Mastermind Meets The MS660

[stihl038x2]

I'm only on my second one but been cutting the chamber on a 2* taper. Been thinking of trying a taper on the crown with a 2* difference between the squish band, like 4* on the squish band and 2* on the piston.

The last one I did was a MS660, flat for ~.075" then 2* taper to combustion chamber. I like the idea of tapering the piston top too, I'll probably try one at some point as well. Maybe even taper (to match squish band), up to a pop-up.

Steve

 

[Jacob J.]

IIRC, I think in Jennings book, he states that the dome piston/combustion chamber combo makes more HP, due to the increased square area of the piston crown. I'll have to get a lot better on the lathe before I try that out though.

That was one of Jennings' original theories and right around the time he published it, Stihl, Husqvarna, and other OPE manufacturers starting putting domed pistons in their equipment that turned out to be great runners (i.e. 056 Super/Magnum.)

I would "think" anything you can do to help get flow to the combustion chamber would be a good thing. But I've seen some CRAZY wide and flat squish bands, with tiny combustion chambers that were serious runners.

That's the configuration of the Sachs, JLO, and Italian kart engines from the 70's and 80's- engines that won championships for years.

The last one I did was a MS660, flat for ~.075" then 2* taper to combustion chamber. I like the idea of tapering the piston top too, I'll probably try one at some point as well. Maybe even taper (to match squish band), up to a pop-up.

Steve

Kawasaki had such a set-up on some of their mid-range bikes in the 70's and they were decent runners.

 

[wigglesworth]

I'm thinking under compression though. Plus a tapered crown has more surface area, more area for the combustion to push on, more HP.

True, the more surface area the better. But from what I've seen, I can't tell much difference between flat or tapered. But ya never know till u try. Give her a whirl.

To my simple mind, as long as your not getting crazy carbon spots on the crown, and are showing good flow patterns from the transfers to the chamber, then your good to go. I've pretty much given up on using popups, cause every one I've done has had bad carbon problems, and terrible flow patterns.

 

[parrisw]

The last one I did was a MS660, flat for ~.075" then 2* taper to combustion chamber. I like the idea of tapering the piston top too, I'll probably try one at some point as well. Maybe even taper (to match squish band), up to a pop-up.

Steve

Some reading I've done, and it makes sense to me is that you want the squish band tapered 2-4* more then the piston.

 

[Tzed250]

I'm thinking under compression though. Plus a tapered crown has more surface area, more area for the combustion to push on, more HP.

While the piston may have more area, it won't result in any more force being applied to the connecting rod. Any force by compressed gas on the crown is perpendicular to the surface, and if the surface is not perpendicular to the con rod then the resultant force will be less. Cosine of 2 deg. is .99939 so the difference is small, but there. The biggest problem with domes is the the increased area for heat transfer into the piston crown. It is all a big balancing act.

 

[Mastermind]

I'm just a hack that likes pie.

I do taper the squish band a fuzz though

 

[Tzed250]

I'm just a hack that likes pie.

I do taper the squish band a fuzz though

I love me some pie!!

(and a tapered squish band)

 

[Simonizer]

I'm thinking under compression though. Plus a tapered crown has more surface area, more area for the combustion to push on, more HP.

LMAO! You are joking right?

 

[parrisw]

While the piston may have more area, it won't result in any more force being applied to the connecting rod. Any force by compressed gas on the crown is perpendicular to the surface, and if the surface is not perpendicular to the con rod then the resultant force will be less. Cosine of 2 deg. is .99939 so the difference is small, but there. The biggest problem with domes is the the increased area for heat transfer into the piston crown. It is all a big balancing act.

Fair enough, guess my thought process was off.

 

[parrisw]

LMAO! You are joking right?

LMAO, bring one of your painted saws down and see how its stacks up against mine. hahahahahahahahahahahahahahahahaha

 

[Simonizer]

LMAO, bring one of your painted saws down and see how its stacks up against mine. hahahahahahahahahahahahahahahahaha

Well if you can bend the laws of physics, I believe you can do just about anything! lol.

 

[gink595]

I remember reading something about squish velocity and it said that with small bore motors the squish velocity is better with a flat wide squish band. ONce you got into bigger motors it was better with a tapered squish. I suppose fuel make s a difference in there too.

 

[morgaj1]

I'm just a hack that likes pie.

I do taper the squish band a fuzz though

Here we go with the rhubarb stuff again... Hey, I got some blueberries ready to pick. You want me to get the wife to make you a cobbler?

 

[parrisw]

Well if you can bend the laws of physics, I believe you can do just about anything! lol.

Ok, I'll refrain from posting my thoughts then. Nobody gets ahead without thinking and experimenting.

Don't see you posting much useful info around here.

 

[Jacob J.]

Ok, I'll refrain from posting my thoughts then. Nobody gets ahead without thinking and experimenting.

Don't see you posting much useful info around here.

I doubt that Simon moves out of his comfort zone much anymore.

 

[Simonizer]

I doubt that Simon moves out of his comfort zone much anymore.

My comfort zone does not include teaching physics to mechanics. It is like trying to fill a 45 gallon-drum with an eye-dropper.

 

[morgaj1]

My comfort zone does not include teaching physics to mechanics. It is like trying to fill a 45 gallon-drum with an eye-dropper.

Wow, what a bunch of self righteous BS. I think these hillbillies (which is what is said before you edited it), build some fine saws. It is through experimentation and vocalizing thought processes that information is shared and gained. But, I guess you have all the knowledge you need, so this discourse is beneath you :msp_tongue:

 

[Jacob J.]

My comfort zone does not include teaching physics to mechanics. It is like trying to fill a 45 gallon-drum with an eye-dropper.

Every time there's a discussion (dating back to when you first joined here) of two-cycle theory, you only offer ridicule and self-aggrandizing malarkey. I have never once seen you say something technically proficient or enlightening.

 

[Simonizer]

Wow, what a bunch of self righteous BS. I think these hillbillies (which is what is said before you edited it), build some fine saws. It is through experimentation and vocalizing thought processes that information is shared and gained. But, I guess you have all the knowledge you need, so this discourse is beneath you :msp_tongue:

I changed it because Will is not a hillbilly. I have modded over 2000 saws now and I do agree trial and error is important in perfecting a design. I am very pompous when it comes to physics and misconceptions because most clowns stick by their beliefs. Will backed down and admitted he jumped the gun. That is admirable and he is welcome here in August to eat steak and drink cold beer during our loggersports. I am trying to become a less arrogant person but it is difficult sometimes.

 

[Simonizer]

Every time there's a discussion (dating back to when you first joined here) of two-cycle theory, you only offer ridicule and self-aggrandizing malarkey. I have never once seen you say something technically proficient or enlightening.

What would you like to know for free?