I'll be watching this, as I have another 372 stock, also 2 more oem p&c's to work with, and windowed & unwindowed pistons.
372 Pston swap/mod
- Thread starter trappermike
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I forgot about lighter, that has to be good.
I’m still skeptical about the windows giving better lower transfer flow being a big deal, imho there’s not much going on there at bdc anyway and the stock piston really isn’t in the way for very long as a percentage on the cycle.
I don’t see a good way to test for it either, too many other changes going on at the same time.
I’m still skeptical about the windows giving better lower transfer flow being a big deal, imho there’s not much going on there at bdc anyway and the stock piston really isn’t in the way for very long as a percentage on the cycle.
I don’t see a good way to test for it either, too many other changes going on at the same time.
Huskybill
Addicted to ArboristSite
I think the windowless piston sends more flow up to the transfer ports over the windowed piston. It’s like getting more compression in the crankcase when the piston goes down.
I think, I haven’t looked at it yet but check the intake port side of the piston to see if the piston in tdc leaves any skirt in the intake port? If so I would scribe a line matching the top of the port to the piston. Then grind out the upside halfmoon shape. I’m not sure if the piston skirt clears the port in tdc.
I mention this because the Kawasaki pistons in the bikes have this cut out. It actually speeds up the intake flow earlier/sooner. The raised exhaust port is only half the job. The intake port needs work too so it’s balanced. As the bad air gets exhausted the intake pours in. The port timings are sped up, smoothed out.
Remember to make little suttle changes at first. I did this on the bikes one change at a time. Then seen the results, on the last ported bike I did every change I done to each bike. It was a screamer.
I think, I haven’t looked at it yet but check the intake port side of the piston to see if the piston in tdc leaves any skirt in the intake port? If so I would scribe a line matching the top of the port to the piston. Then grind out the upside halfmoon shape. I’m not sure if the piston skirt clears the port in tdc.
I mention this because the Kawasaki pistons in the bikes have this cut out. It actually speeds up the intake flow earlier/sooner. The raised exhaust port is only half the job. The intake port needs work too so it’s balanced. As the bad air gets exhausted the intake pours in. The port timings are sped up, smoothed out.
Remember to make little suttle changes at first. I did this on the bikes one change at a time. Then seen the results, on the last ported bike I did every change I done to each bike. It was a screamer.
Yotaismygame
Juiced Saws #GetJuiced
Huskybill, the intake skirt does clear at TDC. Here is stock duration numbers with a 040” pop up, which leaves .100” above the ring
Intake 167.......97
Exhaust 154......103
Transfers 107....127
Blowdown of 24.
I don’t know very much about port timing in relation to what makes power but the numbers I’ve seen other guys get with 372 pistons they usually have close to the same intake and exhaust durations, and about 22-24 blown down. I’m curious if adding some JB Weld to the floor of the intake to shorten duration to let’s say around 160 would be beneficial.
I ran the big pop up in the stock jug and it performed rather well. Compression didn’t feel all that high. I’m wondering with a single ring if comp stays low until it’s fully seated. Sadly I don’t have enough wood around to be running the saw enough to figure this out. Can’t be cutting up all my wood with one saw. Need it for further testing.
Tonight I’m going to do a very conservative port job and see what happens. Hopefully it will turn out like my first build that I sold to a friend.
Personally I don’t think the 268 mod is all that great for someone trying to port several 372s but for someone with limited tools and knowledge I think it’s great. Easy bolt on power
Next week I’ll drop off my other pistons at the machine shop and continue testing.
Intake 167.......97
Exhaust 154......103
Transfers 107....127
Blowdown of 24.
I don’t know very much about port timing in relation to what makes power but the numbers I’ve seen other guys get with 372 pistons they usually have close to the same intake and exhaust durations, and about 22-24 blown down. I’m curious if adding some JB Weld to the floor of the intake to shorten duration to let’s say around 160 would be beneficial.
I ran the big pop up in the stock jug and it performed rather well. Compression didn’t feel all that high. I’m wondering with a single ring if comp stays low until it’s fully seated. Sadly I don’t have enough wood around to be running the saw enough to figure this out. Can’t be cutting up all my wood with one saw. Need it for further testing.
Tonight I’m going to do a very conservative port job and see what happens. Hopefully it will turn out like my first build that I sold to a friend.
Personally I don’t think the 268 mod is all that great for someone trying to port several 372s but for someone with limited tools and knowledge I think it’s great. Easy bolt on power
Next week I’ll drop off my other pistons at the machine shop and continue testing.
Yotaismygame
Juiced Saws #GetJuiced
I’d like to do one thing at a time and test with porting this jug tonight, but I’ve had the jug off this saw about ten times in the last 7 days and quite frankly I’m tired of taking it apart.
