Real talk about saw porting theory (no arguments)

[GoBigBlue1984]

Al posted the link but here it was originally when I cleaned it up minimally. As far as I know this piston is still in the logging woods in a 395. I opened up the windows quite about and ground out what I could from where I could. I weighed the popup piston against s stock piston and the modded piston was heavier but the difference was negligible.I ended up cutting the squish band and leaving the popup .050 tall. That 395 was pulling 215 when I reassembled the saw. In retrospect I won't build another with so much compression but it's easy to go overboard on a 395 in terms of compression.

 

[jmssaws]

View attachment 62640
Al posted the link but here it was originally when I cleaned it up minimally. As far as I know this piston is still in the logging woods in a 395. I opened up the windows quite about and ground out what I could from where I could. I weighed the popup piston against s stock piston and the modded piston was heavier but the difference was negligible.I ended up cutting the squish band and leaving the popup .050 tall. That 395 was pulling 215 when I reassembled the saw. In retrospect I won't build another with so much compression but it's easy to go overboard on a 395 in terms of compression.

The last 395 I did a few weeks ago I used a 040 popup and a 010 band cut,I didn't check compression buy I'm guessing 220lb.
For some reason a 394/5 likes compression where a lot of other saws don't.
I usually build them in the 210-225 lb range,I've went to 235lb and still it just got stronger but a stihl you get past 200 it hurts it.
Those are chamber cut saws though,a 040 band cut will make 215+lb and a 040 popup I just did is 205lb.

 

[jmssaws]

A 394/5 that was going to the log woods I would never cut that much out,I try and stay under 200lb.

 

[XP_Slinger]

View attachment 62640
Al posted the link but here it was originally when I cleaned it up minimally. As far as I know this piston is still in the logging woods in a 395. I opened up the windows quite about and ground out what I could from where I could. I weighed the popup piston against s stock piston and the modded piston was heavier but the difference was negligible.I ended up cutting the squish band and leaving the popup .050 tall. That 395 was pulling 215 when I reassembled the saw. In retrospect I won't build another with so much compression but it's easy to go overboard on a 395 in terms of compression.

Awesome work on that piston. Especially from Al building it up so uniformly with no pits. I wish you had that weights of the pistons still, I'm curious what the "negligible" difference was. No attitude implied, I just like numbers. I would imagine this method of gaining compression is less popular because of the extra effort it takes when compared to quicker method of cutting the base and squish band.

 

[GoBigBlue1984]

its actually faster to cut a popup than it is to cut a band. It doesn't take long to do either but even if I don't have a split bushing already cut to size I'll wrap a piston in Emory cloth and cut a popup in 5 minutes in a 3 jaw. A simple .035-.045 popup and base cut to set squish at .020-.025 runs pretty darn well.

 

[GoBigBlue1984]

Awesome work on that piston. Especially from Al building it up so uniformly with no pits. I wish you had that weights of the pistons still, I'm curious what the "negligible" difference was. No attitude implied, I just like numbers. I would imagine this method of gaining compression is less popular because of the extra effort it takes when compared to quicker method of cutting the base and squish band.

Yes, Al did a fantastic job building that piston up.

 

[jmssaws]

Every tiny bit of piston weight lost is free power.
A 035 popup on a 372 piston is several grams of weight lost and every little bit adds up

 

[Dub11]

Every tiny bit of piston weight lost is free power.
A 035 popup on a 372 piston is several grams of weight lost and every little bit adds up

So I take it on a 372 pop up your removing material never adding?

 

[jmssaws]

So I take it on a 372 pop up your removing material never adding?

I'm not adding no,I want to remove material so I can lower the cylinder.

 

[jake wells]

Yes, Al did a fantastic job building that piston up.

answer you're phone dammit.

 

[Al Smith]

You could write a book on various shapes for pistons .A raised dome piston is not a new innovation .Some might be under the opinion it's done to raise the compression but if you think about it you are removing metal in effect and as compared to a flat top it really doesn't .It isn't like installing custom pistons in a Chevy 350 that you can just buy them,you have to make them for a chainsaw .Of course if you have a bucket full of money Wisco will make them for you for a price .Last I heard the minimum order was 12 at say a couple hundred a piece .It's an option for the rich and famous .

All that BS aside I like them but it wasn't done to raise the compression it was done to change the flow over the top of the piston to improve scavanging .Which like anything could be a subject of debate .After spending nearly 40 years of a nearly 50 year career in the automotive engine industry I think I've seen just about every piston shape known by now .A raised dome is just one of many .Of course those are 4 cycle engines not one in the same but the flow patterns can be similar depending on combustion chamber design and valve placement.

Then again and once again when you cut to the chase and the smoke clears designs and theory make very little difference in the overall schemes of things.Whatever works ,works .

 

[XP_Slinger]

That's what has been rattling around in my head is how piston crown profile affects scavenging. And you're right, theory is just theory until it's applied and results are acquired. Makes for interesting discussion, thanks for your valuable input.

Jason, I had not thought about how cutting a pop up by removing material alters timing which contributes to the overall timing goal. And that without it, you may not be able to grind enough to get your desired numbers. Makes perfect sense and I really appreciate you sharing that info. It's all coming together for me now.

Learning a lot, thanks to all those with experience that are contributing to this thread.

 

[jmssaws]

That's what has been rattling around in my head is how piston crown profile affects scavenging. And you're right, theory is just theory until it's applied and results are acquired. Makes for interesting discussion, thanks for your valuable input.

Jason, I had not thought about how cutting a pop up by removing material alters timing which contributes to the overall timing goal. And that without it, you may not be able to grind enough to get your desired numbers. Makes perfect sense and I really appreciate you sharing that info. It's all coming together for me now.

Learning a lot, thanks to all those with experience that are contributing to this thread.

A popup leaves the exhaust and transfers the same with equal amounts from it and the base.
If you already have to raise the exhaust and transfers which on most saws you do then it's a pretty good choice.

They say it's inconvenient if the owner needs a new piston but if he needs a new piston I'll be the one doing it anyway most likely.

 

[junkman]

What about flame travel flat top vs pop up ?

 

[Al Smith]

Brian W AKA Timber Wolf did a research on a pop up some time ago concerning the flame propagation .The pop up by the way he did on a table top milling machine with a rotary table before he owned a lathe.I forgot the exact conclusions but it was favorable .Whatever happen to him I'll never know but that ole boy did a lot of research with documentation to verify the results .I think he just got annoyed with internet nonsense .It's been maybe 8 years since I talked with him on the phone .

 

[Dub11]

Brian W AKA Timber Wolf did a research on a pop up some time ago concerning the flame propagation .The pop up by the way he did on a table top milling machine with a rotary table before he owned a lathe.I forgot the exact conclusions but it was favorable .Whatever happen to him I'll never know but that ole boy did a lot of research with documentation to verify the results .I think he just got annoyed with internet nonsense .It's been maybe 8 years since I talked with him on the phone .

I have read at least a dozen threads over the last few months that have brought up Timber Wolf and his absence we should send out a search party.

 

[mdavlee]

I used to have Brian's email. I'll try to get a hold of him.

 

[Al Smith]

I don't know what happen to Frank (Crofter ) either ,Brians dad .He was kind of like myself an old junk yard dawg .

 

[Czed]

After porting How much are you guy's advancing the timing on the 372s if you do at all.

 

[jmssaws]

After porting How much are you guy's advancing the timing on the 372s if you do at all.

I don't at all.