust out with a fresh charge of fuel.

 

The shorter blowdown is, the more fuel can get pumped in to the cylinder, too short and the exhaust pressure will not have dissipated and exhasut will be blown in to the transfers.

 

This is one area that will be adjusted and will aim for circa 20 degrees once complete.

 

The plan is to now strip it down and turn a pop up and lower the cylinder base and then modify the ports to widen them and lower the blowdown figure.

 

The exhaust port can be raised a few degrees to raise the powerband and the pop up will ensure the compression won't be compromised.

[spudulike]

Thought that last post would cause stunned silence:lol: Took the cylinder off and found the piston as expected - two big scallops out of each side of both skirts.

 

These scallops are probably to reduce friction but they also halt any widening of ports so have ordered a Meteor one without the cut outs.

 

The picture below shows the difference in skirts with the outline of the same port on the skirt showing the Meteor one has much more potential.

 

The pic is from an 064 done some time ago.

[spudulike]

Started work on an 064 that has rolled in with an oil leak and some porting work.

 

Have measured the squish and is over 1mm - lots of potential there.

 

The piston won't leave much for widening the ports so may have to just re-profile them.

 

Timing figs were

 

Exhaust Duration - 163

Transfer Duration - 119

Inlet Duration - 160

Blowdown - 22

 

I will take the exhaust port up a little and take the blowdown down to around 20 degees and see what that gives.

 

Surprising the difference between the 066 and 064 figures.

[D A Tree Surgeons]

Started work on an 064 that has rolled in with an oil leak and some porting work.

 

Have measured the squish and is over 1mm - lots of potential there.

 

The piston won't leave much for widening the ports so may have to just re-profile them.

 

Timing figs were

 

Exhaust Duration - 163

Transfer Duration - 119

Inlet Duration - 160

Blowdown - 22

 

I will take the exhaust port up a little and take the blowdown down to around 20 degees and see what that gives.

 

Surprising the difference between the 066 and 064 figures.

 

Hi Steve

What can you safely bring the squish down to and is it the same for all models:confused1: also do you make paper gasket or gasket seal

cheers

[spudulike]

Hi Steve

What can you safely bring the squish down to and is it the same for all models:confused1: also do you make paper gasket or gasket seal

cheers

 

0.5mm is the lowest figure you would want to use - I either use liquid gasket if I can just remove the base gasket or a custom gasket made out of gasket paper to achieve near this figure.

 

Pretty much the same for all models and all models are slightly different.

[D A Tree Surgeons]

0.5mm is the lowest figure you would want to use - I either use liquid gasket if I can just remove the base gasket or a custom gasket made out of gasket paper to achieve near this figure.

 

Pretty much the same for all models and all models are slightly different.

 

Thank you :thumbup:

[sloth]

Their design doesn't lend themselves to easy porting but tryto open up the muffler and retune the carb if necessary - that would be he easiest mod.

 

I believe yo can fit the 390 top end on the saw but it is a job to remove the cylinder as it is a clam type engine!

 

Cheers spud, muff mod it is then. No sense buying new bits for it as it's hardly done anything. Better off selling it and getting something else if starts to really bother me. Any idea on second hand value? I think its a 2007 model, ms290. Still on original 18" bar, paint not even all worn off and only on its 3rd or 4th chain.