Questions about Squish

[MrGiggles]

I'd recommend doing a search for threads on degreeing a cylinder. It's important to be thinking in terms of port duration in terms of degrees of crankshaft rotation rather than changes in port height. Depending on your engines stroke and crankshaft/conrod geometry 15-20 thousandths will equate to roughly 4-6 degrees of intake duration. Whether or not that is a big difference depends on what your existing durations are. Lets look just at tranfer durations. You'll find that for a saw between 110 and 120 degrees is in general your ideal range. Some saws will tollerate a bit less or a bit more, but you should generally be shooting for that range for a non piped saw.

If your cylinder as is has say 116 or 118 duration, then loosing 6 deg will put you at 110-112 which wouldn't have a hugely adverse affect. But if you're only at 110 deg and drop the duration to 104 you'll probably see a much larger negative affect of that change in port duration.

As far as how much pressure you gain for a given cut of the cylinder, it depends on where you start. If you're cylinder is producing 130 psi, cutting 10 though off the cylinder will have a smaller affect than if you're starting at 160psi.

In short to see the greatest gains in performance you should simultaneously increase your compression ratio and idealize your port timing. Solely cutting the cylinder to bump up compression will adversly affect your port timings. How adversly they are affected depends on what they were before doing the work.

maybe i'm confused, but if you lower the cylinder base by removing the gasket it isn't going to expose the top of the piston but just the opposite, the piston top will be further above the top of the intake port, inside the cylinder. Theoretically, couldn't you measure the gasket's thickness, throw it away, and then raise the top of the intake port by an amount equal to the gasket thickness without emotional stress. if you slipped and cut too much, then you could have a problem.

I thought about that too.

I think what I am going to do is put it back together without the base gasket and caber ring and run it until it quits. It ran good enough before so it can only be better now.

Do I need to prepare the cylinder at all for the new ring?

 

[heyduke]

I thought about that too.

I think what I am going to do is put it back together without the base gasket and caber ring and run it until it quits. It ran good enough before so it can only be better now.

Do I need to prepare the cylinder at all for the new ring?

Yeah, I'd lose the gasket, seal things up with an appropriate compound. The top end will probably outlast you. Let us know what your compression tester reads.

I don't hone the cylinder but some do. With an Asian after market cylinder I'd worry about how thick chrome or nikosil was.

good luck

 

[mdavlee]

The port timing difference will be hard to notice. The compression increase should give a bump in torque. Don't worry about raising the intake back up. The intake opens on the bottom and more than likely the bottom if the skirt won't quite reach the top of the intake.

 

[heyduke]

Don't worry about raising the intake back up. The intake opens on the bottom and more than likely the bottom if the skirt won't quite reach the top of the intake.

Easy to observe...

 

[heyduke]

The intake opens on the bottom and more than likely the bottom if the skirt won't quite reach the top of the intake.

And you can get a good idea about potential opportunities/hazards by tracing the intake and exhaust ports on the piston skirts with a sharpie before you make any changes.

 

[MrGiggles]

And you can get a good idea about potential opportunities/hazards by tracing the intake and exhaust ports on the piston skirts with a sharpie before you make any changes.

The other alternative is cutting the cylinder .015 and tossing the base gasket.. My gut says that it's not going to be enough to really affect anything but give me a good bump in compression. .035 total squish gives me a good margin of error to avoid detonation.

 

[MrGiggles]

I threw it together today without the gasket and new ring. Compression after running felt about the same when it was hot after running as it was before when it was cold. So I would guess it improved some. Seems to run about the same but it's 90* with 80% humidity and none of them run great.

 

[tolman_paul]

The new ring will probably need a few tanks of fuel before it properly seats and compression should come up a bit.

 

[MrGiggles]

I threw it together today without the gasket and new ring. Compression after running felt about the same when it was hot after running as it was before when it was cold. So I would guess it improved some. Seems to run about the same but it's 90* with 80% humidity and none of them run great.