Crankcase port and polish?

[kevlar]

My question: is there gais to be made by removing material from the crankcase(in this case 660)and repolishing?I would think a little bigger intake charge can't hurt-but that's why I am asking.

 

[joesawer]

the intake charge volume is mostly affected by the displaced volume of the piston not the crank case volume. IMO increasing the crank case volume will usually hurt the performance as it will reduce the compression of the charge before the transfers open. decreasing the crank case volume has long been accepted as an easy way to gain power.

 

[kevlar]

Very interesting! Thanks.

 

[drmiller100]

what joe said.
you are pumping out of hte crankcase up into the combustion chamber.
the smaller the crankcase, and the larger the transfers, the more power per revolution.

 

[Goicoechea]

A newbie question to follow:

Does the crank case volume limit the amount of charge you can transfer to the cylinder?

Thanks for letting me eves drop.

 

[Crofter]

The maximum power produced by a given displacement cylinder depends on the the amount of charge burned and the mean effective pressure, period. not the size of its crankcase! High crankcase compression ratio has some advantage for scavenging efficiency esp. at low rpm., but the smaller (relatively) crancase is not as efficient in the volume of charge trapped, especially on a piped saw. Take a 440 and a 372 and measure them up and compare on the size of the cranckase volumes and transfer duct cross section. See how long you hang onto the idea that smaller crankase volume is "better" Start modifying them and see which one starts to leave its torque behind! Same thing pretty much goes for the 7900 and 460.

 

[timberwolf]

One thing you can be sure of is that on a stock saw with the exception of the case where different bore jugs are put on the same base the crank case volume is engineered to be bang on from the factory. In cases like the 372/365 the case volume seems beter suited to the 372 and fitting the 365 jug on it is more an after thought. having a 48 mm piston compressing the base vs a 50 mm piston will hurt the output potential some. Stihl 290/390 same story.

Once you start to modify the saw all bets are off, the ideal case volume is going to depend on a lot of things, intake duration, transfer port design, intake design, the size of the carb and RPM. Most cases on muffler smaller is better, but on pipe larger has potential too depending on how the rest of the saw is built. Key is that the porting work and base volume work together. Big base volume, long intake duration and small transfers don't go together ect....

 

[Mad Professor]

I've not done saws but many MX bikes. Polish the case to get better/smooth flow and match cases to gasket/transfer ports.

Hogging out the case will only weaken it.

 

[timberwolf]

Good idea on matching up the case to the jug. The saw cases are in general pretty good as far as finish (baked on paint of some sort). Because saws are short stroke compaired to bikes the cases are much tighter in general, esp the older stihls with 2 transfer ports. Also bikes are designed to run with a pipe to aid with transfer flow where saws are designed to run on muffler so depend on base compression a lot more for the transfer port flow.

 

[kevlar]

Good idea on matching up the case to the jug. The saw cases are in general pretty good as far as finish (baked on paint of some sort). Because saws are short stroke compaired to bikes the cases are much tighter in general, esp the older stihls with 2 transfer ports. Also bikes are designed to run with a pipe to aid with transfer flow where saws are designed to run on muffler so depend on base compression a lot more for the transfer port flow.

This is sort of where my question came from,I was looking at the 066 case and thinking that could be matched better,then when it was apart I was going to remove the rest of the finish and smooth it out.Not wanting to remove a lot of material but some.Would there be any gains made from polishing this bottom half?

 

[slabmaster]

Looks like i will be finding out what happens to a saw when base volume changes.I'm in the process of building a 064 hybred saw with a 288 piston & 066 jug.The piston has wider skirts so i will be able to widen the ports.I'll have a smaller case volume with a lowered intake with more duration as the jug will have to be lowered.

 

[huskydave]

Sounds interesting keep us posted.

 

[tree monkey]

i beat this around in my head for a year, finley did it. bored out a 440 case, port matched it and put it together. no port changes. power didn't change mutch, a little more. but it runs longer on a tank of gas. going to port it someday, don't have time now.

 

[slabmaster]

i beat this around in my head for a year, finley did it. bored out a 440 case, port matched it and put it together. no port changes. power didn't change mutch, a little more. but it runs longer on a tank of gas. going to port it someday, don't have time now.

So your saying the added volume gave you more power? Did you measure how much volume you added?

 

[tree monkey]

i had a guy bore it out on a mill till we found a hole.patched the hole and put it together. no idea how mutch we took out.

a small gain in power. runs about 10 min longer on a tank of gas.

later scott