Yotaismygame
Juiced Saws #GetJuiced
More food for thought.
I’m wondering if cutting a .030” pop up instead of a .040” would be better for these reasons: could still set the squish anywhere you need by going with a thicker than stock gasket that would get you to .020”. That would raise the jug up which would shorten intake duration and lengthen exhaust duration. As long as that gets you about 180 comp without raising or lowering ports I’m thinking that would be a good thing. Might be something worth testing.
I’m wondering if cutting a .030” pop up instead of a .040” would be better for these reasons: could still set the squish anywhere you need by going with a thicker than stock gasket that would get you to .020”. That would raise the jug up which would shorten intake duration and lengthen exhaust duration. As long as that gets you about 180 comp without raising or lowering ports I’m thinking that would be a good thing. Might be something worth testing.
Are you doing this all on one saw?
Aiming the transfers for better scavenging might be helpful, but it's harder than widening an intake or exhaust port and it adds another variable. It's the last thing I do after my other numbers seem really good.
Yotaismygame
Juiced Saws #GetJuiced
Yessir. One of the jugs I screwed up I did extend aux transfer towards the intake. No clue if it was good or not.
Today I ported another jug and I widened the exhaust but kept it conservative, lowered the transfer wall about 3mm from flush with the cylinder base, smoothed out inside of the lower transfers some, set squish to .020”, ran a oem 268 piston with 040” pop up and caber ring. Compression after cutting was 195psi! I didn’t touch the intake, and I’m using a XT carb. Pretty strong saw. I’ll cut some more tomorrow with it.
If compression continues to rise I’m going to raise the squish to .030” so it will raise the exhaust and shorten intake duration. Trapper mike told me 170 intake duration was too much. Nothing I can do about that, but he said to raise the exhaust up to the mid 160s and it should make a lot of power up top.
Super interesting. Windowed 268 piston?
You're thinking the intake timing is too long and reducing case compression? Also creating spit-back?
These are the numbers before or after you ported this last jug? The intake confused me at first.
Here are mine, maybe they're helpful.
Widened exhaust to 1.330", blended transfers, reduced transfer walls, no popup, squish set at .024.
2188 carb with gutted XT intake horn.
Exhaust: 96
Trans: 122
Intake: 80
Unfortunately this saw is at my Dad's place in WV so no further info is available. It was strong when I left it, but I felt like it had more potential. I'm building up another right now, but it's slow going. All your information is really helpful!
You're thinking the intake timing is too long and reducing case compression? Also creating spit-back?
These are the numbers before or after you ported this last jug? The intake confused me at first.
Here are mine, maybe they're helpful.
Widened exhaust to 1.330", blended transfers, reduced transfer walls, no popup, squish set at .024.
2188 carb with gutted XT intake horn.
Exhaust: 96
Trans: 122
Intake: 80
Unfortunately this saw is at my Dad's place in WV so no further info is available. It was strong when I left it, but I felt like it had more potential. I'm building up another right now, but it's slow going. All your information is really helpful!
Yotaismygame
Juiced Saws #GetJuiced
Windowed 268 piston.
Glad to help.
This morning I raised my squish up, barely raised the exhaust. After a quick run in the garage compression is around 185. Most likely the finished product.
I’ve been thinking if the pop was .030 that would leave the piston heaight above the ring not including the pop up at .110 instead of .100. Then you could raised the jug up to maintain proper squish. This would shorten intake duration a bit. Same could be done with a .020 pop up. Basically I’d like the the intake and exhaust durations a little closer to each other. Simply raising the exhaust up is okay to a point but I wouldn’t want less than 175psi comp. with a 030 pop up you might be able to raise the exhaust some and still have decent compression. Another option which I find very interesting is using a hyway 268 pop up piston. It already has .020 pop up added to a stock piston. You could turn down .020 making a .040 pop up, leaving .120 above the ring. You would need to raise the jug up around .020. I think this would make a better intake duration, allow you to raise the exhaust up adequately, and maintain 180 comp or higher.
Glad to help.
This morning I raised my squish up, barely raised the exhaust. After a quick run in the garage compression is around 185. Most likely the finished product.
I’ve been thinking if the pop was .030 that would leave the piston heaight above the ring not including the pop up at .110 instead of .100. Then you could raised the jug up to maintain proper squish. This would shorten intake duration a bit. Same could be done with a .020 pop up. Basically I’d like the the intake and exhaust durations a little closer to each other. Simply raising the exhaust up is okay to a point but I wouldn’t want less than 175psi comp. with a 030 pop up you might be able to raise the exhaust some and still have decent compression. Another option which I find very interesting is using a hyway 268 pop up piston. It already has .020 pop up added to a stock piston. You could turn down .020 making a .040 pop up, leaving .120 above the ring. You would need to raise the jug up around .020. I think this would make a better intake duration, allow you to raise the exhaust up adequately, and maintain 180 comp or higher.
