Spuds Porting and Tuning Thread

[Sawchip]

Been porting another 357XP yesterday, turned out well, thought I would show a comparison of the standard exhaust port and the modded one. Lots can be done to these saws resulting in 30%+ gains.

Question if you don't mind me asking....is it a width change in your recipe on the port/ports in general or is it combined with any hight changes. I know there is a lot to describe if you gave a full description including transfer work (if done as well) so just a outline would be cool .

[spudulike]

Question if you don't mind me asking....is it a width change in your recipe on the port/ports in general or is it combined with any hight changes. I know there is a lot to describe if you gave a full description including transfer work (if done as well) so just a outline would be cool .

 

The amount a port can be modified is dictated by the width of the piston skirt, the ring ends and the maximum known safety margin that you work within.

 

The height of the port is only changed if you want to change the revs which the power kicks in, the shape of the port is often changed if there is little width change that can be done.

 

The transfer timing is changed if the manufacturer may have left a bit on the table or the exhaust roof has changed shape to the degree less blow-down time is required.

[Sawchip]

The amount a port can be modified is dictated by the width of the piston skirt, the ring ends and the maximum known safety margin that you work within.

 

The height of the port is only changed if you want to change the revs which the power kicks in, the shape of the port is often changed if there is little width change that can be done.

 

The transfer timing is changed if the manufacturer may have left a bit on the table or the exhaust roof has changed shape to the degree less blow-down time is required.

Yea Im with you about the general rules....I was more interested if you have found the 357 responds to anything in particular or a general re working of the ports. Of corse the 357 "recipe" is one you have refined over the years so there is experience gained by the numbers you'v ported.

[spudulike]

Yea Im with you about the general rules....I was more interested if you have found the 357 responds to anything in particular or a general re working of the ports. Of corse the 357 "recipe" is one you have refined over the years so there is experience gained by the numbers you'v ported.

 

Tuning is generally picking up gains in compression, widening/reshaping ports and then timing changes if required and always in this order. Fortunately, the 357XP has the first two in abundance and the port figures are pretty good as they are.

[Sawchip]

Tuning is generally picking up gains in compression, widening/reshaping ports and then timing changes if required and always in this order. Fortunately, the 357XP has the first two in abundance and the port figures are pretty good as they are.

Do you find they will run without a base gasket ( about 20 thou is what i look for ) without any turning work ...wondered if they will would you rase the port hight to readjust or take the extra blow down. There a speed freek so i was thinking you would be looking for revs .

[spudulike]

Do you find they will run without a base gasket ( about 20 thou is what i look for ) without any turning work ...wondered if they will would you rase the port hight to readjust or take the extra blow down. There a speed freek so i was thinking you would be looking for revs .

 

The base gasket can be dropped while still maintaining enough squish clearance and the port timing on these machines is pretty wild compared to many other machines. Blow down is around 13 - 14 degrees so is already extremely low and had to recheck the durations twice as they are just different from the typical MS660/460 type of machine.

 

On this machine, it had stuffers on the crank lobes and a lot of transfer velocity hence the wild durations so avoid opening up the lowers or doing anything to alter this velocity. I have found standard timing works and the increase in port width/shape, increased compression and an aggressive muffler mod hits the nail on the head for me.

 

Experiment yourself and see what you get.

[Sawchip]

The base gasket can be dropped while still maintaining enough squish clearance and the port timing on these machines is pretty wild compared to many other machines. Blow down is around 13 - 14 degrees so is already extremely low and had to recheck the durations twice as they are just different from the typical MS660/460 type of machine.

 

On this machine, it had stuffers on the crank lobes and a lot of transfer velocity hence the wild durations so avoid opening up the lowers or doing anything to alter this velocity. I have found standard timing works and the increase in port width/shape, increased compression and an aggressive muffler mod hits the nail on the head for me.

 

Experiment yourself and see what you get.

Thanks Spud, thats real helpful as so far a 357 hasn't been in my possession to do more than service work on so always handy to have a good overview. Would very much like one as a personal saw but the dosh is a bit much for a likely candidate to work over at present. Appreciate the info.

[Tom D]

Final put the 7901 together, awesome, great pickup and strong in the cut. Cheers Spud!

[spudulike]

Final put the 7901 together, awesome, great pickup and strong in the cut. Cheers Spud!

 

Thought it should be good, seemed a lot better than I imagined it would be, especially after seeing the damage the circlip had done prior to me having it.

 

Glad you are happy

[Tom D]

I had the chance to run the ported Makita 7901 against the Dolmar 7910, there was a marked difference, we think there was a difference before. I wondered how much of this is down to the porting and how much is down to the different coil and stuff on the newer Dolmar.

What were you able to do to the Makita? I'd like the same done to the dolly if possible. I am wondering if the Makita is now running the same coil with the limiter anyway as we couldn't

Source the original type. If it is running the limited coil then the porting has made a big difference.

I'll ask Shavey about the other coil...