372 Pston swap/mod

[Deleted member 150358]

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!

The long intake should allow more fuel and build more torque in theory, allowing the rpms to be raised by raising the exhaust. Your experiments should help you build a better work saw keeping the saw less peaky, better cooling and lubricated for longevity.

Thanks for taking us along for the ride. Not many build threads lately.

 

[Yotaismygame]

The long intake should allow more fuel and build more torque in theory, allowing the rpms to be raised by raising the exhaust. Your experiments should help you build a better work saw keeping the saw less peaky, better cooling and lubricated for longevity.

Thanks for taking us along for the ride. Not many build threads lately.

I think the issue is that the intake is too much longer than the exhaust duration. Mike said to raise the exhaust way up and it should make more power up top but would lose torque. And that I don't want that. If I can get the intake floor up, and not have to raise the exhaust at all I'm thinking a better work saw will come out of it. That's my thinking with this next piston with a .030 pop up.

I might make a build thread for a 288xp i'm about to start porting. I started a thread on here asking for porting information and nobody commented. I also made one over on O..P....E and that thread is starting to take off.

Currently I'm porting a friends shindaiwa 695. Should be getting a cylinder back from the machine shop today.

 

[Yotaismygame]

By the way if anyone wants me to port there saw I would gladly do it for a small fee.

 

[BGE541]

I think the issue is that the intake is too much longer than the exhaust duration. Mike said to raise the exhaust way up and it should make more power up top but would lose torque. And that I don't want that. If I can get the intake floor up, and not have to raise the exhaust at all I'm thinking a better work saw will come out of it. That's my thinking with this next piston with a .030 pop up.

I might make a build thread for a 288xp i'm about to start porting. I started a thread on here asking for porting information and nobody commented. I also made one over on O..P....E and that thread is starting to take off.

Currently I'm porting a friends shindaiwa 695. Should be getting a cylinder back from the machine shop today.

It’s not an issue of duration it’s all about timing... depending on case compacity and desired power you can drop a intake pretty dang low and make good power across the board.

With the losing torque that’s an issue of compression when talking about raising the exhaust roof but again if you are doing a pop up you should be able to maintain or increase compression while improving (raising) exhaust port timing.

 

[Yotaismygame]

The intake duration is an issue because I’m going for a work saw. To get the exhaust to match the intake it’s going to be one peaky saw. It’s not so mismatched though that it makes a bad saw. Last two I built were plenty strong. 167 intake duration is loooong.

 

[Leafy]

I think you'll be fine, especially if you stick with the tiny stock carb.

 

[Yotaismygame]

Doesn't sound like you guys read the thread at all....

 

[huskihl]

Idk. I dont see many 372 cranks going bad from running a stock "heavy" piston. Don't the 268 skirt widths limit widening the exhaust port too?
I could see doing this if a guy wasn't going to do any port work. The main benefit is the added 12ish° of intake duration. But it seems pretty easy if you're going to port it anyway, to just lower the intake floor to 78.
With what you guys are spending on pistons and machine work, I'd think you could send out your cylinder and have the squish and base cut. Your exhaust would be down around 106 opening and you could tailor it to whatever size bar you wanted to run. Lots of 372's have 225 lbs of compression and loads of torque. And still turn 15k. Or you could lessen the compression and add some up by raising it up a little.

 

[Yotaismygame]

Just another way of doing it. For $60 though you can make a pretty strong saw. If you want to get more involved you’ll make an even stronger saw.

I see this as a great drop in piston for instant gains and high compression or part of a simple recipe to make a pretty strong saw.

 

[Yotaismygame]

I don’t know what kind of extra work Trappermike was doing but the way he talks about it, it sounds like he knew a thing or two about making power with the 268 piston.

 

[huskihl]

I don’t know what kind of extra work Trappermike was doing but the way he talks about it, it sounds like he knew a thing or two about making power with the 268 piston.

Yeah I'm not sure. I never saw any pictures of his work so it's hard to say if he was actually doing it or repeating what he'd heard. He did leave a nice drawing of how we made it's pop-up piston though.

I've also seen quite noticeable gains by only having the base and squish cut. There's a decent bump in compression, a lower exhaust for torque, combined with the added intake duration wakes up most saws.
As far as what he saw at bottom dead center, I have no idea how he came to that conclusion. The bottom of the piston sides do come down below the transfer cut out, but only .080 or so. Cutting the base and squish and removing the gasket gets that down to . 020, and that much is easily removed from the piston to line things up once your gasket matching is done. And besides, there's about 1/2 to 5/8 of an inch below that cutout and above the bearing pockets where the air has lots of room to flow.
I always figured that given the way a 372 runs, maybe it's got something to do with the design of the factory piston. I've actually tried that piston in other bottom fed motors with respectable results

 

[Ketchup]

Do you guys think raising the exhaust helps more with the 268 popup, or widening?

 

[huskihl]

Do you guys think raising the exhaust helps more with the 268 popup, or widening?

My ported exhaust heights are always lower than where they were stock. I try to to keep the roof down for torque and make them as wide as I can, up to 70% if skirt width allows, for RPM's.

 

[Leafy]

People even see gains with this piston by just stacking up base gaskets until the saw assembles. It's only like 6 bucks for the piston.

 

[Yotaismygame]

Do you guys think raising the exhaust helps more with the 268 popup, or widening?

I saw the most power when raising the exhaust until compression reached about 185. Intake duration is long with this piston. Need the exhaust to match. I also Widened the exhaust as much as possible. Plenty of skirt width on the exhaust side. The intake however can only be widened at most 2mm. Last one I built I didn't even touch the intake and didn't notice any loss in power. Last time I was experimenting with the 268 piston I didnt have a degree wheel so I can't comment on numbers. But I have one now and use it regularly. I'll be building another one this month and will for sure get some numbers for you guys.

 

[Yotaismygame]

Yeah I'm not sure. I never saw any pictures of his work so it's hard to say if he was actually doing it or repeating what he'd heard. He did leave a nice drawing of how we made it's pop-up piston though.

In another thread he mentioned building 100's of 372's with this piston and raced them too.

 

[Yotaismygame]

The cylinder on this saw I had originally hogged the intake out a bit too much so I had to expoxy it, hoping that would change how the saw ran for the better. I also put the hyway mega pop up in it which I think is a 050 pop up with still .1” above the ring. The exhaust on this cylinder is high. I would guess 97 or so. Never did get around to degreeing it. I also opened up the lowers pretty good, and stretched the uppers towards the intake. Compression feels on the weak side, like 150. I don’t know if it’s because of the ring or the exhaust being so high. I think if compression was closer to 180 with this high exhaust it would be bad ass. Overall it seemed to cut pretty good. The wood I have is pretty much all cut up so it’s not the greatest test wood.

It’s a jred 2065 I got super cheap. Put a screw up 372 cylinder on it with some tweaks to it. And that 268 hyway mega pop up piston.

 

[old guy]

The high ex. port does rob comp. I went up .080 on mine, .037 pop up, squish .023, compression is in the 150s.
This set up will not 4 stroke above 13000, plug color is rich brown, it has unlimited coil, what kinda revs is that jred turning?

 

[Yotaismygame]

I’m not sure about the rpms. This was my first time running it. With the uppers stretched the saw sounds different at idle. I noted this last time I had this cylinder on a 372.

 

[old guy]

Got me a ° wheel and put it on this 372 today, raising the ex. port .080 has put the ex. opening at 90° atd., as near as I can tell the transfers open at 120°, this leaves blowdown at 30°, that seems like a lot. Any comments.
I did another comp. check, & that's up to 160 lbs. after 3 tanks of fuel.