Yotaismygame
Juiced Saws #GetJuiced
With that hyway setup maybe something like this could be feasible
Durations:
Ex 160
IN 163
Compression 180.
Blow down would be increased also since the exhaust roof would be higher.
Sounds better than stock numbers at 167& 154
Durations:
Ex 160
IN 163
Compression 180.
Blow down would be increased also since the exhaust roof would be higher.
Sounds better than stock numbers at 167& 154
Hmm. The skirt seems like a limiting factor on the 268 piston. Without knowing stock piston numbers fo sure, it seems like an .020 popup
could be cut and still have an intake duration around 180 after dropping the jug. At that point it's probably more practical to just bore the jug for a tight squish. I guess that's the standard strategy.
Would stacking gaskets with a 268 popup increase case volume? Does that help balance a longer intake?
I'm also wondering about the relationship of compression and exhaust width. Could the exhaust go higher if it was stock width? Is it a direct relationship?
Lots to think about.
could be cut and still have an intake duration around 180 after dropping the jug. At that point it's probably more practical to just bore the jug for a tight squish. I guess that's the standard strategy.
Would stacking gaskets with a 268 popup increase case volume? Does that help balance a longer intake?
I'm also wondering about the relationship of compression and exhaust width. Could the exhaust go higher if it was stock width? Is it a direct relationship?
Lots to think about.
Yotaismygame
Juiced Saws #GetJuiced
I need to take a break from this 268 experiment and get back to building other saws. I’ve got a jred 2065 I’m going to toss a 50mm jug on and do more 268 experimenting. Will be my test mule/ firewood saw
Yotaismygame
Juiced Saws #GetJuiced
Just finished up running the current version of this saw and it’s real strong. Instant throttle response. Decent rpms in the cut. Not weak down low like other jugs I ported. I haven’t checked squish but it should be around .028. Compression I’ll check when I get home but I’m guessing about 185psi. Definitely seems like the closer I get the exhaust and intake durations to each other the better. I could probably raise the exhaust just a little more and make a gain but I’m happy enough for now.
Checked compression and it’s 190 psi
Checked compression and it’s 190 psi
Huskybill
Addicted to ArboristSite
I like to arc the top of the exhaust port. I would do a 1/16” in the upper center of the exhaust port and arc it to about 1/32 on each side, on a smaller cc saw. Just go slow and work your way porting. Square up and ramp the transfer ports. Don’t forget to match the base gasket to the cylinder and block. You could push up the 1/16” to 3/32” in the exhaust port. Advance the timing a little, all the changes will work together to improve performance. Write down which saw and the exact changes you did. Keep a record.
Just want to say what a rad thread this is. Thanks @trappermike for sharing your experience and approach to porting. @Yotaismygame, you really illustrated this style of porting well: pop-up, medium compression and subtle port work. It's different than what a lot of the guys on here are doing and it's helped me understand at a whole new level.
I wish there was an opportunity to run Yota's saw against the standard deck and bore port jobs!
I wish there was an opportunity to run Yota's saw against the standard deck and bore port jobs!
Yotaismygame
Juiced Saws #GetJuiced
I'm having fun too. Porting knowledge is a funny thing. Some people guard it with their life and others can't wait to tell someone. There is lots of information out there it just takes hours of searching to find it.
I have a piston getting machined right now down to .030 instead of 040. I'm thinking if I maintain .025 squish with a .030 pop up it will move the cylinder up .010. Leaving the exhaust duration the same but shortening intake. From there I hope all I need to do is work the lower transfers over some and widen the exhaust. In theory it should make the strongest saw I've built yet with a 268 piston. With a smaller pop up comp should end up in a good spot with out raising the exhaust and the intake will benefit from the intake moving up.
A way to experiment with this would be to add JB weld to the floor of the intake to shorten duration. If the saw gets stronger great, if it doesn't just grind it off and move on.
BTW: Somone buy my rebuilt 372!
I have a piston getting machined right now down to .030 instead of 040. I'm thinking if I maintain .025 squish with a .030 pop up it will move the cylinder up .010. Leaving the exhaust duration the same but shortening intake. From there I hope all I need to do is work the lower transfers over some and widen the exhaust. In theory it should make the strongest saw I've built yet with a 268 piston. With a smaller pop up comp should end up in a good spot with out raising the exhaust and the intake will benefit from the intake moving up.
A way to experiment with this would be to add JB weld to the floor of the intake to shorten duration. If the saw gets stronger great, if it doesn't just grind it off and move on.
BTW: Somone buy my rebuilt 372!
You said that well..
